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 THE WORKS 
 
 OB' 
 
 FRANCIS MAITLAND BALFOUR 
 
 VOL. I. 
 
 SEPARATE MEMOIRS. 
 
 Uottiron : 
 
 MACMILLAN AND CO. 
 1885 
 
THE WORKS 
 
 OF 
 
 FRANCIS MAITLAND BALFOUR. 
 
 VOL. I. 
 
 jttemorfal Oftu'ttom 
 
PRINTED BY C. J. CLAY, M.A. AND SON, 
 AT THE UNIVERSITY PRESS. 
 
Memorial (Station, 
 
 THE WORKS 
 
 OF 
 
 FRANCIS MAITLAND BALFOUR, 
 
 M.A., LL.D., F.R.S., 
 
 FELLOW OF TRINITY COLLEGE, 
 
 AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF 
 
 CAMBRIDGE. 
 
 EDITED BY 
 M. FOSTER, F.R.S., 
 
 PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE; 
 
 AND 
 
 ADAM SEDGWICK, M.A., 
 
 FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE. 
 
 VOL. I. 
 
 SEPARATE MEMOIRS. 
 
 MACMILLAN AND CO. 
 1885 
 
[The Right of Translation is reserved.] 
 
5\2 
 v. I 
 
 PREFACE. 
 
 UPON the death of Francis Maitland Balfour, a desire 
 very naturally arose among his friends and admirers to 
 provide some memorial of him. And, at a public meet- 
 ing held at Cambridge in October 1882, the Vice- 
 Chancellor presiding, and many distinguished men of 
 science being present, it was decided to establish a 
 ' Balfour Fund ' the proceeds of which should be applied : 
 firstly to maintain a studentship, the holder of which 
 should devote himself to original research in Biology, 
 especially in Animal Morphology, and secondly, ' by 
 occasional grants of money, to further in other ways 
 original research in the same subject '. The sum of 
 ^8446 was subsequently raised; this was, under certain 
 conditions, entrusted to and accepted by the University of 
 Cambridge; and the first 'Balfour student' was appointed 
 in October 1883. 
 
 The publication of Balfour's works in a collected form 
 was not proposed as an object on which part of the fund 
 should be expended, since his family had expressed their 
 wish to take upon themselves the charge of arranging for 
 a memorial edition of their brother's scientific writings. 
 
 B. b 
 
 M345578 
 
PREFACE. 
 
 That edition, with no more delay than circumstances 
 have rendered necessary, is now laid before the public. 
 It comprises four volumes. 
 
 The first volume contains, in chronological order, 
 all Balfour's scattered original papers, including those 
 published by him in conjunction with his pupils, as well 
 as the Monograph on the Elasmobranch Fishes. The 
 last memoir in the volume, that on the Anatomy and 
 Development of Peripatus Capensis, was published after 
 his death, from his notes and drawings, with additions 
 by Prof. Moseley and Mr Adam Sedgwick, who prepared 
 the manuscript for publication. To the volume is pre- 
 fixed an introductory biographical notice. 
 
 The second and third volumes are the two volumes of 
 the Comparative Embryology reprinted from the original 
 edition without alteration, save the correction of obvious 
 misprints and omissions. 
 
 The fourth volume contains the plates illustrating the 
 memoirs contained in Vol. I. We believe that we are 
 consulting the convenience of readers in adopting this 
 plan, rather than in distributing the plates among the 
 memoirs to which they belong. To assist the reader the 
 explanations of these plates have been given twice : at 
 the end of the memoir to which they belong (in the 
 case of the Monograph on Elasmobranch Fishes at the 
 end of each separate chapter), and in the volume of 
 plates. 
 
 All the figures of these plates had to be redrawn on 
 the stone, and our best thanks are due to the Cambridge 
 Scientific Instrument Company for the pains which they 
 have taken in executing this work. We are also indebted 
 to the Committee of Publication of the Zoological Society 
 for the gift of electrotypes of the wood-cuts illustrating 
 memoir no. xx. of Vol. i. 
 
PREFACE. iii 
 
 Several photographs of Balfour, taken at different 
 times of his life, the last shortly before his death, are in 
 the possession of his relatives and friends ; but these, in 
 the opinion of many, leave much to be desired. 
 
 There is also a portrait of him in oils painted since 
 his death by Mr John Collier, A.R.A., and Herr Hilde- 
 brand of Florence has executed a posthumous bust in 
 bronze*. The portrait which forms the frontispiece of 
 Vol. i. has been drawn on stone by Mr E. Wilson of 
 the Cambridge Scientific Instrument Company, after the 
 latest photograph. Should it fail, in the eyes of those 
 who knew Balfour well, to have reproduced with com- 
 plete success his features and expression, we would ven- 
 ture to ask them to bear in mind the acknowledged 
 difficulties of posthumous portraiture. 
 
 * In possession of the family. Copies also exist in the Library of 
 Trinity College, and in the Morphological Laboratory, at Cambridge. 
 
TABLE OF CONTENTS. 
 
 PAGE 
 
 PREFACE i 
 
 INTRODUCTION i 
 
 1872 
 
 I. On some points in the Geology of the East Lothian 
 
 Coast. By G. W. and F. M. BALFOUR ... 25 
 
 1873 
 
 II. The development and growth of the layers of the blasto- 
 derm. With Plate i 29 
 
 III. On the disappearance of the Primitive Groove in the 
 
 Embryo Chick. With Plate I 41 
 
 IV. The development of the blood-vessels of the Chick. 
 
 With Plate 2 47 
 
 1874 
 
 V. A preliminary account of the development of the Elasmo- 
 
 branch Fishes. With Plates 3 and 4 ... 60 
 
 1875 
 
 VI. A comparison of the early stages in the development of 
 
 Vertebrates. With Plate 5 112 
 
 VII. On the origin and history of the urinogenital organs of 
 
 Vertebrates 135 
 
 VIII. On the development of the spinal nerves in Elasmobranch 
 
 Fishes. With Plates 22 and 23 .... 168 
 
VI 
 
 TABLE OF CONTENTS. 
 
 1876 
 IX. On the spinal nerves of Amphioxus . 
 
 187678 
 
 X. A Monograph on the development of Elasmobranch 
 Fishes. With Plates 621 
 
 1878 
 
 XI. On the phenomena accompanying the maturation and 
 impregnation of the ovum ...... 
 
 XII. On the structure and development of the vertebrate ovary. 
 With Plates 24, 25, 26 
 
 1879 
 
 XIII. On the existence of a Head-kidney in the Embryo Chick, 
 
 and on certain points in the development of the 
 Miillerian duct. By F. M. BALFOUR and A. SEDGWICK. 
 With Plates 27 and 28 
 
 XIV. On the early development of the Lacertilia, together with 
 
 some observations on the nature and relations of the 
 
 primitive Streak. With Plate 29 .... 
 
 XV. On certain points in the Anatomy of Peripatus Capensis . 
 
 XVI. On the morphology and systematic position of the 
 
 Spongida 
 
 1880 
 
 XVII. Notes on the development of the Araneina. With Plates 
 
 30, 3i, 32 
 
 XVIII. On the spinal nerves of Amphioxus 
 
 XIX. Address to the Department of Anatomy and Physiology 
 of the British Association for the Advancement of 
 Science 
 
 PAGE 
 197 
 
 203 
 
 521 
 549 
 
 618 
 
 644 
 
 657 
 
 661 
 
 668 
 696 
 
 1881 
 
 XX. On the development of the skeleton of the paired fins of 
 Elasmobranchii, considered in relation to its bearings 
 on the nature of the limbs of the Vertebrata. With 
 
 Plate 33 714 
 
 XXI. On the evolution of the Placenta, and on the possibility of 
 employing the characters of the Placenta in the classi- 
 fication of the Mammalia 734 
 
TABLE OF CONTENTS. Vll 
 
 PAGE 
 
 1882 
 
 XXII. On the structure and development of Lepidosteus. By 
 F. M. BALFOUR and W. N. PARKER. With Plates 
 3442 "738 
 
 XXIII. On the nature of the organ in Adult Teleosteans and 
 
 Ganoids which is usually regarded as the Head-kidney 
 
 or Pronephros 848 
 
 XXIV. A renewed study of the germinal layers of the Chick. By 
 
 F. M. BALFOUR and F. DEIGHTON. With Plates 
 
 43, 44, 45 8 54 
 
 POSTHUMOUS, 1883 
 
 XXV. The Anatomy and Development of Peripatus Capensis. 
 Edited by H. N. MOSELEY and A. SEDGWICK. With 
 Plates 4653 871 
 
FRANCIS MAITLAND BALFOUR, the sixth child and third 
 son of James Maitland Balfour of Whittinghame, East Lothian, 
 and Lady Blanche, daughter of the second Marquis of Salisbury, 
 was born at Edinburgh, during a temporary stay of his parents 
 there, on the loth November, 1851. He can hardly be said to 
 have known his father, who died of consumption in 1856, at the 
 early age of thirty-six, and who spent the greater part of the last 
 two years of his life at Madeira, separated from the younger chil- 
 dren who remained at home. He fancied at one time that he had 
 inherited his father's constitution ; and this idea seems to have 
 spurred him on to achieve early what he had to do. But, 
 though there was a period soon after he went to College, during 
 which he seemed delicate, and the state of his health caused 
 considerable anxiety to his friends, he eventually became fairly 
 robust, and that in spite of labours which greatly taxed his 
 strength. 
 
 The early years of his life were spent chiefly at Whitting- 
 hame under the loving care of his mother. She made it a point 
 to attempt to cultivate in all her children some taste for natural 
 science, especially for natural history, and in this she was 
 greatly helped by the boys' tutor, Mr J. W. Kitto. They were 
 encouraged to make collections and to form a museum, and 
 the fossils found in the gravel spread in front of the house 
 served as the nucleus of a geological series. Frank soon be- 
 came greatly interested in these things, and indeed they may be 
 said to have formed the beginnings of his scientific career. At 
 all events there was thus awakened in him a love for geology, 
 which science continued to be his favorite study all through his 
 B. I 
 
INTRODUCTION. 
 
 boyhood, and interested him to the last. He was most assiduous 
 in searching for fossils in the gravel and elsewhere, and so great 
 was his love for his collections that while as yet quite a little 
 boy the most delightful birthday present he could think of was 
 a box with trays and divisions to hold his fossils and specimens. 
 His mother, thinking that his fondness for fossils was a passing 
 fancy and that he might soon regret the purchase of the box, 
 purposely delayed the present. But he remained constant to 
 his wish and in time received his box. He must at this time 
 have been about seven or eight years old. In the children's 
 museum, which has been preserved, there are specimens labelled 
 with his childish round-hand, such as a piece of stone with the 
 label " marks of some shels ;" and his sister Alice, who was at 
 that time his chief companion, remembers discussing with him 
 one day after the nursery dinner, when he was about nine years 
 old, whether it were better to be a geologist or a naturalist, he 
 deciding for the former on the ground that it was better to do 
 one thing thoroughly than to attempt many branches of science 
 and do them imperfectly. 
 
 Besides fossils, he collected not only butterflies, as do most 
 boys at some time or other, but also birds ; and he with his 
 sister Alice, being instructed in the art of preparing and pre- 
 serving skins, succeeded in making a very considerable collec- 
 tion. He thus acquired before long not only a very large but 
 a very exact knowledge of British birds. 
 
 In the more ordinary work of the school-room he was some- 
 what backward. This may have been partly due to the great 
 difficulty he had in learning to write, for he was not only left- 
 handed but, in his early years, singularly inapt in acquiring 
 particular muscular movements, learning to dance being a great 
 trouble to him. Probably however the chief reason ^ was that he 
 failed to find any interest in the ordinary school studies. He 
 fancied that the family thought him stupid, but this does not 
 appear to have been the case. 
 
 In character he was at this time quick tempered, sometimes 
 even violent, and the energy which he shewed in after life even 
 thus early manifested itself as perseverance, which, when he was 
 crossed, often took on the form of obstinacy, causing at times 
 no little trouble to his nurses and tutors. But he was at the 
 
INTRODUCTION. 
 
 same time warm-hearted and affectionate ; full of strong im- 
 pulses, he disliked heartily and loved much, and in his affections 
 was wonderfully unselfish, wholly forgetting himself in his 
 thought for others, and ready to do things which he disliked to 
 please those whom he loved. Though, as we have said, some- 
 what clumsy, he was nevertheless active and courageous ; in 
 learning to ride he shewed no signs of fear, and boldly put his 
 pony to every jump which was practicable. 
 
 In 1 86 1 he was sent to the Rev. C. G. Chittenden's prepara- 
 tory school at Hoddesden in Hertfordshire, and here the quali- 
 ties which had been already visible at home became still more 
 obvious. He found difficulty not only in writing but also in 
 spelling, and in the ordinary school-work he took but little 
 interest and made but little progress. 
 
 In 1865 he was moved to Harrow and placed in the house 
 of the Rev. F. Rendall. Here, as at Hoddesden, he did not 
 shew any great ability in the ordinary school studies, though as 
 he grew older his progress became more marked. But happily 
 he found at Harrow an opportunity for cultivating that love of 
 scientific studies which was yearly growing stronger in him. 
 Under the care of one of the Masters, Mr G. Griffith, the boys 
 at Harrow were even then taught the elements of natural 
 science. The lessons were at that time, so to speak, extra- 
 academical, carried on out of school hours ; nevertheless, many 
 of the boys worked at them with diligence and even enthusiasm, 
 and among these Balfour became conspicuous, not only by his 
 zeal but by his ability. Griffith was soon able to recognize the 
 power of his new pupil, and thus early began to see that the 
 pale, earnest, somewhat clumsy-handed lad, though he gave no 
 promise of being a scholar in the narrower sense of the word, 
 had in him the makings of a man of science. Griffith chiefly 
 confined his teaching to elementary physics and chemistry with 
 some little geology, but he also encouraged natural history 
 studies and began the formation of a museum of comparative 
 anatomy. Balfour soon began to be very zealous in dissecting 
 animals, and was especially delighted when the Rev. A. C. 
 Eaton, the well-known entomologist, on a visit to Harrow, 
 initiated Griffith's pupils in the art of dissecting under water. 
 The dissection of a caterpillar in this way was probably an 
 
 I 2 
 
INTRODUCTION. 
 
 epoch in Balfour's life. Up to that time his rough examination 
 of such bodies had revealed to him nothing more than what in 
 school-boy language he spoke of as " squash ;" but when under 
 Eaton's deft hands the intricate organs of the larval Arthropod 
 floated out under water and displayed themselves as a labyrinth 
 of threads and sheets of silvery whiteness a new world of obser- 
 vation opened itself up to Balfour, and we may probably date 
 from this the beginning of his exact morphological knowledge. 
 
 While thus learning the art of observing, he was at the same 
 time developing his power of thinking. He was by nature fond 
 of argument, and defended with earnestness any opinions which 
 he had been led to adopt. He was very active in the Harrow 
 Scientific Society, reading papers, taking part in the discussions, 
 and exhibiting specimens. He gained in 1867 a prize for an 
 essay on coal, and when, in 1868, Mr Leaf offered a prize (a micro- 
 scope) "for the best account of some locality visited by the writer 
 during the Easter Holidays," two essays sent in, one by Balfour, 
 the other by his close friend, Mr Arthur Evans, since well known 
 for his researches in Illyria, were found to be of such unusual 
 merit that Prof. Huxley was specially requested to adjudicate 
 between them. He judged them to be of equal merit, and a 
 prize was given to each. The subject of Balfour's essay was 
 " The Geology and Natural History of East Lothian." When 
 biological subjects were discussed at the Scientific Society, 
 Balfour appears to have spoken as a most uncompromising 
 opponent of the views of Mr Charles Darwin, little thinking that 
 in after life his chief work would be to develope and illustrate 
 the doctrine of evolution. 
 
 The years at Harrow passed quickly away, Balfour making 
 fair, but perhaps not more than fair, progress in the ordinary 
 school learning. In due course however he reached the upper 
 sixth form, and in his last year, became a monitor. At the 
 same time his exact scientific knowledge was rapidly increasing. 
 Geology still continued to be his favorite study, and in this he 
 made no mean progress. During his last years at Harrow he 
 and his brother Gerald worked out together some views concern- 
 ing the geology of their native county. These views they 
 ultimately embodied in a paper, which was published in their 
 joint names in the Geological Magazine for 1872, under the title 
 
INTRODUCTION. 5 
 
 of " Some Points in the Geology of the East Lothian Coast," 
 and which was in itself a work of considerable promise. Geology 
 however was beginning to find a rival in natural history. Much 
 of his holiday time was now spent in dredging for marine animals 
 along the coast off Dunbar. Each specimen thus obtained was 
 carefully determined and exact records were kept of the various 
 ' finds,' so that the dredgings (which were zealously continued 
 after he had left Harrow and gone to Cambridge) really con- 
 stituted a serious study of the fauna of this part of the coast. 
 They also enabled him to make a not inconsiderable collection 
 of shells, in the arrangement of which he was assisted by 
 his sister Evelyn, of Crustacea and of other animals. 
 
 Both to the masters and to his schoolfellows he became known 
 as a boy of great force of character. Among the latter his scrupu- 
 lous and unwavering conscientiousness made him less popular 
 perhaps than might have been expected from his bright kindly 
 manner and his unselfish warmheartedness. In the incidents of 
 school life a too strict conscience is often an inconvenience, and 
 the sternness and energy with which Balfour denounced acts of 
 meanness and falsehood were thought by some to be unnecessarily 
 great. He thus came to be feared rather than liked by many, 
 and comparatively few grew to be sufficiently intimate with him 
 to appreciate the warmth of his affections and the charm of his 
 playful moments. 
 
 At the Easter of 1870 he passed the entrance examination 
 at Trinity College, Cambridge, and entered into residence in the 
 following October. His college tutor was Mr J. Prior, but he 
 was from the first assisted and guided in his studies by his 
 friend, Mr Marlborough Pryor, an old Harrow boy, who in the 
 same October had been, on account of his distinction in Natural 
 Science, elected a Fellow of the College, in accordance with 
 certain new regulations which then came into action for the first 
 time, and which provided that every three years one of the 
 College Fellowships should be awarded for excellence in some 
 branch or branches of Natural Science, as distinguished from 
 mathematics, pure or mixed. During the whole of that year 
 and part of the next Mr Marlborough Pryor remained in resi- 
 dence, and his influence in wisely directing Balfour's studies had 
 a most beneficial effect on the latter's progress. 
 
INTRODUCTION. 
 
 During his first term Balfour was occupied in preparation 
 for the Previous Examination ; and this he successfully passed at 
 Christmas. After that he devoted himself entirely to Natural 
 Science, attending lectures on several branches. During the 
 Lent term he was a very diligent hearer of the lectures on 
 Physiology which I was then giving as Trinity Praelector, 
 having been appointed to that post in the same October that 
 Balfour came into residence. At this time he was not very 
 strong, and I remember very well noticing among my scanty 
 audience, a pale retiring student, whose mind seemed at times 
 divided between a desire to hear the lecture and a feeling that 
 his frequent coughing was growing an annoyance to myself 
 and the class. This delicate-looking student, I soon learnt, was 
 named Balfour, and when the Rev. Coutts Trotter, Mr Pryor 
 and myself came to examine the candidates for the Natural 
 Science Scholarships which were awarded at Easter, we had no 
 difficulty in giving the first place to him. In point of knowledge, 
 and especially in the thoughtfulness and exactitude displayed in 
 his papers and work, he was very clearly ahead of his com- 
 petitors. 
 
 During the succeeding Easter term and the following winter 
 he appears to have studied physics, chemistry, geology and 
 comparative anatomy, both under Mr Marlborough Pryor and 
 by means of lectures. He also continued to attend my lectures, 
 but though I gradually got to know him more and more we 
 did not become intimate until the Lent term of 1872. He had 
 been very much interested in some lectures on embryology 
 which I had given, and, since Marlborough Pryor had left or was 
 about to leave Cambridge, he soon began to consult me a good 
 deal about his studies. He commenced practical histological 
 and embryological work under me, and I remember very vividly 
 that one day when we were making a little excursion in search 
 of nests and eggs of the stickleback in order that he might study 
 the embryology of fishes, he definitely asked my opinion as 
 to whether he might take up a scientific career with a fair chance 
 of success. I had by this time formed a very high opinion 
 of his abilities, and learning then for the first time that he had 
 an income independent of his own exertions, my answer was 
 very decidedly a positive one. Soon after, feeling more and 
 
INTRODUCTION. 
 
 more impressed with his power and increasingly satisfied both 
 with his progress in biological studies and his sound general 
 knowledge of other sciences, anxious also, it may be, at the 
 same time that as much original inquiry as possible should be 
 carried on at Cambridge in my department, I either suggested 
 to him or acquiesced in his own suggestion that he should at 
 once set to work on some distinct research ; and as far as I 
 remember the task which I first proposed to him was an investi- 
 gation of the layers of the blastoderm in the chick. It must 
 have been about the same time that I proposed to him to join 
 me in preparing for publication a small work on Embryology, 
 the materials for this I had ready to hand in a rough form as 
 lectures which I had previously given. To this proposal he 
 enthusiastically assented, and while the lighter task of writing 
 what was to be written fell to me, he undertook to work over 
 as far as was possible the many undetermined points and un- 
 satisfactory statements across which we were continually coming. 
 
 During his two years at College his health had improved ; 
 though still hardly robust and always in danger of overwork- 
 ing himself, he obviously grew stronger. He rejoiced exceed- 
 ingly in his work, never tiring of it, and was also making his 
 worth felt among his fellow students, and especially perhaps 
 among those of his own college whose studies did not lie in 
 the same direction as his own. At this time he must have 
 been altogether happy, but a sorrow now came upon him. His 
 mother, to whom he was passionately attached, and to whose 
 judicious care in his early days not only the right development 
 of his strong character but even his scientific leanings were 
 due, had for some time past been failing in health, though her 
 condition caused no immediate alarm. In May 1872, however, 
 she died quite suddenly from unsuspected heart disease. Her 
 loss was a great blow to him, and for some time afterward I 
 feared his health would give way ; but he bore his grief quietly 
 and manfully and threw himself with even increased vigour 
 into his work. 
 
 During the academic session of 1872 3, he continued steadily 
 at work at his investigations, and soon began to make rapid 
 progress. At the beginning he had complained to me about 
 what he considered his natural clumsiness, and expressed a fear 
 
INTRODUCTION. 
 
 that he should never be able to make satisfactory microscopic 
 sections ; as to his being able to make drawings of his dissec- 
 tions and microscopical preparations, he looked upon that at 
 first as wholly impossible. I need hardly say that in time he 
 acquired great skill in the details of microscopical technique, 
 and that his drawings, if wanting in so-called artistic finish, were 
 always singularly true and instructive. While thus struggling 
 with the details which I could teach him, he soon began to 
 manifest qualities which no teacher could give him. I remember 
 calling his attention to Dursy's paper on the Primitive Streak, 
 and suggesting that he should work the matter over, since if 
 such a structure really existed, it must, most probably, have 
 great morphological significance. I am free to confess that I 
 myself rather doubted the matter, and a weaker student might 
 have been influenced by my preconceptions. Balfour, however, 
 thus early had the power of seeing what existed and of refusing 
 to see what did not exist. He was soon able to convince me 
 that Dursy's streak was a reality, and the complete working 
 out of its significance occupied his thoughts to the end of his 
 days. 
 
 The results of these early studies were made known in three 
 papers which appeared in the Quarterly Journal of Microscopical 
 Science for July 1873, and will be found in the beginning of this 
 volume. The summer and autumn of that year were spent partly 
 in a visit to Finland, in company with his friend and old school- 
 fellow Mr Arthur Evans, and partly in formal preparation for the 
 approaching Tripos examination. Into this preparation Balfour 
 threw himself with characteristic energy, and fully justified my 
 having encouraged his spending so much of the preceding time 
 in original research, not only by the rapidity with which he 
 accumulated the stock of knowledge of various kinds necessary 
 for the examination but also by the manner in which he acquitted 
 himself at the trial itself. At that time the position of the 
 candidates in the Natural Sciences Tripos was determined by 
 the total number of marks, and Balfour was placed second, the 
 first place being gained by H. Newell Martin of Christ's College, 
 now Professor at Baltimore, U.S.A. In the examination, in 
 which I took part, Balfour did not write much, and he had 
 not yet learnt the art of putting his statements in the best 
 
INTRODUCTION. 
 
 possible form ; he won his position chiefly by the firm thought 
 and clear insight which was present in almost all his answers. 
 
 The examination was over in the early days of Dec. 1873 
 and Balfour was now free to devote himself wholly to his 
 original work. Happily, the University had not long before 
 secured the use of two of the tables at the then recently founded 
 Stazione Zoologica at Naples. And upon the nomination of the 
 University, Balfour, about Christmas, started for Naples in 
 company with his friend Mr A. G. Dew-Smith, also of Trinity 
 College. The latter was about to carry on some physiological 
 observations ; Balfour had set himself to work out as completely 
 as he could the embryology of Elasmobranch fishes, about which 
 little was at that time known, but which, from the striking 
 characters of the adult animals could not help proving of in- 
 terest and importance. 
 
 From his arrival there at Christmas 1873 until he left in 
 June 1874, he worked assiduously, and with such success, that 
 as the result of the half-year's work he had made a whole series 
 of observations of the greatest importance. Of these perhaps 
 the most striking were those on the development of the urogenital 
 organs, on the neurenteric canal, on the development of the 
 spinal nerves, on the formation of the layers and on the phe- 
 nomena of segmentation, including a history of the behaviour 
 of nuclei in cell division. He returned home laden with facts 
 and views both novel and destined to influence largely the 
 progress of embryology. 
 
 In August of the same year he attended the meeting of 
 the British Association for the Advancement of Science at 
 Belfast ; and the account he then gave of his researches formed 
 one of the most important incidents at the Biological Section 
 on that occasion. 
 
 In the September of that year the triennial fellowship for 
 Natural Science was to be awarded at Trinity College, and 
 Balfour naturally was a candidate. The election was, according 
 to the regulations, to be determined partly by the result of an 
 examination in various branches of science, and partly by such 
 evidence of ability and promise as might be afforded by original 
 work, published or in manuscript. He spent the remainder of 
 the autumn in preparation for this examination. But when the 
 
10 INTRODUCTION. 
 
 examination was concluded it was found that in his written 
 answers he had not been very successful ; he had not even acquitted 
 himself so well as in the Tripos of the year before, and had the 
 election been determined by the results of the examination 
 alone, the examiners would have been led to choose the gentle- 
 man who was Balfour's only competitor. The original work 
 however which Balfour sent in, including a preliminary account 
 of the discoveries made at Naples, was obviously of so high a 
 merit and was spoken of in such enthusiastic terms by the 
 External Referee Prof. Huxley, that the examiners did not hesi- 
 tate for a moment to neglect altogether the formal written 
 answers (and indeed the papers of questions were only intro- 
 duced as a safeguard, or as a resource in case evidence of 
 original power should be wanted) and unanimously recom- 
 mended him for election. Accordingly he was elected Fellow 
 in the early days of October. 
 
 Almost immediately after, the little book on Embryology 
 appeared, on which he and I had been at work, he doing 
 his share even while his hands and mind were full of the Elas- 
 mobranch inquiry. The title-page was kept back some little 
 time in order that his name might appear on it with the 
 addition of Fellow of Trinity, a title of which he was then, and 
 indeed always continued to be, proud. He also published in 
 the October number of the Quarterly Journal of Microscopi- 
 cal Science a preliminary account of his Elasmobranch re- 
 searches. 
 
 He and his friends thought that after these almost inces- 
 sant labours, and . the excitement necessarily contingent upon 
 the fellowship election, he needed rest and change. Ac- 
 cordingly on the 1 7th of October he started with his friend 
 Marlborough Pryor on a voyage to the west coast of South 
 America. They travelled thither by the Isthmus of Panama, 
 visited Peru and Chili, and returned home along the usual 
 route by the Horn ; reaching England some time in Feb. 
 
 I875- 
 
 Refreshed by this holiday, he now felt anxious to complete 
 as far as possible his Elasmobranch work, and very soon after 
 his return home, in fact in March, made his way again to 
 Naples, where he remained till the hot weather set in in May. 
 
INTRODUCTION. 1 1 
 
 On his return to Cambridge, he still continued working on 
 the Elasmobranchs, receiving material partly from Naples, 
 partly from the Brighton Aquarium, the then director of which, 
 Mr Henry Lee, spared no pains to provide him both with embryo 
 and adult fishes. While at Naples, he communicated to the 
 Philosophical Society at Cambridge a remarkable paper on 
 "The Early Stages of Vertebrates," which was published in 
 full in the Quarterly Journal of Microscopical Science, July, 
 1875 ; he also sent me a paper on "The Development of 
 the Spinal Nerves", which I communicated to the Royal 
 Society, and which was subsequently published in the Phi- 
 losophical Transactions of 1876. He further wrote in the course 
 of the summer and published in the Journal of Anatomy and 
 Physiology in October, 1875, a detailed account of his "Obser- 
 vations and Views on the Development of the Urogenital 
 Organs." 
 
 Some time in August of the same year he started in 
 company with Mr Arthur Evans and Mr J. F. Bullar for a 
 second trip to Finland, the travellers on this occasion making 
 their way into regions very seldom visited, and having to 
 subsist largely on the preserved provisions which they carried 
 with them, and on the produce of their rods and guns. From 
 a rough diary which Balfour kept during this trip it would 
 appear that while enjoying heartily the fun of the rough tra- 
 velling, he occupied himself continually with observations on 
 the geology and physical phenomena of the country, as well 
 as on the manners, antiquities, and even language of the 
 people. It was one of his characteristic traits, a mark of the 
 truly scientific bent of his mind, of his having, as Dohrn soon 
 after Balfour's first arrival at Naples said, 'a real scientific 
 head,' that every thing around him wherever he was, incited 
 him to careful exact observation, and stimulated him to 
 thought. 
 
 In the early part of the Long Vacation of the same year 
 he had made his first essay in lecturing, having given a short 
 course on Embryology in a room at the New Museums, 
 which I then occupied as a laboratory. Though he afterwards 
 learnt to lecture with great clearness he was not by nature 
 a fluent speaker, and on this occasion he was exceedingly 
 
12 INTRODUCTION. 
 
 nervous. But those who listened to him soon forgot these 
 small defects as they began to perceive the knowledge and 
 power which lay in their new teacher. 
 
 Encouraged by the result of this experiment, he threw 
 himself, in spite of the heavy work which the Elasmobranch 
 investigation was entailing, with great zeal into an arrange- 
 ment which Prof. Newton, Mr J. W. Clark and myself had 
 in course of the summer brought about, that he and Mr A. 
 Milnes Marshall, since Professor at Owens College, Manchester, 
 should between them give a course on Animal Morphology, 
 with practical instruction, Prof. Newton giving up a room in 
 the New Museums for the purpose. 
 
 In the following October (1875) upon Balfour's return from 
 Finland, these lectures were accordingly begun and carried 
 on by the two lecturers during the Michaelmas and Lent 
 Terms. The number of students attending this first course, 
 conducted on a novel plan, was, as might be expected, small, 
 but the Lent Term did not come to an end before an en- 
 thusiasm for morphological studies had been kindled in the 
 members of the class. 
 
 The ensuing Easter term (1876) was spent by Balfour at 
 Naples, in order that he might carry on towards completion 
 his Elasmobranch work. He had by this time determined 
 to write as complete a monograph as he could of the develop- 
 ment of these fishes, proposing to publish it in instalments 
 in the Journal of Anatomy and Physiology, and subsequently 
 to gather together the several papers into one volume. The 
 first of these papers, dealing with the ovum, appeared in Jan. 
 1876; most of the numbers of the Journal during that and 
 the succeeding year contained further portions ; but the com- 
 plete monograph did not leave the publisher's hands until 1878. 
 
 He returned to England with his pupil and friend Mr J. F. 
 Bullar some time in the summer ; on their way home they 
 passed through Switzerland, and it was during the few days which 
 he then spent in sight of the snow-clad hills that the begin- 
 nings of a desire for that Alpine climbing, which was destined 
 to be so disastrous, seem to have been kindled in him. 
 
 In October, 1876, he resumed the lectures on Morphology, 
 taking the whole course himself, his colleague, Mr Marshall, 
 
INTRODUCTION. 13 
 
 having meanwhile left Cambridge. Indeed, from this time on- 
 ward, he may be said to have made these lectures, in a certain 
 sense, the chief business of his life. He lectured all three terms, 
 devoting the Michaelmas and Lent terms to a systematic course 
 of Animal Morphology, and the Easter term to a more eTerfient- 
 ary course of Embryology. These lectures were given under 
 the auspices of Prof. Newton ; but Balfour's position was before 
 long confirmed by his being made a Lecturer of Trinity College, 
 the lectures which he gave at the New Museums, and which 
 were open to all students of the University, being accepted in a 
 liberal spirit by the College as equivalent to College Lectures. 
 He very soon found it desirable to divide the morphological 
 course into an elementary and an advanced course, and to 
 increase the number of his lectures from three to four a week. 
 Each lecture was followed by practical work, the students dis- 
 secting and examining microscopically, an animal or some 
 animals chosen as types to illustrate the subject-matter of the 
 lecture ; and although Balfour had the assistance at first of 
 one 1 , and ultimately of several demonstrators, he himself 
 put his hand to the plough, and after the lecture always spent 
 some time in the laboratory among his pupils. Had Balfour 
 been only an ordinary man, the zeal and energy which he threw 
 into his lectures, and into the supervision of the practical work, 
 added to the almost brotherly interest which he took in the 
 individual development of every one of the pupils who shewed 
 any love whatever for the subject, would have made him a most 
 successful teacher. But his talents and powers were such as 
 could not be hid even from beginners. His extensive and 
 exact knowledge, the clearness with which in spite of, or shall I 
 not rather say, by help of a certain want of fluency, he explained 
 difficult and abstruse matters, the trenchant way in which he lay 
 bare specious fallacies, and the presence in almost his every word 
 of that power which belongs only to the man who has thought 
 out for himself everything which he says, these things aroused 
 and indeed could hardly fail to arouse in his hearers feelings 
 which, except in the case of the very dullest, grew to be those of 
 
 1 His first Demonstrator up to Christmas 1877, was Mr J. F. Bullar. In Jan. 
 1878, Mr Adam Sedgwick took the post of Senior Demonstrator, and held it until 
 Balfour's death. 
 
14 INTRODUCTION. 
 
 enthusiasm. His class, at first slowly, but afterwards more 
 rapidly, increased in numbers, and, what is of more import- 
 ance, grew in quality. The room allotted to him soon became 
 far too small and steps were taken to provide for him, for 
 myself, whose wants were also urgent, and for the biological 
 studies generally, adequate accommodation ; but it was not 
 until Oct. 1877 that we were able to take possession of the 
 new quarters. 
 
 Even this new accommodation soon became insufficient, 
 and in the spring of 1 882 a new morphological laboratory was 
 commenced in accordance with plans suggested by himself. 
 He was to have occupied them in the October term, 1883, but 
 did not live to see them finished. 
 
 As might have been expected from his own career, he 
 regarded the mere teaching of what is known as a very small 
 part of his duties as Lecturer ; and as soon as any of his pupils 
 became sufficiently advanced, he urged or rather led them to 
 undertake original investigations ; and he had the satisfaction 
 before his death of seeing the researches of his pupils (such 
 as those by Messrs. Bullar, Sedgwick, Mitzikuri, Haddon, Scott, 
 Osborne, Caldwell, Heape, Weldon, Parker, Deighton and others) 
 carried to a successful end. In each of these inquiries he himself 
 took part, sometimes a large part, generally suggesting the 
 problem to be solved, indicating the methods, and keeping a 
 close watch over the whole progress of the study. Hence in 
 many cases the published account bears his name as well as 
 that of the pupil. 
 
 In the year 1878 his Monograph on Elasmobranch Fishes was 
 published as a complete volume, and in the same year he received 
 the honour of being elected a Fellow of the Royal Society, 
 a distinction which now-a-days does not often fall to one so 
 young. No sooner was the Monograph completed than in 
 spite of the labours which his lectures entailed, he set himself 
 to the great task of writing a complete treatise on Comparative 
 Embryology. This not only laid upon him the heavy burden 
 of gathering together the observations of others, enormous in 
 number and continually increasing, scattered through many 
 journals and books, and recorded in many different languages, 
 as well as of putting them in orderly array, and of winnowing 
 
INTRODUCTION. 15 
 
 out the grain from the chaff (though his critical spirit found 
 some relief in the latter task), but also caused him much labour, 
 inasmuch as at almost every turn new problems suggested them- 
 selves, and demanded inquiry before he could bring his mind 
 to writing about them. This desire to see his way straight 
 before him, pursued him from page to page, and while it has 
 resulted in giving the book an almost priceless value, made 
 the writing of it a work of vast labour. Many of the ideas 
 thus originated served as the bases of inquiries worked out by 
 himself or his pupils, and published in the form of separate 
 papers, but still more perhaps never appeared either in the 
 book or elsewhere and were carried with him undeveloped and 
 unrecorded to the grave. 
 
 The preparation of this work occupied the best part of his 
 time for the next three years, the first volume appearing in 
 1880, the second in 1881. 
 
 In the autumn of 1880, he attended the Meeting at Swansea 
 of the British Association for the Advancement of Science, 
 having been appointed Vice-President of the Biological Sec- 
 tion with charge of the Department of Anatomy and Physio- 
 logy. At the Meetings of the Association, especially of late 
 years, much, perhaps too much, is expected in the direction 
 of explaining the new results of science in a manner inter- 
 esting to the unlearned. Popular expositions were never 
 very congenial to Balfour, his mind was too much occupied 
 with the anxiety of problems yet to be solved ; he was there- 
 fore not wholly at his ease, in his position on this occasion. 
 Yet his introductory address, though not of a nature to interest 
 a large mixed audience, was a luminous, brief exposition of 
 the modern development and aims of embryological investi- 
 gation. 
 
 During these years of travail with the Comparative Em- 
 bryology the amount of work which he got through was a 
 marvel to his friends, for besides his lectures, and the re- 
 searches, and the writing of the book, new labours were de- 
 manded of him by the University for which he was already 
 doing so much. Men at Cambridge, and indeed elsewhere as 
 well, soon began to find out that the same clear insight which 
 was solving biological problems could be used to settle knotty 
 
16 INTRODUCTION. 
 
 questions of policy and business. Moreover he united in a 
 remarkable manner, the power of boldly and firmly asserting 
 and maintaining his own views, with a frank courteousness 
 which went far to disarm opponents. Accordingly he found 
 himself before long a member of various Syndicates, and indeed 
 a very great deal of his time was thus occupied, especially 
 with the Museums and Library Syndicates, in both of which 
 he took the liveliest interest. Besides these University duties 
 his time and energy were also at the service of his College. 
 In the preparation of the New Statutes, with which about this 
 time the College was much occupied, the Junior Fellows of the 
 College took a conspicuous share, and among these Junior 
 Fellows Balfour was perhaps the most active ; indeed he was 
 their leader, and he threw himself into the investigation of 
 the bearings and probable results of this and that proposed 
 new statute with as much zeal as if he were attacking some 
 morphological problem. 
 
 While he was in the midst of these various labours, his 
 friends often feared for his strength, for though gradually im- 
 proving in health after his first year at Cambridge, he was not 
 robust, and from time to time he seemed on the point of break- 
 ing down. Still, hard as he was working, he was in reality 
 wisely careful of himself, and as he grew older, paid more and 
 more attention to his health, daily taking exercise in the form 
 either of bicycle rides or of lawn-tennis. Moreover he continued 
 to spend some part of his vacations in travel. Combining business 
 with pleasure, he made frequent visits to Germany and France, 
 and especially to Naples. The Christmas of 1876 7 he spent 
 in Greece, that of 1878 9 at Ragusa, where his old school-fellow 
 and friend Mr Arthur Evans was at that time residing, and the 
 appointment of his friend Kleinenberg to a Professorship at 
 Messina led to a journey there. Early in the long vacation of 
 1880, he went with his sister, Mrs H. Sidgwick, and her husband 
 to Switzerland, and was joined there for a short time by his friend 
 and pupil Adam Sedgwick. During this visit he took his first 
 lessons in Alpine climbing, making several excursions, some of 
 them difficult and dangerous ; and the love of mountaineering 
 laid so firm a hold upon him, that he returned to Switzerland 
 later on in the autumn of the same year, in company with his 
 
INTRODUCTION. 
 
 brother Gerald, and spent some weeks near Zermatt in systematic 
 climbing, ascending, among other mountains, the Matterhorn and 
 the Weisshorn. In the following summer, 1 88 1, he and his brother 
 Gerald again visited the Alps, dividing their time between the 
 Chamonix district and the Bernese Oberland. On this occa- 
 sion some of the excursions which they made were of extreme 
 difficulty, and such as needed not only great presence of mind 
 and bodily endurance, but also skilful and ready use of the 
 limbs. As a climber indeed Balfour soon shewed himself 
 fearless, indefatigable, and expert in all necessary movements 
 as well as full of resources and expedients in the face of diffi- 
 culties, so much so that he almost at once took rank among 
 the foremost of distinguished mountaineers. In spite of his 
 apparent clumsiness in some matters, he had even as a lad 
 proved himself to be a bold and surefooted climber. More- 
 over he had been perhaps in a measure prepared for the 
 difficulties of Alpine climbing by his experience in deer- 
 stalking. This sport he had keenly and successfully pur- 
 sued for many years at his brother's place in Rosshire. When 
 however about the year 1877, the question of physiological 
 experiments on animals became largely discussed in public, he 
 felt that to continue the pursuit of this or any other sport 
 involving, for the sake of mere pleasure, the pain and death of 
 animals, was inconsistent with the position which he had warmly 
 taken up, as an advocate of the right to experiment on animals ; 
 and he accordingly from that time onward wholly gave it up. 
 
 His fame as an investigator and teacher, and as a man of 
 brilliant and powerful parts, was now being widely spread. 
 Pupils came to him, not only from various parts of England, 
 but from America, Australia and Japan. At the York Meeting 
 of the British Association for the Advancement of Science, in 
 August, 1 88 1, he was chosen as one of the General Secretaries. 
 In April, 1881, the honorary degree of LL.D. was conferred 
 upon him by the University of Glasgow, and in November of 
 the same year the Royal Society gave him one of the Royal 
 Medals in recognition of his embryological discoveries, and at 
 the same time placed him on its Council. 
 
 At Cambridge he was chosen, in the autumn of 1880, Presi- 
 dent of the Philosophical Society, and in the December of that 
 B. 2 
 
1 8 INTRODUCTION. 
 
 year a brilliant company were gathered together at the Annual 
 Dinner to do honour to their new young President. Otherwise 
 nothing as yet had been done for him in his own University in the 
 way of recognition of his abilities and services ; and he still re- 
 mained a Lecturer of Trinity College, giving lectures in a Uni- 
 versity building. An effort had been made by some of his friends 
 to urge the University to take some step in this direction ; 
 but it was thought at that time impossible to do anything. 
 In 1 88 1 a great loss fell upon the sister University of Oxford 
 in the death of Prof. George Rolleston ; and soon after very 
 vigorous efforts were made to induce Balfour to become a 
 candidate for the vacant chair. The prospect was in many 
 ways a tempting one, and Balfour seeing no very clear way in 
 the future for him at his own University, was at times inclined 
 to offer himself, but eventually he decided to remain at Cam- 
 bridge. Hardly had this temptation if we may so call it been 
 overcome when a still greater one presented itself. Through 
 the lamented death of Sir Wyville Thomson in the winter of 
 1 88 1 2, the chair of Natural History at Edinburgh, perhaps 
 the richest and most conspicuous biological chair in the 
 United Kingdom, became vacant. The post was in many ways 
 one which Balfour would have liked to hold. The teaching 
 duties were it is true laborious, but they had in the past been 
 compressed into a short time, occupying only the summer 
 session and leaving the rest of the year free, and it seemed 
 probable that this arrangement might be continued with him. 
 The large emolument would also have been grateful to him 
 inasmuch as he would have felt able to devote the whole of it 
 to scientific ends ; and the nearness to Whittinghame. his native 
 place and brother's home, added to the attractions ; but what 
 tempted him most was the position which it would have given 
 him, and the opportunities it would have afforded, with the 
 rich marine Fauna of the north-eastern coast close at hand, 
 to develope a large school of Animal Morphology. The existing 
 Professors at Edinburgh were most desirous that he should join 
 them, and made every effort to induce him to come. On the 
 part of the Crown, in whose hands the appointment lay, not 
 only were distinct offers made to him, but he was repeatedly 
 pressed to accept the post. Nor was it until after a considerable 
 
INTRODUCTION. 1 9 
 
 struggle that he finally refused, his love for his own University 
 in the end overcoming the many inducements to leave; he 
 elected to stay where he was, trusting to the future opening 
 up for him some suitable position. In this decision he was 
 undoubtedly influenced by the consideration that Cambridge, 
 besides being the centre of his old friendships, had become as it 
 were a second home for his own family. By the appointment of 
 Lord Rayleigh to the chair of Experimental Physics his sister 
 Lady Rayleigh had become a resident, his sister Mrs Sidgwick had 
 lived there now for some years, and his brother Gerald generally 
 spent the summer there; their presence made Cambridge 
 doubly dear to him. 
 
 At the close of the Michaelmas term, with feelings of relief 
 at having completed his Comparative Embryology, the prepara- 
 tion of the second volume of which had led to almost 
 incessant labour during the preceding year, he started to 
 spend the Christmas vacation with his friend Kleinenberg at 
 Messina. Stopping at Naples on his way thither he found his 
 pupil Caldwell, who had been sent to occupy the University 
 table at the Stazione Zoologica, lying ill at Capri, with what 
 proved to be typhoid fever. The patient was alone, without 
 any friend to tend him, and his mother who had been sent for 
 had not yet arrived. Accordingly Balfour (with the kindness 
 all forgetful of himself which was his mark all his life 
 through) stayed on his journey to nurse the sick man until 
 the mother came. He then went on to Messina, and there 
 seemed to be in good health, amusing. himself with the ascent 
 of Etna. Yet in January, soon after his return home, he com- 
 plained of being unwell, and in due time distinct symptoms of 
 typhoid fever made their appearance. The attack at first pro- 
 mised to be severe, but happily the crisis was soon safely passed 
 and the convalescence was satisfactory. 
 
 While yet on his sick bed, a last attempt was made to 
 induce him to accept the Edinburgh offer, and for the last time 
 he refused. These repeated offers, and the fact that the dangers 
 of his grave illness had led the University vividly to realize 
 how much they would lose if Balfour were taken away from 
 them, encouraged his friends to make a renewed effort to gain 
 for him some adequate position in the University. This time 
 
 2 2 
 
20 INTRODUCTION. 
 
 the attempt was successful, and the authorities took a step, 
 unusual but approved of by the whole body of resident members 
 of the University ; they instituted a new Professorship of 
 Animal Morphology, to be held by Balfour during his life or 
 as long as he should desire, but to terminate at his death or 
 resignation unless it should be otherwise desirable. Accordingly 
 in May, 1882, he was admitted into the Professoriate as Pro- 
 fessor of Animal Morphology. 
 
 During his illness his lectures had been carried on by his 
 Demonstrator, Mr Adam Sedgwick, who continued to take his 
 place during the remainder of that Lent Term and during the 
 ensuing Easter Term. The spring Balfour spent partly in the 
 Channel Islands with his sister Alice, partly in London with 
 his eldest brother, but in the course of the Easter Term 
 returned to Cambridge and resumed his work though not his 
 lectures. His recovery to health was steady and satisfactory, 
 the only drawback being a swelling over the shin-bone of one 
 leg, due to a blow on the rocks at Sark ; otherwise he was 
 rapidly becoming strong. He himself felt convinced that a visit 
 to the Alps, with some mountaineering of not too difficult a 
 kind, would complete his restoration to health. In this view 
 many of his friends coincided ; for the experience of former 
 years had shewn them what a wonderfully beneficial effect the 
 Alpine air and exercise had upon his health. He used to go 
 away pale, thin and haggard, to return bronzed, clear, firm and 
 almost stout ; nor was there anything in his condition which 
 seemed to forbid his climbing, provided that he was cautious 
 at the outset. Accordingly, early in June he left Cambridge 
 for Switzerland, having long ago, during his illness in fact, en- 
 gaged his old guide, Johann Petrus, whom he had first met in 
 1880, and who had always accompanied him in his expeditions 
 since. 
 
 His first walking was in the Chamonix district ; and here he 
 very soon found his strength and elasticity come back to him. 
 Crossing over from Montanvert to Courmayeur, by the Col du 
 Geant, he was attracted by the peak called the Aiguille Blanche 
 de Peuteret, a virgin peak, the ascent of which had been before 
 attempted but not accomplished. Consulting with Petrus he 
 determined to try it, feeling that the fortnight, which by this 
 
INTRODUCTION. 21 
 
 time he had spent in climbing, had brought back to him his old 
 vigour, and that his illness was already a thing of the past. 
 
 There is no reason to believe that he regarded the expedition 
 as one of unusual peril ; and an incident which at the time of his 
 death was thought by some to indicate this was in reality 
 nothing more than a proof of his kindly foresight. The guide 
 Petrus was burdened by a debt on his land amounting to 
 about i$o. In the previous year Balfour and his brother had 
 come to know of this debt ; and, seeing that no Alpine ascent 
 is free from danger, that on any expedition some accident 
 might carry them off, had conceived the idea of making 
 some provision for Petrus' family in case he might meet 
 with sudden death in their service. This suggestion of 
 the previous year Balfour carried out on this occasion, and 
 sent home to his brother Gerald a cheque of 150 for this 
 purpose. But the cheque was sent from Montanvert before he 
 had even conceived the idea of ascending the Aiguille Blanche. 
 It was not a provision for any specially dangerous ascent, and 
 must be regarded as a measure prompted not by a sense of coming 
 peril but rather by the donor's generous care for his servant. 
 
 On Tuesday afternoon, July 18, he and Petrus, with a porter 
 to carry provisions and firing to their sleeping-place on the 
 rocks, set out from Courmayeur, the porter returning the same 
 night. They expected to get back to Courmayeur some time 
 on the Thursday, but the day passed without their appearing. 
 This did not cause any great anxiety because it was 
 supposed that they might have found it more convenient to 
 pass over to the Chamonix side than to return to Cour- 
 mayeur. When on Friday however telegrams dispatched to 
 Chamonix and Montanvert brought answers that nothing had 
 been seen of them, it became evident .that some accident had 
 happened, and an exploring party set out for the hills. It was 
 not until early on the Sunday morning that this search party 
 found the bodies, both partly covered with snow, lying on the 
 Glacier de Fresney, below the impassable icefall which sepa- 
 rates the upper basin of the glacier from the lower portion, 
 and at the foot of a couloir which descends by the side of the 
 icefall. Their tracks were visible on the snow at the top of 
 the couloir. Balfour's neck was broken, and his skull fractured 
 
22 INTRODUCTION. 
 
 in three places ; Petrus' body was also fractured in many 
 places. The exact manner of their death will never be known, 
 but there can be no doubt that, in Balfour's case at all 
 events, it was instantaneous, and those competent to form a 
 judgment are of opinion that they were killed by a sudden fall 
 through a comparatively small height, slipping on the rocks as 
 they were descending by the side of the ice-fall, and not precipi- 
 tated from the top of the couloir. There is moreover indirect 
 evidence which renders it probable that in the fatal fall Petrus 
 slipped first and carried Balfour with him. Whether they had 
 reached the summit of the Aiguille and were returning home 
 after a successful ascent or whether they were making their way 
 back disheartened and wearied with failure, is not and perhaps 
 never will be known. Since the provisions at the sleeping-place 
 were untouched, the deaths probably took place on Wednesday 
 the I9th. The bringing down the bodies proved to be a task of 
 extreme difficulty, and it was not till Wednesday the 26th that 
 the remains reached Courmayeur, where M. Bertolini, the master 
 of the hotel, and indeed everyone, not least the officers of a 
 small body of Italian troops stationed there, shewed the greatest 
 kindness and sympathy to Balfour's brothers, Gerald and Eus- 
 tace, who hastened to the spot as soon as the news of the terrible 
 disaster was telegraphed home. Mr Walter Leaf also and Mr 
 Conway, friends of Balfour, the former a very old one, who had 
 made their way to Courmayeur from other parts of Switzerland 
 as soon as they heard of the accident, rendered great assistance. 
 The body was embalmed, brought to England, and buried at 
 Whittinghame on Saturday, Aug. 5, the Fellows of Trinity 
 College holding a service in the College Chapel at the same 
 time. 
 
 In person he was tall, being fully six feet in height, well 
 built though somewhat spare. A broad forehead overhanging 
 deeply set dark brown eyes whose light shining from beneath 
 strongly marked eye-brows told all the changes of his moods, 
 slightly prominent cheek-bones, a pale skin, at times in- 
 clined to be even sallow, dark brown hair, allowed to grow on 
 the face only as a small moustache, and slight whiskers, made 
 up a countenance which bespoke at once strength of character 
 and delicacy of constitution. It was an open countenance, hiding 
 
INTRODUCTION. 23 
 
 nothing, giving sign at once, both when his body was weary or 
 weak, and when his mind was gladdened, angered or annoyed. 
 
 The record of some of his thoughts and work, all that 
 he had given to the world will be found in the following 
 pages. But who can tell the ideas which had passed into Iiis 
 quick brain, but which as yet were known only to himself, of 
 which he had given no sign up to that sad day on which he 
 made the fatal climb? And who can say whither he might 
 not have reached had he lived, and his bright young life ripened as 
 years went on ? This is not the place to attempt any judg- 
 ment of his work : that may be left to other times, and to 
 other hands; but it may be fitting to place here on record 
 a letter which shews how much the greatest naturalist of this 
 age appreciated his younger brother. Among Balfour's papers 
 was found a letter from Charles Darwin, acknowledging the 
 receipt of Vol. II. of the Comparative Embryology in the fol- 
 lowing words : 
 
 "July 6, 1881. 
 
 DOWN, BECKENHAM, KENT. 
 
 MY DEAR BALFOUR, 
 
 I thank you heartily for the present of your grand 
 book, and I congratulate you on its completion. Although I read 
 almost all of Vol. I. I do not feel that I am worthy of your present, 
 unless indeed the fullest conviction that it is a memorable work makes 
 me worthy to receive it. 
 
 * * * * * 
 
 Once again accept my thanks, for I am proud to receive a book 
 from you, who, I know, will some day be the chief of the English 
 Biologists. 
 
 Believe me, 
 
 Yours sincerely, 
 
 CHARLES DARWIN." 
 
 The loss of him was a manifold loss. He is mourned, 
 and will long be mourned, for many reasons. Some miss only 
 the brilliant investigator ; others feel that their powerful and 
 sympathetic teacher is gone ; some look back on his memory 
 
24 INTRODUCTION. 
 
 and grieve for the charming companion whose kindly courtesy 
 and bright wit made the hours fly swiftly and pleasantly along ; 
 and to yet others is left an aching void when they remember 
 that they can never again lean on the friend whose judgment 
 seemed never to fail and whose warm-hearted affection was 
 a constant help. And to some he was all of these. At the 
 news of his death the same lines came to the lips of all of 
 us, so fittingly did Milton's words seem to speak our loss and 
 grief 
 
 "For Lycidas is dead, dead ere his prime, 
 . Young Lycidas, and hath not left his peer." 
 
 M. FOSTER. 
 
I. ON SOME POINTS IN THE GEOLOGY OF THE EAST 
 LOTHIAN COAST \ 
 
 By G. W. and F. M. BALFOUR, Trinity College, Cambridge. 
 
 THE interesting relation between the Porphyrite of Whit- 
 berry Point, at the mouth of the Tyne. near Dunbar, and the 
 adjacent sedimentary rocks, was first noticed, we believe, by 
 Professor Geikie, who speaks of it in the Memoirs of the Geologi- 
 cal Siirvey of East Lothian, pages 40 and 31, and again in the 
 new edition of Jukes's Geology, p. 269. The volcanic mass 
 which forms the point consists of a dark felspathic base with 
 numerous crystals of augite: it is circular in form, and is exposed 
 for two-thirds of its circumference in a vertical precipice facing 
 the sea, about twenty feet in height. 
 
 The rock is traversed by numerous joints running both in a 
 horizontal and in a vertical direction. The latter are by far the 
 most conspicuous, and give the face of the cliff, when seen from 
 a distance, a well-marked columnar appearance, though the 
 columns themselves are not very distinct or regular. They are 
 quadrangular in form, and are evidently produced by the inter- 
 section at right-angles of the two series of vertical joints. 
 
 It is clear that the face of the precipice has been gradually 
 receding in proportion as it yielded to the action of the waves ; 
 and that at a former period the volcanic rock extended con- 
 siderably further than at present over the beds which are seen 
 to dip beneath it. These latter consist of hard fine-grained 
 calcareous sandstones belonging to the Lower Carboniferous 
 formation. Their colour varies from red to white, and their 
 prevailing dip is in a N.W. direction, with an average inclination 
 of 12 20. If the volcanic mass is a true intrusive rock, we 
 should naturally expect the strata which surround it to dip away 
 in all directions, the amount of their inclination diminishing in 
 
 1 From the Geological Magazine, Vol. ix. No. 4. April, 1872. 
 
26 
 
 GEOLOGY OF THE EAST LOTHIAN COAST. 
 
 proportion to their distance from it. We find, however, that the 
 case is precisely the reverse : as the beds approach the base of 
 the cliff, they dip towards it from every side at perpetually in- 
 creasing angles, until at the point of junction the inclination 
 amounts in places to as much as 55 degrees. The exact amount 
 of dip in the various positions will be seen on referring to the 
 accompanying map. 
 
 FIG. I. 
 
 FIG. i. MAP OF STRATA. AT WHITBERRY POINT. Scale, 6 in. to the mile. 
 
 A. Lava sheet. B. Sandstone Beds, dipping from every side towards the lava. 
 CC. Line of Section along which Fig. 2 is supposed to be drawn. 
 
 We conceive that the phenomenon is to be explained by 
 supposing the orifice through which the lava rose and overflowed 
 the surface of the sedimentary strata to have been very much 
 smaller in area than the extent of igneous rock at present visible ; 
 and that the pressure of the erupted mass on the soft beds be- 
 neath, aided perhaps by the abstraction of matter from below, 
 caused them to incline towards the central point at a gradually 
 increasing angle. The diagram, fig. 2, will serve further to 
 illustrate this hypothesis. A is the neck or orifice by which the 
 melted matter is supposed to ascend. C shews the sheet of lava 
 after it has overspread the surface of the sandstone beds B, so as 
 to cause .them to assume their present inclination. The dotted 
 
GEOLOGY OF THE EAST LOTHIAN COAST. 27 
 
 lines represent the hypothetical extension of the igneous mass 
 and sandstones previous to the denudation which they have 
 suffered from the action of the waves. 
 
 Professor Geikie, in his admirable treatise on the Geology of 
 the county 1 , adopts a view on this subject which is somewhat 
 different from that which is suggested in this paper. He con- 
 siders that the whole mass is an intrusive neck of rock with 
 perpendicular sides; and that it once filled up an orifice through 
 the surrounding sedimentary strata, of which it is now the only 
 remnant. 
 
 LEVEL OF 
 
 SHORE 
 
 FIG. 2 
 
 FIG. i. VERTICAL SECTION THROUGH CC. DIAGRAM (Fio. i). 
 
 A. Orifice by which the lava ascended. B. Sandstone Beds. B'. Hypothetical 
 extension of ditto. C. Sheet of lava spread over the sandstones /?. C. Hypo- 
 thetical extension of ditto. 
 
 He admits that the inclination of the sandstone beds towards 
 the igneous mass in the centre is a phenomenon that is some- 
 what difficult to explain, and suggests that a subsequent contrac- 
 tion of the column may have tended to produce such a result. 
 To use his own words: "In the case of a solid column of felstone 
 or basalt, the contraction of the melted mass on cooling may 
 have had some effect in dragging down the sides of the orifice 2 ." 
 
 But, apart from other objections, it is scarcely conceivable 
 that this result should have been produced by the contraction of 
 the column. 
 
 In his recent edition of Jukes's Manual of Geology (p. 269), 
 in which he also refers to this instance, he states that in other 
 cases of "necks" it is found to be an almost invariable rule, "that 
 
 1 Memoirs of Geological Survey of Scotland, sheet 33, pp. 40, 41. 
 
 2 Note on p. 41 of Mem. Geol. Survey of East Lothian. 
 
28 GEOLOGY OF THE EAST LOTHIAN COAST. 
 * 
 
 strata are bent down so as to k dip into the neck all round its 
 margin." We are not aware to what other instances Prof. Geikie 
 may allude; but on referring to his Memoir on the Geology of 
 East Lothian, we find that he states in the cases of 'North 
 Berwick Law' and 'Traprain' (which he compares with the 
 igneous mass at Whitberry Point), that the beds at the base of 
 these two necks, where exposed, dip away from them, and that 
 at a high angle. 
 
 In support of the hypothesis which we have put forward, the 
 following arguments may be urged : 
 
 (1) That in one place at least the sedimentary strata are 
 seen to be actually dipping beneath the superincumbent basalt; 
 and that the impression produced by the general relation of the 
 two rocks is, that they do so everywhere. 
 
 (2) Since the columns into which the lava is split are verti- 
 cal, the cooling surface must have been horizontal : the mass 
 must, therefore, have formed a sheet, and not a dyke; for, in the 
 latter case, the cooling surfaces would have been vertical. 
 
 (3) It is difficult to conceive, on the supposition that the 
 volcanic rock is a neck with perpendicular sides, that the marine 
 denudation should have uniformly proceeded only so far as to 
 lay bare the junction between the two formations. We should 
 have expected that in many places the igneous rock itself would 
 have been cut down to the general level, whereas the only signs 
 of such an effect are shown in a few narrow inlets where the 
 rock was manifestly softer than in the surrounding parts. 
 
 The last objection is greatly confirmed by the overhanging 
 cliffs and numerous blocks of porphyrite which lie scattered on 
 the beach, as if to attest the former extension of that ancient 
 sheet of which these blocks now form but a small remnant. In- 
 deed, the existence of such remains appears sufficient of itself to 
 condemn any hypothesis which presumes the present face of the 
 cliff to have formed the original boundary of the mass. 
 
 It may be fairly objected to our theory, as Prof. Geikie him- 
 self has suggested, that the high angle at which the strata dip is 
 difficult to account for. But, in fact, this steep inclination con- 
 stitutes the very difficulty which any hypothesis on the subject 
 must be framed to explain; and it is a difficulty which is not 
 more easily solved by Prof. Geikie's theory than by our own. 
 
II. THE DEVELOPMENT AND GROWTH OF THE LAYERS 
 
 OF THE BLASTODERM 1 . 
 
 With Plate I. figs, i 5 and 9 12. 
 
 THE following paper deals with the changes which take place 
 in the cells of the blastoderm of the hen's egg during the first 
 thirty or forty hours of incubation. The subject is one which 
 has, as a general rule, not been much followed up by embryo- 
 logists, but is nevertheless of the greatest interest, both in refer- 
 ence to embryology itself, and to the growth and changes of 
 protoplasm exhibited in simple embryonic cells. I am far from 
 having exhausted the subject in this paper, and in some cases I 
 shall be able merely to state facts, without being able to give 
 any explanation of their meaning. 
 
 My method of investigation has been the examination of 
 sections and surface views. For hardening the blastoderm I 
 have employed, as usual, chromic acid, and also gold chloride. 
 It is, however, difficult to make sections of blastoderms hardened 
 by this latter reagent, and the sections when made are not in all 
 cases satisfactory. For surface views I have chiefly used silver 
 nitrate, which brings out the outlines of the cells in a manner 
 which leaves nothing to be desired as to clearness. If the out- 
 lines only of the cells are to be examined, a very short immersion 
 (half a minute) of the blastoderm in a half per cent, solution of 
 silver nitrate is sufficient, but if the immersion lasts for a longer 
 period the nuclei will be brought out also. For studying the 
 latter, however, I have found it better to employ gold chloride 
 or carmine in conjunction with the silver nitrate. 
 
 My observations begin with the blastoderm of a freshly laid 
 egg. The appearances presented by sections of this have been 
 accurately described by Peremeschko, " Ueber die Bildung der 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xin., 1873. 
 
30 DEVELOPMENT AND GROWTH OF 
 
 Keimblatter im Hiihnerei," Sitzungsberichte der K. Akademie der 
 Wissenscliaften in Wien, 1868. Oellacher, " Untersuchung uber 
 die Furchung und Blatterbildung im Hiihnerei," Studien aus dem 
 Institut filr Experim, PatJwlogie in Wien, 1870 (pp. 54 74), and 
 Dr Klein, Ixiii. Bande der Sitz. der K. Acadamie der Wiss. in 
 Wien, 1871. 
 
 The unincubated blastoderm (PI. I, fig. i) consists of two 
 layers. The upper layer is composed of a single row of columnar 
 cells. Occasionally, however, the layer may be two cells thick. 
 The cells are filled with highly refracting spherules of a very 
 small size, and similar in appearance to the finest white yolk 
 spherules, and each cell also contains a distinct oval nucleus. 
 This membrane rests with its extreme edge on the white yolk, 
 its central portion covering in the segmentation cavity. From 
 the very first it is a distinct coherent membrane, and exhibits 
 with silver nitrate a beautiful hexagonal mosaic of the outlines 
 (PI. I. fig. 6) of the cells. The diameter of the cells when 
 viewed from above is from ^Vo FcW f an mcn - The under 
 layer is very different from this : it is composed of cells which 
 are slightly, if at all, united, and which vary in size and appear- 
 ance, and in which a nucleus can rarely be seen. The cells 
 of which it is composed fill up irregularly the segmentation 
 cavity, though a distinct space is even at this time occasionally 
 to be found at the bottom of it. Later, when the blastoderm 
 has spread and the white yolk floor has been used as food, 
 a considerable space filled with fluid may generally be found. 
 
 The shape of the floor of the cavity varies considerably, 
 but it is usually raised in the middle and depressed near the 
 circumference. In this case the under layer is perhaps only 
 two cells deep at the centre and three or four cells deep near 
 the circumference. 
 
 The cells of which this layer is composed vary a good deal 
 in size ; the larger cells being, however, more numerous in 
 the lower layers. In addition, there are usually a few very large 
 cells quite at the bottom of the cavity, occasionally separated 
 from the other cells by fluid. They were called formative cells 
 (Bildungselemente) by Peremeschko (loc. cit.) ; and, according 
 to Oellacher's observations (loc. cit.), some of them, at any rate, 
 fall to the bottom of the segmentation cavity during the later 
 
THE LAYERS OF THE BLASTODERM. 31 
 
 stages of segmentation. They do not differ from the general 
 lower layer cells except in size, and even pass into them by 
 insensible gradations. All the cells of the lower layer are 
 granular, and are filled with highly refracting spherules precisely 
 similar to the smaller white yolk spherules which line the bottom 
 of the segmentation cavity. 
 
 The size of the ordinary cells of the lower layer varies 
 from ^J^ Y^ of an inch. The largest of the formative 
 cells come up to ^ of an inch. It will be seen from this 
 description that, morphologically speaking, we cannot attach 
 much importance to the formative cells. The fact that they 
 broke off from the blastoderm, towards the end of the seg- 
 mentation even if we accept it as a normal occurrence, rather 
 than the result of manipulation is not of much importance, and, 
 except in size, it is impossible to distinguish these cells from 
 other cells of the lower layer of the blastoderm. 
 
 Physiologically, however, as will be afterwards shewn, they 
 are of considerable importance. 
 
 The changes which the blastoderm undergoes during the 
 first three or four hours of incubation are not very noticeable. 
 At about the sixth or eighth hour, or in some cases consider- 
 ably earlier, changes begin to take place very rapidly. These 
 changes result in the formation of a hypoblast and mesoblast, 
 the upper layer of cells remaining comparatively unaltered 
 as the epiblast. 
 
 To form the hypoblast a certain number of the cells of the 
 lower layer begin to undergo remarkable changes. From being 
 spherical and, as far as can be seen, non-nucleated, they become 
 (vide fig. 2 //) flattened and nucleated, still remaining granular, 
 but with fewer spherules. 
 
 Here, then, is a direct change, of which all the stages can be 
 followed, of a cell of one kind into a cell of a totally different 
 character. The new cell is not formed by a destruction of 
 the old one, but directly from it by a process of metamorphosis. 
 These hypoblast cells are formed first at the centre and later 
 at the circumference, so that from the first the cells at the 
 circumference are less flattened and more granular than the 
 cells at the centre. A number of cells of the original lower 
 layer are enclosed between this layer and the epiblast; and, 
 
32 DEVELOPMENT AND GROWTH OF 
 
 in addition to these, the formative cells (as has been shewn by 
 Peremeschko, Oellacher, and Klein, whose observations I can 
 confirm) begin to travel towards the circumference, and to pass 
 in between the epiblast and hypoblast. 
 
 Both the formative cells, and the lower layer cells enclosed 
 between the hypoblast and epiblast, contribute towards the 
 mesoblast, but the mode in which the mesoblast is formed is 
 very different from that in which the hypoblast originates. 
 
 It is in this difference of formation that the true distinction be- 
 tween the mesoblast and hypoblast is to be looked for, rather than 
 in the original difference of the cells from which they are derived. 
 
 The cells of the mesoblast are formed by a process which 
 seems to be a kind of free cell formation. The whole of the 
 interior of each of the formative cells, and of the other cells 
 which are enclosed between the epiblast and the hypoblast, 
 become converted into new cells. These are the cells of the 
 mesoblast. I have not been able perfectly to satisfy myself 
 as to the exact manner in which this takes place, but I am 
 inclined to think that some or all of the spherules which are 
 contained in the original cells develop into nuclei for the new 
 cells, the protoplasm of the new cells being formed from that 
 of the original cells. 
 
 The stages of formation of the mesoblast cells are shewn 
 in the section (PI. I, fig. 2), taken from the periphery of a 
 blastoderm of eight hours. 
 
 The first formation of the mesoblast cells takes place in 
 the centre of the blastoderm, and the mass of cells so formed 
 produces the opaque line known as the primitive streak. This 
 is shown in PI. I, fig. 9. 
 
 One statement I have made in the above description in 
 reference to the origin of the mesoblast cells, viz. that they are 
 only partly derived from the formative cells at the bottom 
 of the segmentation cavity, is to a certain extent opposed to 
 the statements of the three investigators above mentioned. 
 They state that the mesoblast is entirely derived from the 
 formative cells. It is not a point to which I attach much im- 
 portance, considering that I can detect no difference between 
 these, cells and any other cells of the original lower layer except 
 that of size ; and even this difference is probably to be explained 
 
THE LAYERS OF THE BLASTODERM. 33 
 
 by their proximity to the white yolk, whose spherules they 
 absorb. But my reason for thinking it probable that these cells 
 alone do not form the mesoblast are, ist. That the mesoblast 
 and hypoblast are formed nearly synchronously, and except at 
 the centre a fairly even sprinkling of lower layer cells isTrom 
 the first to be distinguished between the epiblast and hypoblast. 
 2nd. That if some of the lower layer cells are not converted into 
 mesoblast, it is difficult to see what becomes of them, since they 
 appear to be too numerous to be converted into the hypoblast 
 alone. 3rd. That the chief formation of mesoblast at first takes 
 place in the centre, while if the formative cells alone took part in 
 its formation, it would be natural to expect that it would begin 
 to be formed at the periphery. 
 
 Oellacher himself has shewn (Zeitschrift fiir wissenschaftliche 
 Zoologie, 1873, " Beitrage zur Entwick. Gesch. der Knochen- 
 fische") that in osseous fishes the cells which break away from 
 the blastoderm take no share in the formation of the mesoblast, 
 so that we can derive no argument from the formation of the 
 mesoblast in these animals, for believing that in the chick it 
 is derived only from the formative cells. 
 
 In the later stages, however, from the twelfth to the twenty- 
 fifth hour, the growth of the mesoblast depends almost entirely 
 on these cells, and Peremeschko's discovery of the fact is of 
 great value. 
 
 Waldeyer (Henle und v. Pfeufer's Zeitschrift, xxxiv. Band, 
 fur 1869) has given a different account of the origin of the 
 layers. There is no doubt, however, in opposition to his stat.e- 
 ' ments and drawings, that from the very first the hypoblast is 
 distinct from the mesoblast, which is, indeed, most conspicu- 
 ously shewn in good sections ; and his drawings of the deriva- 
 tion of the mesoblast from the epiblast are not very correct. 
 
 The changes which have been described are also clearly 
 shewn by means of silver nitrate. Whereas, at first this reagent 
 brought out no outline markings of cells in the lower layer, 
 by the eighth to the twelfth hour the markings (PI. I, fig. 3) 
 are very plain, and shew that the hypoblast is a distinct coherent 
 membrane. 
 
 In section, the cells of the hypoblast appear generally very 
 thin and spindle shaped, but the outlines brought out by the 
 
 B- 3 
 
34 DEVELOPMENT AND GROWTH OF 
 
 silver nitrate shew that they are much expanded horizontally, 
 but very irregular as to size, varying even within a small area 
 from ffaq ffa of an inch in the longest diameter. 
 
 At about the twelfth hour they are uniformly smaller a 
 short way from each extremity of its longer axis than over 
 the rest of the blastoderm. 
 
 It is, perhaps, fair to conclude from this that growth is 
 most rapid at these parts. 
 
 At this time the hypoblast, both in sections and from a 
 surface view after treatment with silver nitrate, appears to 
 end abruptly against the white yolk. The surface view also 
 shews that its cells are still filled with highly refractive globules, 
 making it difficult to see the nucleus. In some cases I thought 
 that I could (fig. 3, a) make out that it was hour-glass shaped, 
 and some cells certainly contain two nuclei. Some of the cells 
 (fig- 3, b) shew re-entrant curves, which prove that they have 
 undergone division. 
 
 The cells of the epiblast, up to the thirteenth hour, have 
 chiefly undergone change in becoming smaller. 
 
 In surface views they are about ^ OT of an inch in diameter 
 over the centre of the pellucid area, and increase to -^^ of 
 an inch over the opaque area. 
 
 In the centre of the pellucid area the form of the epiblast 
 cells is more elongated vertically and over the opaque area more 
 flattened than was the case with the original upper layer cells. 
 In the centre the epiblast is two or three cells deep. 
 
 Before going on to the further changes of the blastodermic 
 cells it will be well to say a few words in reference to the origin 
 of the mesoblast. 
 
 From the description given above it will be clear that in 
 the chick the mesoblast has an independent origin ; it can 
 be said neither to originate from the epiblast nor from the 
 hypoblast. It is formed coincidently with the latter out of 
 apparently similar segmentation cells. The hypoblast, as has 
 been long known, shews in the chick no trace of its primitive 
 method of formation by involution, neither does the mesoblast 
 shew any signs, of its primitive mode of formation. In so 
 excessively highly differentiated a type as birds we could hardly 
 expect to find, and certainly do not find, any traces of the 
 
THE LAYERS OF THE BLASTODERM. 35 
 
 primitive origin of the mesoblast, either from the epiblast or 
 hypoblast, or from both. In the chick the mesoblast cells 
 are formed directly from the ultimate products of segmentation. 
 From having a secondary origin in most invertebrates the 
 mesoblast comes to have, in the chick, a primary origin from the 
 segmentation spheres, precisely as we find to be the case with 
 the nervous layer in osseous fishes. It is true we cannot tell 
 which segmentation-cells will form the mesoblast, and which the 
 hypoblast ; but the mesoblast and hypoblast are formed at the 
 same time, and both of them directly from segmentation spheres. 
 
 The process of formation of the mesoblast in Loligo, as 
 observed by Mr Ray Lankester (Annals and Magazine of Natural 
 History, February, 1873), is still more modified. Here the 
 mesoblast arises independently of the blastoderm, and by a 
 process of free cell-formation in the yolk round the edge of the 
 blastoderm. If Oellacher's observations in reference to the 
 origin of formative cells are correct, then the modes of origin 
 of the mesoblast in Loligo and the chick would have nothing in 
 common ; but if the formative cells are in reality derived from 
 the white yolk, and also are alone concerned in the formation of 
 the mesoblast, then the modes of formation of the mesoblast in 
 the chick would be substantially the same as that observed 
 by Mr Ray Lankester in Loligo. 
 
 No very important changes take place in the ac.tual forms 
 of the cells during the next few hours. A kind of fusion takes 
 place between the epiblast and the mesoblast along the line 
 of the primitive streak forming the axis-string of His ; but the 
 line of junction between the layers is almost always more or less 
 visible in sections. In any case it does not appear that there is 
 any derivation of mesoblast cells from the epiblast ; and since 
 the fusion only takes place in the region of the primitive groove, 
 and not in front, where the medullary groove arises (see succeed- 
 ing paper), it cannot be considered of any importance in reference 
 to the possible origin of the Wolfftan duct, &c., from the epiblast 
 (as mooted by Waldeyer, Eierstock und Ei, Leipzig, 1870). 
 The primitive groove, as can be seen in sections, begins to 
 appear very early, generally before the twelfth hour. The 
 epiblast spreads rapidly over the white yolk, and the area 
 pellucida also increases in size. 
 
 32 
 
36 DEVELOPMENT AND GROWTH OF 
 
 From the mesoblast forming at first only a small mass of 
 cells, which lies below the primitive streak, it soon comes to 
 be the most important layer of the blastoderm. Its growth 
 is effected by means of the formative cells. These cells are 
 generally not very numerous in an unincubated blastoderm, 
 but rapidly increase in numbers, probably by division ; at the 
 same time they travel round the edge of, and in some cases 
 through, the hypoblast, and then become, converted in the 
 manner described into mesoblast cells. They act as carriers 
 of food from the white yolk to the mesoblast till, after the 
 formation of the vascular area, they are no longer necessary. 
 The numerous cases in which two nucleoli and even two nuclei 
 can be seen in one cell prove that the mesoblast cells also 
 increase by division. 
 
 The growth of the hypoblast takes place in a very different 
 way. It occurs by a direct conversion, cell for cell, of the 
 white yolk spheres into hypoblast cells. This interpretation 
 of the appearances, which I will describe presently, was first 
 suggested to me by Dr Foster, from an examination of some 
 of my specimens of about thirty-six hours, prepared with silver 
 nitrate. Where there is no folding at the junction between the 
 pellucid and opaque areas, there seems to be a perfect continuity 
 in the silver markings and a gradual transition in the cells, from 
 what would be undoubtedly called white yolk spheres, to as 
 undoubted hypoblast cells (vide PI. I, fig. 5). In passing from 
 the opaque to the pellucid areas the number of white yolk 
 spherules in each cell becomes less, but it is not till some way 
 into the pellucid area that they quite cease to be present. I at 
 first thought that this was merely due to the hypoblast cells 
 feeding on the white yolk sphericles, but the perfect continuity 
 of the cells, and the perfect gradation in passing from the white 
 yolk cells to the hypoblast, proves that the other interpretation is 
 the correct one, viz. that the white yolk spheres become directly 
 converted into the hypoblast cells. This is well shewn in 
 sections (vide PI. I, fig. 4) taken from embryos of all ages 
 from the fifteenth to the thirty-sixth hour and onwards. But 
 it is, perhaps, most easily seen in embryos of about twenty 
 hours. In such an embryo there is a most perfect gradation : 
 the cells of the hypoblast become, as they approach the edge 
 
THE LAYERS OF THE BLASTODERM. 37 
 
 of the pellucid area, broader, and are more and more filled 
 with white yolk sphericles, till at the line of junction it is quite 
 impossible to say whether a particular cell is a white-yolk cell 
 (sphere) or a hypoblast cell. The white-yolk cells near the 
 line of junction can frequently be seen to possess nuclei. At 
 first the hypoblast appears to end abruptly against the white 
 yolk ; this state of things, however, soon ends, arid there super- 
 venes a complete and unbroken continuity between the hypo- 
 blast and the white yolk. 
 
 Of the mode of increase of the epiblast I have but little 
 to say. The cells undoubtedly increase entirely by division, 
 and the new material is most probably derived directly from 
 the white yolk. 
 
 Up to the sixth hour the cells of the upper layer retain 
 their early regular hexagonal pattern, but by the twelfth hour 
 they have generally entirely lost this, and are irregularly shaped 
 and very angular. The cells over the centre of the pellucid 
 area remain the smallest up to the twenty-fifth hour or later, 
 while those over the rest of the pellucid area are uniformly 
 larger. 
 
 In the hypoblast the cells under the primitive groove, and 
 on each side as far as the fold which marks off the exterior 
 limit of the proto-vertebrae, are at the eighteenth hour consider- 
 ably smaller than any other cells of this layer. 
 
 In all the embryos between the eighteenth and twenty-third 
 hour which I have examined for the purpose, I have found 
 that at about two-thirds of the distance from the anterior end 
 of the pellucid area, and just external to the side fold, there 
 is a small space on each side in which the cells are considerably 
 larger than anywhere else in the hypoblast. These larger 
 cells, moreover, contain a greater number of highly refractive 
 spherules than any other cells. It is not easy to understand 
 why growth should have been less rapid here than elsewhere, 
 as the position does not seem to correspond to any feature 
 in the embryo. In some specimens the hypoblast cells at 
 the extreme edge of the pellucid area are smaller than the 
 cells immediately internal to them. At about the twenty- third 
 hour these cells begin rapidly to lose the refractive spherules 
 they contained in the earlier stages of incubation, and come 
 
38 DEVELOPMENT AND GROWTH OF 
 
 to consist of a nucleus surrounded simply by granular proto- 
 plasm. 
 
 At about this period of incubation the formative cells are 
 especially numerous at the periphery of the blastoderm, and, 
 no doubt, become converted into the mass of mesoblast which 
 is found at about the twenty-fifth hour in the region of the 
 vascular area. Some of them are lobate, and appear as if 
 they were undergoing division. At this time also the greatest 
 number of formative cells are to be found at the bottom of the 
 now large segmentation cavity. 
 
 In embryos of from thirty to forty hours the cells of the 
 hypoblast have, over the central portion of the pellucid area, 
 entirely lost their highly refractive spherules, and in the fresh 
 state are composed of the most transparent protoplasm. When 
 treated with reagents they are found to contain an oval nucleus 
 with one or sometimes two nucleoli, imbedded in a considerable 
 mass of protoplasm. The protoplasm appears slightly granular 
 and generally contains one or two small vacuoles. I have already 
 spoken of the gradation of the hypoblast at the edge of the 
 blastoderm into white yolk. I have, therefore, only to mention 
 the variations in the size of its cells in different parts of the 
 pellucid area. The points where the cells are smallest seem 
 generally to coincide with the points of maximum growth. Over 
 the embryo the cells are more regular than elsewhere. They 
 are elongated and arranged transversely to the long axis of 
 the embryo. They are somewhat hexagonal in shape, and not 
 unlike the longer pieces in the dental plate of a Myliobatis 
 (PI. I, fig. 10). This regularity, however, is much more marked 
 in some specimens than in others. These cells are about 4oVo tn 
 of an inch in breadth, and y^^th in length. On each side of the 
 embryo immediately external to the proto-vertebrae the cells are 
 frequently about the same size as those over the embryo itself. 
 In the neck, however, and near the end of the sinus rhomboidalis, 
 they are considerably smaller, about ^y^th inch each way. The 
 reason of this small size is not very clear, but probably shews 
 that the greatest growth is taking place at these two points. 
 The cells, again, are very small at the head fold, but are very 
 much larger in front of this larger, in fact, than any other cells 
 of the hypoblast. Outside the embryo they gradually increase 
 
THE LAYERS OF THE BLASTODERM. 39 
 
 in size towards the edge of the pellucid area. Here they are 
 about jtj^th of an inch in diameter, irregular in shape and rather 
 angular. 
 
 The outlines of the cells of the epiblast at this time are 
 easily distinguished from the cells of the hypoblast by "being 
 more elongated and angular ; they are further distinguished 
 by the presence of numerous small oval cells, frequently at the 
 meeting point of several cells, at other times at points along the 
 lines of junction of two cells (PI. I, fig. 12). These small cells 
 look very like the smaller stomata of endothelial membranes, 
 but are shewn to be cells by possessing a nucleus. There is 
 considerable variation in size in the cells in different parts of the 
 epiblast. Between the front lobes of the brain the cells are very 
 small, ^oijth inch, rising to ^oVtfth on each side. They are about 
 the latter size over the greater part of the embryo. But over 
 the sinus rhomboidalis they fall again to from -^^ to ^Vu tn 
 inch. This is probably to be explained by the growth of the 
 medullary fold at this point, which pushes back the primitive 
 groove. At the sides of the head the cells are larger than any- 
 where else in the epiblast, being here about y^^th inch in 
 diameter. I at present see no explanation of this fact. At the 
 periphery of the pellucid area and over the vascular area the 
 cells are y^Vrrth to aoVoth mcn ^ n diameter, but at the periphery 
 of the opaque area they are smaller again, being about the 
 jniVoth of an inch. This smaller size at the periphery of the 
 area opaca is remarkable, since in the earlier stages the most 
 peripheral epiblast cells were the largest. It, perhaps, implies 
 that more rapid growth is at this time taking place in that part 
 of the epiblast which is spreading over the yolk sac. 
 
4O DEVELOPMENT AND GROWTH OF THE BLASTODERM. 
 
 EXPLANATION OF PLATE I. Figs. 15 and 912. 
 
 Fig. i. Section through an unincubated blastoderm, shewing the upper layer, 
 composed of a single row of columnar cells, and the lower layer, composed of several 
 rows of rounded cells in which no nucleus is visible. Some of the " formative cells," 
 at the bottom of the segmentation cavity, are seen at (6). 
 
 Fig. 2. Section through the periphery of an eight hours' blastoderm, shewing the 
 epiblast (/), the hypoblast (h), and the mesoblast commencing to be formed (c), partly 
 by lower-layer cells enclosed between the epiblast and hypoblast, and partly by 
 formative cells. Formative cells at the bottom of the segmentation cavity are seen 
 at b. At s is one of the side folds parallel to the primitive groove. 
 
 Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated with 
 silver nitrate, shewing the great variation in the size of the cells at this period. An 
 hour-glass shaped nucleus is seen at a. 
 
 Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for cell the 
 junction between the hypoblast (//) and white-yolk spheres (w). 
 
 Fig- 5- Junction between the white-yolk spheres and the hypoblast cells at the 
 passage from the area pellucida to the area opaca. The specimen was treated with 
 silver nitrate to bring out the shape of the cells. The line of junction between the 
 opaque and pellucid areas passes diagonally. 
 
 Fig. 9. Section through the primitive streak of an eight hours' blastoderm. The 
 specimen shews the mesoblast very much thickened in the immediate neighbourhood 
 of the primitive streak, but hardly formed at all on each side of the streak. It also 
 shews the primitive groove just beginning to be formed (pr), and the fusion between 
 the epiblast and the mesoblast under the primitive groove. The hypoblast is com- 
 pletely formed in the central part of the blastoderm. At f is seen one of the side 
 folds parallel to the primitive groove. Its depth has been increased by the action of 
 the chromic acid. 
 
 Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo, treated 
 with silver nitrate, shewing the regularity and elongated shape of the cells over the 
 embryo and the smaller cells on each side. 
 
 Fig. n. Epiblast cells from an unincubated blastoderm, treated with silver 
 nitrate, shewing the regular hexagonal shape of the cells and the small spherules 
 they contain. 
 
 Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated with silver 
 nitrate, shewing the small rounded cells frequently found at the meeting-points of 
 several larger cells which are characteristic of the upper layer. 
 
III. ON THE DISAPPEARANCE OF THE PRIMITIVE GROOVE 
 IN THE EMBRYO CHICK *. 
 
 With Plate I, figs. 68 and 1319. 
 
 THE investigations of Dursy (Der Primitivstreif des Hiihn- 
 chciis, von Dr E. Dursy. Lahr, 1866) on the primitive groove, 
 shewing that it is a temporary structure, and not connected with 
 the development of the neural canal, have in this country either 
 been ignored or rejected. They are, nevertheless, perfectly 
 accurate ; and had Dursy made use of sections to support his 
 statements I do not think they would so long have been denied. 
 In Germany, it is true, Waldeyer has accepted them with a few 
 modifications, but I have never seen them even alluded to in any 
 English work. The observations which I have made corro- 
 borating Dr Dursy may, perhaps, under these circumstances be 
 worth recording. 
 
 After about twelve hours of incubation the pellucid area of 
 a hen's egg has become somewhat oval, with its longer axis 
 at right angles to the long axis of the egg. Rather towards 
 the hinder (narrower) end of this an opaque streak has appeared, 
 with a somewhat lighter line in the centre. A section made at 
 the time shews that the opaque streak is due partly to a thicken- 
 ing of the epiblast, but more especially to a large collection 
 of the rounded mesoblast cells, which along this opaque line 
 form a thick mass between the epiblast and the hypoblast. 
 The mesoblast cells are in contact with both hypoblast and 
 epiblast, and appear to be fused with the latter. The line of 
 junction between them can, however, almost always be made 
 out. 
 
 Soon after the formation of this primitive streak a groove is 
 formed along its central line by a pushing inwards of the epiblast. 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xm, 1873. 
 
42 PRIMITIVE GROOVE IN THE EMBRYO CHICK. 
 
 The epiblast is not thinner where it lines the groove, but the 
 mass of mesoblast below the groove is considerably thinner 
 than at its two sides. This it is which produces the peculiar 
 appearance of the primitive groove when the blastoderm is 
 viewed by transmitted light as a transparent line in the middle 
 of an opaque one. 
 
 This groove, as I said above, is placed at right angles to 
 the long axis of the egg, and nearer the hind end, that is, the 
 narrower end of the pellucid area. It was called " the primitive 
 groove" by the early embryologists, and they supposed that 
 the neural canal arose from the closure of its edges above. 
 It is always easy to distinguish this groove, in transverse sections, 
 by several well-marked characters. In the first place, the 
 epiblast and mesoblast always appear more or less fused together 
 underneath it ; in the second place, the epiblast does not become 
 thinner where it lines the groove ; and . in the third place, the 
 mesoblast beneath it never shews any signs of being differentiated 
 into any organ. 
 
 As Dursy has pointed out, there is frequently to be seen 
 in fresh specimens, examined as transparent objects, a narrow 
 opaque line running down the centre of this groove. I do not 
 know what this line is caused by, as there does not appear 
 to be any structural feature visible in sections to which it can 
 correspond. 
 
 From the twelfth to the sixteenth hour the primitive groove 
 grows rapidly, and by the sixteenth hour is both absolutely 
 and considerably longer than it was at the twelfth hour, and 
 also proportionately longer as compared with the length of the 
 pellucid area. 
 
 There is a greater interval between its end and that of the 
 pellucid area in front than behind. 
 
 At about the sixteenth hour, or a little later, a thickening 
 of the mesoblast takes place in front of the primitive groove, 
 forming an opaque streak, which in fresh specimens looks like a 
 continuation from the anterior extremity of the primitive groove 
 (vide PI. I, fig. 8). From hardened specimens, however, it is 
 easy to see that the connection of this streak with the primitive 
 groove is only an apparent one. Again, it is generally possible 
 to see that in the central line of this streak there is a narrow 
 
PRIMITIVE GROOVE IN THE EMBRYO CHICK. 43 
 
 groove. I do not feel certain that there is no period when this 
 groove may not be present, but its very early appearance has 
 not been recognized either by Dursy or by Waldeyer. More- 
 over, both these authors, as also His, seem to have mistaken 
 the opaque streak spoken of above for the notochord. This, 
 however, is not the case, and the notochord does not make 
 its appearance till somewhat later. The mistake is of very 
 minor importance, and probably arose in Dursy's case from 
 his not sufficiently making use of sections. At about the time 
 the streak in front of the primitive groove makes its appearance 
 a semicircular fold begins to be formed near the anterior ex- 
 tremity of the pellucid area, against which the opaque streak, 
 or as it had, perhaps, better be called, " the medullary streak," 
 ends abruptly. 
 
 This fold is the head fold, and the groove along the me- 
 dullary streak is the medullary groove, which subsequently forms 
 the cavity of the medullary or neural canal. 
 
 Everything which I have described above can without diffi- 
 culty be made out from the examination of fresh and hardened 
 specimens under the simple microscope ; but sections bring out 
 still more clearly these points, and also shew other features 
 which could not have been brought to light without their aid. 
 In PI. I, figs. 6 and 7, two sections of an embryo of about 
 eighteen hours are shewn. The first of these passes through the 
 medullary groove, and the second of them through the extreme 
 anterior end of the primitive groove. The points of difference 
 in the two sections are very obvious. 
 
 From fig. 6 it is clear that a groove has already been formed 
 in the medullary streak, a fact which was not obvious in the 
 fresh specimen. In the second place the mesoblast is thickened 
 both under the groove and also more especially in the medullary 
 folds at the sides of the groove ; but shews hardly a sign of the 
 differentiation of the notochord. So that it is clear that the 
 medullary streak is not the notochord, as was thought to be the 
 case by the authors above mentioned. In the third place there 
 is no adhesion between the epiblast and the mesoblast. In all 
 the sections I have cut through the medullary groove I have 
 found this feature to be constant ; while (for instance, as in 
 PL I, figs. 7, 9, 17) all sections through the primitive groove 
 
44 PRIMITIVE GROOVE IN THE EMBRYO CHICK. 
 
 shew most clearly an adhesion between the epiblast and meso- 
 blast. This fact is both strongly confirmatory of the separate 
 origins of the medullary and primitive grooves, and is also 
 important in itself, as leaving no loophole for supposing that 
 in the region of embryo there is any separation of the cells 
 from the epiblast to form the mesoblast. 
 
 By this time the primitive groove has attained its maximum 
 growth, and from this time begins both absolutely to become 
 smaller, and also gradually to be pushed more and more back- 
 wards by the growth of the medullary groove. 
 
 The specimen figured in PL I, fig. 18, magnified about ten 
 diameters, shews the appearance presented by an embryo of 
 twenty-three hours. The medullary groove (me) has become 
 much wider and deeper than it was in the earlier stage ; the 
 medullary folds (A) are also broader and more conspicuous. 
 The medullary groove widens very much posteriorly, and also 
 the medullary folds separate far apart to enclose the anterior 
 end of the primitive groove (pr). 
 
 All this can easily be seen with a simple microscope, but the 
 sections taken from the specimen figured also fully bear out the 
 interpretations given above, and at the same time shew that 
 the notochord has at this age begun to appear. The sections 
 marked 13 17 pass respectively through the lines with corre- 
 sponding numbers in fig. 18. Section I (fig. 13) passes through 
 the middle of the medullary canal. 
 
 In it the following points are to be noted, (i) That the 
 epiblast becomes very much thinner where it lines the me- 
 dullary canal (me), a feature never found in the epiblast lining 
 the primitive groove. (2) That the mesoblast is very much 
 thickened to form the medullary folds at A, A, while there is 
 no adherence between it and the epiblast, below the primitive 
 groove. (3) The notochord (ch) has begun to be formed, though 
 its separation from the rest of the mesoblast is not as yet very 
 distinct 1 . 
 
 In fig. 14 the medullary groove has become wider and the 
 medullary folds broader, the notochord has also become more 
 expanded: the other features are the same as in section I. In 
 the third section (fig. 15) the notochord is still more expanded; 
 
 1 In the figure the notochord has been made too distinct. 
 
PRIMITIVE GROOVE IN THE EMBRYO CHICK. 4$ 
 
 the bottom of the now much expanded medullary groove has 
 become raised to form the ridge which separates the medullary 
 from the primitive groove. The medullary folds are also flatter 
 and broader than in the previous section. Section 4 (fig. 16) 
 passes through the anterior end of the primitive groove. Here 
 the notochord is no longer visible, and the adherence between 
 the mesoblast and epiblast below the primitive groove comes 
 out in marked contrast with the entire separation of the two 
 layers in the previous sections. 
 
 The medullary folds (A] are still visible outside the raised 
 edges of the primitive groove, and are as distinctly as possible 
 separate and independent formations, having no connection with 
 the folds of the primitive groove. In the last section (fig. 17), 
 which is taken some way behind section 4, no trace of the 
 medullary folds is any longer to be seen, and the primitive 
 groove has become deeper. This series of sections, taken in 
 conjunction with the specimen figured in fig. 18, must remove all 
 possible doubt as to the total and entire independence of the 
 primitive and medullary grooves. They arise in different parts 
 of the blastoderm ; the one reaches its maximum growth before 
 the other has commenced to be formed ; and finally, they are 
 distinguished by almost every possible feature by which two 
 such grooves could be distinguished. 
 
 Soon after the formation of the notochord, the proto-vertebrae 
 begin to be formed along the sides of the medullary groove (PI. 
 I, fig. 19, pv). Each new proto-vertebra (of those which are 
 formed from before backwards) arises just in front of the an- 
 terior end of the primitive groove. As growth continues, the 
 primitive groove becomes pushed further and further back, and 
 becomes less and less conspicuous, till at about thirty-six hours 
 only a very small and curved remnant is to be seen behind the 
 sinus rhomboidalis ; but even up to the forty-ninth Dursy has 
 been able to distinguish it at the hinder end of the embryo. 
 
 The primitive groove in the chick is, then, a structure which 
 appears very early, and soon disappears without entering di- 
 rectly into the formation of any part of the future animal, and 
 without, so far as I can see, any function whatever. It is clear, 
 therefore, that the primitive groove must be the rudiment of 
 some ancestral feature ; but whether it is a rudiment of some 
 
46 PRIMITIVE GROOVE IN THE EMBRYO CHICK. 
 
 structure which is to be found in reptiles, or whether of some 
 earlier form, I am unable to decide. It is just possible that it 
 is the last trace of that involution of the epiblast by which the 
 hypoblast is formed in most of the lower animals. The fact that 
 it is formed in the hinder part of the pellucid area perhaps tells 
 slightly in favour of this hypothesis, since the point of involution 
 of the epiblast not unfrequently corresponds with the position of 
 the anus. 
 
 EXPLANATION OF PLATE I. Figs. 68 and 1319. 
 
 Figs. 6 and 7 are sections through an embryo rather earlier than the one drawn 
 in fig. 8. Fig. 6 passes through the just commencing medullary groove (nid), 
 which appears in fresh specimens, as in fig. 8, merely as an opaque streak coming 
 from the end of the primitive groove. The notochord is hardly differentiated, but the 
 complete separation of mesoblast and hypoblast under the primitive groove is clearly 
 shewn. Fig. 7 passes through the anterior end of the primitive groove (pr), and 
 shews the fusion between the mesoblast and epiblast, which is always to be found 
 under the primitive groove. 
 
 Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a transparent 
 object. Primitive groove (pr). Medullary groove (md), which passes off from the 
 anterior end of the primitive groove, and is produced by the thickening of the meso- 
 blast. Headfold (//). 
 
 Figs. 13 17 are sections through the blastoderm, drawn in fig. 18 through the 
 lines i, 2, 3, 4, 5 respectively. 
 
 The first section (fig. 13) passes through the true medullary groove (me); the two 
 medullary folds (A, A) are seen on each side with the thickened mesoblast, and the 
 mesoblast cells are beginning to form the notochord (nc) under the medullary groove. 
 There is no adherence between the mesoblast cells and the epiblast under the me- 
 dullary groove. 
 
 The second (fig. 14) section passes through the medullary groove where it has 
 become wider. Medullary folds, A, A ; notochord, ch. 
 
 In the third section (fig. 15) the notochord (ch) is broader, and the epiblast is 
 raised in the centre, while the medullary folds are seen far apart at A. 
 
 In section fig. 16 the medullary folds (A) are still to be seen enclosing the anterior 
 end of the primitive groove (pr). Where the primitive groove appears there is a 
 fusion of the epiblast and mesoblast, and no appearance of the notochord. 
 
 In the last section, fig. 17, no trace is to be seen of the medullary folds. 
 
 Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is 
 twenty-three hours old ; fig. 19 twenty-five hours. They both shew how the medullary 
 canal arises entirely independently of the primitive groove and in front of it, and also 
 how the primitive groove gets pushed backwards by the growth of the medullary 
 groove, pv, Proto-vertebrse ; other references as above. Fig. 18 is the blastoderm from 
 which sections figs. 13 17 were cut. 
 
IV. THE DEVELOPMENT OF THE BLOOD-VESSELS OF 
 
 THE CHICK 1 . 
 With Plate II. 
 
 THE development of the first blood-vessels of the yolk-sac 
 of the chick has been investigated by a large number of ob- 
 servers, but with very discordant results. A good historical 
 resume of the subject will be found in a paper of Dr Klein 
 (liii. Band der K. Akad. der Wissensch. Wien], its last in- 
 vestigator. 
 
 The subject is an important one in reference to the homo- 
 logies of the blood-vascular system of the vertebrata. As I 
 shall shew in the sequel (and on this point my observations 
 agree with those of Dr Klein), the blood-vessels of the chick 
 do not arise as spaces or channels between the cells of the 
 mesoblast ; on the contrary, they arise as a network formed by 
 the united processes of mesoblast-cells, and it is through these 
 processes, and not in the spaces between them, that the blood 
 flows. It is, perhaps, doubtful whether a system of vessels 
 arising in this way can be considered homologous with any 
 vascular system which takes its origin from channels hollowed 
 out in between the cells of the mesoblast. 
 
 My own researches chiefly refer to the development of the 
 blood-vessels in the pellucid area. I have worked but very 
 slightly at their development in the vascular area ; but, as far 
 as my observations go, they tend to prove that the mode of 
 their origin is the same, both for the pellucid and the vascular 
 area. 
 
 The method which I have principally pursued has been to 
 examine the blastoderm from the under surface. It is very 
 difficult to obtain exact notions of the mode of development of 
 
 1 From the Quarterly Journal of Microscopical Science^ Vol. XIII, 1873. 
 
48 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 the blood-vessels by means of sections, though these come in as 
 a valuable confirmation of the other method. 
 
 For the purpose of examination I have employed (i) fresh 
 specimens ; (2) specimens treated with spirit, and then mounted 
 in glycerine ; (3) specimens treated with chloride of gold for about 
 half a minute, and then mounted in glycerine ; and (4) specimens 
 treated with osmic acid. 
 
 All these methods bring out the same appearances with 
 varying clearness ; but the successful preparations made by 
 means of the gold chloride are the best, and bring out the 
 appearances with the greatest distinctness. 
 
 The first traces of the blood-vessels which I have been able 
 to distinguish in the pellucid area are to be seen at about the 
 thirtieth hour or slightly earlier, at about the time when there 
 are four to five proto-vertebrae on each side. 
 
 Fig. i shews the appearance at this time. Immediately 
 above the hypoblast there are certain cells whose protoplasm 
 sends out numerous processes. These processes vary consider- 
 ably in thickness and size, and quickly come in contact with 
 similar processes from other cells, and unite with them. 
 
 I have convinced myself, by the use of the hot stage, that 
 these processes continually undergo alteration, sometimes uniting 
 with other processes, sometimes becoming either more elongated 
 and narrower or broader and shorter. In this way a network of 
 somewhat granular protoplasm is formed with nuclei at the 
 points from which the processes start. 
 
 From the first a difference may be observed in the character 
 of this network in different parts of the pellucid area. In the 
 anterior part the processes are less numerous and thicker, the 
 nuclei fewer, and the meshes larger ; while in the posterior part 
 the processes are generally very numerous, and at first thin, the 
 meshes small, and the nuclei more frequent. As soon as this 
 network commences to be formed the nuclei begin to divide. 
 I have watched this take place with the hot stage. It begins 
 by the elongation of the nucleus and division of the nucleolus, 
 the parts of which soon come to occupy the two ends of the 
 nucleus. The nucleus becomes still longer and then narrows 
 in the centre and divides. By this means the nuclei become 
 much more numerous, and are found in almost all the larger 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 49 
 
 processes. Whether they are carried out into the processes 
 by the movement of the surrounding protoplasm, or whether 
 they move through the protoplasm, I have been unable to 
 determine ; the former view, however, seems to be the most 
 probable. 
 
 It is possible that some nuclei arise spontaneously in the 
 protoplasm, but I am much more inclined to think that they 
 are all formed by the division of pre-existing nuclei a view 
 favoured by the number of nuclei which are seen to possess two 
 nucleoli. Coincidently with the formation of the new nuclei 
 the protoplasm of the processes, as well as that surrounding the 
 nuclei at the starting-points of the processes, begins to increase 
 in quantity. 
 
 At these points the nuclei also increase more rapidly than 
 elsewhere, but at first the resulting nuclei seem to be all of the 
 same kind. 
 
 In the anterior part of the pellucid area (fig. 4) the increase 
 in the number of nuclei and in the amount of protoplasm at the 
 starting-points of the protoplasm is not very great, but in the 
 posterior part the increase in the amount of the protoplasm at 
 these points is very marked, and coincidently the increase in 
 number of the nuclei is also great. This is shewn in figs. 2 
 and 3. These are both taken from the tail end of an embryo 
 of about thirty-three hours, with seven or eight proto-vertebrae. 
 Fig. 3 shews the processes beginning to increase in thickness, 
 and also the protoplasm at the starting-points increasing in 
 quantity ; at the same time the nuclei at these points are be- 
 ginning to become more numerous. Fig. 3 is taken from a 
 slightly higher level, i. e. slightly nearer the epiblast. In it 
 the protoplasm is seen to have increased still more in quantity, 
 and to be filled with nuclei. These nuclei have begun to be 
 slightly coloured, and one of them is seen to possess two 
 nucleoli. 
 
 Very soon after this a change in the nuclei begins to be 
 observed, more especially in the hinder part of the embryo. 
 While before this time they were generally elongated, some of 
 them now become more nearly circular. In addition to this, 
 they begin to have a yellowish tinge, and the nuclei, when 
 treated with gold (for in the fresh condition it is not easy to 
 B. 4 
 
5O DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 see them distinctly), have a more jagged and irregular appear- 
 ance than the nucleoli of the other nuclei. 
 
 This change takes place especially at the starting-points of 
 the processes, so that the appearance presented (fig. 5) is that 
 of spherical masses of yellowish nuclei connected with other 
 similar spherical masses by protoplasmic processes, in which 
 nuclei of the original type are seen imbedded. These masses 
 are surrounded by a thin layer of protoplasm, at the edge of 
 which a normal nucleus may here and there be detected, as at 
 fig. 5 a and a, the latter possessing two nucleoli. Some of 
 these processes are still very delicate, and it is exceedingly 
 probable that they undergo further changes of position before 
 the final capillary system is formed. 
 
 These differentiated nuclei are the first stage in the forma- 
 tion of the blood-corpuscles. From their mode of formation 
 it is clear that the blood-corpuscles of the Sauropsida are to be 
 looked upon as nuclei containing nucleoli, rather than as cells 
 containing nuclei ; indeed, they seem to be merely ordinary 
 nuclei with red colouring matter. 
 
 This would make them truly instead of only functionally 
 homologous with the red corpuscles of the Mammalia, and 
 would well agree with the fact that the red corpuscles of 
 Mammalia, in their embryonic condition, possess what have 
 previously been called nuclei, but which might perhaps more 
 properly be called nucleoli. 
 
 In the anterior part of the blastoderm the processes, as I 
 have stated, are longer and thinner, and the spaces enclosed 
 between them are larger. This is clearly brought out in 
 PI. 2, fig. 4. But, besides these large spaces, there are 
 other smaller spaces, such as that at v. It is, on account of 
 the transparency of the protoplasm, very difficult to decide 
 whether these are vacuoles or simply spaces enclosed by the 
 processes, but I am inclined to think that they are merely 
 spaces. The difficulty of exactly determining this point is 
 increased by the presence of numerous white-yolk spherules 
 in the hypoblast above, which considerably obscure the view. 
 At about the same time that the blood-corpuscles appear in 
 the posterior end of the pellucid area, or frequently a little 
 later, they begin to be formed in the anterior part also. The 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 51 
 
 masses of them are, however, far smaller and far fewer than 
 in the posterior part of the embryo. It is at the tail end of 
 the pellucid area i that the chief formation of blood-corpuscles 
 takes place. 
 
 The part of the pellucid area intermediate in position be- 
 tween the anterior and posterior ends of the embryo is likewise 
 intermediate as regards the number of corpuscles formed and 
 the size of the spaces between the processes ; the spaces being 
 here larger than at the posterior extremity, but smaller than 
 the spaces in front. Close to the sides of the embryo the spaces 
 are, however, smaller than in any other part of the pellucid 
 area. It is, however, in this part that the first formation of 
 blood-corpuscles takes place, and that the first complete capil- 
 laries are formed. 
 
 We have then somewhat round protoplasmic masses filled 
 with blood-corpuscles and connected by means of processes, a 
 few of which may begin to contain blood-corpuscles, but the 
 majority of which only contain ordinary nuclei. The next 
 changes to be noticed take place in the nuclei which were not 
 converted into blood-corpuscles, but which were to be seen in 
 the protoplasm surrounding the corpuscles. They become more 
 numerous and smaller, and, uniting with the protoplasm in 
 which they were imbedded, become converted into flat cells 
 (spindle-shaped in section), and in a short time form an entire 
 investment for the masses of blood-corpuscles. The same 
 change also occurs in the protoplasmic processes which con- 
 nect the masses of corpuscles. In the case of those processes 
 which contain no corpuscles the greater part of their protoplasm 
 seems to be converted into the protoplasm of the spindle-shaped 
 cells. The nuclei arrange themselves so as completely to sur- 
 round the exterior of the protoplasmic processes. In this way 
 each process becomes converted into a hollow tube, completely 
 closed in by cells formed from the investment of the original 
 nuclei by the protoplasm which previously formed the solid 
 processes. The remainder of the protoplasm probably becomes 
 fluid, and afterwards forms the plasma in which the corpuscles 
 float. While these changes are taking place the formation of 
 the blood-corpuscles does not stand still, and by the time a 
 system of vessels, enclosed by cellular walls, is formed out of 
 
 42 
 
52 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 the protoplasmic network, a large number of the connecting 
 processes in this network have become filled with blood-cor- 
 puscles. The appearances presented by the network at a 
 slightly later stage than this is shewn in PI. 2, fig. 6, but in 
 this figure all the processes are seen to be filled with blood- 
 corpuscles. 
 
 This investment of the masses of corpuscles by a cellular 
 wall occurs much earlier in some specimens than in others, both 
 in relation to the time of incubation and to the completion of 
 the network. It is generally completed in some parts by the 
 time there are eight or nine proto-vertebrae, and is almost 
 always formed over a great part of the pellucid area by the 
 thirty-sixth hour. The formation of the corpuscles, as was 
 pointed out above, occurs earliest in the central part of the 
 hour-glass shaped pellucid area, and latest in its anterior part. 
 In the hinder part of the pellucid area the processes, as well 
 as their enlarged starting-points, become entirely filled with 
 corpuscles ; this, however, is by no means the case in its an- 
 terior part. Here, although the corpuscles are undoubtedly 
 developed in parts as shewn in fig. 7, yet a large number of 
 the processes are entirely without them. Their development, 
 moreover, is in many cases very much later, When the de- 
 velopment has reached the stage described, very little is re- 
 quired to complete the capillary system. There are always, of 
 course, a certain number of the processes which end blindly, 
 and others are late in their development, and are not by this 
 time opened ; but, as a general rule, when the cellular invest- 
 ment is formed for the masses of corpuscles, there is completed 
 an open network of tubes with cellular walls, which are more or 
 less filled with corpuscles. These become quickly driven into 
 the opaque area in which at that time more corpuscles may 
 almost always be seen than in the pellucid area. 
 
 By the formation of a network of this kind it is clear that 
 there must result spaces enclosed between the walls of the 
 capillaries ; these spaces have under the microscope somewhat 
 the appearance of being vesicles enclosed by walls formed of 
 spindle-shaped cells. In reality they are only spaces enclosed 
 at the sides, and, as a general rule, not above and below. 
 They have been mistaken by some observers for vesicles in 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 53 
 
 which the corpuscles were supposed to be developed, and to 
 escape by the rupture of the walls into the capillary spaces 
 between. This mistake has been clearly pointed out by Klein 
 (loc. cit.). 
 
 At the time when these spaces are formed, and especially 
 in the hinder two-thirds of the pellucid area, and in the layer 
 of blood-vessels immediately above the hypoblast, a formation 
 takes place which forms in appearance a secondary investment 
 of the capillaries. Dr Klein was the first to give a correct ac- 
 count of this formation. It results from the cells of the meso- 
 blast in the meshes of the capillary system. Certain of these 
 cells become flattened, and send out fine protoplasmic processes. 
 They arrange themselves so as completely to enclose the spaces 
 between the capillaries, forming in this way vesicles. 
 
 Where seen on section (vide fig. 6) at the edge of the vesicles 
 these cells lining the vesicles appear spindle-shaped, and look 
 like a secondary investment of the capillaries. This investment 
 is most noticeable in the hinder two-thirds of the pellucid area ; 
 but, though less conspicuous, there is a similar formation in its 
 anterior third, where there would seem to be only veins present. 
 Dr Klein (loc. cit., fig. 12) has also drawn this investment in the 
 anterior third of the pellucid area. He has stated that the 
 vessels in the mesoblast between the splanchnopleure and the 
 somatopleure, and which are enclosed by prolongations from the 
 former, do not possess this secondary investment ; he has also 
 stated that the same is true for the sinus terminalis ; but I am 
 rather doubtful whether the generalisation will hold, that veins 
 and arteries can from the first be distinguished by the latter 
 possessing this investment. I am also rather doubtful whether 
 the spaces enclosed by the protoplasmic threads between the 
 splanchnopleure and somatopleure are the' centres of vessels at 
 all, since I have never seen any blood-corpuscles in them. 
 
 It is not easy to learn from sections much about the first 
 stages in the formation of the capillaries, and it is impossible 
 to distinguish between a completely-formed vessel and a mere 
 spherical space. The fine protoplasmic processes which connect 
 the masses of corpuscles can rarely be seen in sections, except 
 when they pass vertically, as they do occasionally (vide PI. 2. 
 fig- 9) i tne opaque area, joining the somatopleure and the 
 
54 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 splanchnopleure. Dr Klein considers these latter processes to 
 be the walls of the vessels, but they appear rather to be the 
 processes which will eventually become new capillaries. 
 
 From sections, however, it is easy to see that the appear- 
 ances of the capillaries in the vascular area are similar to the 
 appearances in the pellucid area, from which it is fair to con- 
 clude that their mode of formation is the same in both. It is 
 also easy to see that the first formation of vessels occurs in the 
 splanchnopleure, and that even up to the forty-fifth hour but few 
 or no vessels are found in the somatopleure. The mesoblast of 
 the somatopleure is continued into the opaque area as a single 
 layer of spindle-shaped cells. 
 
 Sections clearly shew in the case of most of the vessels that 
 the secondary investment of Klein is present, even in the case of 
 those vessels which lie immediately under the somatopleure. 
 
 In reference to the origin of particular vessels I have not 
 much to say. Dr Klein's account of the origin of the sinus 
 terminalis is quite correct. It arises by a number of the 
 masses of blood-corpuscles, similar to those described above, 
 becoming connected together by protoplasmic processes. The 
 whole is subsequently converted into a continuous vessel in the 
 usual way. 
 
 From the first the sinus terminalis possesses cellular walls, 
 as is clear from its mode of origin. I am inclined to think 
 that Klein is right in saying that the aortae arise in a similar 
 manner, but I have not worked out their mode of origin very 
 fully. 
 
 It will be seen from the account given above that, in refer- 
 ence to the first stages in the development of the blood-vessels, 
 my observations differ very considerably from those of Dr Klein ; 
 as to the later stages, however, we are in tolerable agreement. 
 We are in agreement, moreover, as to the fundamental fact that 
 the blood-vessels are formed by a number of cells becoming 
 connected, and by a series of changes converted into a network 
 of vessels, and that they are not in the first instance merely 
 channels between the cells of the mesoblast. 
 
 By the forty-fifth hour colourless corpuscles are to be found 
 in the blood whose exact origin I could not determine ; pro- 
 bably they come from the walls of the capillaries. 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 55 
 
 In the vessels themselves the coloured corpuscles undergo 
 increase by division, as has already been shewn by Remak. 
 Corpuscles in the various stages of division may easily be found. 
 They do not appear to show very active amoeboid movementsjn 
 the vessels, though their movements are sometimes very active 
 when removed from the body. 
 
 To recapitulate some of the cells of the mesoblast of the 
 splanchnopleure send out processes, these processes unite with 
 the processes from other cells, and in this way a network is 
 formed. The nuclei of the original cells divide, and at the 
 points from which the processes start their division is especially 
 rapid. Some of them acquire especially at these points a red 
 colour, and so become converted into blood-corpuscles ; the 
 others, together with part of the protoplasm in which they are 
 imbedded, become converted into an endothelium both for the 
 processes and the masses of corpuscles ; the remaining proto- 
 plasm becomes fluid, and thus the original network of the cells 
 becomes converted into a network of hollow vessels, filled with 
 fluid, in which corpuscles float. 
 
 In reference to the development of the heart, my observa- 
 tions are not quite complete. It is, however, easy to prove 
 from sections (vide figs. 10 and II, PL 2) that the cavity of the 
 heart is produced by a splitting or absorption of central cells 
 of the thickened mesoblast of the splanchnopleure, while its 
 muscular walls are formed from the remaining cells of this 
 thickened portion. It is produced in the following way : 
 When the hypoblast is folded in to form the alimentary canal 
 the mesoblast of the splanchnopleure follows it closely, and 
 where the splanchnopleure turns round to assume its normal 
 direction (fig. 11) its mesoblast becomes thickened. This thick- 
 ened mass of mesoblast is, as can easily be seen from figs. 10 
 and 11, PI. 2, entirely distinct from the mesoblast which forms 
 the outside walls of the alimentary canal. At the point where 
 this thickening occurs an absorption takes place to form the 
 cavity of the heart. The method in which the cavity is formed 
 can easily be seen from figs. 10 and n. It is in fig. 11 shewn 
 as it takes place in the mesoblast on each side, the folds 
 of the splanchnopleure not having united in the middle line ; 
 and hence a pair of cavities are formed, one on each side. It 
 
56 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 is, however, probable that, in the very first formation of the 
 heart, the cavity is single, being formed after the two ends of 
 the folded mesoblast have united (vide h z, fig. 10). In some 
 cases the two folds of the mesoblast appear not at first to 
 become completely joined in the middle line ; in this case the 
 cavity of the heart is still complete from side to side, but the 
 mesoblast-cells which form its muscular walls are deficient 
 above. By the process of absorption, as I said, a cavity is 
 produced in the thickened part of the mesoblast of the splanch- 
 nopleure, a cavity which is single in front, but becomes divided 
 further behind, where the folds of the mesoblast have not united, 
 into two cavities, to form the origin of the omphalomeseraic 
 veins. As the folding proceeds backwards the starting-point 
 of the omphalomeseraic veins is also pushed backwards, and 
 the cavities which were before separated become joined to- 
 gether. From its first formation the heart is lined internally 
 by an endothelium ; this is formed of flattened cells, spindle- 
 shaped in section. The exact manner of the origin of this 
 lining I have not been able to determine ; it is, however, probable 
 that some of the central mesoblast-cells are directly converted 
 into the cells of the endothelium. 
 
 I have obtained no evidence enabling me to determine 
 whether Dr Klein is correct in stating that the cells of the 
 mesoblast in the interior of the heart become converted partly 
 into blood-corpuscles and partly into a cellular lining forming 
 the endothelium of the heart, in the same way that the blood- 
 vessels in the rest of the blastoderm are formed. But I should 
 be inclined to think that it is very probable certainly more 
 probable than that the cavity of the heart is formed by a pro- 
 cess of splitting taking place. Where I have used the word 
 " absorption " in speaking of the formation of the cavity of the 
 heart, I must be understood as implying that certain of the 
 interior cells become converted into the endothelium, while 
 others either form the plasma or become blood-corpuscles. 
 
 The originally double formation of the hinder part of the 
 heart probably explains Dr Afanassiev's statement (Bulletin de 
 rAcadem. Impe'riale de St Petersb., torn, xiii, pp. 321 335), that 
 he finds the endothelium of the heart originally dividing its 
 interior into two halves ; for when the partition of the mesoblast 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 57 
 
 which separated at first the two halves of the heart became 
 absorbed, the endothelium lining of each of the originally sepa- 
 rate vessels would remain complete, dividing the cavity of the 
 heart into two parts. The partition in the central line is, Jiow- 
 ever, soon absorbed. 
 
 The account given above chiefly differs from that of Remak 
 by not supposing that the mesoblast-cells which form the heart 
 are in any way split off from the wall of the alimentary canal. 
 
 There can be no doubt that His is wrong in supposing that 
 the heart originates from the mesoblast of the splanchnopleure 
 and somatopleure uniting to form its walls, thus leaving a cavity 
 between them in the centre. The heart is undoubtedly formed 
 out of the mesoblast of the splanchnopleure only. 
 
 Afanassiev's observations are nearer to the truth, but there 
 are some points in which he has misinterpreted his sections. 
 
 Sections PI. 2, figs. 10 and 11, explain what I have just said 
 about the origin of the heart. Immediately around the noto- 
 chord the mesoblast is not split, but a very little way outside it 
 is seen to be split into two parts so and sp ; the former of these 
 follows the epiblast, and together with it forms the somatopleure, 
 which has hardly begun to be folded at the line where the sec- 
 tions are taken. The latter (sp) forms with the hypoblast (liy} 
 the splanchnopleure, and thus has become folded in to form 
 the walls of the alimentary canal (d). In fig. 11 the folds have . 
 not united in the central line, but in fig. 10 they have so united. 
 In fig. n, where the mesoblast, still following the hypoblast, 
 turns back to assume its normal direction, it is seen to be 
 thickened and to have become split, so that a cavity (of) (of 
 the omphalomeseraic vein) is formed in it on each side, lined by 
 endothelium. 
 
 In the section immediately behind section fig. 11 the meso- 
 blast was thickened, but had not become split. 
 
 In fig. 10 the hypoblast folds are seen to have united in the 
 centre, so as to form a completely closed digestive canal (d) ; the 
 folds of the mesoblast have also united, so that there is only a 
 single cavity in the heart (hz), lined, as was the case with the 
 omphalomeseraic veins, by endothelium. 
 
 In conclusion, I have to thank Dr Foster for his assistance 
 and suggestions throughout the investigations which have formed 
 
58 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 
 
 the subject of these three short papers, and which were well 
 carried on in the apartments used by him as a Physiological 
 Laboratory. 
 
 EXPLANATION OF PLATE 2. 
 
 Fig. i is taken from the anterior part of the pellucid area of a thirty hours' chick, 
 with four proto-vertebrse. At n is a nucleus with two nucleoli. 
 
 Figs. 2 and 3 are taken from the posterior end of the pellucid area of a chick 
 with eight proto-vertebras. In fig. 3 the nuclei are seen to have considerably in- 
 creased in number at the points of starting of the protoplasmic processes. At n is 
 seen a nucleus with two nucleoli. 
 
 Fig. 4 is taken from the anterior part of the pellucid area of an embryo of thirty- 
 six hours. It shews the narrow processes characteristic of the anterior part of the 
 pellucid area, and the fewer nuclei. Small spaces, which have the appearance of 
 vacuoles, are shewn at v. 
 
 Fig. 5 is taken from the posterior part of the pellucid area of a thirty-six hours' 
 embryo. It shews the nuclei, with somewhat irregular nucleoli, which have begun 
 to acquire the red colour of blood-corpuscles ; the protoplasmic processes con- 
 taining the nuclei ; the nuclei in the protoplasm surrounding the corpuscles, as 
 shewn at a, a'. 
 
 Fig. 6 shews fully formed blood-vessels, in part filled with blood-corpuscles and 
 in part empty. The walls of the capillaries, formed of cells, spindle-shaped in sec- 
 tion, are shewn, and also the secondary investment of Klein at k, and at b is seen a 
 narrow protoplasmic process filled with blood-corpuscles. 
 
 Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six hours' 
 embryo. It shews a collection of nuclei which are beginning to become blood- 
 corpuscles. 
 
 Figs, i 5 are drawn with an ^ object-glass. Fig. 6 is on a much smaller scale. 
 Fig. 7 is intermediate. 
 
 Fig. 8. A transverse section through the dorsal region of a forty-five hours' em- 
 bryo ; ao, aorta with a few blood-corpuscles, v, Blood-vessels, all of them being 
 formed in the splanchnopleure, and all of them provided with the secondary invest- 
 ment of Klein ; /, e, pellucid area ; o, p, opaque area. 
 
 Fig. 9. Small portion of a section through the opaque area of a thirty-five hours' 
 embryo, showing protoplasmic processes, with nuclei passing from the somatopleure 
 to the splanchnopleure. 
 
 Fig. 10. Section through the heart of a thirty-four hours' embryo, a. Alimen- 
 tary canal ; hb, hind brain ; nc, notochord ; e, epiblast ; s, o, mesoblast of the soma- 
 topleure ; sp, mesoblast of the splanchnopleure ; hy, hypoblast ; hz, cavity of the 
 heart. 
 
DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 59 
 
 Fig. ii. Section through the same embryo as fig. 10, and passing through the 
 orifice of the omphalo-meseraic vein, of, Omphalo-meseraic vein ; other references 
 as above. 
 
 These two sections shew that the heart is entirely formed from the mesoblast of 
 the splanchnopleure, and that it is formed by the splitting of that part of the meso- 
 blast which has turned to assume its normal direction after being folded in to form 
 the muscular wall of the alimentary canal. In fig. 1 1 the cavities so formed on each 
 side have not yet united, but in fig. 10 they have united. When the folding be- 
 comes more complete the cavities (of, of) in fig. 1 1 will unite, and in this way the 
 origin of the omphalo-meseraic veins will be carried further backwards. In the sec- 
 tion immediately behind section 1 1 the mesoblast had become thickened, but had not 
 split. 
 
V. A PRELIMINARY ACCOUNT OF THE DEVELOPMENT OF 
 THE ELASMOBRANCH FiSHES 1 . 
 
 With Plates 3 and 4. 
 
 DURING the spring of the present year I was studying at 
 the Zoological Station, founded by Dr Dohrn at Naples, and 
 entirely through its agency was supplied with several hundred 
 eggs of various species of Dog-fish (Selachii) a far larger 
 number than any naturalist has previously had an opportunity 
 of studying.' The majority of the eggs belonged to an oviparous 
 species of Mustelus, but in addition to these I had a considerable 
 number of eggs of two or three species of Scyllium, and some of 
 the Torpedo. Moreover, since my return to England, Professor 
 Huxley has most liberally given me several embryos of Scy Ilium 
 stellare in a more advanced condition than I ever had at Naples, 
 which have enabled me to fill up some lacunae in my observa- 
 tions. 
 
 On many points my investigations are not yet finished, but I 
 have already made out a number of facts which I venture to 
 believe will add to our knowledge of vertebrate embryology ; 
 and since it is probable that some time will elapse before I am 
 able to give a complete account of my investigations, I have 
 thought it worth while preparing a preliminary paper in which I 
 have briefly, but I hope in an intelligible manner, described some 
 of the more interesting points in the development of the Elas- 
 mobranchii. The first-named species (Mustelus sp.?) was alone 
 used for the early stages, for the later ones I have also employed 
 the other species, whose eggs I have had ; but as far as I have 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xiv. 1874. 
 Read in Section D, at the Meeting of the British Association at Belfast. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 6l 
 
 seen at present, the differences between the various species in 
 early embryonic life are of no importance. 
 
 Without further preface I will pass on to my investigations. 
 
 The Egg-shell. 
 
 In the eggs of all the species of Dog-fishes which I have ex- 
 amined the yolk lies nearest that end of the quadrilateral shell 
 which has the shortest pair of strings for attachment. This is 
 probably due to the shape of the cavity of the shell, and is 
 certainly not due to the presence of any structures similar to 
 chalazae. 
 
 The Yolk. 
 
 The yolk is not enclosed in any membrane comparable to 
 the vitelline membrane of Birds, but lies freely in a viscid albu- 
 men which fills up the egg-capsule. It possesses considerable 
 consistency, so that it can be removed into a basin, in spite of 
 the absence of a vitelline membrane, without falling to pieces. 
 This consistency is not merely a property of the yolk-sphere as 
 a whole, but is shared by every individual part of it. 
 
 With the exception of some finely granular matter around 
 the blastoderm, the yolk consists of rather small, elliptical, highly 
 refracting bodies, whose shape is very characteristic and renders 
 them easily recognizable. A number of striae like those of 
 muscle are generally visible on most of the spherules, which give 
 them the appearance of being in the act of breaking up into a 
 series of discs; but whether these striae are normal, or produced 
 by the action of water I have not determined. 
 
 Position of the Blastoderm. . 
 
 The blastoderm is always situated, immediately after impreg- 
 nation, near the pole of the yolk which lies close to the end of 
 the egg-capsule. Its position varies a little in the different 
 species and is not quite constant in different eggs of the same 
 species. But this general situation is quite invariable. It is of 
 about the same proportional size as the blastoderm of a bird. 
 
 Segmentation. 
 
 In a fresh specimen, in which segmentation has only just 
 commenced, the blastoderm or germinal disc appears as a circu- 
 
62 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 lar disc, distinctly marked off by a dark line from the rest of the 
 yolk. This line, as is proved by sections, is the indication of a 
 very shallow groove. The appearance of sharpness of distinc- 
 tion between the germ and the yolk is further intensified by 
 their marked difference of colour, the germ itself being usually 
 of a darker shade than the remainder of the yolk ; while around 
 its edge, and apparently sharply separated from it by the groove 
 before mentioned, is a ring of a different shade which graduates 
 at its outer border into the normal shade of the yolk. 
 
 These appearances are proved by transverse sections to be 
 deceptive. There is no sharp line either at the sides or below 
 separating the blastoderm from the yolk. In the passage be- 
 tween the fine granular matter of the germ to the coarser yolk- 
 spheres every intermediate size of granule is present; and, 
 though the space between the two is rather narrow, in no sense' 
 of the word can there be said to be any break or line between 
 them. 
 
 This gradual passage stands in marked contrast with what 
 we shall find to be the case at the close of the segmentation. 
 In the youngest egg which I had, the germinal disc was already 
 divided into four segments by two furrows at right angles. 
 These furrows, however, did not reach its edge; and from my 
 sections I have found that they were not cut off below by any 
 horizontal furrow. So that the four segments were continuous 
 below with the remainder of the germ without a break. 
 
 In the next youngest specimen which I had, there were 
 already present eighteen segments, somewhat irregular in size, 
 but which might roughly be divided into an outer ring of larger 
 spheres, separated, as it were, by a circular furrow from an inner 
 series of smaller segments. The furrows in this case reached 
 quite to the edge of the germinal disc. 
 
 The remarks I made in reference to the earlier specimen 
 about the separation of the germ from the yolk apply in every 
 particular to the present one. The external limit of the blasto- 
 derm was not defined by a true furrow, and the segmentation 
 furrows still ended below without meeting any horizontal fur- 
 rows, so that the blastoderm was not yet separated by any line 
 from the remainder of the yolk, and the segments of which it 
 was composed were still only circumscribed upon five sides. In 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 63 
 
 this particular the segmentation in these animals differs materi- 
 ally from that in the Bird, where the horizontal furrows appear 
 very early. 
 
 In each segment a nucleus was generally to be seen in sec- 
 tions. I will, however, reserve my remarks upon the nature of 
 the nuclei till I discuss the nuclei of the blastoderm as a whole. 
 
 For some little time the peripheral segments continue larger 
 than the more central ones, but this difference of size becomes 
 less and less marked, and before the segments have become too 
 small to be seen with the simple microscope, their size appears 
 to be uniform over the whole surface of the blastoderm. 
 
 In the blastoderms somewhat older than the one last de- 
 scribed the segments have already become completely separate 
 masses, and each of them already possesses a distinct nucleus. 
 They form a layer one or two segments deep. The limits of the 
 blastoderm are not, however, defined by the already completed 
 segments, but outside these new segments continue to be formed 
 around nuclei which appear in the yolk. At this stage there is, 
 therefore, no line of demarcation between the germ and the yolk, 
 but the yolk is being bored into, so to speak, by a continuous 
 process of fresh segmentation. 
 
 The further segmentation of the already existing spheres, 
 and the formation of new ones from the yolk below and to the 
 sides, continues till the central cells acquire their final size, the 
 peripheral ones being still large, and undefined towards the yolk. 
 These also soon reach the final size, and the blastoderm then 
 becomes rounded off towards the yolk and sharply separated 
 from it. 
 
 The Nuclei of tJie Yolk. 
 
 Intimately connected with the segmentation is the appear- 
 ance and history of a number of nuclei which arise in the yolk 
 surrounding the blastoderm 
 
 When the horizontal furrows appear which first separate the 
 blastoderm from the yolk, the separation does not occur along 
 the line of passage from the fine to the coarse yolk, but in the 
 former at some distance from this line. 
 
 The blastoderm thus rests upon a mass of finely granular 
 material, from which, however, it is sharply separated. At this 
 
64 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 time there appear in this finely granular material a number of 
 nuclei of a rather peculiar character. 
 
 They vary immensely in size from that of an ordinary 
 nucleus to a size greater than the largest blastoderm-cell. 
 
 In PI. 3, fig. i, n, is shewn their distribution in this finely 
 granular matter and their variation in size. But whatever may 
 be their size, they always possess the same characteristic struc- 
 ture. This is shewn in PI. 3, figs. I and 2, ;/. 
 
 They are rather irregular in shape, with a tendency when 
 small to be roundish, and are divided by a number of lines into 
 distinct areas, in each of which a nucleolus is to be seen. The 
 lines dividing them into these areas have a tendency (in the 
 smaller specimens) to radiate from the centre, as shewn in PI. 3, 
 fig. I. 
 
 . These nuclei colour red with haematoxylin and carmine and 
 brown with osmic acid, while the nucleoli or granules contained 
 in the areas also colour very intensely with all the three above- 
 named reagents. 
 
 With such a peculiar structure, in favourable specimens these 
 nuclei are very easily recognised, and their distribution can be 
 determined without difficulty. They are not present alone in 
 the finely granular yolk, but also in the coarsely granular yolk 
 adjoining it. They form very often a special row, sometimes 
 still more markedly than in PI. 3, fig. i, along the floor 
 of the segmentation cavity. They are not, however, found 
 alone in the yolk. All the blastoderm-cells in the earlier stages 
 possess precisely similar nuclei ! From the appearance of the 
 first nucleus in a segmentation-sphere till a comparatively late 
 period in development, every nucleus which can be distinctly 
 seen is found to be of this character. In PL 3, fig. 2, this is 
 very distinctly shewn. 
 
 (i) We have, then, nuclei of this very peculiar character 
 scattered through the subgerminal granular matter, and also 
 universally present in the cells of the blastoderm. (2) These 
 nuclei are distributed in a special manner under the floor of 
 the segmentation cavity on which new cells are continually 
 appearing. Putting these two facts together, there would be 
 the strongest presumption that these nuclei do actually become 
 the nuclei of cells which enter the blastoderm, and such is 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 65 
 
 actually the case. In my account of the segmentation I have, 
 indeed, already mentioned this, and I will return to it, but 
 before doing so will enter more fully into the distribution of 
 these nuclei in the yolk. 
 
 They appear in small numbers around the blastoderm at 
 the close of segmentation, and round each one of them there 
 may at this time be seen in osmic acid specimens, and 
 with high powers, a fine network similar to but finer than 
 that represented in PI. 3, fig. 2. This network cannot, as 
 a general rule, be traced far into the yolk, but in some 
 exceptionally thin specimens it may be seen in any part of 
 the fine granular yolk around the blastoderm, the meshes of 
 the network being, however, considerably coarser between than 
 around the nuclei. This network may be seen in the fine 
 granular material around the germ till the latest period of 
 which I have yet cut sections of the blastoderm. In the later 
 specimens, indeed, it is very much more distinctly seen than 
 in the earlier, owing to the fact that in parts of the blastoderm, 
 especially under the embryo, the yolk-granules have disap- 
 peared partly or entirely, leaving only this fine network with 
 the nuclei in it. 
 
 A specimen of this kind is represented in PI. 3, fig. 2, 
 where the meshes of the network are seen to be finer 
 immediately around the nuclei, and coarser in the intervals. 
 The specimen further shows in the clearest manner that this 
 network is not divided into areas, each representing a cell and 
 each containing a nucleus. I do not know to what extent this 
 network extends into the yolk. I have never yet seen the 
 limits of it, though it is very common to see the coarsest yolk- 
 granules lying in its meshes. Some of these are shewn in 
 PL 3, fig. 2,yk. 
 
 This network of lines 1 (probably bubbles) is characteristic of 
 many cells, especially ova. We are, therefore, forced to believe 
 that the fine granular and probably coarser granular yolk of 
 this meroblastic egg consists of an active organized basis with 
 
 1 The interpretation of this network is entirely due to Dr Kleinenberg, who sug- 
 gested it to me on my shewing him a number of specimens exhibiting the nuclei and 
 network. 
 
 B. 5 
 
66 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 passive yolk-spheres imbedded in it. The organized basis is 
 especially concentrated at the germinal pole of the egg, but 
 becomes less and less in quantity, as compared with the yolk- 
 spheres, the further we depart from this. 
 
 Admitting, as I think it is necessary to do, the organized 
 condition of the whole yolk-sphere, there are two possible views 
 as to its nature. We may either take the view that it is one 
 gigantic cell, the ovum, which has grown at the expense of the 
 other cells of the egg-follicle, and that these cells in becoming 
 absorbed have completely lost their individuality; or we may 
 look upon the true formative yolk (as far as we can separate it 
 from the remainder of the food-yolk) as the remains of one cell 
 (the primitive ovum), and the remainder of the yolk as a body 
 formed from the coalescence of the other cells of the egg-follicle, 
 which is adherent to, but has not coalesced with, the primitive 
 ovum, the cells in this case not having completely lost their 
 individuality ; and to these cells, the nuclei, I have found, must 
 be supposed to belong. 
 
 The former view I think, for many reasons, the most pro- 
 bable. The share of these nuclei in the segmentation, and the 
 presence of similar nuclei in the cells of the germ, both support 
 it, and are at the same time difficulties in the way of the other 
 view. Leaving this question which cannot be discussed fully in 
 a preliminary paper like the present one, I will pass on to 
 another important question, viz. : 
 
 How do these nuclei originate ? Are they formed by the 
 division of the pre-existing nuclei, or by an independent for- 
 mation ? It must be admitted that many specimens are strongly 
 in favour of the view that they increase by division. In 
 the first place, they are often seen "two together;" examples 
 of this will be seen in PI. 3, fig. I. In the second place, 
 I have found several specimens in which five or six appear 
 close together, which look very much as if there had been an 
 actual division into six nuclei. It is, however, possible in 
 this case that the nuclei are really connected below and only 
 appear separate, owing to the crenate form of the mass. 
 Against this may be put the fact that the division of a 
 nucleus is by no means so common as has been sometimes 
 supposed, that in segmentation it has very rarely been ob- 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 67 
 
 served that the nucleus of a sphere first divides 1 , and that 
 then segmentation takes place, but segmentation generally 
 occurs and then a new nucleus arises in each of the newly 
 formed spheres. Such nuclei as I have described are rare^ 
 they have, however, been observed in the egg of a Neplielis 
 (one of the Leeches), and have in that case been said to 
 divide. Dr Kleinenberg, however, by following a single egg 
 through the whole course of its development, has satisfied 
 himself that this is not the case, and that, further, these nuclei 
 in Nephelis never form the nuclei of newly developing cells. 
 
 I must leave it an open question, and indeed one which can 
 hardly be solved from sections, whether these nuclei arise freely 
 or increase by division, but I am inclined to believe that both 
 processes may possibly take place. In any case their division 
 does not appear to determine the segmentation or segregation 
 of the protoplasm around them. 
 
 As was mentioned in my account of the segmentation, these 
 nuclei first appear during that process, and become the nuclei 
 of the freshly formed segmentation spheres. At the close of 
 segmentation a few of them are still to be seen around the 
 blastoderm, but they are not very numerous. 
 
 From this period they rapidly increase in number, up to the 
 commencement of the formation of the embryo as a body dis- 
 tinct from the germ. Though before this period they probably 
 become the nuclei of veritable cells which enter the germ, it is 
 not till this period, when the growth of the blastoderm becomes 
 very rapid and it commences to spread over the yolk, that these 
 new cells are formed in large numbers. I have many speci- 
 mens of this age which shew the formation of these new cells 
 with great clearness. This is most distinctly to be seen imme- 
 diately below the embryo, where the yolk-spherules are few 
 in number. At the opposite end of the blastoderm I believe 
 that more of these cells are formed, but, owing to the presence of 
 numerous yolk-spherules, it is much more difficult to make cer- 
 tain of this. 
 
 1 Kowalevsky (" Beitrage zur Entwickelungsgeschichte der Holothurien, " Mt- 
 moirs de fAc. Imp. de St Petersbourg, vii ser., Vol. xi. 1867) describes the division 
 of nuclei during segmentation in the Holothurians, and other observers have described 
 it elsewhere. 
 
 52 
 
68 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 As to the final destination of these cells, my observations 
 are not yet completed. Probably a large number of them are 
 concerned in the formation of the vascular system, but I will 
 give reasons later on for believing that some of them are con- 
 cerned in the formation of the walls of the digestive canal and 
 of other parts. 
 
 I will conclude my account of these nuclei by briefly 
 summarizing the points I have arrived at in reference to 
 them. 
 
 A portion, or more probably the whole, of the yolk of the 
 Dog-fish consists of organized material, in which nuclei ap- 
 pear and increase either by division or by a process of in- 
 dependent formation, and a great number of these subse- 
 quently become the nuclei of cells formed around them, 
 frequently at a distance from the germ, which then travel up 
 and enter it. 
 
 The formation of cells in the yolk, apart from the general 
 process of segmentation, has been recognised by many ob- 
 servers. Kupffer (Archiv. fur Micr. Anat., Bd. IV. 1868) and 
 Owsjannikow ('' Entwickelung der Coregonus," Bulletin der 
 Akad. St Petersburg^ Vol. XIX.) in osseous fishes 1 , Ray Lan- 
 kester (Annals and Mag. of Nat. Hist. Vol. XI. 1873, p. 81) in 
 Cephalopoda, Gotte (Archiv. fur Micr. Anat. Vol. X.) in the 
 chick, have all described a new formation of cells from the 
 so-called food-yolk. The organized nature of the whole 
 or part of this, previous to the formation of the cells from 
 it, has not, however, as a rule, been distinctly recognised. 
 In the majority of cases, as, for instance, in Loligo, the 
 nucleus is not the first thing to be formed, but a plastide is 
 first formed, in which a nucleus subsequently makes its ap- 
 pearance. 
 
 1 Gotte, at the end of a paper on "The Development of the Layers in the Chick " 
 (Archiv. Jiir Micr. Anat., Vol. X. 1873, P- J 9 6 ). mentions that the so-called cells in 
 Osseous fishes which Oellacher states to have migrated into the yolk, and which are 
 clearly the same as those mentioned by Owsjannikow, are really not cells, but large 
 nuclei. If this statement is correct the phenomena in Osseous fishes are precisely the 
 same as those I have described in the Dog-fish. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 69 
 
 Formation of tJie Layers. 
 
 Leaving these nuclei, I will now pass on to the formation 
 of the layers. 
 
 At the close of segmentation the surface of the blasto- 
 derm is composed of cells of a uniform size, which, however, 
 are too small to be seen by the aid of the simple micro- 
 scope. 
 
 The cells of this uppermost layer are somewhat columnar, 
 and can be distinguished from the remainder of the cells of the 
 blastoderm as a separate layer. This layer forms the epiblast ; 
 and the Dog-fish agree with Birds, Batrachians, and Osseous 
 fish in the very early differentiation of it. 
 
 The remainder of the cells of the blastoderm form a 
 mass, many cells deep, in which it is impossible as yet or 
 till a very considerably later period to distinguish two layers. 
 They may be called the lower layer cells. Some of them 
 near the edge of this mass are still considerably larger than 
 the rest, but they are, as a whole, of a fairly uniform size. 
 Their nuclei are of the same character as the nuclei in the 
 yolk. 
 
 There is one point to be noticed in the shape of the blas- 
 toderm as a whole. It is unsymmetrical, and a much larger 
 number of its cells are found collected at one end than at the 
 other. This absence of symmetry is found in all sections 
 which are cut parallel to the long axis of the egg-capsule. 
 The thicker end is the region where the embryo will subse- 
 quently appear. 
 
 This very early appearance of distinction in the blasto- 
 derm between the end at which the embryo will appear, and 
 the non-embryonic end is important, especially as it shews 
 the affinity of the modes of .development of Osseous fishes 
 and the Elasmobranchii. Oellacher (Zeitschrift fur Wiss. Zoo- 
 logie, Vol. XXXIII. 1873) has shewn, and, though differing from 
 him on many other points, on this point Gotte (Arch, fur Micr. 
 Anat. Vol. IX. 1873) agrees with him, that a similar absence of 
 symmetry by which the embryonic end of the germ is marked 
 off, occurs almost immediately after the end of segmentation 
 in Osseous fishes. In the early stages of development there are 
 
7O DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 a number of remarkable points of agreement between the 
 Osseous fish and the Dog-fish, combined with a number of 
 equally remarkable points of difference. Some of these I shall 
 point out as I proceed with my description. 
 
 The embryonic end of the germ is always the one which 
 points towards the pole of the yolk farthest removed from the 
 egg-capsule. 
 
 The germ grows, but not very rapidly, and without other- 
 wise undergoing any very appreciable change, for some time. 
 
 The growth at these early periods appears to be particularly 
 slow, especially when compared with the rapid manner in 
 which some of the later stages of the development are passed 
 through. 
 
 The next important change which occurs is the formation of 
 the so-called " segmentation cavity." 
 
 This forms a very marked feature throughout the early 
 stages. It appears, however, to have somewhat different re- 
 lations to the blastoderm than the homologous structure in 
 other vertebrates. In its earliest stage which I have observed, 
 it appears as a small cavity in the centre of the lower layer 
 cells. This grows rapidly, and its roof becomes composed 
 of epiblast and only a thin lining of " lower layer " cells, 
 while its floor is formed by the yolk (PL 3, fig. 3, s g}. In 
 the next and third stage (PI. 3, fig. 4, s g] its floor is 
 formed by a thin layer of cells, its roof remaining as before. 
 It has, however, become a less conspicuous formation than 
 it was ; and in the last (fourth) stage in which it can be 
 distinguished it is very inconspicuous, and almost filled up 
 by cells. 
 
 What I have called the second stage corresponds to a period 
 in which no trace of the embryo is to be seen. In the third 
 stage the embryonic end of the blastoderm projects outwards 
 to form a structure which I shall speak of as the " embryonic 
 rim," and in the fourth and last stage a distinct medullary 
 groove is formed. For a considerable period during the second 
 stage the segmentation cavity remains of about the same size ; 
 during the third stage it begins to be encroached upon, and 
 becomes smaller both absolutely, and relatively to the increased 
 size of the germ. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. /I 
 
 The segmentation cavity of the Dog-fish most nearly agrees 
 with that of Osseous fishes in its mode of formation and re- 
 lation to the embryo. 
 
 Dog-fish resemble Osseous fish in the fact that their em- 
 bryos are entirely formed from a portion of the germ which 
 does not form part of the roof of the segmentation cavity, so 
 that the cells forming the roof of the segmentation cavity 
 take no share at any time in the formation of their embryos. 
 They further agree with Osseous fish (always supposing that 
 the descriptions of Oellacher, loc. cit., and Gotte, Archiv. fur 
 Micr. Anat. Bd. IX. are correct) in the floor of the segmen- 
 tation cavity being formed at one period by yolk. Toge- 
 ther with these points of similarity there are some important 
 differences. 
 
 (1) The segmentation cavity in the Osseous fish from the 
 first arises as a cavity between the yolk and the blastoderm, and 
 its floor is never at any period covered with cells. In the Dog- 
 fish> as we have said above, both in the earlier and later periods 
 the floor is covered with cells. 
 
 (2) The roof in the Dog-fish is invariably formed by the 
 epiblast and a row of flattened lower layer cells. 
 
 According to both Gotte and Oellacher the roof of the 
 segmentation cavity in Osseous fishes is in the earlier stages 
 formed alone of the two layers which correspond with the 
 single layer forming the epiblast in the Dog-fish. In Osseous 
 fishes it is very difficult to distinguish the various layers, 
 owing to the similarity of their component cells. In Dog- 
 fish this is very easy, owing to the great distinctness of the 
 epiblast, and it appears to me, on this account, very probable 
 that the two above-named observers may be in error as to 
 the constitution of its roof in the Osseous fish. With both 
 the Bird and the Frog the segmentation cavity of the Dog- 
 fish has some points of agreement, and some points of differ- 
 ence, but it would take me too far from my present subject to 
 discuss them. 
 
 When the segmentation cavity is first formed, no great 
 changes have taken place in the cells forming the blastoderm. 
 The upper layer the epiblast is composed of a single layer 
 of columnar cells, and the remainder of the cells of blastoderm, 
 
72 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 forming the lower layer, are of a fairly uniform size, and poly- 
 gonal from mutual pressure. The whole edge of the blastoderm 
 is thickened, but this thickening is especially marked at its 
 embryonic end. 
 
 This thickened edge of the blastoderm is still more conspi- 
 cuous in the next and second stage (PI. 3, fig. 3). 
 
 In the second stage the chief points of progress, in addi- 
 tion to the increased thickness of the edge of the blastoderm, 
 are 
 
 (1) The increased thickness and distinctness of the epiblast, 
 caused by its cells becoming more columnar, though it remains 
 as a one-cell-thick layer. 
 
 (2) The disappearance of the cells from the floor of the seg- 
 mentation cavity. 
 
 The lower layer cells have undergone no important changes, 
 and the blastoderm has increased very little if at all in size. 
 
 From PL 3, fig. 3, it is seen that there is a far larger 
 collection of cells at the embryonic than at the opposite end. 
 
 Passing over some rather unimportant stages, I will come to 
 the next important one. 
 
 The general features of this (the third) stage in a surface 
 view are 
 
 (1) The increase in size of the blastoderm. 
 
 (2) The diminution in size of the segmentation cavity, both 
 relatively and absolutely'. 
 
 (3) The appearance of a portion of the blastoderm pro- 
 jecting beyond the rest over the yolk. This projecting rim 
 extends for nearly half the circumference of the yolk, but is 
 most marked at the point where the embryo will shortly appear. 
 I will call it the " embryonic rim." 
 
 These points are still better seen from sections than from 
 surface views, and will be gathered at once from an inspection 
 of PL 3, fig- 4- 
 
 The epiblast has become still more columnar, and is 
 markedly thicker in the region where the embryo will ap- 
 pear. But its most remarkable feature is that at the outer 
 edge of the " embryonic rim" (e r) it turns round and becomes 
 continuous with the lower layer cells. This feature is most im- 
 portant, and involves some peculiar modifications in the develop- 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 73 
 
 ment. I will, however, reserve a discussion of its meaning till 
 the next stage. 
 
 The only other important feature of this stage is the ap- 
 pearance of a layer of cells on the floor of the segmentation 
 cavity. 
 
 Does this layer come from an ingrowth from the thickened 
 edge of the blastoderm, or does it arise from the formation of 
 new cells in the yolk ? 
 
 It is almost impossible to answer this question with cer- 
 tainty. The following facts, however, make me believe that 
 the newly formed cells do play an important part in the forma- 
 tion of this layer. 
 
 (1) The presence at an earlier date of almost a row of nuclei 
 under the floor of the segmentation cavity (PI. 3, fig. i). 
 
 (2) The presence on the floor of the cavity of such large cells 
 as those represented in fig. i, b d, cells which are very different, 
 as far as the size and granules are concerned, from the remain- 
 der of the cells of the blastoderm. 
 
 On the other hand, from this as well as other sections, I 
 have satisfied myself that there is a distinct ingrowth of cells 
 from the embryonic swelling. It is therefore most probable 
 that both these processes, viz. a fresh formation and an ingrowth, 
 have a share in the formation of the layer of cells on the floor 
 of the segmentation cavity. 
 
 In the next stage we find the embryo rising up as a distinct 
 body from the blastoderm, and I shall in future speak of the 
 body, which now becomes distinct as the embryo. It cor- 
 responds with what Kupffer (loc. tit.} in his paper on the 
 "Osseous Fishes" has called the "embryonic keel." This 
 starting-point for speaking of the embryo as a distinct body is 
 purely arbitrary and one merely of convenience. If I wished to 
 fix more correctly upon a period which could be spoken of as 
 marking the commencing formation of the embryo, I should 
 select the time when structures first appear to mark out the 
 portion of the germ from which the embryo becomes formed ; 
 this period would be in the Elasmobranchii, as in the Osseous 
 fish, at the termination of segmentation, when the want of sym- 
 metry between the embryonic end of the germ and the opposite 
 end first appears. 
 
74 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 I described in the last stage the appearance of the " embry- 
 onic rim." It is in the middle point of this, where it projects 
 most, that the formation of the embryo takes place. There 
 appear two parallel folds extending from the edge of the 
 blastoderm towards the centre, and cut off at their central end 
 by another transverse fold. These three folds raise up, be- 
 tween them, a flat broadish ridge, "tfie embryo" (PI. 3, fig. 5). 
 The head end of the embryo is the end nearest the centre of 
 the blastoderm, the tail end being the one formed by its (the 
 blastoderm's) edge. 
 
 Almost from its first appearance this ridge acquires a 
 shallow groove the medullary groove (PL 3, fig. 5, m g) 
 along its middle line, where the epiblast and hypoblast are 
 in absolute contact (vide fig. 6 a, 7 a, 7 b, &c.) and where the 
 mesoblast (which is already formed by this stage) is totally 
 absent. This groove ends abruptly a little before the front 
 end of the embryo, and is deepest in the middle and wide and 
 shallow behind. 
 
 Oh each side of it is a plate of mesoblast equivalent to the 
 combined vertebral and lateral plates of the Chick. These, 
 though they cannot be considered as entirely the cause of the 
 medullary groove, may perhaps help to make it deeper. In 
 the parts of the germ outside the embryo the mesoblast is 
 again totally absent, or, more correctly, we might say that 
 outside the embryo the lower layer cells do not become differ- 
 entiated into hypoblast and mesoblast, and remain continu- 
 ous only with the lower of the two layers into which the 
 lower layer cells become differentiated in the body of embryo. 
 This state of things is not really very different from what 
 we find in the Chick. Here outside the embryo (i.e. in 
 the opaque area) there is a layer of cells in which no dif- 
 ferentiation into hypoblast and mesoblast takes place, but the 
 layer remains continuous rather with the hypoblast than the 
 mesoblast. 
 
 There is one peculiarity in the formation of the mesoblast 
 which I wish to call attention to, i.e. its formation as two 
 lateral masses, one on each side of the middle line, but not 
 continuous across this line (vide figs. 6 a and 6 b, and 7 a and 
 7 b}. Whether this remarkable condition is . the most primi- 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 75 
 
 tive, i.e. whether, when in the stage before this the mesoblast 
 is first formed, it is only on each side of the middle line that 
 the differentiation of the lower layer cells into hypoblast and 
 mesoblast takes place, I do not certainly know, but it is un- 
 doubtedly a very early condition of the mesoblast. The con- 
 dition of the mesoblast as two plates, one on each side of the 
 neural canal, is precisely similar to its embryonic condition in 
 many of the Vermes, e.g. Euaxes and Lunibricus. In these there 
 are two plates of mesoblast, one on each side of the nervous 
 cord, which are known as the Germinal streaks (Keimstreifen) 
 (vide Kowalevsky " Wurmern u. Arthropoden " ; Me"m. de I'Acad. 
 Imp. St Peter sbourg, 1871). 
 
 From longitudinal sections I have found that the segmen- 
 tation cavity has ceased by this stage to have any distinct 
 existence, but that the whole space between the epiblast and 
 the yolk is filled up with a mass of elongated cells, which 
 probably are solely concerned in the formation of the vas- 
 cular system. The thickened posterior edge of the blastoderm 
 is still visible. 
 
 At the embryonic end of the blastoderm, as I pointed out 
 in an earlier stage, the epiblast and the lower layer cells are 
 perfectly continuous. 
 
 Where they join the epiblast, the lower layer cells become 
 distinctly divided, and this division commenced even in the 
 earlier stage, into two layers ; a lower one, more directly 
 continuous with the epiblast, consisting of ceHs somewhat 
 resembling the epiblast-cells, and an upper one of more flat- 
 tened cells (PI. 3, fig. 4, m). The first of these forms the 
 hypoblast, and the latter the mesoblast. They are indicated by 
 hy and m in the figures. The hypoblast, as I said before, re- 
 mains continuous with the whole of the rest of lower layer cells 
 of the blastoderm (vide fig. 7 b). This division into hypoblast 
 and mesoblast commences at the earlier stage, but becomes 
 much more marked during this one. 
 
 In describing the formation of the hypoblast and meso- 
 blast in this way I have assumed that they are formed out 
 of the large mass of lower layer cells which underlie the epi- 
 blast at the embryonic end of the blastoderm. But there 
 is another and, in some ways, rather a tempting view, viz. 
 
76 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 to suppose that the epiblast, where it becomes continuous with 
 the hypoblast, in reality becomes involuted, and that from 
 this involuted epiblast are formed the whole mesoblast and 
 hypoblast. 
 
 In this case we would be compelled to suppose that the mass 
 of lower layer cells which forms the embryonic swelling is used 
 as food for the growth of the involuted epiblast, or else em- 
 ployed solely in the growth over the yolk of the non-embryonic 
 portion of the blastoderm ; but the latter possibility does not 
 seem compatible with my sections. 
 
 I do not believe that it is possible, from the examination of 
 sections alone, to decide which of these two views (viz. whether 
 the epiblast is involuted, or whether it becomes merely conti- 
 nuous with the lower layer cells) is the true one. The question 
 must be decided from other considerations. 
 
 The following ones have induced me to take the view that 
 there is no involution, but that the mesoblast and hypoblast are 
 formed from the lower layer cells. 
 
 (1) That it would be rather surprising to find the mass of 
 lower layer cells which forms the " embryo swelling " playing no 
 part in the formation of embryo. 
 
 (2) That the view that it is the lower layer cells from which 
 the hypoblast and mesoblast are derived agrees with the mode 
 of formation of these two layers in the Bird, and also in the 
 Frog ; since although, in the latter animal, there is an involu- 
 tion, this is not of the epiblast, but of the larger cells of the 
 lower pole of the yolk, which in part correspond with what 
 I have called the lower layer cells in the Dog-fish. 
 
 If the view be accepted that it is from the lower layer cells 
 that the hypoblast and mesoblast are formed, it becomes ne- 
 cessary to explain what the continuity of the hypoblast with 
 the epiblast means. 
 
 The explanation of this is, I believe, the keystone to the 
 whole position. The vertebrates may be divided as to their 
 early development into two classes, viz. those with koloblastic 
 ova, in which the digestive canal is formed by an involution with 
 the presence of an "anus of Rusconi" 
 
 This class includes "Amphioxus," the " Lamprey," the "Stur- 
 geon," and " Batrachians." 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 77 
 
 The second class are those with meroblastic ova and no anus 
 of Rusconi, and with an alimentary canal formed by the infold- 
 ing of the sheet of hypoblast, the digestive canal remaining in 
 communication with the food-yolk for the greater part of em- 
 bryonic life by an umbilical canal. 
 
 This class includes the " Elasmobranchii," "Osseous fish," 
 " Reptiles," and " Aves." 
 
 The mode of formation of the alimentary canal in the first 
 class is clearly the more primitive ; and it is equally clear that 
 its mode of formation in the second class is an adaptation due 
 to the presence of the large quantity of food-yolk. 
 
 In the Dog-fish I believe that we can see, to a certain extent, 
 how the change from the one to the other of these modes of de- 
 velopment of the alimentary canal took place. 
 
 In all the members of the first class, viz. " A mphioxus" the 
 "Lamprey," the "Sturgeon," and the "Batrachians," the epiblast 
 becomes continuous with the hypoblast at the so-called " anus 
 of Rusconi," and the alimentary canal, potentially in all and 
 actually in the Sturgeon (vide Kowalevsky, Owsjannikow, and 
 Wagner, Bulletin der Acad. d. St Petersbourg, Vol. xiv. 1870, 
 " Entwicklung der Store "), communicates freely at its ex- 
 treme hind end with the neural canal. The same is the case 
 in the Dog-fish. In these, when the folding in to form the 
 alimentary canal on the one hand, and the neural on the 
 other, takes place, the two foldings unite at the corner, where 
 the epiblast and hypoblast are in continuity, and place the two 
 tubes, the neural and alimentary, in free communication with 
 each other 1 . 
 
 There is, however, nothing corresponding with the " anus of 
 Rusconi," which merely indicates the position of the involution 
 of the digestive canal, and subsequently completely closes up, 
 though it nearly coincides in position with the true anus in the 
 Batrachians, &c. 
 
 This remarkable point of similarity between the Dog-fish's 
 development and the normal mode of development in the first 
 class (the holoblastic) of vertebrates, renders it quite clear 
 that the continuity of the epiblast and hypoblast in the Dog- 
 
 1 This has been already made out by Kowalevsky, " Wurmern u. Arthropoden, " 
 lot. cit. 
 
78 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 fish is really the remnant of a more primitive condition, when 
 the alimentary canal was formed by an involution. Besides 
 the continuity between neural and alimentary canals, we have 
 other remnants of the primitive involution. Amongst these 
 the most marked is the formation of the embryonic rim, 
 which is nothing less than the commencement of an involu- 
 tion. Its form is due to the flattened, sheet-like condition 
 of the germ. In the mode in which the alimentary canal is 
 closed in front I shall shew there are indications of the 
 primitive mode of formation of the alimentary canal ; and in 
 certain peculiarities of the anus, which I shall speak of later, 
 we have indications of the primitive anus of Rusconi ; and 
 finally, in the general growth of the epiblast (small cells of the 
 upper pole of the Batrachian egg) over the yolk (lower pole of 
 the Batrachian egg), we have an example of the manner in 
 which the primitive involution, to form the alimentary canal, 
 invariably disappears when the quantity of yolk in an egg 
 becomes very great. 
 
 I believe that in the Dog-fish we have before our eyes 
 one of the steps by which a direct mode of formation comes 
 to be substituted for an indirect one by involution. We find, 
 in fact, in the Dog-fish, that the cells from which are derived 
 the mesoblast and hypoblast come to occupy their final position 
 in the primitive arrangement of the cells during segmentation, 
 and not by a subsequent and secondary involution. 
 
 This change in the mode of formation of the alimentary 
 canal is clearly a result of change of mechanical conditions from 
 the presence of the large food-yolk. 
 
 Excellent parallels to it will be found amongst the Mollusca. 
 In this class the presence or absence of food-yolk produces not 
 very dissimilar changes to those which are produced amongst 
 vertebrates from the same cause. 
 
 The continuity of the hypoblast and epiblast at the em- 
 bryonic rim is a remnant which, having no meaning or function, 
 except in reference to the earlier mode of development, is 
 likely to become lost, and in Birds no trace of it is any longer 
 to be found. 
 
 I will not in the present preliminary paper attempt hypo- 
 thetically to trace the steps by which the involution gradually 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 79 
 
 disappeared, though I do not think it would be very difficult to 
 do so. Nor will I attempt to discuss the question whether the 
 condition with a large amount of food-yolk (as seems more 
 probable) was twice acquired once by the Elasmobranchii-and 
 Osseous fishes, and once by Reptiles and Birds or whether only 
 once, the Reptiles and Birds being lineal descendants of the 
 Dog-fish. 
 
 In reference to the former point, however, I may mention 
 that the Batrachians and Lampreys are to a certain extent 
 intermediate in condition between the Ampkioxus'&oA. the Dog- 
 fishes, since in them the yolk becomes divided during segmen- 
 tation into lower layer cells and epiblast, but a modified invo- 
 lution is still retained, while the Dog-fish may be looked upon 
 as intermediate between Birds and Batrachians, the continuity 
 at the hind end between the epiblast and hypoblast being 
 retained by them, though not the involution. 
 
 It may be convenient here to call attention to some of the 
 similarities and some of the differences which I have not yet 
 spoken of between the development of Osseous fish and the 
 Dog-fish in the early stages. The points of similarity are (i) 
 The swollen edge of the blastoderm. (2) The embryo-swelling. 
 (3) The embryo-keel. (4) The spreading of the blastoderm 
 over the yolk-sac from a point corresponding with the position 
 of the embryo, and not with the centre of the germ. The growth 
 is almost nothing at that point, and most rapid at the opposite 
 pole of the blastoderm, being less and less rapid along pouits 
 of the circumference in proportion to their proximity to the 
 embryonic swelling. (5) The medullary groove. 
 
 In external appearance the early embryos of Dog-fish and 
 Teleostei are very similar ; some of my drawings could almost 
 be substituted for those given by Oellacher. This similarity is 
 especially marked at the first appearance of the medullary 
 groove. In the Dog-fish the medullary groove becomes con- 
 verted into the medullary canal in the same way as in Birds 
 and all other vertebrates, except Osseous fishes, where it comes 
 to nothing, and is, in fact, a rudimentary structure. But in 
 spite of Oellacher's assertions to the contrary, I am convinced 
 from the similarity of its position and appearance to the true 
 medullary groove in the Dog-fish, that the groove which appears 
 
So DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 in Osseous fishes is the true medullary groove ; although Oel- 
 lacher and Kuppfer appear to have conclusively proved that it 
 does not become converted into the medullary canal. The 
 chief difference between the Dog-fish and Osseous fish, in ad- 
 dition to the point of difference about the medullary groove, is 
 that the epiblast is in the Dog-fish a single layer, and not 
 divided into nervous and epidermic layers as in Osseous fish, 
 and this difference is the more important, since, throughout the 
 whole period of development till after the commencement of 
 the formation of the neural canal, the epiblast remains in Dog- 
 fish as a one-cell-deep layer of cells, and thus the possibility 
 is excluded of any concealed division into a neural and epi- 
 dermic layer, as has been supposed to be the case by Strieker 
 and others in Birds. 
 
 Development of the Embryo, 
 
 After the embryo has become definitely established, for 
 some time it grows rapidly in length, without externally under- 
 going other important changes, with the exception of the ap- 
 pearance of two swellings, one on each side of its tail. 
 
 These swellings, which I will call the Caudal lobes (figs. 8 
 and 9, t s), are also found in Osseous fishes, and have been 
 called by Oellacher the Embryonal saum. They are caused by 
 a thickening of mesoblast on each side of the hind end of the 
 embryo, at the edge of the embryonic rim, and form a very 
 conspicuous feature throughout the early stages of the develop- 
 ment of the Dog-fish, and are still more marked in the Torpedo 
 (PI. 3, fig. 9). Although from the surface the other changes 
 which are visible are very insignificant, sections shew that the 
 notochord is commencing to be formed. 
 
 I pointed out that beneath the medullary groove the epiblast 
 and hypoblast were not separated by any interposed mesoblast. 
 Along the line (where the mesoblast is deficient) which forms 
 the long axis of the embryo, a rod-like thickening of the hypo- 
 blast appears (PI. 3, figs, ^a and jb, ch and ch'), first at the 
 head end of the embryo, and gradually extends backwards. This 
 is the rudiment of the notochord ; it remains attached for some 
 time to the hypoblast, and becomes separated from it first at 
 
DKVKLOPMKXT OF THE EI.ASMOBRAXCH FISHKS. 8l 
 
 the head end of the embryo, and the separation is then carried 
 backwards. This thickening of the hypoblast projects up and 
 comes in contact with the epiblast, and in the later stages with 
 bad (especially chromic-acid) specimens the line of separation 
 between the epiblast and the thickening may become a little 
 obscured, and might possibly lead to the supposition that a 
 structure similar to that which has been called the "axis cord" 
 was present. In all my best (osmic-acid) specimens the line of 
 junction is quite clear ; and any one who is aware how easily 
 two separate masses of cells may be made indistinguishably 
 to fuse together from simple pressure will not be surprised to 
 find the occasional obscurity of the line of junction between the 
 epiblast and hypoblast. In the earlier stage of the thickening 
 there is never in the osmic-acid preparations any appearance of 
 fusion except in very badly prepared ones. Its mode of for- 
 mation will be quite clear without further description from 
 an inspection of PI. 3, figs, "a and jb, cli and ell . Both are 
 taken from one embryo. In fig. 7^, the most anterior of the 
 two, the notochord has become quite separated from the hypo- 
 blast. In fig. 7 a, ch, there is only a very marked thickening of 
 hypoblast, which reaches up to the epiblast, but the thickening 
 is still attached to the hypoblast. Had I had space to insert 
 a drawing of a third section of the same embryo there would 
 only have been a slight thickening of the hypoblast. In the 
 earlier stage it will be seen, by referring to figs. 6a and 6b, that 
 there is no sign of a thickening of the hypoblast. My numerous 
 sections (all made from embryos hardened in osmic acid) shew- 
 ing these points are so clear that I do not think there can 
 be any doubt whatever of the notochord being formed as a 
 thickening of the hypoblast. Two interpretations of this seem 
 possible. 
 
 I mentioned that the mesoblast appeared to be primitively 
 formed as two independent sheets, split off, so to speak, from llic 
 hypoblast, one on e"ach side of the middle line of the embryo. 
 If we looked upon the notochord as a third median sheet of 
 mesoblast, split off from the hypoblast somewhat later than the 
 other two, we should avoid having to admit its hypoblastic origin. 
 
 Professor Huxley, to whom I have shewn my specimens, 
 strongly advocates this view. 
 
 H. 6 
 
82 DEVELOPMENT OF THE- ELASMOBRANCH FISHES. 
 
 The other possibility is that the notochord is primitively a 
 true hypoblastic structure which has only by adaptation become 
 an apparently mesoblastic one in the higher vertebrates. In 
 favour of this view are the following considerations : 
 
 (i) That this is the undoubtedly natural interpretation of 
 the sections. (2) That the notochord becomes separated from 
 the hypoblast after the latter has acquired its typical structure, 
 and differs in that respect from the two lateral sheets of meso- 
 blast, which are formed coincidently with the hypoblast by a 
 homogeneous mass of cells becoming differentiated into two 
 distinct layers. (3) That the first mode of looking at the matter 
 really proves too much, since it is clear that by the same method 
 of reasoning we could prove the mesoblastic origin of any organ 
 derived from the hypoblast and budded off into the mesoblast. 
 We would merely have to assert that it was really a mass of 
 mesoblast budded off from the hypoblast rather later than the 
 remainder of the mesoblast. Still, it must be admitted that the 
 first view I have suggested is a possible, not to say a probable 
 one, though the mode of arguing by which it can be upheld 
 may be rather dangerous if generally applied. We ought not, 
 however, for that reason necessarily to reject it in the present 
 case. As Mr Ray Lankester pointed out to me, if we accept 
 the hypoblastic origin of the notochord, we should find a partial 
 parallel to it in the endostyle of Tunicates, and it is perhaps 
 interesting to note in reference to it that the notochord is the 
 only imsegmentcd portion of the axial skeleton. 
 
 Whether the strong a priori difficulties of the hypoblastic 
 origin of the notochord are sufficient to counterbalance the 
 
 O 
 
 natural interpretation of my sections, cannot, I think, be decided 
 from the single case of the Dog-fish. It is to be hoped that 
 more complete investigations of the Lamprey, &c., may throw 
 further light upon the question. 
 
 Whichever view of the primitive origin of the notochord 
 is the true one, its apparent origin is very instructive as illus- 
 trating the possible way in which an organ might come to 
 change the layer to which it primarily belonged. 
 
 If the notochord is a true mesoblastic structure, it is easy 
 to be seen how, by becoming separated from the hypoblast a 
 little later than is the case with the Dog-fish, its mesoblastic 
 
DKVKI.OI'MF.XT OF THK ELASMO6RANCH FISIIKS. 8} 
 
 origin would become lost ; while if, on the other hand, it is 
 primitively a hypoblastic structure, we see from higher verte- 
 brates how, by becoming separated from the hypoblast rather 
 earlier than in the Dog-fish, viz. at the same time as the_rest 
 of the mesoblast, its primitive derivation from the hypoblast 
 has become concealed. 
 
 The view seemingly held by many embryologists of the 
 present day, that an organ, when it was primitively derived from 
 one layer, can never be apparently formed in another layer, 
 appears to me both unreasonable on ei priori grounds, and also 
 unsupported by facts. 
 
 I see no reason for doubting that the embryo in the earliest 
 periods of development is as subject to the laws of natural 
 selection as is the animal at any other period. Indeed, there 
 appear to me grounds for the thinking that it is more so. The 
 remarkable differences in allied species as to the amount of 
 food-yolk, which always entail corresponding alterations in the 
 development the different modes of segmentation in allied 
 species, such as are found in the Amphipoda and Isopoda the 
 suppression of many stages in freshwater species, which are 
 retained in the allied marine species are all instances of modifi- 
 cations due to natural selection affecting the earliest stages of 
 development. If such points as these can be affected by natural 
 selection I see no reason why the arrangement of individual 
 cells (or rather primitive elements) should not also be modified ; 
 why, in fact, a mass of cells which was originally derived from 
 one layer, but in the course of development became budded off 
 from that layer and entered another layer, should not by a series 
 of small steps cease ever to be attached to the original layer, 
 but from the first moment it can be distinguished should be 
 found as a separate mass in the second layer. 
 
 The change of layers will, of course, only take place where 
 some economy is effected by it. The variations in the mode of 
 development of the nervous system may probably be explained 
 in this way. 
 
 If we admit that organs can undergo changes, as to the 
 primitive layer from which they arc derived, important conse- 
 quences must follow. 
 
 It will, for instance, by no means be sufficient evidence of 
 
 62 
 
84 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 two organs not being homologous that they are not developed 
 from the same layer. It renders the task of tracing out the 
 homologies from development much more difficult than if the 
 ordinary view of the invariable correspondence of the three 
 layers throughout the animal kingdom be accepted. Although 
 I do not believe that this correspondence is invariable or exact, 
 I think that we both find and should expect to find that it is, 
 roughly speaking, fairly so. 
 
 Thus, the muscles, internal skeleton, and connective tissue 
 are always placed in the adult between the skin (epidermis) and 
 the epithelium of the alimentary canal. 
 
 We should therefore expect to find them, and, as a matter 
 of fact, we always do find them, developed from a middle layer 
 when this is present. 
 
 The upper layer must always and does always form the 
 epidermis, and similarly the lower layer or hypoblast must form 
 a -part of the epithelium of the alimentary canal. A full dis- 
 cussion of this question would, however, lead me too far away 
 from my present subject. 
 
 The only other point of interest which I can touch on in 
 this stage is the commencing closure of the alimentary canal 
 in the region of the head. This is shewn in PI. 3, figs. 6a, 66, 
 jb, 11. a. From these figures it can be seen that the closing 
 does not take place as much by an infolding as by an ingrowth 
 from the side walls of the alimentary canal towards the middle 
 line. In this abnormal mode of closing of the alimentary canal 
 we have again, I believe, an intermediate stage between the 
 mode of formation of the alimentary canal in the Frog and 
 the typical folding in which occurs in Birds. There is, how- 
 ever, another point in reference to it which is still more inter- 
 esting. The cells to form the ingrowth from the bottom (ven- 
 tral) wall of the alimentary canal are derived by a continuous 
 fresh formation from the yolk, being formed around the nuclei 
 spoken of above (vide p. 63 et seq.). All my sections shew 
 this with more or less clearness, especially those a little later 
 than fig. 6b, in which the lower wall of the alimentary canal is 
 nearly completed. This is the more interesting since, from the 
 mode of formation of the alimentary canal in the Batrachians, 
 &c., we might expect that the cells from the yolk would take 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 85 
 
 a share in its formation in the Dog-fish. I have not as yet 
 made out for certain the share which is taken by these freshly 
 formed cells of the yolk in the formation of any other organ. 
 
 By the completion of its lower wall in the way described, 
 the throat early becomes a closed tube, its closing taking place 
 before any other important changes are visible in the embryo 
 from surface views. 
 
 A considerable increase in length is attained before other 
 changes than an increase in depth of the medullary groove and 
 a more complete folding off of the embryo from the blastoderm 
 take place. The first important change is the formation of the 
 protovertebrae. 
 
 These are formed by the lateral plates of mesoblast, which 
 I said were equivalent at once to the vertebral and lateral 
 plates in the Bird, becoming split by transverse divisions into 
 cubical masses. 
 
 At the time when this occurs, and, indeed, up till a con- 
 siderably later period, the mesoblast is not split into somato- 
 pleure and splanchnopleure, and it is not divided into vertebral 
 and lateral plates. The transverse lines of division of the proto- 
 vertebras do not, however, extend to the outer edge of the 
 undivided lateral plates. 
 
 The differences between this mode of formation of the pro- 
 tovertebrae and that occurring in Birds are too obvious to require 
 pointing out. I will speak of them more fully when I have 
 given the whole history of the protovertebras of the Dog-fish. 
 
 I will only now say that I have had in the early stages 
 to investigate the formation of the protovertebras entirely by 
 means of sections, the objects being too opaque to be other- 
 wise studied. 
 
 The next change of any importance is the commencement 
 of the formation of the head. The region of the head first 
 becomes distinguishable by the flattening out of the germ at 
 its front end. 
 
 The flattened-out portion of the germ grows rapidly, and 
 forms a spatula-like termination to the embryo (PI. 3, fig. 8). 
 
 In the region of the head the medullary groove is at first 
 totally absent (vide section, PI. 3, fig. 8. a), 
 
 Indeed, as can be seen from fig. 8 b, the laminae dorsales, so 
 
86 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 far from bending up at this stage, actually bend down in the 
 opposite direction. 
 
 I am at present quite unable even to form a guess what this 
 peculiar feature of the brain means. It, no doubt, has some 
 meaning in reference to the vertebrate ancestry if we could only 
 discover it. The peculiar spatula-like flattened condition of the 
 head is also (vide loc. ant. cit.) apparently found in the Stur- 
 geons ; it must therefore almost undoubtedly be looked upon as 
 not merely an accidental peculiarity. 
 
 While these changes have been taking place in the head not 
 less important changes have occurred in the remainder of the 
 body. In the first place the two caudal lobes have increased 
 in size, and have become, as it were, pushed in together, leaving 
 a groove between them (fig. 8, / s). They are very conspicuous 
 objects, and, together with the spatula-like head, give the whole 
 embryo an alm6st comical appearance. The medullary canal 
 has by this time become completely closed in the region of the 
 tail (figs. 8 and 8 U]. 
 
 It is still widely open in the region of the back, and, though 
 more nearly closed again in the neck, is, as I have said, flattened 
 out to nothing in the head. 
 
 The groove 1 between the two caudal lobes must not be con- 
 fused (as may easily be done) with the medullary groove, which 
 by the time the former groove has become conspicuous is a 
 completely closed canal. 
 
 The vertebral plates are not divided (vide fig. 7) into a 
 somatopleuric and splanchnoplcuric layer by this stage, except 
 in the region of the head (vide fig. 8 , //), where there is a 
 distinct space between the two layers, which is undoubtedly 
 homologous with the pleuro-peritoneal cavity of the hinder 
 portion of the body. 
 
 It is probably the same cavity which Oellacher (loc. cit.) calls 
 in Osseous fishes the pericardial cavity. In the Dog-fish, at 
 least, it has no connection with the pericardium. Of its subse- 
 quent history I shall say a few words when I come to speak of 
 the later stages. 
 
 1 This groove is the only structure which it seems possible to compare with the 
 so-called "primitive groove" of Birds. It is, however, doubtful whether they are 
 really homologous. 
 
DEVELOPMENT OF THE ELASMOBKANCH FISHES. 8/ 
 
 The embryo does not take more than twenty-four hours in 
 passing from this stage, when the head is a flat plate, to the 
 stage when the whole neural canal (including the region of 
 the head) is closed in. The other changes, in addition to the 
 closing in of the neural canal, are therefore somewhat insig- 
 nificant. The folding off of the embryo from the germ has, 
 however, progressed considerably, and a portion of the hind 
 gut is closed in below. This is accomplished, not by a tail-fold^ 
 as in Birds, but by two lateral folds, which cause the sides of 
 the body to meet and coalesce below. At the extreme hind 
 end, where the epiblast is continuous with the hypoblast, the 
 lateral folds turn round, so to speak, and become continuous 
 with the medullary folds, so that when the various folds meet 
 each other an uninterrupted canal is found passing round from 
 the neural into the alimentary canal, and placing these two in 
 communication at the tail end of the body. Since I have 
 already mentioned this, and spoken of its significance, I will not 
 dwell on it further here. 
 
 The cranial flexure commences coincidently with the closing 
 in of the neural canal in the region of the brain, and the divi- 
 sion into fore, mid, and hind brain becomes visible at the same 
 time as or even before the closing of the canal occurs. The 
 embryo has now become more or less transparent, and proto- 
 vertebrae, of which about twenty are present, can noiv be 
 seen in the fresh specimens. The heart, however, is not yet 
 formed. 
 
 Up to this period, a period at which the embryo becomes 
 very similar in external appearance to any other vertebrate em- 
 bryo, I have followed in my description a chronological order. 
 I shall now cease to do so, since it would be too long for a pre- 
 liminary notice of this kind, but shall confine myself to the 
 history of a few organs whose development is either more im- 
 portant or more peculiar than that of the others. 
 
 The Protovertebra. 
 
 I have thought it worth while to give a short history of 
 the development of the protovertebrae, firstly, because it is 
 very easy to follow this in the Dog-fish, and, secondly, because 
 
DEVELOPMENT OF THE ELASMOBKANCH FISHES. 
 
 I believe that the Dog-fish have more nearly retained the pri- 
 mitive condition of the protovertebrae than any other vertebrate 
 whose embryology has hitherto been described with sufficient 
 detail. 
 
 I intend to describe, at the same time, the development of 
 the spinal nerves. 
 
 I left each lateral mass of mesoblast in my last stage as 
 a plate which had not yet become split into a somatic and 
 a splanchnic sheet (PI. 3, fig. 8 a, v p), but which had be- 
 come cut by transverse lines (not, indeed, extending to the 
 outer limit of the sheet, but as yet not cut off by longitudi- 
 nal lines of cleavage) into segments, which I called proto- 
 vertebrae. 
 
 This sheet of mesoblast is fairly thick at its proximal (upper) 
 end, but thins off laterally to a sheet two cells deep, and its 
 cells are so arranged as to foreshadow its subsequent splitting 
 into somatic and splanchnic sheets. Its upper (proximal) end 
 is at this stage level with the bottom of the neural canal, but 
 soon begins to grow upwards, and at the same time the splitting 
 into somatopleure and splanchnopleure commences (PI. 3, fig. 10, 
 so and sp}. 
 
 The separation between the two sheets is first visible in its 
 uppermost part, and thence extends outwards. By this means 
 each of the protovertebrae becomes divided into two sheets, 
 which are only connected at their upper ends and outside the 
 region of the body. I speak of the whole lateral sheet as being 
 composed of protovertebrae, because at this time no separa- 
 tion into vertebral and lateral plates can be seen ; but I may 
 anticipate matters by saying that only the upper portion of the 
 sheet from the level of the top of the digestive canal, becomes 
 subsequently the true protovertebrae. From this it is clear that 
 the pleuro-peritoneal cavity extends primitively quite up to the 
 top of the protovertebrae ; and that thus a portion of a sheet of 
 mesoblast, at first perfectly continuous with the splanchnic sheet 
 from which is derived the muscular wall of the alimentary canal, 
 is converted into a part of the voluntary muscular system of the 
 body, having no connection whatever with the involuntary mus- 
 cular system of. the digestive tract. 
 
 The pleuro-peritoneal cavity is first distinctly formed at a 
 
DEVELOPMENT OF THK ELASMOBRANCH FISHES. 89 
 
 time when only two visceral clefts are present. Before the 
 appearance of a third visceral cleft in a part of the innermost 
 layer of each protovertebrae (which may be called the splanchnic 
 layer, from its being continuous with the mesoblast of _the^ 
 splanchnopleure), opposite the bottom of the neural tube, some 
 of the cells commence to become distinguishable from the rest, 
 and to form a separate mass. This mass becomes much more 
 distinct a little later, its cells being characterised by being 
 spindle-shaped, and having an elongated nucleus which be- 
 comes deeply stained by reagents (PI. 4, fig. \\,mp'}. Coin- 
 cidently with its appearance the young Dog-fish commences 
 spontaneously to move rapidly from side to side with a kind 
 of serpentine motion, so that, even if I had not traced the 
 development of this differentiated mass of cells till it becomes 
 a band of muscles close to the notochord, I should have had 
 little doubt of its muscular nature. It is indicated in figs, n, 
 12, 13, by the letters mp'. Its early appearance is most pro- 
 bably to be looked upon as an adaptation consequent upon the 
 respiratory requirements of the young Dog-fish necessitating 
 movements within the egg. 
 
 Shortly after this date, at a period when three visceral clefts 
 are present, I have detected the first traces of the spinal nerves. 
 At this time they appear in sections as small elliptical masses 
 of cells, entirely independent of the protovertebrae, and closely 
 applied to the upper and outer corners of the involuted epiblast 
 of the neural canal (PI. 4, fig. n t spn}. These bodies are far 
 removed from any mesoblastic structures, and at the same time 
 the cells composing them are not similar to the cells composing 
 the walls of the neural canal, and are not attached to these, 
 though lying in contact with them. I have not, therefore, suf- 
 ficient evidence at present to enable me to say with any cer- 
 tainty where the spinal nerves are derived from in the Dog-fish. 
 They may be derived from the involuted epiblast of the neural 
 canal, and, indeed, this is the most natural interpretation of 
 their position. 
 
 On the other hand, it is possible that they are formed from 
 wandering cells of the mesoblast a possibility which, with our 
 present knowledge of wandering cells, must not be thrown aside 
 as altogether improbable. 
 
90 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 In any case, it is clear that the condition in the Bird, where 
 the spinal nerves are derived from tissue of the protovertebrae, 
 is not the primitive one. Of this, however, I will speak again 
 when I have concluded my account, of the development of the 
 protovertebrae. 
 
 About the same time that the first rudiments of the nerves 
 appear, the division of the mesoblast of the sides of the body 
 into a vertebral and a lateral portion occurs. This division first 
 appears in the region where the oviduct (M tiller's duct) is formed 
 (PL 4, fig. 11,0V). 
 
 At this part opposite the level of the dorsal aorta the two 
 sheets, viz. the splanchnic and the somatic, unite together, and 
 thus each lateral sheet of mesoblast becomes divided into an 
 upper portion (fig. u, nip), split up by transverse partitions into 
 protovertebrae, and a lower portion not so split, but consisting of 
 an outer layer, the true somatopleure, and an inner layer, the 
 true splanchnopleure. These two divisions of the primitive plate 
 are thus separated by the line at which a fusion between the 
 mesoblast of the somatopleure and splanchnopleure takes place. 
 The mass of cells resulting from the fusion at this point cor- 
 responds with the intermediate cell-mass of Birds (vide Waldeyer, 
 Eierstock tmd Ei). 
 
 At the same time, in the upper of these two sheets (the pro- 
 tovertebrae), the splanchnic layer sends a growth of cells in- 
 wards towards the notochord and the neural canal. This growth 
 is the commencement of the large quantity of mesoblastic 
 tissue around the notochord, which is in part converted into 
 the axial skeleton, and in part into the connective tissue ad- 
 joining this. 
 
 This mass of cells is at first quite continuous with the 
 splanchnic layer of the protovertebrae, and I see no reason 
 for supposing that it is not derived from the growth of the 
 cells of this layer. The ingrowth to form it first appears a 
 little after the formation of the dorsal aorta ; but, as far as 
 I have been able to see, its cells have no connection with the 
 walls of the aorta. 
 
 What I have said as to the development of the skeleton- 
 forming layer will be quite clear from figs. II and \2a; and 
 from these it will also be clear, especially from fig. 1 1 a, that 
 
DEVKLOPMKXT OF THE ELASMOBRANCH FISHES. QI 
 
 the outermost layer of this mass of cells, which was the primi- 
 tive splanchnic layer of the protovertebrae, still retains its 
 epithelial character, and so can easily be distinguished from 
 those cells which will form the skeleton. In the next stage 
 which I have figured (fig. 12 a), this outer portion of the splanch- 
 nic layer is completely separated from the skeleton-forming 
 cells, and at the same time, having united below as well as 
 above with the outer (somatic) layer of the two layers of which 
 the protovertebrae are formed, the two together form an inde- 
 pendent mass (fig. 12, in p\ similar in appearance and in every 
 way homologous with the muscle-plate of Birds. 
 
 On the inner side of this, which we may now call the muscle- 
 plate, is seen the bundle of earlier-developed muscles (fig. 12, 
 in /') which I spoke of before. 
 
 The section represented in fig. 12 is from a very considerably 
 later embryo than that represented in fig. II, so that the skele- 
 ton-forming cells, few in number in the earlier section, have 
 become very numerous in the later one, and have grown up 
 above the neural canal, and also below the notochord, between 
 the digestive canal and the aorta. They have, moreover, 
 changed their character ; they were round before, now they 
 have become stellate. As to their further history, it need only 
 be said that the layer of them immediately around the noto- 
 chord and neural canal forms the cartilaginous centra and arches 
 of the vertebrse, and that the remaining portion of them, which 
 becomes much more insignificant in size as compared with the 
 muscles, forms the connective tissue of the skeleton and of the 
 parts around and between the muscles. 
 
 A muscle-plate itself is at this stage (shewn in fig. 12) com- 
 posed of an inner and an outer layer of columnar cells (splanchnic 
 and somatic) united at the upper and lower ends of the plate, 
 and on the inner of the two lies the more developed mass of 
 muscles before spoken of (?/'). 
 
 Each of these plates now grows both upwards and down- 
 wards ; and at the same time connective-tissue cells appear 
 between the plates and epidermis ; but from where they come 
 I do not know for certain ; very probably they are derived from 
 the somatic layer of the muscle-plate. 
 
 While the muscle- plates continue to grow both upwards and 
 
92 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 downwards, the cells of which they are composed commence to 
 become elongated and soon acquire an unmistakably muscular 
 character (PI. 4, fig. 13, ;///). 
 
 Before this has occurred the inner mass of muscles has 
 also undergone further development and become a large and 
 conspicuous band of muscles close to the notochord (fig. 
 13, ;/). 
 
 At the same time that the muscle-plates acquire the true 
 histological character of muscle, septa of connective tissue grow 
 in and divide them into a number of distinct segments which 
 subsequently form separate bands of muscle. I will not say 
 more in reference to the development of the muscular system 
 than that the whole of the muscles of the body ^apart from the 
 limbs, the origin of whose muscular system I have not yet in- 
 vestigated) are derived from the muscle-plates which grow up- 
 wards above the neural canal and downwards to the ventral 
 surface of the body. 
 
 During the time the muscle-plates have been undergoing 
 these changes the nerve masses have also undergone develop- 
 mental changes. 
 
 They become more elongated and fibrous, their main attach- 
 ment to the neural tube being still at its posterior (dorsal) 
 surface, near which they first appeared. Later still they become 
 applied closely to the sides of the neural tube and send fibres 
 to it below as well as above. Below (ventral to) the neural 
 tube a ganglion appears, forming only a slight swelling, but 
 containing a number of characteristic nerve-cells. The ganglion 
 is apparently formed just below the junction of the anterior and 
 posterior roots, though probably the fibres of the two roots do 
 not mix till below it. 
 
 The main points which deserve notice in the development 
 of the protovertebrae are 
 
 (i) That at the time when the mesoblast becomes split 
 horizontally into somatopleure and splanchnopleure the verte- 
 bral and lateral plates are one, and the splitting extends to the 
 very top of the vertebral or muscle-plate, so that the future 
 muscle-plates are divided into a splanchnic and somatic layer, 
 the space between which is at first continuous with the pleuro- 
 peritoneal cavity. 
 
DEVELOPMENT OK THE EEASMOBRANTH FISHES. 93 
 
 (2) That the following parts are respectively formed by the 
 vertebral and lateral plates : 
 
 (#) Vertebral plate. From the splanchnic layer of this, or 
 from cells which appear close to and continuous with it,_the 
 skeleton, and connective tissue of the upper part of the body, 
 are derived. 
 
 The remainder of the plate, consisting of a splanchnic and 
 somatic layer, is entirely converted into the muscles of the trunk, 
 all of which are derived from it. 
 
 (b) Between the vertebral plate and the lateral plate is a 
 mass of cells where, as I mentioned above, the mesoblast of the 
 somatopleure and splanchnopleure fuse together. This mass of 
 cells is the equivalent of the intermediate cell mass of Birds 
 (vide Waldeyer, Eierstock nnd Ei). 
 
 From it are derived the Wolffian bodies and duct, the 
 oviduct, the ovaries and the testis, and the connective tissue of 
 the parts adjoining these. 
 
 (<:) The lateral plate. From the somatic layer of this is 
 derived the connective tissue of the ventral half of the body ; the 
 mesoblast of the limbs, including probably the muscles, and 
 certainly the skeleton. From its splanchnic layer are derived 
 the muscles and connective tissue of the alimentary canal. 
 
 (3) The spinal nerves are developed independently of the 
 protovertebrae, so that the protovertebrae of the Elasmobranchii 
 do not appear to be of such a complicated structure as the proto- 
 vertebrae of Birds. 
 
 TJie Digestive Canal. 
 
 I do not intend to enter into the whole history of the di- 
 gestive canal, but to confine myself to one or two points of 
 interest connected with it. These fall under two heads : 
 
 (1) The history of the portion of the digestive canal be- 
 tween the anus and the end of the tail where the digestive canal 
 opens into the neural canal. 
 
 (2) Certain less well-known organs derived from the di- 
 gestive canal. 
 
94 DEVELOPMENT OF THK ELASMOBKANCH FISHES. 
 
 The anus is a rather late formation, but its position becomes 
 very early marked out by the hypoblast of the digestive canal 
 approaching at that point close to the .surface, whilst receding 
 to some little distance from it on either side. The portion of 
 the digestive tract I propose at present dealing with is that 
 between this point, which I will call, for the sake of brevity, the 
 anus and the hind end of the body. This portion of the canal 
 is at first very short ; it is elliptical in section, and of rather a 
 larsrer bore than the remainder of the canal. Its diameter be- 
 
 o 
 
 comes, however, slightly less as it approaches the tail, dilating 
 again somewhat at its extreme end. It is lined by a markedly 
 columnar epithelium. Though at first very short, its length 
 increases with the growth of the tail, but at the same tfme its 
 calibre continually becomes smaller as compared with the re- 
 remainder of the alimentary canal. 
 
 It commences to become smaller, first of all, near, though 
 not quite, at its extreme hind end, and thus becomes of a conical 
 shape; the base of the cone being just behind the anus, while 
 the apex of the cone is situated a short distance from the hind 
 end of the embryo. The extreme hind end, however, at the 
 same time does not diminish in size, and becomes relatively 
 (if not also absolutely) much larger in diameter than it was 
 at first, as compared with the remainder of the digestive canal. 
 It becomes, in fact, a vesicle or vesicular dilatation at the end 
 of a conical canal. 
 
 Just before the appearance of the external gills this part of 
 the digestive canal commences to atrophy. It begins to do so 
 close to the terminal vesicle, which, however, still remains as 
 or more conspicuous than it was before. The lumen of the 
 canal becomes smaller and smaller, and finally it becomes a 
 solid string of cells, and these also soon become indistinguishable 
 and not a trace of the canal is left. 
 
 Almost the whole of it has disappeared before the vesicle 
 begins to atrophy, but very shortly after all trace of the rest 
 of the canal has vanished the terminal vesicle also vanishes. 
 This occurs just about the time or shortly after the appearance 
 of the external gills there being slight differences probably in 
 this respect in the different species. 
 
 In this history there are two points of especial interest : 
 
DEVELOPMENT OF THE EEASMOHRAXCII EISHI.S. 95 
 
 (1) The terminal vesicle. 
 
 (2) The disappearance of a large and well-developed por- 
 tion of the alimentary canal. 
 
 The interest in the terminal vesicle lies in the possibility of 
 its being some rudimentary structure. 
 
 In Osseous fishes Kupffer has described the very early 
 appearance of a vesicle near the tail end, which he doubtfully 
 speaks of as the " allantois." The figure he gives of it in his 
 earlier paper (Arclriv. fiir Micro. Ana/. Vol. II. pi. xxiv, fig. 2) 
 bears a very strong resemblance to my figures of this vesicle at 
 the time when the hind end of the alimentary canal is com- 
 mencing to disappear ; and I feel fairly confident that it is the 
 same structure as I have found in the Dog-fish : but until the 
 relations of the Kupffer's vesicle to the alimentary canal are 
 known, any comparison between it and the terminal vesicle in 
 the Dog-fish must be to a certain extent guess-work. 
 
 I have, however, been quite unsuccessful in finding any other 
 vesicular structure which can possibly correspond to the so-called 
 allantoic vesicle of Osseous fish. 
 
 The disappearance of a large portion of the alimentary canal 
 behind the anus is very peculiar. In order, however, to under- 
 stand the whole difficulties of the case I shall be obliged to 
 speak 'of the relations of the anus of the Dog-fish to the anus of 
 Rusconi in the Lamprey, &c. 
 
 In those vertebrates whose alimentary canal is formed by 
 an involution, the anus of Rusconi represents the opening of this 
 involution, and therefore the point where the alimentary canal 
 primitively communicates with the exterior. When, however, 
 the " anus of Rusconi " becomes closed, the wall of the alimentary 
 canal still remains at that point in close juxtaposition to the 
 surface, and the new and final anus is formed at or close to that 
 point. In the Dog-fish, although the anus of Rusconi is not 
 present, still, during the closing of the alimentary canal, the point 
 which would correspond with this becomes marked out by the 
 alimentary canal there approaching the surface, and it is at this 
 point that the involution to form the true anus subsequently 
 appears. 
 
 The anus in the Dog-fish has thus, more than a mere secon- 
 dary significance. It corresponds with the point of closing of 
 
96 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 the primitive involution. If it was not for this peculiarity of the 
 vertebrate anus we would naturally suppose, from the disap- 
 pearance of a considerable portion of the alimentary canal lying 
 behind its present termination, that in the adult the alimentary 
 canal once extended much farther back than at present, and 
 that the anus we now find was only a secondary anus, and not 
 the primitive one. It is perhaps possible that this hinder portion 
 of the alimentary canal is a result of the combined growth of 
 the tail and the persisting continuity (at the end of the body) of 
 the epiblast with the hypoblast. 
 
 Whichever view is correct, it may be well to mention, in 
 order to shew that the difficulty about the anus of Rusconi is 
 no mere visionary one, that Gotte (" Untersuchung iiber die 
 Entwickelung der Bombinator igneus," ArcJiiv. fur Micro. 
 Anat., vol. v. 1869) has also described the disappearance of the 
 hind portion of the alimentary canal in Batrachians, a rudiment 
 (according to him) remaining in the shape of a lymphatic trunk. 
 
 It is, perhaps, possible that we have a further remnant of 
 this " hind portion " of the alimentary canal amongst the higher 
 vertebrates in the " allantois." 
 
 Organs developed from the Digestive Canal. 
 
 In reference to the development of the liver, pancreas, &c., 
 as far as my observations have at present gone, the Dog-fish 
 presents no features of peculiar interest. The liver is developed 
 as in the Bird, and independently of the yolk. 
 
 There are, however, two organs derived from the hypoblast 
 which deserve more attention. Immediately under the noto- 
 chord, and in contact with it (vide PL 3, fig. 10; 4, 11 and I2,;r), 
 a small roundish (in section) mass of cells is to be seen in most 
 of the sections. 
 
 Its mode of development is shewn in fig. 10, x. That section 
 shows a mass of cells becoming pinched off from the top of the 
 alimentary canal. By this process of pinching ofif from the 
 alimentary canal a small rod-like body close under the noto- 
 chord is formed. It persists till after the appearance of the 
 external gills, but later than that I have not hitherto succeeded 
 in finding any trace of it 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 97 
 
 It was first seen by Gotte (loc. cit.) in the Batrachians, and 
 he gave a correct account of its development, and added that it 
 became the thoracic duct. 
 
 I have not myself worked out the later stages in the de-_ 
 velopment of this body with sufficient care to be in a position 
 to judge of the correctness of Gotte's statements as to its final 
 fate. If it is true that it becomes the thoracic duct it is very 
 remarkable, and ought to throw some light upon the homologies 
 of the lymphatic system. 
 
 Some time before the appearance of the external gills another 
 mass of cells becomes, I believe, constricted off from the part 
 of the alimentary canal in the neighbourhood of the anus, and 
 forms a solid rod composed at first of dark granular cells lying 
 between the Wolffian ducts. I have not followed out its de- 
 velopment quite completely, but I have very little doubt that 
 it is really constricted off from a portion of the alimentary canal 
 chiefly in front of the point where the anus appears, but also, 
 I believe, from a small portion behind this. 
 
 Though the cells of which it is composed are at first columnar 
 and granular (fig. 12, s u, r), they soon begin to become altered, 
 and in the latter stage of its development the body forms a 
 conspicuous rounded mass of cells with clear protoplasm, and 
 each provided with a large nucleus. Later still it becomes 
 divided into a number of separate areas of cells by septa of 
 connective tissue, in which (the septa) capillaries are also present. 
 Since I have not followed it to its condition in the adult, I 
 cannot make any definite statements as to the fate of this 
 body ; but I think that it possibly becomes the so-called supra- 
 renal organ, which in the Dog-fish forms a yellowish elongated 
 body lying between the two kidneys. 
 
 The development of tlic Wolffian Duct and Body and of t/ie 
 
 Oviduct. 
 
 The development of the Wolffian duct and the Oviduct in 
 the various classes of vertebrates is at present involved in some 
 obscurity, owing to the very different accounts given by different 
 observers. 
 
 B. 7 
 
98 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 The manner of development of these parts in the Dog-fish 
 is different from anything that previous investigators have met 
 with in other classes, but I believe that it gives a clearer insight 
 into the true constitution of these parts than vertebrate embryo- 
 logy has hitherto supplied, and at the same time renders easier 
 the task of understanding the differences in the modes of de- 
 velopment in the different classes. 
 
 I shall commence with a simple description of the observed 
 facts, and then give my view as to their meaning. At about 
 the time of the appearance of the third visceral cleft, and a 
 short way behind the point up to which the alimentary canal 
 is closed in front, the splanchnopleure and somatopleure fuse 
 together opposite the level of the dorsal aorta. 
 
 From the mass of cells formed by this fusion a solid knob 
 rises up towards the epiblast (PI. 4, fig. 1 1 b, o v\ and from 
 this knob a solid rod of cells grows backwards towards the 
 tail (fig. nc, o i>) very closely applied to the epiblast. This 
 description will be rendered clear by referring to figs, lib and c. 
 Fig. 1 1 b is a section at the level of the knob, and fig. 1 1 c is a 
 section of the same embryo a short way behind this point. So 
 closely does the rod of cells apply itself to the epiblast that 
 it might very easily be supposed to be derived from it. Such, 
 indeed, was at first my view till I cut a section passing through 
 the knob. In order, however, to avoid all possibility of mistake 
 I made sections of a large number of embryos of about the age 
 at which this appears, and invariably found the large knob in 
 front, and from it the solid string growing backwards. 
 
 This string is the commencement of the Oviduct or Mutter s 
 duct, which in the Dog-fish as in the Batrachians is the first 
 portion of the genito-urinary system to appear, and is in the 
 Dog-fish undoubtedly at first solid. All my specimens have 
 been hardened with osmic acid, and with specimens hardened 
 with this reagent it is quite easy to detect even the very smallest 
 hole in a mass of cells. 
 
 As a solid string or rod of cells the Oviduct remains for 
 some time ; it grows, indeed, rapidly in length, the extreme hind 
 end of the rod being very small and the front end continuing 
 to remain attached to the knob. The knob, however, travels 
 inwards and approaches nearer and nearer to the true pleuro- 
 
DEVELOPMENT OF THE KI.ASMO15RANCH FISIIKS. 99 
 
 peritoneal cavity, always remaining attached to the intermediate 
 cell mass. 
 
 At about the time when five visceral clefts are present the 
 Oviduct first begins to get a lumen and to open at its front end 
 into the pleuro-peritoneal cavity. The cells of the rod are first 
 of all arranged in an irregular manner, but gradually become 
 columnar and acquire a radiating arrangement around a central 
 point. At this point, where the ends of all the cells meet, a 
 very small hole appears, which gradually grows larger and 
 becomes the cavity of the duct (fig. 12, o v). The hole first 
 makes its appearance at the anterior end of the duct, and then 
 gradually extends backwards, so that the hind end is still without 
 a lumen, when the lumen of the front end is of a considerable 
 size. 
 
 At the front knob the same alteration in the cells takes 
 place as in the rest of the duct, but the cells become deficient 
 on the side adjoining the pleuro-peritoneal cavity, so that 
 an opening is formed into the pleuro-peritoneal cavity, which 
 soon becomes of a considerable size. Soon after its first for- 
 mation, indeed, the opening becomes so .large that it may 
 be met in from two to three consecutive sections if these are 
 very thin. 
 
 Thus is formed the lumen of the Oviduct. The duct still, 
 at this age, ends behind without having become attached to 
 the cloaca, so that at this time the Oviduct is a canal closed 
 behind, but communicating in front by a large opening with the 
 pleuro-peritoneal cavity. 
 
 It has during this time been travelling downwards, and is 
 now much nearer the pleuro-peritoneal cavity than the epiblast. 
 
 It may be well to point out that the mode of development 
 which I have described is really not very different from an invo- 
 lution, and must, in fact, be only looked upon as a modification 
 of an involution. Many examples from all classes in the animal 
 kingdom could be selected to exemplify how an involution may 
 become simply a solid thickening. In the Osseous fish nearly 
 all the organs which are usually formed by an involution have 
 undergone this change in their mode of development. I shall 
 attempt to give reasons later on for the solid form having been 
 acquired in this particular case of the Oviduct. 
 
 72 
 
100 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 At about the time when a lumen appears in the Oviduct the 
 first traces of the Wolffian duct become visible. 
 
 At intervals along the whole length, between the front and 
 hind ends of the Oviduct, involutions arise from the pleuro- 
 peritoneal cavity (fig. 12, a,pwd] on the inside (nearer the 
 middle line) of the Oviduct. The upper ends of these numerous 
 involutions unite together and form a string of cells, at first 
 solid, but very soon acquiring a lumen, and becoming a duct 
 which communicates (as it clearly must from its mode of for- 
 mation), at numerous points with the pleuro-peritoneal cavity. 
 It is very probable that there is one involution to each seg- 
 ment of the body between the front and hind ends of the 
 Oviduct. This duct is the Wolffian duct, which thus, toge- 
 ther with the Oviduct, is formed before the appearance of the 
 external gills. 
 
 For a considerable period the front end of the Oviduct does 
 not undergo important changes ; the hind end, however, comes 
 into connection with the extreme end of the alimentary canal. 
 The two Oviducts do not open together into the cloaca, though, 
 as my sections prove, their openings are very close together. 
 The whole Oviduct, as might be expected, shares in the general 
 growth, and its lumen becomes in both sexes very considerably 
 greater than it was before. 
 
 It is difficult to define the period at which I find these 
 changes accomplished without giving drawings of the whole 
 embryo. The stage is one considerably after the external 
 gills have appeared, but before the period at which the growth 
 of the olfactory bulbs renders the head of an elongated shape. 
 
 During the same period the Wolffian duct has undergone 
 most important changes. It has commenced to bud off diverti- 
 cula, which subsequently become the tubules of the Wolffian 
 body (vide fig. 13, wd}. I am fairly satisfied that the tubules 
 are really budded off, and are not formed independently in the 
 mesoblast. The Dog-fish agrees so far with Birds, where I have 
 also no doubt the tubules of the Wolffian body are formed as 
 diverticula from the Wolffian duct. 
 
 The Wolffian ducts have also become much longer than the 
 Oviduct, and are now found behind the anus, though they do not 
 extend as far forward as does the Oviduct. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. IOI 
 
 They have further acquired a communication with the 
 Oviduct, in the form of a narrow duct passing from each of them 
 into an Oviduct a short way before the latter opens into the 
 cloacal dilatation of the alimentary canal. 
 
 The canals formed by the primitive involution leading from 
 the pleuro-peritoneal cavity into the Wolffian duct have become 
 much more elongated, and at the same time narrower. One of 
 these is shewn in fig. i^^piud. 
 
 Any doubt which could possibly be entertained as to the 
 true character of the ducts whose development I have described 
 is entirely removed by the development of the tubules of the 
 Wolffian body. In the still later stage than this further proofs 
 are furnished involving the function of the Oviduct. At the 
 period when the olfactory lobes have become so developed as to 
 render the head of the typical elongated shape of the adult, I find 
 that the males and females can be distinguished by the presence 
 in the former of the clasping appendages 1 . I find at this stage 
 that in the female the front ends of the Oviducts have approached 
 the middle line, dilated considerably, and commenced to exhibit 
 at their front ends the peculiarities of the adult. In the male 
 they are much less conspicuous, though still present. 
 
 At the same time the tubules of the Wolffian body become 
 much more numerous, the Malpighian tufts appear, and the ducts 
 cease almost, if not entirely, to communicate with the pleuro- 
 peritoneal cavity. I have not made out anything very definitely 
 as to the development of the Malpighian tufts, but I am inclined 
 to believe that they arise independently in the mesoblast of the 
 intermediate cell mass. 
 
 The facts which I have made out in reference to the develop- 
 ment of the Wolffian duct, especially of its arising as a series 
 of involutions from the pleuro-peritoneal cavity, will be found, 
 I believe, of the greatest importance in understanding the true 
 constitution of the Wolffian body. To this I will return directly, 
 but first wish to clear the ground by insisting upon one preli- 
 minary point. 
 
 From their development the Oviduct and Wolffian body 
 appear to stand to each other in the relation of the Wolffian 
 
 1 Fur the specimens of this age I am indebted to Profes-ior Huxley. 
 
IO2 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 duct being the equivalent to a series, so to speak, of Ovi- 
 ducts. 
 
 I pointed out before that the mode of development of the 
 Oviduct could only be considered as a modification of a simple 
 involution from the pleuro-peritoneal cavity. Its development, 
 both in the Birds and in the Batrachians as an involution, still 
 more conclusively proves the truth of this view. 
 
 The explanation of its first appearing as a solid rod of cells 
 which keeps close to the epiblast is, I am inclined to think, 
 the following. Since the Oviduct had to grow a long way 
 backwards from its primitive point of involution, it was clearly 
 advantageous for it not to bore its way through the mesoblast 
 of the intermediate cell mass, but to pass between this and the 
 epiblast. This modification having been adopted, was followed 
 by the knob forming the origin of the duct coming to be placed 
 at the outside of the intermediate cell mass rather than close 
 to the pleuro-peritoneal cavity, a change which necessitated 
 the mode of development by an involution being dropped and 
 the solid mode of development substituted for it, a lumen being 
 only subsequently acquired. 
 
 In support of the modification in the development being due 
 to this cause is the fact that in Birds a similar modification has 
 taken place with the t Wolffian duct. The Wolffian duct there 
 arises differently from its mode of development in all the lower 
 vertebrates as a solid rod close to the epiblast 1 , instead of as an 
 involution. 
 
 If the above explanation about the Oviduct be correct, then 
 it is clear that similar causes have produced a similar modifica- 
 tion in development (only with a different organ) in Birds ; while, 
 at the same time, the primitive mode of origin of the Oviduct 
 (Miiller's duct) has been retained by them. 
 
 The Oviduct, then, may be considered as arising by an invo- 
 lution from the pleuro-peritoneal cavity. 
 
 The Wolffian duct arises by a series of such involutions, 
 all of which are behind (nearer the tail) the involution to form 
 the Oviduct. 
 
 1 If Roiniti's observations (Archives fiir J///v. Anatoin. Vol. ix. p. 200) are 
 correct, then the ordinary view of the Wolffian duct arising in Birds as a solid rod at 
 the outer corner of the protovertebne will have to be abandoned. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISH1 -. 103 
 
 The natural interpretation of these facts is that in the place 
 of the Oviduct and Wolffian body there were primitively a series 
 of similar bodies (probably corresponding in number with the 
 vertebral segments), each arising by an involution from the 
 pleuro-peritoneal cavity ; and that the first of these subsequenTly" 
 became modified to carry eggs, while the rest coalesced to form 
 the Wolffian duct. 
 
 If we admit that the Wolffian duct is formed by the coa- 
 lescence of a series of similar organs, we shall only have to 
 extend the suggestion of Gegenbaur as to the homology of 
 the Wolffian body in order to see its true nature. Gegen- 
 baur looks upon the whole urino-genital system as homolo- 
 gous with a pair of segmental organs. Accepting its homology 
 with the segmental organs, its development in Elasmobranchii 
 proves that it is not one pair, but a series of pairs of segmental 
 organs with which the urino-genital system is homologous. The 
 first of these have become modified so as to form the Ovi- 
 ducts, and the remainder have coalesced to form the Wolffian 
 ducts. 
 
 The part of a segmental organ which opens to the exterior 
 appears to be lost in the case of all but the last one, where this 
 part is still retained, and serves as the external opening for all. 
 
 Whether the external opening of the first segmental organ 
 (Oviduct) is retained or not is doubtful. Supposing it has been 
 lost, we must look upon the external opening for the Wolffian 
 body as serving also for the Oviduct. In the case of all other 
 vertebrates whose development has been investigated (but the 
 Elasmobranchii), the Wolffian duct arises by a single invo- 
 lution, or, what is equivalent to it, the other involutions having 
 disappeared. This even appears to be the case in the Mar- 
 sipobranchii. In the adult Lamprey the Wolffian duct ter- 
 minates at its anterior end by a large ciliated opening into 
 the pleuro-peritoneal cavity. It will, perhaps, be found, when 
 the development of the Marsipobranchii is more carefully 
 studied, that there are primitively a number of such open- 
 ings 1 . The Oviduct, when present, arises in other vertebrates 
 
 1 While correcting the proofs of this paper I have come across a memoir of W. 
 Miiller (" Ueber die Persistenz der Urniere bei Myxine Glutinosa," Jcnaische Zeit- 
 schrift, Vol. vn. 1873), in which he mentions that in Myxine the upper end of the 
 
104 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 as a single involution, strongly supporting the view that its 
 mode of formation in the Dog-fish is fundamentally merely an 
 involution. 
 
 The duct of the testes is, I have little doubt, derived from 
 the anterior part of the Wolffian body ; if so, it must be looked 
 upon as not precisely equivalent to the Oviduct, but rather 
 to a series of coalesced organs, each equivalent to the Oviduct. 
 The Oviduct is in the Elasmobranchii, as in other vertebrates, 
 primitively developed in both sexes. In the male, however, 
 it atrophies. I found it still visible in the male Torpedos, 
 though much smaller than in the females near the close of intra- 
 uterine life. 
 
 Whether or^ not these theoretical considerations as to the 
 nature of the Wolffian body and Oviduct are correct, I believe 
 that the facts. I have brought to light in reference to the develop- 
 ment of these parts in the Dog-fish will be found of service 
 to every one who is anxious to discover the true relations of 
 these parts. 
 
 Before leaving the subject I will say one on two words 
 about the development of the Ovary. In both sexes the ger- 
 minal epithelium (fig. 13) becomes thickened below the Oviduct, 
 and in both sexes a knob (in section but really a ridge) comes 
 to project into the pleuro-peritoneal cavity on each side of 
 the mesentery (fig. 13, pov}. In both sexes, but especially 
 the females, the epithelium on the upper surface of this ridge 
 becomes very much thickened, whilst subsequently it elsewhere 
 atrophies. In the females, however, the thickened epithe- 
 lium on the knob grows more and more conspicuous, and de- 
 velops a number of especially large cells with large nuclei, 
 precisely similar to Waldeyer's (loc. cit.} "primitive ova" of the 
 Bird. In the male the epithelium on the ridge, though con- 
 taining primitive ova, is not as conspicuous as in the female. 
 Though I have not worked out the matter further than this at 
 present, I still have no doubt that these projecting ridges be- 
 come the Ovaries. 
 
 Wolffian duct communicates by numerous openings with the pleuro-peritoneal cavity ; 
 this gives to the suggestion in the text a foundation of fact. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 105 
 
 The Head. 
 
 The study of the development of the parts of the head, on 
 account of the crowding of organs which occurs there, always 
 presents greater difficulties to the investigator than that_of the 
 remainder of the body. My observations upon it are cor- 
 respondingly incomplete. I have, however, made out a few 
 points connected with it in reference to some less well-known 
 organs, which I have thought it worth while calling attention to 
 in this preliminary account. 
 
 The continuation of the Plenro-peritoneal Cavity into the 
 
 Head. 
 
 In the earlier part of this paper (p. 86) I called attention 
 to the extension of the separation between somatopleure and 
 splanchnopleure into the head, forming a space continuous with 
 the pleuro-peritoneal cavity (PI. 3, fig. 8 a, pp'} ; this be- 
 comes more marked in the next stage, and, indeed, the pleuro- 
 peritoneal cavity is present for a considerable time in the head 
 before it becomes visible elsewhere. At the time of the appear- 
 ance of the second visceral cleft it has become for the most 
 part atrophied, but there persist two separated portions of it in 
 front of the first cleft, and also remnants of it less well marked 
 between and behind the two clefts. The visceral clefts neces- 
 sarily divide it into separate parts^ 
 
 The two portions in front of the first visceral cleft remain 
 very conspicuous till the appearance of the external gills, and 
 above the hinder one of the two the fifth nerve bifurcates. 
 
 These two are shewn as they appear in a surface view in 
 fig. 14, //. They are in reality somewhat flattened spaces, 
 lined by a mesoblastic epithelium ; the epithelium on the inner 
 surface of the space corresponding to the splanchnopleure, and 
 that on the outer to the somatopleure. 
 
 I have not followed the history of these later than the time 
 of the appearance of the external gills. 
 
 The presence of the pleuro-peritoneal cavity in the head is 
 interesting, as shewing the fundamental similarity between the 
 head and the remainder of the body. 
 
106 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 TJie Pituitary Body. 
 
 All my sections seem to prove that it is .a portion of the 
 epiblastic involution to form the mouth which is pinched off to 
 form the pituitary body, and not a portion of the hypoblast of 
 the throat. Since Gotte (Archiv. filr Micr. Anat. Bd. IX.) has 
 also found that the same is the case with the Batrachians and 
 Mammalia, I have little doubt it will be found to be universally 
 the case amongst vertebrates. 
 
 Probably the observations which lead to the supposition that 
 it was the 'throat which was pinched off to form the pituitary 
 body were made after the opening between the mouth and throat 
 was completed, when it would naturally be impossible to tell 
 whether the pinching off was from the epiblast of the mouth 
 involution or the hypoblast of the throat. 
 
 The Cranial Nerves. 
 
 The cranial nerves in their early condition are so clearly 
 visible that I have thought it worth while giving a figure of 
 them, and calling attention to some points about their embry- 
 onic peculiarities. 
 
 From my figure (14) it will be seen that there is behind the 
 auditory vesicle a nervous tract, from which four nerves descend, 
 and that each of these nerves is distributed to the front portion 
 of a visceral arch. When the next and last arch (in this species) 
 is developed, a branch from this nervous mass will also pass 
 down to it. That each of these is of an equal morphological 
 value can hardly be doubted. 
 
 The nerve to the third arch becomes the glosso-pharyngeal 
 (fig. 14, g I), the nerves to the other arches become the bran- 
 chial branches of the vagus nerve (fig. 14, v g). Thus the 
 study of their development strongly supports Gegenbaur's view 
 of the nature of the vagus and glosso-pharyngeal, viz. that 
 the vagus is a compound nerve, each component part of it which 
 goes to an arch being equivalent to one nerve, such as the 
 glosso-pharyngeal. 
 
 Of the nerves in front of the auditory sac the posterior is 
 the seventh nerve (fig. 14, Vll). Its mode of distribution to 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. IO/ 
 
 the second arch leaves hardly a doubt that it is equivalent to 
 one such nerve as those distributed to the posterior arches. 
 Subsequently it acquires another branch, passing forwards to- 
 wards the arch in front. 
 
 The most anterior nerve is the fifth (fig. 14, v), of which 
 two branches are at this stage developed. The natural inter- 
 pretation of its present condition is, that it is equivalent to 
 two nerves, but the absence of relation in its branches to any 
 visceral clefts renders it more difficult to determine the mor- 
 phology of the fifth nerve than of the other nerves. The front 
 branch of the two is the ophthalmic branch of the adult, and the 
 hind branch the inferior maxillary branch. The latter branch 
 subsequently gives off low down, i.e. near its distal extremity, 
 another branch, the superior maxillary branch. 
 
 In its embryonic condition this latter branch does not appear 
 like a third branch of the fifth, equivalent to the seventh or the 
 glosso-pharyngeal nerves, but rather resembles the branch of 
 the seventh nerve which passes to the arch in front, which also 
 is present in all the other cranial nerves. 
 
 Modes of Preparation. 
 
 Before concluding I will say one or two words as to my 
 modes of preparation. 
 
 I have used picric and chromic acids, both applied in the 
 usual way ; but for the early stages I have found osmic acid by 
 far the most useful reagent. I placed the object to be hardened, 
 in osmic acid (half per cent.) for two hours and a half, and then 
 for twenty four in absolute alcohol. 
 
 I then embedded and cut sections of it in the usual way, 
 without staining further. 
 
 I found it advantageous to cut sections of these embryos 
 immediately after hardening, since if kept for long in the 
 absolute alcohol the osmic acid specimens are apt to become 
 brittle. 
 
IO8 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 LIST OF WORKS REFERRED TO. 
 
 Gegenbaur. Anat. der Wirbelthiere, III Heft, Leipzig, 1873. 
 
 A. Gotte. Archiv. fur Micr. Anat., Vol. x. 1873. "Der Keim der Forelleneies," 
 
 Archiv.filr. Micr. Anat., Vol. IX. 1873. " Untersuchung liber die Entwickelung 
 
 der Bombinator igneus," Archiv. filr Micr. Anat., Vol. V. 1869. " Kurze Mit- 
 
 theilungen aus der Entwickelungsgeschichte der Unke," Archiv. fur Micr. Anat., 
 
 Vol. IX. 1873. 
 Kupffer. Archiv. fur Micr. Anat., Vol. n. 1866, p. 473. Ibid. Vol. IV. 1868, 
 
 p. 209. 
 Kowalevsky. " Entwickelungsgeschichte der Holothurien," Memoirs de fAcad. 
 
 Imper. des Sciences de St Petersboiirg, vii ser. Vol. xi. 1867. 
 Kowalevsky, Owsjannikow, uncl Wagner. "Entwickelung der Store," Bulletin der 
 
 K. Acad. St Petersbourg, Vol. XIV. 1873. 
 Kowalevsky. " Embryologische Studien an Wurmern und Arthropoden," Memoirs 
 
 de rAcad. Imper. des Sciences de St Petersbourg, Vol. xvi. 1871. 
 E. Ray Lankester. Annals and Mag. of Nat. History, Vol. xi. 1873, P- 81. 
 W. Mliller. " Ueber die Persistenz der Urniere bei Myxine Glutinosa," Jenaische 
 
 Zeitschrift, Vol. vii. 1873. 
 
 Oellacher. Zeitschrift fur IViss. Zoologie, Vol. XXIII. 1873. 
 Owsjannikow. "Entwickelung der Coregonus," Eul. der K. Akad. St Petersbourg, 
 
 Vol. xix. 
 
 Romiti. Archiv. filr Micr. Anat., Vol. IX. 1873, 
 Waldeyer. EierstocR u. Eie. 
 
 EXPLANATION OF PLATES 3 AND 4. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 a I. Alimentary canal, a o. Dorsal aorta, an v. Auditory vesicle, b d. Forma- 
 tive cell probably derived from the yolk, ca v. Cardinal vein, c h. Notochord. 
 c H . Thickening of hypoblast to form the notochord. e b. Line indicating the edge 
 of the blastoderm, e p. Epiblast. e p' . Epidermis, e r. Embryonic rim. e s. Em- 
 bryonic swelling, g I. Glosso-pharyngeal nerve, h. Head. ///. Heart, h y. Hypo- 
 blast. //. Lower layer cells. I y. Line of separation between the blastoderm and 
 the yolk. m. Mesoblast. m c. Medullary canal, m g. Medullary groove, m p. 
 Muscle-plate, m p'. Early formed mass of muscles, n. Peculiar nuclei formed in 
 the yolk. '. Similar nuclei in the cells of the blastoderm, n a. Cells which help 
 to close in the alimentary canal, and which are derived from the yolk, n y. Network 
 of lines present in the food-yolk, o I. Olfactory pit. op. Eye. ov. Oviduct, p n. 
 Pineal gland, p ov. Projection which becomes the ovary. //. Pleuro-peritoneal 
 cavity. //' Remains of pleuro-peritoneal cavity in the head, pr v. Protovertebrse. 
 p wd. Primary points of involution from the pleuro-peritoneal cavity by the coalesc- 
 ence of which the Wolffian duct is formed, s g. Segmentation cavity, j o. Somato- 
 pleure. so s. Stalk connecting embryo with yolk-sac, s p. Splanchnopleure. sp n. 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. 109 
 
 Spinal nerve, su r. Supra-renal body, t s. Caudal lobes, v. Blood-vessel, v g. 
 Vagus nerve, v. Fifth nerve, vn. Seventh nerve, v c , 1,2, 3, c. ist, 2nd and 
 3rd &c. visceral clefts, v p. Vertebral plates, iv d. Wolffian duct. x. Peculiar body 
 underlying the notochord derived from the hypoblast. y k. Yolk spherules. 
 All the figures were drawn with the Camera Lucida. 
 
 Plate 3. 
 
 Fig. i. Section parallel with the long axis of the embryo through a blastoderm, 
 in which the floor of the segmentation cavity (s g) is not yet completely lined by cells. 
 The roof of the segmentation cavity is broken. (Magnified 60 diam.) The section is 
 intended chiefly to illustrate the distribution of nuclei () in the yolk under the blasto- 
 derm. One of the chief points to be noticed in their distribution is the fact that they 
 form almost a complete layer under the floor of the segmentation cavity. This pro- 
 bably indicates that the cells whose nuclei they become take some share in forming 
 the layer of cells which subsequently (vide fig. 4) forms the floor of the cavity. 
 
 Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the time 
 of the first appearance of the medullary groove. (Magnified 300 diam.) 
 
 The specimen is taken from a portion of the blastoderm which will form part of 
 the embryo. It shews two large nuclei of the yolk () and the network in the yolk 
 between them ; this network is seen to be closer around the nuclei than in the inter- 
 vening space. The specimen further shews that there are no areas representing cells 
 around the nuclei. 
 
 Fig. 3. Section parallel with the long axis of the embryo through a blastoderm, 
 in which the floor of the segmentation cavity is not yet covered by a complete layer of 
 cells. (Magnified 60 diam.) 
 
 It illustrates (i) the characters of the epiblast, (2) the embryonic swelling (e s), 
 (3) the segmentation cavity (sg). It should have been drawn upon the same scale as 
 fig. 4 ; the line above it represents its true length upon this scale. 
 
 Fig. 4. Longitudinal section through a blastoderm at the time of the first appear- 
 ance of the embryonic rim, and before the formation of the medullary groove. 
 (Magnified 45 diam.) 
 
 It illustrates (i) the embryonic rim, (2) the continuity of epiblast and hypoblast at 
 edge of this, (3) the continual differentiation of the lower layer cells, to form, on the 
 one hand, the hypoblast, which is continuous with the epiblast, and on the other the 
 mesoblast, between this and the epiblast; (4) the segmentation cavity, whose floor of 
 cells is now completed. 
 
 N.B. The cells at the embryonic end of the blastoderm have been made rather 
 too large. 
 
 Fig. 5. Surface view of the blastoderm shortly after the appearance of the me- 
 dullary groove. To shew the relation of the embryo to the blastoderm. 
 
 Fig. 6 a and b. Two transverse sections of the same embryo, shortly after the 
 appearance of the medullary groove. (Magnified 96 diam.) 
 
 a. In the region of the groove. It shews (i) the two masses of mesoblast on 
 each side, and the deficiency of the mesoblast underneath the medullary groove; 
 (2) the commencement of the closing in of the alimentary canal below, chiefly from 
 cells (n a) derived from the yolk. 
 
 b. Section in the region of the head where the medullary groove is deficient, 
 other points as above. 
 
IIO DEVELOPMENT OF THE ELASMOBRANCH FISHES. 
 
 Fig. 7 a and K. Two transverse sections of an embryo about the age or rather 
 younger than that represented in fig. 5. (Magnified 96 diam.) 
 
 a. Section nearer the tail ; it shews the thickening of the hypoblast to form the 
 notochord (ch r ). 
 
 In b the thickening has become completely separated from the hypoblast as the 
 notochord. In a the epiblast and hypoblast are continuous at the edge of the section, 
 owing to the section passing through the embryonic rim. 
 
 Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (i) the 
 flattened head (k) where the medullary groove is deficient, (2) the caudal lobes, with 
 a groove between them; it also shews that at this point, the medullary groove has 
 become roofed over and converted into a canal. 
 
 Fig. 8 a. Transverse section of fig. 8, passing through the line a. (Magnified 
 90 diam.) The section shews (i) the absence of the medullary groove in the head 
 and the medullary folds turning dovvn at this time instead of upwards; (2) the pre- 
 sence of the pleuro-peritoneal cavity in the head (//); (3) the completely closed 
 alimentary canal (a /). 
 
 Fig. 8 b. Transverse section of fig. 8, through the line b. (Magnified 90 diam.) 
 It shews (i) the neural canal completely formed ; (2) the vertebral plates of mesoblast 
 not yet split up into somatopleure and splanchnopleure. 
 
 Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object a 
 little older than the embryo represented in fig. 8. (Magnified 20 diam.) The internal 
 anatomy has hardly altered, with the exception of the medullary folds having closed 
 over above the head and the whole embryo having become more folded off from the 
 germ. 
 
 The two caudal lobes, and the very marked groove between them, are seen at t s. 
 The front end of the notochord became indistinct, and I could not see its exact 
 termination. The epithelium of the alimentary canal (a 1} is seen closely underlying 
 the notochord and becoming continuous with the epiblast at the hind end of the 
 notochord. 
 
 The first visceral cleft (i v c] and eye (op) are just commencing to be formed, and 
 the cranial flexure has just appeared. 
 
 Fig. 10. Section through the dorsal region of an embryo somewhat older than 
 the one represented in fig. 9. (Magnified 96 diam.) 
 
 It shews (i) the formation by a pinching off from the top of the alimentary canal of 
 a peculiar body which underlies the notochord (x)\ (i) the primitive extension of the 
 pleuro-peritoneal cavity up to the top of the vertebral plates. 
 
 Plate 4. 
 
 Fig. 1 1 a, b, and c. Three sections closely following each other from an embryo 
 in which three visceral clefts are present; a is the most anterior of the three. (Mag- 
 nified 96 diam.) In all of these the muscle-plates are shewn at in p. They have 
 become separated from the lateral plates in b and c, but are still continuous with them 
 in a. The early formed mass of muscles is also shewn in all the figures (m p'). 
 
 The figures further shew (i) the formation of the spinal nerves (sp n) as small 
 bodies of cells closely applied to the upper and outer edge of the neural canal. 
 
 (2) The commencing formation of the cells which form the axial skeleton from 
 the inner (splanchnopleuric) layer of the muscle-plate. Sections /' and c are given 
 more especially to shew the mode of formation of the oviduct (ov). 
 
DEVELOPMENT OF THE ELASMOBRANCH FISHES. I I I 
 
 In b it is seen as a solid knob (ov), arising from the point where the somatopleure 
 and splachnopleure unite, and in c (the section behind l>) as a solid rod (ov) closely ap- 
 plied to the epiblast, which has grown backwards from the knob seen in b. 
 
 N.B. In all three sections only one side is completed. 
 
 Fig. 120 and b. Two transverse sections of an embryo just before the appearance 
 of the external gills. (Magnified 96 diam.) 
 
 In a there is seen to be an involution on each side (p w d), while b is a section 
 from the space between two involutions from the pleuro-peritoneal cavity, so that the 
 Wolffian duct (at first solid) (w d) is not connected as in a with the pleuro-peritoneal 
 cavity. The further points shewn in the sections are 
 
 (1) The commencing formation of the spiral valve (a I). 
 
 (2) The supra- renal body (si/ >). 
 
 (3) The oviduct (ov), which has acquired a lumen. 
 
 (4) The increase in length of the muscle-plates, the spinal nerve 1 :, &c. 
 
 Fig. 13. Section through the dorsal region of an embryo in which the external 
 gills are of considerable length. (Magnified 40 diam.) The chief points to be 
 noticed : 
 
 (1) The formation of the Wolffian body by outgrowths from the Wolffian 
 duct (w d). 
 
 (2) One of the still continuing connections (primitive involutions) between 
 the Wolffian duct and the pleuro-peritoneal cavity (/ w d). 
 
 (3) The oviduct largely increased in size (ov). 
 
 N.B. On the left side the oviduct has been accidentally made too small. 
 
 (4) The growth downwards of the muscle-plate to form the muscles of the 
 abdomen. 
 
 (?) The formation of an outgrowth on each side of the mesentery (/ ov), which 
 will become the ovary. 
 
 (6) The spiral valve (a /). 
 
 Fig. 14. Transparent view of the head of an embryo shortly before the appear- 
 ance of the external gills. (Magnified 20 diam.) The chief points to be noticed are 
 
 (1) The relation of the cranial nerves to the visceral clefts and the manner in 
 which the glosso-pharyngeal (g /) and vagus (v g) are united. 
 
 (2) The remnants of the pleuro-peritoneal cavity in the head (p p). 
 
 (3) The eye (of). The stalk, as well as the bulb of the eye, are supposed to be 
 in focus, so that the whole eye has a somewhat peculiar appearance. 
 
VI. A COMPARISON OF THE EARLY STAGES IN THE DE- 
 VELOPMENT OF VERTEBRATES 1 . 
 
 With Plate 5. 
 
 IF the genealogical relationships of animals are to be mainly 
 or largely determined on embryological evidence, it becomes a 
 matter of great importance to know how far evidence of this 
 kind is trustworthy. 
 
 The dependence to be placed on it has been generally 
 assumed to be nearly complete. Yet there appears to be no 
 a priori reason why natural selection should not act during the 
 embryonic as well as the adult period of life ; and there is no 
 question that during their embryonic existence animals are 
 more susceptible to external forces than after they have be- 
 come full grown : indeed, an immense mass of evidence could 
 be brought to shew that these forces do act upon embryos, 
 and produce in them great alterations tending to obscure the 
 genealogical inferences to be gathered from their developmental 
 histories. Even the time-honoured layers form to this no ex- 
 ception. In Elasmobranchs, for instance, we find the notochord 
 derived from the hypoblast and the spinal ganglia derived from 
 the involuted epiblast of the neural canal, whilst in the higher 
 vertebrates both of these organs are formed in the mesoblast. 
 Such instances are leading embryologists to recognise the fact 
 that the so-called layers are not quite constant and must not be 
 absolutely depended upon in the determination of homologies. 
 But though it is necessary to recognise the fact that great 
 changes do occur in animals during their embryonic life, it is 
 not necessary to conclude that all embryological evidence is 
 thereby vitiated ; but rather it becomes incumbent on us to 
 attempt to determine which embryological features are an- 
 cestral and which secondary. For this purpose it is requisite 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xv. 1875. 
 
EARLY STAGES IN DEVELOPMENT OF VERTEBRATES. 113 
 
 to ascertain what are the general characters of secondary 
 features and how they are produced. Many vertebrates have 
 in the first stages of their development a number of secondary 
 characters which are due to the presence of food material in 
 the ovum; the present essay is mainly an attempt to indicate 
 how those secondary characters arose and to trace their gradual 
 development. At the same time certain important ancestral 
 characters of the early phases of the development of verte- 
 brates, especially with reference to the formation of the 
 hypoblast and mesoblast, are pointed out and their meaning 
 discussed. 
 
 There are three orders of vertebrates of which no mention 
 has been made, viz., the Mammals, the Osseous fishes, and the 
 Reptiles. The first of these have been passed over because the 
 accounts of their development are not sufficiently satisfactory, 
 though as far as can be gathered from BischofFs account of the 
 dog and rabbit there would be no difficulty in shewing their 
 relations with other vertebrates. 
 
 We also require further investigations on Osseous fishes, but 
 it seems probable that they develop in nearly the same manner 
 as the Elasmobranchs. 
 
 With reference to Reptiles we have no satisfactory investi- 
 gations. 
 
 Amphioxus is the vertebrate whose mode of development in 
 its earliest stages is simplest, and the modes of development of 
 other vertebrates are to be looked upon as modifications of this 
 due to the presence of food material in their ova. It is not 
 necessary to conclude from this that Amphioxus was the an- 
 cestor of our present vertebrates, but merely that the earliest 
 stages of development of this vertebrate ancestor were similar to 
 those of Amphioxus. 
 
 The ovum of Amphioxus contains very little food material 
 and its segmentation is quite uniform. The result of segmenta- 
 tion is a vesicle whose wall is formed of a single layer of cells. 
 These are all of the same character, and the cavity of the vesicle 
 called the segmentation cavity is of considerable size. A section 
 of the embryo, as we may now call the ovum, is represented in 
 Plate 5, fig. A I. 
 
 u. 8 
 
114 KARLY STAGES IN THE 
 
 The first change which occurs is the pushing in of one half 
 of the wall of the vesicle towards the opposite half. At the 
 same time by the narrowing of its mouth the hollow hemisphere 
 so formed becomes again a vesicle 1 . 
 
 Owing to its mode of formation the wall of this secondary 
 vesicle is composed of two layers which are only separated by a 
 narrow space, the remnant of the segmentation cavity. 
 
 Two of the stages in the formation of the secondary vesicle 
 by this process of involution are shewn in Plate X, fig. A II, 
 and A III. In the second of these the general growth has been 
 very considerable, rendering the whole animal much larger than 
 before. The cavity of this vesicle, A in, is that of the com- 
 mencing alimentary canal whose final form is due to changes of 
 shape undergone by this primitive cavity. The inner wall of the 
 vesicle becomes converted into the wall of the alimentary canal 
 or hypoblast, and also into part or the whole of the mesoblast. 
 
 During the involution the cells which are being involuted 
 undergo a change of form, and before the completion of the 
 process have acquired a completely different character to the 
 cells forming the external wall of the secondary vesicle or 
 epiblast. This change of character in the cells is already well 
 marked in fig. A II. It is of great importance, since we shall 
 find that some of the departures from this simple mode of de- 
 velopment, which characterise other vertebrates, are in part due 
 to the distinction between the hypoblast and epiblast cells 
 appearing during segmentation, and not subsequently as in 
 Amphioxus during the involution of the hypoblast. 
 
 Kowalevsky (Entwickelungeschichte des Amphioxus) originally 
 believed that the narrow mouth of the vesicle (according to 
 Mr Lankester's terminology blastopore) became the anus of the 
 adult. He has since, and certainly correctly, given up this 
 view. The opening of the involution becomes closed up and 
 the adult anus is no doubt formed as in all other vertebrates by 
 a pushing in from the exterior, though it probably corresponds 
 in position very closely with the point of closing up of the 
 original involution. 
 
 1 I have been able to make at Naples observations which confirm the account of 
 the invagination of Amphioxus as given by Kowalevsky, though my observations are 
 not nearly so complete as those of the Russian naturalist. 
 
DEVELOPMENT OF VERTEBRATES. 115 
 
 The mode of formation of the mesoblast is not certainly 
 known in Amphioxus ; we shall find, however, that for all other 
 vertebrates it arises from the cells which are homologous with 
 the involuted cells of this animal. 
 
 Since food material is a term which will be very often em- 
 ployed, it will be well to explain exactly the sense in which 
 it will be used. It will be used only with reference to those 
 passive highly refractive particles which are found embedded in 
 most ova. 
 
 In some eggs, of which the hen's egg may be taken as a 
 familiar example, the yolk-spherules or food material form the 
 larger portion of the ovum, and a distinction is frequently made 
 between the germinal disc and the yolk. 
 
 This distinction is, however, apt to lead to a misconception 
 of the true nature of the egg. There are strong grounds for 
 believing that the so-called yolk, equally with the germinal disc, 
 is composed of an active protoplasmic basis endowed with the 
 power of growth, in which passive yolk-spherules are embedded ; 
 but that the part ordinarily called the yolk contains such a 
 preponderating amount of yolk-spheruies that the active basis 
 escapes detection, and does not exhibit the same power of 
 growth as the germinal disc. 
 
 With the exception of mammals, whose development requires 
 to be more completely investigated, Amphioxus is as far as we 
 know the only vertebrate whose ovum does not contain a large 
 amount of food material. 
 
 In none of these (vertebrate) yolk-containing ova is the food 
 material distributed uniformly. It is always concentrated much 
 more at one pole than at the other, and the pole at which it is 
 most concentrated may be conveniently called the lower pole of 
 the egg. 
 
 In eggs in which the distribution of food material is not 
 uniform segmentation does not take place with equal rapidity 
 through all parts of the egg, but its rapidity is, roughly speaking, 
 inversely proportional to the quantity of food material. 
 
 When the quantity of food material in a part of the egg 
 becomes very great, segmentation does not occur at all ; and 
 even in those cases where the quantity of food yolk is not 
 too great to prevent segmentation the resulting segmentation 
 
 82 
 
Il6 EARLY STAGES IN THE 
 
 spheres are much larger than where the yolk-granules are more 
 sparsely scattered. 
 
 The Frog is the vertebrate whose development comes nearest 
 to that of Amphioxus, as far as the points we are at present 
 considering are concerned. But it will perhaps facilitate the 
 understanding of their relations shortly to explain the dia- 
 grammatic sections which I have given of an animal supposed 
 to be intermediate in its development between the Frog and 
 Amphioxus. Plate 5, fig. B I, represents a longitudinal section 
 of this hypothetical egg at the close of segmentation. The 
 lower pole, coloured yellow, represents the part containing more 
 yolk material, and the upper pole, coloured blue, that with 
 less yolk. Owing to the presence of this yolk the lower pole 
 even at the close of segmentation is composed of cells of a 
 different character to those of the upper pole. In this respect 
 this egg can already be distinguished from that of Amphioxus, 
 in which no such difference between the two poles is apparent at 
 the corresponding period (Plate 5, fig. A l). 
 
 The segmentation cavity in this ovum is not quite so large 
 proportionately as in Amphioxus, and the encroachment upon 
 it is due to the larger bulk of the lower pole of the egg. In 
 fig. B II the involution of the lower pole has already com- 
 menced ; this involution is (i) not quite symmetrical, and (2) 
 on the ventral side (the left side) the epiblast cells forming the 
 upper part of the egg are growing round the cells of the lower 
 pole of the egg or lower layer cells. Both of these peculiarities 
 are founded upon what happens in the Frog and the Selachian, 
 but it is to be noticed that the change from the lower layer cells 
 being involuted towards the epiblast cells, to the epiblast cells 
 growing round the lower layer cells, is a necessary consequence 
 of the increased bulk of the latter. 
 
 In this involution not only are the cells of the lower pole 
 pushed on, but also some of those of the upper or yellow por- 
 tion ; so that in this as in all other cases the true distinction 
 between the epiblast and hypoblast does not appear till the 
 involution to form the latter is completed. In the next stage, 
 B ill, the involution has become nearly completed and the 
 opening to the exterior or blastopore quite constricted. 
 
 The segmentation cavity has been entirely obliterated, as 
 
DEVELOPMENT OF VERTEBRATES. 
 
 would have been found to be the case with Amphioxus had the 
 stage a little older than that on Plate 5, A ill, been represented. 
 The cavity marked (#/), as was the case with Amphioxus, is that 
 of the alimentary canal. 
 
 The similarities between the mode of formation of the hypo- 
 blast and alimentary canal in this animal and in Amphioxus are 
 so striking and the differences between the two cases so slight 
 that no further elucidation is required. One or two points need 
 to be spoken of in order to illustrate what occurs in the Frog. 
 When the involution to form the alimentary canal occurs, certain 
 of the lower layer cells (marked hy) become distinguished from 
 the remainder of the lower layer cells as a separate layer and 
 form the hypoblast which lines the alimentary canal. It is to 
 be noticed that the cells which form the ventral epithelium of 
 the alimentary canal are not so soon to be distinguished from 
 the other lower layer cells as those which form its dorsal epi- 
 thelium. This is probably a consequence of the more active 
 growth, indicated by the asymmetry of the involution, on the 
 dorsal side, and is a fact with important bearings in the ova 
 with more food material. The cells marked m and coloured 
 red also become distinguished as a separate layer from the 
 remainder of the hypoblast and form the mesoblast. The re- 
 mainder of the lower layer cells form a mass equivalent to the 
 yolk-sac of many vertebrates, and are not converted directly into 
 the tissues of the animal. 
 
 Another point to be noticed is the different relation of epi- 
 blast cells to the hypoblast cells at the upper and lower side 
 of the mouth of the involution. Above it, on its dorsal side, the 
 epiblast and hypoblast are continuous with one another. On 
 its ventral side they are primitively not so continuous. This 
 is due to the epiblast, as was before mentioned, growing round 
 the lower layer cells on the ventral side, vide B n, and merely 
 remaining continuous with them on the dorsal. The importance 
 of these two points will appear when we come to speak of other 
 vertebrates. 
 
 The next animal whose development it is necessary to speak 
 of is the Frog, and its differences from the mode of development 
 are quite easy to follow and interpret. Segmentation is again 
 not uniform, and results in the formation of an upper layer of 
 
Il8 EARLY STAGES IN THE 
 
 smaller cells and a lower one of larger ; in the centre is a 
 segmentation cavity. The stage at the close of segmentation 
 is represented in c I. From the diagram it is apparent that 
 the lower layer cells occupy a larger bulk than they did in the 
 previous animal (Plate 5, B i), and tend to encroach still more 
 upon the segmentation cavity, otherwise the differences between 
 the two are unimportant. There are, however, two points to be 
 noted. In the first place, although the cells of the upper pole 
 are distinguished in the diagrams from the lower by their 
 colour, it is not possible at this stage to say what will become 
 epiblast and what hypoblast. In the second place the cells 
 of the upper pole or epiblast consist of two layers an outer 
 called the epidermic layer and an inner called the nervous. In 
 the previous cases the epiblast consisted of a single layer of 
 cells. The presence of these two layers is due to a distinction 
 which, arising in most other vertebrates late, in the Frog arises 
 early. In most other vertebrates in the later stages of develop- 
 ment the epiblast consists of an outer layer of passive and an 
 inner of active cells. In the Frog and other Batrachians these 
 two layers become distinguished at the commencement of de- 
 velopment. 
 
 In the next stage (c li) we find that the involution to form 
 the alimentary canal has commenced (#/), but that it is of a 
 very different character to the involution in the previous case. 
 It consists in the growing inwards of a number of cells from the 
 point x (C l) towards the segmentation cavity. The cells which 
 grow in this way are partly the blue cells and partly the 
 smaller yellow ones. At first this involuted layer of cells is only 
 separated by a slit from the remainder of the lower layer cells ; 
 but by the stage represented in C II this has widened into an 
 elongated cavity (al). In its formation this involution pushes 
 backwards the segmentation cavity, which finally disappears in 
 the stage C III. The point x remains practically stationary, 
 but by the general growth of the epiblast, mesoblast and hypo- 
 blast, becomes further removed from the segmentation cavity 
 in C II than in c I. On the opposite side of the embryo to that 
 at which the involution occurs the epiblast cells as before, grow 
 round the lower layer cells. The commencement of this is 
 already apparent in c I, and in c II the process is nearly com- 
 
DEVELOPMENT OF VERTEBRATES. I 19 
 
 pleted, though there is still a small mass of yolk filling up the 
 blastopore. The features of this involution are in the main 
 exaggerations of what was supposed to occur in the previous 
 animal. The asymmetry of the involution is so great that it is 
 completely one-sided and results, in the first instance, in_a mere 
 slit ; and the whole process of enclosing the yolk by epiblasl is 
 effected by the epiblast cells on the side of the egg opposite to 
 the involution. 
 
 The true mesoblast and hypoblast are formed precisely as in 
 the previous case. The involuted cells become separated into 
 two layers, one forming the dorsal epithelium of the alimentary 
 canal, and a layer between this and the epiblast forming the 
 mesoblast. There is also a layer of mesoblast accompanying 
 the epiblast which encloses the yolk, which is derived from the 
 smaller yellow cells at_y (C l). The edge of this mesoblast, m', forms 
 a thickened ridge, a feature which persists in other vertebrates. 
 
 It is a point of some importance for understanding the rela- 
 tion between the mode of formation of the alimentary canal in 
 the Frog and other vertebrates to notice that on the ventral 
 surface the cells which are to form the epithelium of the ali- 
 mentary canal become distinguished as such very much later 
 than do those to form its dorsal epithelium, and are derived not 
 from the involuted cells but from the primitive large yolk-cells. 
 It is indeed probable that only a very small portion of epi- 
 thelium of the ventral wall of the mid-gut is in the end derived 
 from these larger yolk-cells. The remainder of the yolk-cells 
 (c III, and C II, yk) form the yolk mass and do not become 
 directly formed into the tissues of the animal. 
 
 In the last stage I have represented for the frog, cm, there 
 are several features to be noticed. 
 
 The direct connection at their hind-ends between the cavities 
 of the neural and alimentary canals is the most important of 
 these. This is a result of the previous continuity of the epiblast 
 and hypoblast at the point x, and is a feature almost certainly 
 found in Amphioxus, but which I will speak of more fully in 
 my account of the Selachian's development. The opening of 
 the blastopore called the anus of Rusconi is now quite nar- 
 rowed, it does not become the anus of the adult. It may be 
 noticed that at the front end of the embryo the primitive dorsal 
 
120 EARLY STAGES IN THE 
 
 epithelium of the alimentary canal is growing in such a way as 
 to form the epithelium both of the dorsal and ventral surfaces of 
 the fore-gut. 
 
 In spite of various features rendering the development of the 
 Frog more difficult of comprehension than that of most other 
 vertebrates, it is easy to see that the step between it and 
 Amphioxus is not a very great one, and will very likely be 
 bridged over at some future time, when our knowledge of the 
 development of other forms becomes greater. 
 
 From the Frog to the Selachian is a considerable step, but I 
 have again hypothetically sketched a type intermediate between 
 them whose development agrees in some important points with 
 that of Pelobates fuscus as described by Bambeke. The points 
 of agreement, though not obvious at first sight, I shall point out 
 in the course of my description. 
 
 The first stage (D i), at the close of segmentation, deserves 
 careful attention. The segmentation cavity by the increase of 
 the food yolk is very much diminished in size, and, what is still 
 more important, has as it were sunk down so as to be completely 
 within the lower layer cells. The roof of the segmentation cavity 
 is thus formed of epiblast and lower layer cells, a feature which 
 Bambeke finds in Pelobates fuscus and which is certainly found 
 in the Selachians. In the Frog we found that the segmentation 
 cavity began to be encroached on by the lower layer cells, and 
 from this it is only a small step to find these cells creeping still 
 further up and forming the roof of the cavity. In the lower 
 layer cells themselves we find an important new feature, viz. 
 that during segmentation they become divided in two distinct 
 parts one of these where the segments owing to the presence of 
 much food yolk are very large, and the other where the segments 
 are much smaller. 
 
 The separation between these two is rather sharp. Even 
 this separation was foreshadowed in the Frog's egg, in which a 
 number of lower layer cells were much smaller and more active 
 at the two sides of the segmentation cavity than elsewhere. The 
 segmentation cavity at first lies completely within the region of 
 the small spheres. The larger cells serve almost entirely as food 
 yolk. The epiblast, as is normal with vertebrates, consists of a 
 single layer of columnar cells. 
 
DEVELOPMENT OF VERTEBRATES. 121 
 
 In the next stage (D II) the formation of the alimentary canal 
 (al] has commenced, but it is to be observed that there is in this 
 case no true involution. 
 
 As an accompaniment to the encroachment upon the seg- 
 mentation cavity, which was a feature of the last stage, the 
 cells to form the walls of the alimentary canal have come to 
 occupy their final position during segmentation and without the 
 intermediation of an involution, and traces only of the invo- 
 lution, are to be found in (i) a split in the lower layer cells 
 which passes along the line separating the small and the large 
 lower layer cells ; and (2) in the epiblast becoming continuous 
 with the hypoblast on the dorsal side of the mouth of this split. 
 It is even possible that at this point a few cells (though cer- 
 tainly only a very small number) of those marked blue in 
 D I become involuted. This point in this, as in all other cases, 
 is the tail end of the embryo. The other features of this stage 
 are as follows : (i) The segmentation cavity has become smaller 
 and less conspicuous than it was. (2) The epiblast cells have 
 begun to grow round the yolk even in a more conspicuous 
 manner than they did in the Frog, and are accompanied by a 
 layer of mesoblast cells which again becomes thickened at its 
 edge. The mesoblast cells in the region of the body are formed 
 in the same way as before, viz. by the separation of a layer to 
 form the epithelium of the alimentary canal, the other cells 
 remaining as mesoblast ; and as in the Frog, or in a more con- 
 spicuous manner, we find that the dorsal surface only of the 
 alimentary cavity has a wall formed of a distinct layer of cells, 
 but on the ventral side the cavity is at first closed in by the 
 large spheres of the yolk only. The formation of the ali- 
 mentary canal by a split and not by an involution is exactly 
 what Bambeke finds in Pelobates. 
 
 The next stage, D ill, is about an equivalent age to C III in 
 the Frog. It exhibits the same connection between the neural 
 and the alimentary canals as was found there. 
 
 The alimentary canal is beginning to become closed in 
 below, and this occurs near the two ends earlier than in the 
 middle. The cells to form the ventral wall are derived from 
 the large yolk-cells. The non-formation of the ventral wall of 
 the alimentary canal so soon in the middle as at the ends is an 
 
122 EARLY STAGES IN THE 
 
 early trace of the umbilical canal found in Birds and Selachians, 
 by which the alimentary tract is placed in communication with 
 the yolk-sac. The segmentation cavity has by this stage com- 
 pletely vanished, and the epiblast with its accompanying meso- 
 blast has spread completely round the yolk material so as to 
 form the ventral wall of the body. 
 
 Though in some points this manner of development may 
 seem to differ from that of the Frog, there is really a funda- 
 mental agreement between the two, and between this mode of 
 development and that of the Selachians we shall find the agree- 
 ment to be very close. 
 
 After segmentation we find that the egg of a Selachian 
 consists of two parts one of these called the germinal disc 
 or blastoderm, and the other the yolk. The former of these 
 corresponds with the epiblast and the part of the lower pole 
 composed of smaller segments in the last-described egg, and 
 the latter to the larger segments of the lower pole. This latter 
 division, owing to the quantity of yolk which it contains, has not 
 undergone segmentation, but its homology with the larger seg- 
 ments of the previous eggs is proved (i) by its containing a 
 number of nuclei (E I, ;/), which become the nuclei of true cells 
 and enter the blastoderm, and (2) by the presence in it of a 
 number of lines forming a network similar to that of many cells. 
 The segmentation cavity, as before, lies completely within the 
 lower layer cells 
 
 The next stage, E II, is almost precisely similar to the 
 second stage of the last egg. As there, the primitive invo- 
 lution is merely represented by a split separating the yolk and 
 the germinal disc, and on the dorsal side alone is there a true 
 cellular wall for this split, and at the dorsal mouth of the split 
 the alimentary epithelium becomes continuous with the epiblast. 
 
 The segmentation cavity has become diminished, and round 
 the yolk the epiblast, accompanied by a layer of mesoblast, is 
 commencing to grow. In this growth' all parts of the blasto- 
 derm take a share except that part where the epiblast and hypo- 
 blast are continuous. This manner of growth is precisely what 
 occurs in the Frog, though there it is not so easily made out ; 
 and not all the investigators who have studied the Frog have 
 understood the exact meaning of the appearances they have 
 
DEVELOPMENT OF VERTEBRATES. 123 
 
 seen and drawn. This similarity of relation of the epiblast to 
 the yolk in the two cases is a further confirmation of the 
 identity of the Selachian's yolk with the large yolk-spheres of 
 the previous eggs. 
 
 The next stage, E ill, is in many ways identical with Jthe 
 corresponding stage in the last-described egg, and in the same 
 way as in that case the neural and alimentary canals are placed 
 in communication with each other. 
 
 The mode in which this occurs will be easily gathered from 
 a comparison of E II and E III. It is the same for the Sela- 
 chians and Batrachians. The neural canal (n c] is by the stage 
 figured E ill, completely formed in the way so well known in 
 the Bird, and between the roof of the canal and the external 
 epiblast a layer of mesoblast has already grown in. The floor 
 of the neural canal is the same layer marked ep in E II, and 
 therefore remains continuous with the hypoblast at x\ and when 
 by a simultaneous process the roof of the neural canal and the 
 ventral wall of the alimentary become formed by the folding 
 over of one continuous layer (the epiblast and hypoblast con- 
 tinuous at the point x), the two canals, viz. the neural and ali- 
 mentary, are necessarily placed in communication at their hind- 
 ends, as is seen in the diagram. 
 
 There are several important points of difference between 
 E III and D III. In the first place, owing to the larger size of 
 the yolk mass in E III, the epiblast, accompanied by mesoblast, 
 has not proceeded nearly so far round it as in the previous case. 
 It is also worth notice that at the right as well as at the left end 
 of the germinal disc the epiblast is commencing to grow round 
 the yolk. The yolk has, however, become surrounded to a much 
 smaller extent on the right hand than on the left. Since, in the 
 earlier stage, the epiblast became continuous with the hypoblast 
 at x, it is not from sections obvious how this occurs. I have 
 therefore appended a diagram to explain it (E'). The blasto- 
 derm rests like a disc on the yolk and grows over it on all sides, 
 except at the point where the epiblast and hypoblast are con- 
 tinuous (x). This point becomes as it were left in a bay. Next 
 the two sides of the bay coalesce, the bay becomes obliterated, 
 and the effect produced is exactly as if the blastoderm had 
 grown round the yolk at the point x (corresponding with the 
 
124 EARLY STAGES IN THE 
 
 tail of the embryo) as well as everywhere else. It thus comes 
 about that the final point where the various parts of the blasto- 
 derm meet and completely enclose the yolk mass does not cor- 
 respond with the anus of Rusconi of the Frog, but is at some 
 little distance from the hind-end of the embryo. In other 
 words, the position of the blastopore in the Selachian is not 
 the same as in the Frog. 
 
 Another point deserving attention is the formation of the 
 ventral wall of the alimentary canal. This takes place in two 
 ways partly by a folding-in at the sides and end, and partly 
 from cells formed around the nuclei () in the yolk. From 
 these a large portion of the ventral wall of the mid-gut is 
 formed. 
 
 The folding-in of the sheet of hypoblast to assist in the 
 closing-in of the ventral wall of the alimentary canal is a conse- 
 quence of the flattened form of the original alimentary slit which 
 is far too wide to form the cavity of the final canal. In the Bird 
 whose development must next be considered this folding-in is a 
 still more prominent feature in the formation of the alimentary 
 canal. As in the last case, the alimentary canal is widely open 
 in the middle to the yolk at the time when its two ends are 
 closed below and shut off from it ; still later this opening be- 
 comes very narrow and forms the duct of the so-called umbilical 
 cord which places the yolk-sac in communication with the ali- 
 mentary canal. As the young animal becomes larger the yolk- 
 sac ceases to communicate directly with the alimentary canal, 
 and is carried about by it for some time as an appendage and 
 only at a later period shrivels up. 
 
 The mesoblast is formed in a somewhat different way in the 
 Sharks than in other vertebrates. It becomes split off from the 
 hypoblast, not in the form of a single sheet as in other verte- 
 brates, but as two lateral sheets, one on each side of the middle 
 line and separated from one another by a considerable interval ; 
 whilst the notochord is derived not as in other vertebrates from 
 the mesoblast, but from the hypoblast (vide F. M. Balfour, " De- 
 velopment of Selachians 1 ," Journal of Microscopical Science, Oct., 
 1874). 
 
 1 Paper No. V, p. 82 el set/, in this edition. 
 
DEVELOPMENT OF VERTEBRATES. 12$ 
 
 Between the Selachians and the Aves there is a considerable 
 gulf, which it is more difficult satisfactorily to bridge over than 
 in the previous cases ; owing to this I have not attempted to 
 give any intermediate stage between them. 
 
 The first stage of the Bird (F l) is very similar in -many 
 respects to the corresponding stage in the Selachian. The seg- 
 mentation cavity is, however, a less well-defined formation, and 
 it may even be doubted whether a true segmentation cavity, 
 homologous with the segmentation cavity in the previously 
 described eggs, is present. On the floor of the cavity which is 
 formed by the yolk are a few larger cells known as formative 
 cells which, according to Gotte's observations, are derived from 
 the yolk, in a somewhat similar manner to the cells which 
 were formed around the nuclei in the Selachian egg, and 
 which helped to form the ventral wall of the alimentary 
 canal. Another point to be noticed is that the segmentation 
 cavity occupies a central position, and not one to the side as in 
 the Selachian. 
 
 The yolk is proportionately quite as large as in the Sela- 
 chian's egg, but, as in that case, there can be little or no doubt 
 of its being homologous with the largest of the segmentation 
 spheres of the previous eggs. It does not undergo segmentation. 
 The epiblast is composed of columnar cells, and extends a short 
 way beyond the edge of the lower layer cells. 
 
 In the next stage the more important departures from the 
 previous type of development become visible. 
 
 The epiblast spreads uniformly over the yolk-sac and not on 
 the one side only as in the former eggs. 
 
 This is due to the embryo (indicated in F II by a thickening 
 of the cells) lying in the centre and not at the edge of the blasto- 
 derm. A necessary consequence of this is, that the epiblast does 
 not, as in the previous cases, become continuous with the hypo- 
 blast at the tail end of the embryo. This continuity, being of 
 no functional importance, could easily be dispensed with, and 
 the central position of the embryo may perhaps be explained by 
 supposing the process, by which in the Selachian egg the blasto- 
 pore ceases to correspond in position with the opening of the 
 alimentary slit or anus of Rusconi (vide E'), to occur quite early 
 during segmentation instead of at a late period of development. 
 
126 EARLY STAGES IN THE 
 
 For the possibility of such a change in the date of formation, the 
 early appearance of the nervous and epidermic layers in the Frog 
 affords a parallel. 
 
 The epiblast in its growth round the yolk is only partially 
 accompanied by mesoblast, which, however, is thickened at its 
 extreme edge as in the Frog. Owing to the epiblast not be- 
 coming continuous with the hypoblast at the tail end of the 
 embryo, the alimentary slit is not open to the exterior. The 
 hypoblast is formed by some of the lower layer cells becoming 
 distinguished as a separate layer; the remainder of the lower 
 layer cells become the mesoblast. 
 
 The formation of the mesoblast and hypoblast out of the 
 lower layer cells has been accepted for the Bird by most ob- 
 servers, but has been disputed by several, and recently by 
 Kolliker. These have supposed that the mesoblast is derived 
 from the epiblast. I feel convinced that these observers are in 
 the wrong, and that the mesoblast is genuinely derived from the 
 lower layer cells. 
 
 The greater portion of the alimentary cavity consists of the 
 original segmentation cavity (vide diagrams). This feature of 
 the segmentation cavity of Birds sharply distinguishes it from 
 any segmentation cavity of other eggs, and renders it very 
 doubtful whether the similarly named cavities of the Bird and 
 of other vertebrates are homologous. On the floor of the cavity 
 are still to be seen some of the formative cells, but observers 
 have not hitherto found that they take any share in forming the 
 ventral wall of the alimentary canal. 
 
 The features of the next stage are the necessary consequences 
 of those of the last. 
 
 The ventral wall of the alimentary canal is entirely formed 
 by a folding-in of the sheet of hypoblast. 
 
 The more rapid folding-in at the head still indicates the 
 previous more vigorous growth there, otherwise there is very 
 little difference between the forms of the fold at the head and 
 tail. The alimentary canal does not of course, at this or any 
 period, communicate with the neural tube, since the epiblast and 
 hypoblast are never continuous. The other features, such as the 
 growth of the epiblast round the yolk-sac, are merely continua- 
 tions of what took place in the last stage. 
 
DEVELOPMENT OF VERTEBRATES. 12; 
 
 In the development of a yolk-sac as a distinct appendage, 
 and its absorption within the body, at a later period, the bird 
 fundamentally resembles the dog fish. 
 
 Although there are some difficulties in deriving the type of 
 development exhibited by the Bird directly from that of ihe 
 Selachian, it is not very difficult to do so directly from Amphi- 
 oxus. Were the alimentary involution to remain symmetrical 
 as in Amphioxus, and the yolk-containing part of the egg to 
 assume the proportions it does in the Bird, we should obtain a 
 mode of development which would not be very dissimilar to that 
 of the Bird. The epiblast would necessarily overgrow the yolk 
 uniformly on all sides and not in the unsymmetrical fashion of 
 the Selachian egg. A confirmation of this view might perhaps 
 be sought for in the complete difference between the types of 
 circulation of the yolk-sac in Birds and Selachians ; but this is 
 not so important as might at first sight appear, since it is not 
 from the Selachian egg but from some Batrachian that it would 
 be necessary to derive the Reptiles' and Birds' eggs. 
 
 If this view of the Bird's egg be correct, we are compelled to 
 suppose that the line of ancestors of Birds and Reptiles did not 
 include amongst them the Selachians and the Batrachians, or at 
 any rate Selachians and Batrachians which develope on the type 
 we now find. 
 
 The careful investigation of the development of some Rep- 
 tiles might very probably throw light upon this important 
 point. In the meantime it is better to assume that the type 
 of development of Birds is to be derived from that of the Frog 
 and Selachians. 
 
 Summary. If the views expressed in this paper are correct, 
 all the modes of development found in the higher vertebrates are 
 to be looked upon as modifications of that of Amphioxus. It 
 is, however, rather an interesting question whether it is possible 
 to suppose that the original type was not that of Amphioxus, 
 but of some other animal, say, for instance, that of the Frog, and 
 that this varied in two directions, on the one hand towards 
 Amphioxus, in the reverse direction to the course of variation 
 presupposed in the text ; and on the other hand in the direction 
 towards the Selachians as before. 
 
 The answer to this question must in my opinion be in the 
 
128 EARLY STAGES IN THE 
 
 negative. It is quite easy to conceive the food material of the 
 Frog's egg completely vanishing, but although this would entail 
 simplifications of development and possibly even make seg- 
 mentation uniform, there would, as far as I can see, be no 
 cause why the essential features of difference between the 
 Frog's mode of development and that of Amphioxus should 
 change. The asymmetrical and slit-like form of involution on 
 the one side and the growth of the epiblast over the mesoblast 
 on the other side, both characteristics of the present Frog's egg, 
 would still be features in the development of the simplified egg. 
 
 In the Mammal's egg we probably have an example of a 
 Reptile's egg simplified by the disappearance of the food ma- 
 terial ; and when we know more of Mammalian embryology it 
 will be very interesting to trace out the exact manner in which 
 this simplification has affected the development. It is also pro- 
 bable that the eggs of Osseous fish are fundamentally simplified 
 Selachian eggs ; in which case we already know that the dimi- 
 nution of food material has affected but very slightly the funda- 
 mental features of development. 
 
 One common feature which appears prominently in reviewing 
 the embryology of vertebrates as a whole is the derivation of the 
 mesoblast from the hypoblast ; in other words, we find that it is 
 from the layer corresponding to that which becomes involuted 
 in Amphioxus so as to line the alimentary cavity that the meso- 
 blast is split off. 
 
 That neither the hypoblast or mesoblast can in any sense be 
 said to be derived from the epiblast is perfectly clear. When 
 the egg of Amphioxus is in the blastosphere stage we cannot 
 speak of either an epiblast or hypoblast. It is not till the invo- 
 lution or what is equivalent has occurred, converting the single- 
 walled vesicle into a double-walled one, that we can speak of 
 these two layers. It might seem scarcely necessary to insist 
 upon this point, so clear is it without explanation, were it not 
 that certain embryologists have made a confusion about it. 
 
 The derivation of the mesoblast from the hypoblast is the 
 more interesting, since it is not confined to the vertebrates, but 
 has a very wide extension amongst the invertebrates. In the 
 cases (whose importance has been recently insisted upon by 
 Professor Huxley), of the Asteroids, the Echinoids, Sagitta, and 
 
DEVELOPMENT OF VERTEBRATES. 
 
 others, in which the body cavity arises as an outgrowth of the 
 alimentary canal and the somatopleure and splanchnopleure are 
 formed from that outgrowth, it is clear without further remark 
 that the mesoblast is derived from the hypoblast. For the 
 Echinoderms in which the water-vascular system and muscular 
 system arise as a solid outgrowth of the wall of the alimentary 
 canal there can also be no question as to the derivation of the 
 mesoblast from the hypoblast. 
 
 Amongst other worms, in addition to Sagitta, the investi- 
 gations of Kowalevsky seem to shew that in Lumbricus the 
 mesoblast is derived from the hypoblast. 
 
 Amongst Crustaceans, Bobretsky's 1 observations on Oniscus 
 (Zeitsclirift fur wiss. Zoologie, 1874) lead to the same con- 
 clusion. 
 
 In Insects Kowalevsky's observations lead to the conclusion 
 that mesoblast and hypoblast arise from a common mass of 
 cells; Ulianin's observations bring out the same result for the 
 abnormal Poduridae, and Metschnikoff's observations shew that 
 this also holds for Myriapods. 
 
 In Molluscs the point is not so clear. 
 
 In Tunicates, even if we are not to include them amongst 
 vertebrates 2 , the derivation of mesoblast from hypoblast is with- 
 out doubt. 
 
 Without going further into details it is quite clear that the 
 derivation of the mesoblast from the hypoblast is very general 
 amongst invertebrates. 
 
 It will hardly be disputed that primitively the muscular 
 system of the body wall could not have been derived from the 
 layer of cells which lines the alimentary canal. We see indeed 
 in Hydra and the Hydrozoa that in its primitive differentia- 
 tion, as could have been anticipated beforehand, the muscular 
 system of the body is derived from the epiblast cells. What, 
 then, is the explanation of the widespread derivation of the 
 mesoblast, including the muscular system of the body, from the 
 hypoblast ? 
 
 1 He says, p. 182 : " Bevor aber die Halfte der Eioberflache von den Embryonal- 
 zellen bedeckt 1st, kommt die erste gemeinsame Anlage des mittleren imd unteren 
 Keimblattes zum Vorschein." 
 
 - Anton Dohrn, Der Ursprung des Wirbclthieres. Leipzig, 1875. 
 
 I!. 9 
 
130 EARLY STAGES IN THE 
 
 The explanation of it may, I think, possibly be found, and 
 at all events the suggestion seems to me sufficiently plausible to 
 be worth making, in the fact that in many cases, and probably 
 this applies to the ancestors of the vertebrates, the body cavity 
 was primitively a part of the alimentary. 
 
 Mr Lankester, who has already entered into this line of 
 speculation, even suggests (Q. y. of Micr. Science, April, 1875) 
 that this applies to all higher animals. It might then be 
 supposed that the muscular system of part of the alimentary 
 canal took the place of the primitive muscular system of the 
 body ; so that the whole muscular system of higher animals 
 would be primitively part of the muscular system of the di- 
 gestive tract. 
 
 I put this forward merely as a suggestion, in the truth of 
 which I feel no confidence, but which may perhaps induce em- 
 bryologists to turn their attention to the point. If we accept it 
 for the moment, the supplanting of the body muscular system 
 by that of the digestive tract may hypothetically be supposed to 
 have occurred in the following way. 
 
 When the diverticulum or rather paired diverticula were 
 given off from the alimentary canal they would naturally be- 
 come attached to the body wall, and any contractions of their 
 intrinsic muscles would tend to cause movements in the body 
 wall. So far there is no difficulty, but there is a physiological 
 difficulty in explaining .how it can have happened that this 
 secondary muscular system can have supplanted the original 
 muscular system of the body. 
 
 The following suggestions may lessen this difficulty, though 
 perhaps they hardly remove it completely. If we suppose that 
 the animal in which these diverticula appeared had a hard test 
 and was not locomotive, the intrinsic muscular system of the 
 body would naturally completely atrophy. But since the mus- 
 cular system of the diverticula from the stomach would be 
 required to keep up the movement of the nutritive fluid, it 
 would not atrophy, and were the test subsequently to become 
 soft and the animal locomotive, would naturally form the mus- 
 cular system of the body. Or even were the animal locomotive 
 in which the diverticula appeared, it is conceivable that the two 
 systems might at first coexist together; that either (i) subse- 
 
DEVELOPMENT OF VERTEBRATES. 131 
 
 quently owing to the greater convenience of early development, 
 the two systems might acquire a development from the same 
 mass of cells and those the cells of the inner or hypoblast layer, 
 so that the derivation of the body muscles from the hypoblast 
 would only be apparent and not real, or (2) owing tcL their 
 being better nourished as they would necessarily be, and to 
 their possibly easier adaptability to some new form of move- 
 ment of the animal, the muscle-cells of the alimentary canal 
 might become developed exclusively whilst the original mus- 
 cular system atrophied. 
 
 I only hold this view provisionally till some better explana- 
 tion is given of the cases of Sagitta and the Echinoderms, as 
 well as of the nearly universal derivation of the mesoblast from 
 the hypoblast. The cases of this kind may be due to some 
 merely embryonic changes and have no meaning in reference 
 to the adult condition, but I think that we have no right to 
 assume this till some explanation of the embryonic can be 
 suggested. 
 
 For vertebrates, I have shewn that in Selachians the body 
 cavity at first extends quite to the top of what becomes the 
 muscle plate, so that the line or space separating the two layers 
 of the muscle plate (vide Balfour, ' Development of Elasmo- 
 branch Fishes 1 ,' Quart. Journ. of Micro. Science for Oct., 1874. 
 Plate XV, fig. n a, 1 1 b, 12 a, ;///.) is a portion of the original 
 body cavity. If this is a primitive condition, which is by no 
 means certain, we have a condition which we might expect, 
 in which both the inner and the outer wall of the primitive 
 body cavity assists in forming the muscular system of the 
 body. 
 
 It is very possible that the formation of the mesoblast as two 
 masses, one on each side of the middle line as occurs in Sela- 
 chians, and which as I pointed out in the paper quoted above 
 also takes place in some worms, is a remnant of the primitive 
 formation of the body cavity as paired outgrowth of the ali- 
 mentary canal. This would also explain the fact that in Sela- 
 chians the body cavity consists at first of two separate portions, 
 one on each side of the alimentary canal, which only subse- 
 
 1 I'apui XD. V, p. 60 ft -v t y. of tliis edition, pi. 4. lijjs. 1 1 <i, 1 1 A, 1 1 a, ////>. 
 
 C 2 
 
132 EARLY STAGES IN THE 
 
 quently become united below and converted into a single cavity 
 (vide loc. cit.\ Plate XIV, fig. 8 b, //). 
 
 In the Echinoderms we find instances where the body cavity 
 and water-vascular system arise as an outgrowth from the ali- 
 mentary canal, which subsequently becomes constricted off from 
 the latter (Asteroids and Echinoids), together with other instances 
 (Ophiura, Synapta) where the water-vascular system and body 
 cavity are only secondarily formed in a solid mass of mesoblast 
 originally split off from the walls of the alimentary canal. 
 
 These instances shew us how easily a change of this kind 
 may take place, and remove the difficulty of understanding why 
 in vertebrates the body cavity never communicates with the 
 alimentary. 
 
 The last point which I wish to call attention to is the blasto- 
 pore or anus of Rusconi. 
 
 This is the primitive opening by which the alimentary canal 
 communicates with the exterior, or, in other words, the opening 
 of the alimentary involution. It is a distinctly marked structure 
 in Amphioxus and the Batrachians, and is also found in a less 
 well-marked form in the Selachians ; in Birds no trace of it is any 
 longer to be seen. In all those vertebrates in which it is present, 
 it closes up and does not become the anus of the adult. The 
 final anus nevertheless corresponds very closely in position with 
 the anus of Rusconi. Mr Lankester has shewn (Quart. Journ. 
 of Micro. Science for April, 1875) that in invertebrates as well as 
 vertebrates the blastopore almost invariably closes up. It never- 
 theless corresponds as a rule very nearly in position either with 
 the mouth or with the anus. 
 
 If this opening is viewed, as is generally done, as really being 
 the mouth in some cases and the anus in others, it becomes very 
 difficult to believe that the blastopore can in all cases represent 
 the same structure. In a single branch of the animal king- 
 dom it sometimes forms the mouth and sometimes the anus : 
 thus for instance in Lumbricus it is the mouth (according to 
 Kowalevsky), in Palaemon (Bobretzky) the anus. Is it credible 
 that the mouth and anus have become changed, the one for the 
 other ? 
 
 If, on the other hand, we accept the view that the blastopore 
 1 PI. 3 of this edition, fig. 8 /-. />/>. 
 
DEVELOPMENT OF VEKTKHKATES. 133 
 
 never becomes either the one or the other of these openings, it 
 is, I think, possible to account for its corresponding in position 
 with the mouth in some cases or the anus in others. 
 
 That it would soon come to correspond either with the 
 mouth or anus (probably with the earliest formed of these in 
 the embryo), wherever it was primitively situated, follows from 
 the great simplification which would be effected by its doing so. 
 This simplification consists in the greater facility with which the 
 fresh opening of either mouth or anus could be made where the 
 epiblast and hypoblast were in continuity than elsewhere. Even 
 a change of correspondence from the position of the mouth to 
 that of the anus or vice versa could occur. The mode in which 
 this might happen is exemplified by the case of the Selachians. 
 I pointed out in the course of this paper how the final point of 
 envelopment of the yolk became altered in Selachians so as to 
 cease to correspond with the anus of Rusconi ; in other words, 
 how the position of the blastopore became changed. In such a 
 case, if the yolk material again became diminished, the blasto- 
 pore would correspond in position with neither mouth nor anus, 
 and the causes which made it correspond in position with the 
 anus before, would again operate, and make it correspond in 
 position perhaps with the mouth. Thus the blastopore might 
 absolutely cease to correspond in position with the anus and 
 come to correspond in position with the mouth. 
 
 It is hardly possible to help believing that the blastopore 
 primitively represented a mouth. It may perhaps have lost 
 this function owing to an increase of food yolk in the ovum 
 preventing its being possible for the blastopore to develop 
 directly into a mouth, and necessitating the formation of a 
 fresh mouth. If such were the case, there would be no reason 
 why the blastopore should ever again serve functionally as a 
 mouth in the descendants of the animal which developed this 
 fresh mouth. 
 
134 EARLY STAGES IN DEVELOPMENT OF VERTEBRATES. 
 
 EXPLANATION OF PLATE 5. 
 
 COMPLETE LIST OF REFERENCES. 
 
 al. Cavity of alimentary canal, bl. Blastoderm, ch. Notochord. ep. Epiblast. 
 em. Embryo, f. Formative cells, hy. Hypoblast. / /. Lower layer cells. ;//. 
 Mesoblast. . Nuclei of yolk of Selachian egg. n c. Neural canal, s g. Segmenta- 
 tion cavity, x. Point where epiblast and hypoblast are continuous at the mouth of 
 the alimentary involution. This point is always situated at the tail end of the 
 embryo, yk. Yolk. 
 
 Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower 
 layer cells before their separation into hypoblast and mesoblast are also coloured 
 green. 
 
 A I, A II, A ill. Diagrammatic sections of Amphioxus in its early stages (founded 
 upon Kowalevsky's observations). 
 
 B I, B n, B III. Diagrammatic longitudinal sections of an hypothetical animal, 
 intermediate between Amphioxus and Batrachians, in its early stages. 
 
 c I, c II, c III. Diagrammatic longitudinal sections of Bombinator igneus in its 
 early stages (founded upon Gotte's observations). In c in the neural canal is com- 
 pleted, which was not the case in B in. The epiblast in c ill has been diagram- 
 matically represented as a single layer. 
 
 D I, D II, D in. Diagrammatic longitudinal sections of an animal, intermediate 
 between Batrachians and Selachians, in its early stages. 
 
 E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early 
 stages. 
 
 E'. Surface view of the yolk of a Selachian's egg to shew the manner in which it 
 is enclosed by the Blastoderm. The yolk is represented yellow and the Blastoderm 
 blue. 
 
 F I, F 11, F in. Diagrammatic longitudinal sections of a Bird in its early stages. 
 
VII. ON THE ORIGIN AND HISTORY OF THE URINOGENITAL 
 ORGANS OF VERTEBRATES 1 . 
 
 RECENT discoveries 2 as to the mode of development and 
 anatomy of the urinogenital system of Selachians, Amphibians, 
 and Cyclostome fishes, have greatly increased our knowledge 
 of this system of organs, and have rendered more possible a 
 comparison of the types on which it is formed in the various 
 orders of vertebrates. 
 
 1 From the Journal of Anatomy and Physiology, Vol. X. 1875. 
 
 2 The more important of these are: 
 
 Semper Ueber die Stammverwandtschaft der Wirbelthiere u. Anneliden. Cen- 
 tralblatt f. Med. Wiss. 1874, No. 35. 
 
 Semper Segmentalorgane bei ausgewachsenen Haien. Centralblatt f. Med. 
 Wiss. 1874, No. 52. 
 
 Semper Das Urogenitalsystem der hoheren Wirbelthiere. Centralblatt /". Med. 
 Wiss. 1874, No. 59. 
 
 Semper Stammesverwandschaft d. Wirbelthiere u. \Virbellosen. Arbeiten ans 
 Zool. Zootom. Inst. Wiirzburg. II Band. 
 
 Semper Bildung u. Wachstum der Keimdriisen bei den Plagiostomen. Central- 
 blatt f. Med. Wiss. 1875, No. 12. 
 
 Semper Entw. d. Wolf. u. Miill. Gang. Centralblatt f. Med. Wiss. 1875, 
 
 No. 29. 
 
 Alex. Schultz - Phylogenie d. Wirbelthiere. Centralblatt f. Med. Wiss. 1874, 
 
 No. 51. 
 
 Spengel Wimpertrichtern i. d. Amphibienniere. Centralblatt f. Med. Wiss. 
 1875, No. 23. 
 
 M e y er Anat. des Urogenitalsystems der Selachier u. Amphibien. Sitzb. Natnr- 
 
 for. Gesellschaft. Leipzig, 30 April, 1875. 
 
 F. M. Balfour Preliminary Account of development of Elasmobranch fishes. 
 Quart. Journ. of Micro. Science, Oct. 1874. (This edition, Paper V. p. 60 et seq.} 
 W. Miiller Persistenz der Urniere bei Myxine glutinosa. Jenaische Zeitschrijt, 
 
 1873- 
 
 W. Miiller Urogenitalsystem d. Amphioxus u. d. Cyclostomen. Jatmscke Zeit- 
 
 schrift, 1875. 
 
 Alex. Gotte Entwickelnngsgeschichtf d.-r l'nk<- (Rinnbinator igncns}. 
 
136 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 The following paper is an attempt to give a consecutive 
 history of the origin of this system of organs in vertebrates and 
 of the changes which it has undergone in the different orders. 
 
 For this purpose I have not made use of my own observa- 
 tions alone, but have had recourse to all the Memoirs with which 
 I am acquainted, and to which I have access. I have com- 
 menced my account with the Selachians, both because my own 
 investigations have been directed almost entirely to them, and 
 because their urinogenital organs are, to my mind, the most 
 convenient for comparison both with the more complicated and 
 with the simpler types. 
 
 On many points the views put forward in this paper will be 
 found to differ from those which I expressed in my paper 
 (loc. cit^) which give an account of my original 1 discovery of the 
 segmental organs of Selachians, but the differences, with the 
 exception of one important error as to the origin of the Wolffian 
 duct, are rather fresh developments of my previous views from 
 the consideration of fresh facts, than radical changes in them. 
 
 In Selachian embryos an intermediate cell-mass, or middle 
 plate of mesoblast is formed, as in birds, from a partial fusion of 
 the somatic and splanchnic layers of the mesoblast at the outer 
 border of the protovertebrae. From this cell-mass the whole of 
 the urinogenital system is developed. 
 
 At about the time when three visceral clefts have appeared, 
 there arises from the intermediate cell-mass, opposite the fifth 
 protovertebra, a solid knob, from which a column of cells grows 
 backwards to opposite the position of the future anus (fig. i, pd.}. 
 
 This knob projects outwards toward the epiblast, and the 
 column lies at first between the mesoblast and epiblast. The 
 knob and column do not long remain solid. The knob be- 
 coming hollow acquires a wide opening into the pleuroperitoneal 
 or body cavity, and the column a lumen ; so that by the time 
 that five visceral clefts have appeared, the two together form a 
 
 1 These organs were discovered independently by Professor Semper and myself. 
 Professor Semper's preliminary account appeared prior to my own which was pub- 
 lished (with illustrations) in the Quarterly Journal of Mic. Science. Owing to my 
 being in South America, I did not know of Professor Semper's investigations till 
 several months after the publication of my paper. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 137 
 
 spa 
 
 FlG. I. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL 
 
 CLEFTS. 
 
 The sections are to shew the development of the segmental duct (pd) or primi- 
 tive duct of the kidneys. In A (the anterior of the two sections) this appears as a 
 solid knob projecting towards the epiblast. In B is seen a section of the column 
 which has grown backwards from the knob in A. 
 
 spn. rudiment of a spinal nerve ; me. medullary canal ; ch. notochord ; X. 
 string of cells below the notochord ; mp. muscle-plate ; nip', specially developed 
 portion of muscle-plate ; ao. dorsal aorta ; pd. segmental duct. so. somatopleura ; 
 sp. splanchnopleura ; //. pleuroperitoneal or body cavity ; ep. epiblast ; al. ali- 
 mentary canal. 
 
 duct closed behind, but communicating in front by a wide 
 opening with the pleuroperitoneal cavity. 
 
 Before these changes are accomplished, a series of solid? 
 outgrowths of elements of the 'intermediate cell- mass' appear 
 at the uppermost corner of the body-cavity. These soon be- 
 come hollow and appear as involutions from the body-cavity, 
 curling round the inner and dorsal side of the previously formed 
 duct. 
 
 One involution of this kind makes its appearance for each 
 protovertebra, and the first belongs to the protovertebra im- 
 mediately behind the anterior end of the duct whose develop- 
 ment has just been described. In Pristiurus there are in all 
 29 of these at this period. The last two or three arise from 
 that portion of the body-cavity, which at this stage still exists 
 behind the anus. The first-formed duct and the subsequent 
 involutions are the rudiments of the whole of the urinary system. 
 
 1 These outgrowths are at first solid in both Pristiurus, Scyllium and Torpedo, but 
 in Torpedo attain a considerable length before a lumen appears in them. 
 
138 THE URINOGENTTAL ORGANS OF VERTEBRATES. 
 
 The duct is the primitive duct of the kidney 1 ; I shall call it 
 in future the segmental duct ; and the involutions are the com- 
 mencements of the segmental tubes which constitute the body 
 of the kidney. I shall call them in future segmental tubes 
 
 Soon after their formation the segmental tubes become 
 convoluted, and their blind ends become connected with the 
 segmental duct of the kidney. At the same time, or rather 
 before this, the blind posterior termination of each of the seg- 
 mental ducts of the kidneys unites with and opens into one of 
 the horns of the cloaca. At this period the condition of affairs 
 is represented in fig. 2. 
 
 FIG. 2. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN A 
 SELACHIAN EMBRYO. 
 
 pd. segmental duct. It opens at o into the body cavity and at its other extremity 
 into the cloaca ; x. line along which the division appears which separates the seg- 
 mental duct into the Wolffian duct above and the Miillerian duct below ; st. seg- 
 mental tubes. They open at one end into the body-cavity, and at the other into the 
 segmental duct. 
 
 There is at pd, the segmental duct of the kidneys, opening 
 in front (p) into the body-cavity, and behind into the cloaca, and 
 there are a series of convoluted segmental tubes (sf), each 
 opening at one end into the body-cavity, and at the other into 
 the duct (pd). 
 
 The next important change which occurs is the longitudinal 
 division of the segmental duct of the kidneys into Miiller's duct, 
 or the oviduct, and the duct of the Wolffian bodies or Leydig's 
 duct. The splitting 2 is effected by the growth of a wall of cells 
 
 , * This duct is often called either Miiller's duct, the oviduct, or the duct of the 
 primitive kidneys ' Urnierengang.' None of these terms are very suitable. A justifi- 
 cation of the name I have given it will appear from the facts given in the later parts 
 of this paper. In my previous paper I have always called it oviduct, a name which is 
 very inappropriate. 
 
 This splitting was first of all discovered and an account of it published by 
 Semper ( Centralblatt f. Med. Wiss. 1875, No. 29). I had independently made it out 
 
THE UKINOCKXITAI. ORGANS ()K VERTEBRATES. 139 
 
 which divides the duct into two parts (fig. 3, ^ L vd. and md.}. It 
 takes place in such a way that the front end of the segmental 
 duct, anterior to the entrance of the first segmental tube, together 
 with the ventral half of the rest of the duct, is split off from its 
 dorsal half as an independent duct (vide fig. 2, .r). 
 
 The dorsal portion also forms an independent duct, and into 
 it the segmental tubes continue to open. Such at least is the 
 
 tn-p 
 
 FIG. 3. TRANSVERSE SECTION OF A SELACHIAN EMBRYO ILLUSTRATING THE 
 FORMATION OF THE WOLFFIAN AND MiJLLERIAN DUCTS BY THE LONGI- 
 TUDINAL SPLITTING OF THE SEGMENTAL DUCT. 
 
 me. medullary canal ; mp. muscle-plate; c/i. notochord; ao. aorta; cav. car- 
 dinal vein; st. segmental tube. On the one side the section passes through the 
 opening of a segmental tube into the body cavity. On the other this opening is 
 represented by dotted lines, and the opening of the segmental tube into the Wolffian 
 duct has been cut through; wd. Wolffian duct; md. Miillerian duct. The Miil- 
 lerian duct and the Wolffian duct together constitute the primitive segmental duct ; 
 jfr. The germinal ridge with the thickened germinal epithelium : /. liver ; /. intes- 
 tine with spiral valve. 
 
 for the female a few weeks before the publication of Semper's account but have not 
 yet made observations about the point for the male. 
 
 My own previous account of the origin of the Wolffian duct (Quart. Journ. of 
 Micros. Science, Oct. 1874, and this edition, Paper V.), is completely false, and was 
 due to my not having had access to a complete series of my sections when I wrote the 
 paper. 
 
140 THE URTNOGENITAL ORGANS OF VERTEBRATES. 
 
 method of splitting for the female for the male the splitting is 
 according to Professor Semper, of a more partial character, and 
 consists for the most part in the front end of the duct only 
 being separated off from the rest The result of these changes 
 is the formation in both sexes of a fresh duct which carries 
 off the excretions of the segmental involutions, and which I 
 shall call the Wolffian duct while in the female there is formed 
 another complete and independent duct, which I shall call the 
 Miillerian duct, or oviduct, and in the male portions only of 
 such a duct. 
 
 The next change which takes place is the formation of an- 
 other duct from the hinder portion of the Wolffian duct, which 
 receives the secretion of the posterior segmental tubes. This 
 secondary duct unites with the primary or Wolffian duct near 
 its termination, and the primary ducts of the two sides unite 
 together to open to the exterior by a common papilla. 
 
 Slight modifications of the posterior terminations of these 
 ducts are found in different genera of Selachians (vide Semper, 
 Centralblatt fur Med. Wiss. 1874, No. 59), but they are of no 
 fundamental importance. 
 
 These constitute the main changes undergone by the seg- 
 mental duct of the kidneys and the ducts derived from it ; but 
 the segmental tubes also undergo important changes. In the 
 majority of Selachians their openings into the body-cavity, or, 
 at any rate, the openings of a large number of them, persist 
 through life ; but the investigations of Dr Meyer 1 render it 
 very probable that the small portion of each segmental tube 
 adjoining the opening becomes separated from the rest and 
 becomes converted into a sort of lymph organ, so that the open- 
 ings of the segmental tubes in the adult merely lead into lymph 
 organs and not into the gland of the kidneys. 
 
 These constitute the whole changes undergone in the female, 
 but in the male the open ends of a varying number (according 
 to the species) of the segmental tubes become connected with 
 the testis and, uniting with the testicular follicles, serve to carry 
 away the seminal fluid 2 . The spermatozoa have therefore to 
 
 1 Sitzen. der Naturfor. Gesellschaft, Leipzig, 30 April, 1875. 
 
 2 We owe to Professor Semper the discovery of the arrangement of the seminal 
 ducts. Centralblatt f. Med. Wiss. 1875, No. \i. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 141 
 
 pass through a glandular portion of the kidneys before they 
 enter the Wolffian duct, by which they are finally carried away 
 to the exterior. 
 
 In the adult female, then, there are the following parts of 
 the urinogenital system (fig. 4) : 
 
 (i) The oviduct, or M tiller's duct (fig. 4, md.}, split off from 
 the segmental duct of the kidneys. Each oviduct opens at its 
 upper end into the body-cavity, and behind the two oviducts 
 have independent communications with the cloaca. The ovi- 
 ducts serve simply to carry to the exterior the ova, and have no 
 communication with the glandular portion of the kidneys. 
 
 s.f 
 
 FIG. 4. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN 
 AN ADULT FEMALE SELACHIAN. 
 
 Hid. Mullerian duct ; wd. Wolffian duct ; st. segmental tubes ; d. duct of the 
 posterior segmental tubes ; ov. ovary. 
 
 (2) The Wolffian ducts (fig. 4, wd.} or the remainder of the 
 segmental ducts of the kidneys. Each Wolffian duct ends 
 blindly in- front, and the two unite behind to open by a common 
 papilla into the cloaca. 
 
 This duct receives the secretion of the whole anterior end of 
 the kidneys 1 , that is to say, of all the anterior segmental tubes. 
 
 (3) The secondary duct (fig. 4, d.} belonging to the lower 
 portion of the kidneys opening into the former duct near its 
 termination. 
 
 (4) The segmental tubes (fig. 4. st) from whose convolutions 
 and outgrowths the kidney is formed. They may be divided 
 
 1 This upper portion of the kidneys is called Leydig's gland by Semper. It would 
 be letter to call it the Wolftian body, for I shall attempt to shew that it is homologous 
 with the gland so named in Saumpsida and Mammalia. 
 
142 
 
 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 into two parts, according to the duct by which their secretion is 
 carried off. 
 
 In the male the following parts are present : 
 
 (1) The Miillerian duct (fig. 5, md.}, consisting of a small 
 remnant, attached to the liver, which represents the foremost 
 end of the oviduct of the female. 
 
 (2) The Wolffian duct (fig. 5, wd], which precisely corre- 
 sponds to the Wolffian duct of the female, except that, in ad- 
 dition to functioning as the duct of the anterior part of the 
 kidneys, it also serves to carry away the semen. In the female 
 it is straight, but has in the adult male a very tortuous course 
 ^vide fig. 5). 
 
 FIG. 5. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN 
 AN ADULT MALE SELACHIAN. 
 
 md. rudiment of Miillerian duct ; ivd. Wolffian duct, which also serves as vas 
 deferens ; st. segmental tubes. The ends of three of those which in the female 
 open into the body-cavity, have in the male united with the testicular follicles, and 
 serve to carry away the products of the testis ; d. duct of the posterior segmental 
 tubes; t. testis. 
 
 (3) the duct (fig. 5, d.} of the posterior portion of the kid- 
 neys, which has the same relations as in the female. 
 
 (4) The segmental tubes (fig. 5. .$/.) . These have the same 
 relations as in the female, except that the most anterior two, 
 three or more, unite with the testicular follicles, and carry away 
 the semen into the Wolffian duct. 
 
 The mode of arrangement and the development of these 
 parts suggest a number of considerations. 
 
 In the first place it is important to notice that the seg- 
 mental tubes developc primitively as completely independent 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 143 
 
 organs 1 , one of which appears in each segment. If embryology is 
 in any way a repetition of ancestral history, it necessarily follows 
 that these tubes were primitively independent of each other. 
 Ancestral history, as recorded in development, is often, it is true, 
 abridged ; but it is clear that though abridgement might prevent 
 a series of primitively separate organs from appearing as such, 
 yet it would hardly be possible for a primitively compound 
 organ, which always retained this condition, to appear during 
 development as a series of separate ones. These considerations 
 appear to me to prove that the segmented ancestors of verte- 
 brates possessed a series of independent and segmental ex- 
 cretory organs. 
 
 Both Professor Semper and myself, on discovering these 
 organs, were led to compare them and state our belief in their 
 identity with the so-called segmental organs of Annelids. 
 
 This view has since been fairly generally accepted. The 
 segmental organs of annelids agree with those of vertebrates in 
 opening at one end into the body-cavity, but differ in the fact 
 that each also communicates with the exterior by an inde- 
 pendent opening, and that they are never connected with each 
 other. 
 
 On the hypothesis of the identity of the vertebrate segmental 
 tubes with the annelid segmental organs, it becomes essential to 
 explain how the external openings of the former may have 
 become lost. 
 
 This brings us at once to the origin of the segmental duct of 
 the kidneys, by which the secretion of all the segmental tubes 
 was carried to the exterior, and it appears to me that a right 
 understanding of the vertebrate urinogenital system depends 
 greatly upon a correct view of the origin of this duct. I would 
 venture to repeat the suggestion which I made in my original 
 paper (loc. cit.} that this duct is to be looked upon as the most 
 anterior of the segmental tubes which persist in vertebrates. 
 
 1 Further study of my sections has shewn me that the initial independence of 
 these organs is even more complete than might be gathered from the description in 
 my paper (loc. cit.}. I now find, as I before conjectured, that they at first correspond 
 exactly with the muscle-plates, there being one for each muscle-plate. This can be 
 seen in the fresh embryos, but longitudinal sections shew it in an absolutely demon- 
 strable manner. 
 
144 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 In favour of this view are the following anatomical and em- 
 bryological facts, (i) It developes in nearly the same manner 
 as the other segmental tubes, viz. in Selachians as a solid 
 outgrowth from the intermediate cell- mass, which subsequently 
 becomes hollowed so as to open into the body-cavity : and in 
 Amphibians and Osseous and Cyclostome fishes as a direct 
 involution from the body-cavity. (2) In Amphibians, Cyclos- 
 tomes and Osseous fishes its upper end develops a glandular 
 portion, by becoming convoluted in a manner similar to the 
 other segmental tubes. This glandular portion is often called 
 either the head-kidney or the primitive kidney. It is only an 
 embryonic structure, but is important as demonstrating the true 
 nature of the primitive duct of the kidneys. 
 
 We may suppose that some of the segmental tubes first 
 united, possibly in pairs, and that then by a continuation of this 
 process the whole of them coalesced into a common gland. 
 One external opening sufficed to carry off the entire secretion 
 of the gland, and the other openings therefore atrophied. 
 
 This history is represented in the development of the dog- 
 fish in an abbreviated form, by the elongation of the first seg- 
 mental tube (segmental duct of the kidney) and its junction 
 with each of the posterior segmental tubes. Professor Semper 
 looks upon the primitive duct of the kidneys as a duct which 
 arose independently, and was not derived from metamorphosis 
 of the segmental organs. Against this view I would on the one 
 hand urge the consideration, that it is far easier to conceive of 
 the transformation by change of function (comp. Dohrn, Func- 
 tionsweclisel, Leipzig, 1875) of a segmental organ into a segmental 
 duct, than to understand the physiological cause which should 
 lead, in the presence of so many already formed ducts, to the 
 appearance of a totally new one. By its very nature a duct is a 
 structure which can hardly arise de novo. We must even sup- 
 pose that the segmental organs of Annelids were themselves 
 transformations of still simpler structures. On the other hand 
 I would point to the development in this very duct amongst 
 Amphibians and Osseous fishes of a glandular portion similar 
 to that of a segmental tube, as an a posteriori proof of its 
 being a metamorphosed segmental tube. The development in 
 insects of a longitudinal tracheal duct by the coalescence of a 
 
THE URTNOGEXITAL ORGANS OF VERTEBRATES. 145 
 
 series of transverse tracheal tubes affords a parallel to the forma- 
 tion of a duct from the coalescence of a series of segmental 
 tubes. 
 
 Though it must be admitted that the loss of the external 
 openings of the segmental organs requires further working out, 
 yet the difficulties involved in their disappearance are not so 
 great as to render it improbable that the vertebrate segmental 
 organs are descended from typical annelidan ones. 
 
 The primitive vertebrate condition, then, is probably that of 
 an early stage of Selachian development while there is as yet 
 a segmental duct, the original foremost segmental tube open- 
 ing in front into the body-cavity and behind into the cloaca ; 
 with which duct all the segmental tubes communicate. Vide 
 Fig. 2. 
 
 The next condition is to be looked upon as an indirect 
 result of the segmental duct serving as well for the products 
 of the generative organs as the secretions of the segmental tubes. 
 
 As a consequence of this, the segmental duct became split 
 into a ventral portion, which served alone for the ova, and 
 a dorsal portion which received the secretion of the segmental 
 tubes. The lower portion, which we have called the oviduct, 
 in some cases may also have received the semen as well as 
 the ova. This is very possibly the case with Ceratodus (vide 
 Gunther, Trans, of Royal Society, 1871), and the majority of 
 Ganoids (Hyrtl, Dcnkschriften Wien, Vol. VIII.). In the majo- 
 rity of other cases the oviduct exists in the male in a completely 
 rudimentary form ; and the semen is carried away by the same 
 duct as the urine. 
 
 In Selachians the transportation of the semen from the 
 testis to the Wolffian duct is effected by the junction of the 
 open ends of two or three or more segmental tubes with the 
 testicular follicles, and the modes in which this junction is 
 effected in the higher vertebrates seem to be derivatives from 
 this. If the views here expressed are correct it is by a complete 
 change of function that the oviduct has come to perform its 
 present office. And in the bird and higher vertebrates no trace, 
 or only the very slightest (vide p. 165) of the primitive urinary 
 function is retained during embryonic or adult life. 
 
 The last feature in the anatomy of the Selachians which 
 B. 10 
 
146 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 requires notice is the division of the kidney into two portions, 
 an anterior and posterior. The anatomical similarity between 
 this arrangement and that of higher vertebrates (birds, &c.) is very 
 striking. The anterior one precisely corresponds, anatomically, 
 to the Wolffian body, and the posterior one to the true per- 
 manent kidney of higher vertebrates : and when we find that 
 in the Selachians the duct for the anterior serves also for the 
 semen as does the Wolffian duct of higher vertebrates, this 
 similarity seems almost to amount to identity. A discussion of 
 the differences in development in the two cases will come con- 
 veniently with the account of the bird ; but there appear to me 
 the strongest grounds for looking upon the kidneys of Selachians 
 as equivalent to both the Wolffian bodies and the true kidneys 
 of the higher vertebrates. 
 
 The condition of the urinogenital organs in Selachians is by 
 no means the most primitive found amongst vertebrates. 
 
 The organs of both Cyclostomous and Osseous fishes, as well 
 as those of Ganoids, are all more primitive ; and in the majority 
 of points the Amphibians exhibit a decidedly less differentiated 
 condition of these organs than do the Selachians. 
 
 In Cyclostomous fishes the condition of the urinary system 
 is very simple. In Myxine (vide Joh. M tiller Myxinoid fislies, 
 and Wilhelm Miiller, Jenaische Zeitschrift, 1875, Das Urogenital- 
 system des Amphioxus u. d. Cyclostomcn} there is a pair of ducts 
 which communicate posteriorly by a common opening with 
 the abdominal pore. From these ducts spring a series of trans- 
 verse tubules, each terminating in a Malpighian corpuscle. These 
 together constitute the mass of the kidneys. About opposite 
 the gall-bladder the duct of the kidney (the segmental duct) 
 narrows very much, and after a short course ends in a largish 
 glandular mass (the head-kidney), which communicates with the 
 pericardial cavity by a number of openings. 
 
 In Petromyzon the anatomy of the kidneys is fundamentally 
 the same as in Myxine. They consist of the two segmental 
 ducts, and a number of fine branches passing off from these, 
 which become convoluted but do not form Malpighian tufts. 
 The head-kidney is absent in the adult. 
 
 W. Muller (loc. cit.} has given a short but interesting account 
 of the development of the urinary system of Petromyzon. He 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 147 
 
 finds that the segmental ducts develop first of all as simple 
 involutions from the body-cavity. The anterior end of each 
 then developes a glandular portion which comes to communicate 
 by a number of openings with the body-cavity. Subsequently 
 to the development of this glandular portion the remainder of 
 the kidneys appears in the posterior portion of the body-cavity ; 
 and before the close of embryonic life the anterior glandular 
 portion atrophies. 
 
 The comparison of this system with that of a Selachian is 
 very simple. The first developed duct is the segmental duct of 
 a Selachian, and the glandular portion developed at its anterior 
 extremity, which is permanent in Myxine but embryonic in 
 Petromyzon, is, as W. Muller has rightly recognized, equivalent 
 to the head-kidney of Amphibians, which remains undeveloped 
 in Selachians. It is, according to my previously stated view, 
 the glandular portion of the first segmental organ or the seg- 
 mental duct. The series of orifices by which this communicates 
 with the body-cavity are due to the division of the primary 
 opening of the segmental duct. This is shewn both by the facts 
 of their development in Petromyzon given by Muller, as well as 
 by the occurrence of a similar division of the primary orifice in 
 Amphibians, which is mentioned later in this paper. In a note 
 in my original paper (loc. cit.} I stated that these openings 
 were equivalent to the segmental involutions of Selachians. 
 This is erroneous, and was due to my not having understood the 
 description given in a preliminary paper of Muller (JenaiscJie 
 Zeitschrift, 1873). The large development of this glandular 
 mass in the Cyclostome and Osseous fishes and in embryo Am- 
 phibians, implies that it must at one time have been important. 
 Its earlier development than the remainder of the kidneys is 
 probably a result of the specialized function of the first seg- 
 mental organ. 
 
 The remainder of the kidney in Cyclostomes is equivalent to 
 the kidney of Selachians. Its development from segmental in- 
 volutions has not been recognized. If these segmental involu- 
 tions are really absent it may perhaps imply that the simplicity 
 of the Cyclostome kidneys, like that of so many other of their 
 organs, is a result of degeneration rather than a primitive con- 
 dition. 
 
 IO 2 
 
148 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 In Osseous fishes the segmental duct of the kidneys developes, 
 as the observations of Rosenberg 1 (" Teleostierniere," Liang. 
 Disser. Dorpat, 1867) and Oellacher (Zeitschrift fur Wiss. Zool. 
 1873) clearly prove, by an involution from the body-cavity. 
 This involution grows backwards in the form of a duct and 
 opens into the cloaca. The upper end of this duct (the most 
 anterior segmental tube) becomes convoluted, and forms a 
 glandular body, which has no representative in the urinary 
 apparatus of Selachians, but whose importance, as indicating the 
 origin of the segmental duct of the kidneys, I have already 
 insisted upon. 
 
 The rest of the kidney becomes developed at a later period, 
 probably in the same way as in Selachians ; but this, as far as I 
 know, has not been made out. 
 
 The segmental duct of the kidneys forms the duct for this 
 new gland, as in embryo Selachians (Fig. 2), but, unlike what 
 happens in Selachians, undergoes no further changes, with the 
 exception of a varying amount of retrogressive metamorphosis 
 of its anterior end. The kidneys of Osseous fish usually extend 
 from just behind the head to opposite the anus, or even further 
 back than this. They consist for the most part of a broader 
 anterior portion, an abdominal portion reaching from this to the 
 anus, and, as in those cases in which the kidneys extend further 
 back than the anus, of a caudal portion. 
 
 The two ducts (segmental ducts of the kidneys) lie, as a rule, 
 in the lower part of the kidneys on their outer borders, and open 
 almost invariably into a urinary bladder. In some cases they 
 unite before opening into the bladder, but generally have inde- 
 pendent openings. 
 
 This bladder, which is simply a dilatation of the united 
 lower ends of the primitive kidney-ducts, and has no further 
 importance, is almost invariably present, but in many cases lies 
 unsymmetrically either to the right or the left. It opens to the 
 exterior by a very minute opening in the genito-urinary papilla, 
 immediately behind the genital pore. There are, however, a 
 few cases in which the generative and urinary organs have a 
 
 1 I am unfortunately only acquainted with Dr Rosenberg's paper from an ab- 
 stract. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 149 
 
 common opening. For further details vide Hyrtl, Denk. der k. 
 Akad. Wicn, Vol. II. 
 
 It is possible that the generative ducts of Osseous fishes are 
 derived from a splitting from the primitive duct of the kidney, 
 but this is discussed later in the paper. 
 
 In Osseous fishes we probably have an embryonic condition 
 of the Selachian kidneys retained permanently through life. 
 
 In the majority of Ganoids the division of the segmental 
 duct of the kidney into two would seem to occur, and the ventral 
 duct of the two (Mullerian duct), which opens at its upper end 
 into the body-cavity, is said to serve as an excretory duct for 
 both male and female organs. 
 
 The following are the more important facts which are known 
 about the generative and urinary ducts of Ganoids. 
 
 In Spatularia (vide Hyrtl, Geschlechts u. Harnwerkzeuge bei 
 den Ganoiden, Denkschriften der k. Akad. Wien, Vol. VIII.) the 
 following parts are found in the female. 
 
 (1) The ovaries stretching along the whole length of the 
 abdominal cavity. 
 
 (2) The kidneys, which are separate and also extend along 
 the greater part of the abdominal cavity. 
 
 (3) The ureters lying on the outer borders of the kidneys. 
 Each ureter dilates at its lower end into an elongated wide 
 tube, which continues to receive the ducts from the kidneys. 
 The two ureters unite before terminating and open behind 
 the anus. 
 
 (4) The two oviducts (Mullerian ducts). These open widely 
 into the abdominal cavity, at about two-thirds of the distance 
 from the anterior extremity of the body-cavity. Each opens by 
 a narrow pore into the dilated ureter of its side. 
 
 In the male the same parts are found as in the female, but 
 Hyrtl found that the Mullerian duct of the left side at its 
 entrance into the ureter became split into two horns, one of 
 which ended blindly. On the right side the opening of the 
 Mullerian duct was normal. 
 
 In the Sturgeon (vide J. Miiller, Ban u. Grenzen d. Ganoiden, 
 Berlin Akad. 1844; Leydig, Fisclicn u. kept Hie n, and Hyrtl, 
 Ganoiden} the same parts are found as in Spatularia. 
 
150 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 The kidneys extend along the whole length of the body- 
 cavity ; and the ureter, which does not reach the whole length 
 of the kidneys, is a thin-walled wide duct lying on the outer 
 side. On laying it open the numerous apertures of the tubules 
 for the kidney are exposed. The Miillerian duct, which opens 
 in both sexes into the abdominal cavity, ends, according to 
 Leydig, in the cases of some males, blindly behind without 
 opening into the ureter, and Mu'ller makes the same statement 
 for both sexes. It was open on both sides in a female specimen 
 I examined 1 , and Hyrtl found it invariably so in both sexes in 
 all the specimens he examined. 
 
 Both Rathke and Stannius (I have been unable to refer to 
 the original papers) believed that the semen was carried off by 
 transverse ducts directly into the ureter, and most other ob- 
 servers have left undecided the mechanism of the transportation 
 of the semen to the exterior. If we suppose that the ducts 
 Rathke saw really exist they might perhaps be supposed to 
 enter not directly into the ureter, but into the kidney, and 
 be in fact homologous with the vasa efferentia of the Selachians. 
 The frequent blind posterior termination of the Mullerian duct 
 is in favour of the view that these ducts of Rathke are really 
 present. 
 
 In Polypterus (vide Hyrtl, Ganoideii) there is, as in other 
 Ganoids, a pair of Mullerian ducts. They unite at their lower 
 ends. The ureters are also much narrower than in previously 
 described Ganoids and, after coalescing, open into the united 
 oviducts. The urinogenital canal, formed by coalescence of 
 the Mullerian ducts and ureters, has an opening to the exterior 
 immediately behind the anus. 
 
 In Amia (vide Hyrtl) there is a pair of Mullerian ducts 
 which, as well as the ureters, open into a dilated vesicle. This 
 vesicle appears as a continuation of the Mullerian ducts, but 
 receives a number of the efferent ductules of the kidneys. There 
 is a single genito-urinary pore behind the anus. 
 
 In Ceratodus (Giinther, Phil. Trans. 1871) the kidneys are 
 small and confined to the posterior extremity of the abdomen. 
 The generative organs extend however along the greater part of 
 
 1 For this specimen I am indebted to Ur Giinther. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 151 
 
 the length of the abdominal cavity. In both male and female 
 there is a long Mullerian duct, and the ducts of the two sides 
 unite and open by a common pore into a urinogenital cloaca 
 which communicates with the exterior by the same opening 
 as the alimentary canal. In both sexes the Mullerian duct 
 has a wide opening near the anterior extremity of the body- 
 cavity. The ureters coalesce and open together into the urino- 
 genital cloaca dorsal to the Mullerian ducts. It is not abso- 
 lutely certain that the semen is transported to the exterior 
 by the Mullerian duct of the male, which is perhaps merely a 
 rudiment as in Amphibia. Dr Giinther failed however to find 
 any other means by which it could be carried away. 
 
 The genital ducts of Lepidosteus differ in important par- 
 ticulars from those of the other Ganoids (vide M tiller, loc. cit. 
 and Hyrtl, loc. cit.}. 
 
 In both sexes the genital ducts are continuous with the in- 
 vestments of the genital organs. 
 
 In the female the dilated posterior extremities of the ureters 
 completely invest for some distance the generative ducts, whose 
 extremities are divided into several processes, and end in a 
 different way on the two sides. A similar division and asym- 
 metry of the ducts is mentioned by Hyrtl as occurring in 
 the male of Spatularia, and it seems not impossible that on 
 the hypothesis of the genital ducts being segmental tubes these 
 divisions may be remnants of primitive glandular convolu- 
 tions. The ureters in both sexes dilate as in other Ganoids 
 at their posterior extremities, and unite with one another. 
 The unpaired urinogenital opening is situated behind the anus. 
 In the male the dilated portion of the ureters is divided into 
 a series of partitions which are not present in the female. 
 
 Till the embryology of the secretory system of Ganoids has 
 been worked out, the homologies of their generative ducts are 
 necessarily a matter of conjecture. It is even possible that 
 what I have called the Mullerian duct in the male is function- 
 less, as with Amphibians, but that, owing to the true ducts of 
 the testis having been overlooked, it has been supposed to 
 function as the vas deferens. Gunther's (loc. cit.} injection ex- 
 periments on Ccratodus militate against this view, but I do 
 not think they can be considered as conclusive as long as the 
 
152 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 mechanism for the transportation of the semen to the exterior 
 has not been completely made out. Analogy would certainly 
 lead us to expect the ureter to serve in Ganoids as the vas 
 deferens. 
 
 The position of the generative ducts might in some cases 
 lead to the supposition that they are not Miillerian ducts, or, in 
 other words, the most anterior pair of segmental organs but 
 a pair of the posterior segmental tubes. 
 
 What are the true homologies of the generative ducts of 
 Lepidosteus, which are continuous with the generative glands, 
 is somewhat doubtful. It is very probable that they may re- 
 present the similarly functioning ducts of other Ganoids, but 
 that they have undergone further changes as to their anterior 
 extremities. 
 
 It is, on the other hand, possible that their generative ducts 
 are the same structures as those ducts of Osseous fishes, which 
 are continuous with the generative organs. These latter ducts 
 are perhaps related to the abdominal pores, and had best be 
 considered in connection with these; but a completely satisfac- 
 tory answer to the questions which arise in reference to them 
 can only be given by a study of their development. 
 
 In the Cyclostomes the generative products pass out by an 
 abdominal pore, which communicates with the peritoneal cavity 
 by two short tubes 1 , and which also receives the ducts of the 
 kidneys. 
 
 Gegenbaur suggests that these are to be looked upon as 
 Mullerian ducts, and as therefore developed from the segmental 
 ducts of the kidneys. Another possible view is that they are 
 the primitive external openings of a pair of segmental organs. 
 In Selachians there are usually stated to be a pair of abdominal 
 pores. In Scyllium I have only been able to find, on each side, 
 a large deep pocket opening to the exterior, but closed below 
 towards the peritoneal cavity, so that in it there seem to be no 
 abdominal pores 2 . In the Greenland Shark (L&margus Borcalis] 
 
 1 According to M tiller (Myxinoidcn, 1845) there is in Myxine an abdominal pore 
 with two short canals leading into it, and Vogt and Pappenheim (An. Sci. Nat. 
 Part IV. Vol. xi.) state that in Petromyzon there are two such pores, each connected 
 with a short canal. 
 
 - My own rough examination of preserved specimens was hardly sufficient to 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 153 
 
 Professor Turner (Journal of Anat. and Phys. Vol. VIII.) failed 
 to find either oviduct or vas deferens, but found a pair of large 
 open abdominal pores, which he believes serve to carry away 
 the generative products of both sexes. Whether the so-called 
 abdominal pores of Selachians usually end blindly as in Scyl- 
 lium, or, as is commonly stated, open into the body-cavity, 
 there can be no question that they are homologous with true 
 abdominal powers. 
 
 The blind pockets of Scyllium appear very much like the 
 remains of primitive involutions from the exterior, which might 
 easily be supposed to have formed the external opening of a 
 pair of segmental organs, and this is probably the true meaning 
 of abdominal pores. The presence of abdominal pores in all 
 Ganoids in addition to true genital ducts and of these pockets 
 or abdominal pores in Selachians, which are almost certainly 
 homologous with the abdominal pores of Ganoids and Cyclo- 
 stomes, and also occur in addition to true Miillerian ducts, speak 
 strongly against the view that the abdominal pores have any 
 relation to Mullerian ducts. Probably therefore the abdominal 
 pores of the Cyclostomous fishes (which seem to be of the same 
 character as other abdominal pores) are not to be looked on as 
 rudimentary Mullerian ducts. 
 
 We next come to the question which I reserved while speak- 
 ing of the kidneys of Osseous fishes, as to the meaning of their 
 genital ducts. 
 
 In the female Salmon and the male and female Eel, the gen- 
 erative products are carried to the exterior by abdominal pores, 
 and there are no true generative ducts. In the case of most 
 other Osseous fish there are true generative ducts which are 
 continuous with the investment of the generative organs 1 and 
 
 enable me to determine for certain the presence or absence of these pores. Mr Bridge, 
 of Trinity College, has, however, since then commenced a series of investigations on 
 this point, and informs me that these pores are certainly absent in Scyllium as well as 
 in other genera. 
 
 1 The description of the attachment of the vas deferens to the testis in the Carp 
 given by Vogt and Pappenheim (Ann. Scien. Nat. 1859) does not agree with what I 
 found in the Perch (Perca Jluvialis). The walls of the duct are in the Perch con- 
 tinuous with the investment of the testis, and the gland of the testis occupies, as it 
 were, the greater part of the duct ; there is, however, a distinct cavity corresponding 
 to what Vogt and P. call the duct, near the border of attachment of the testis into 
 
154 TI *E URINOGENITAL ORGANS OF VERTEBRATES. 
 
 have generally, though not always, an opening or openings inde- 
 pendent of the ureter close behind the rectum, but no abdominal 
 pores are present. It seems, therefore, that in Osseous fish the 
 generative ducts are complementary to abdominal pores, which 
 might lead to the view that the generative ducts were formed 
 by a coalescence of the investment of the generative glands with 
 the short duct of abdominal pore. 
 
 Against this view there are, however, the following facts : 
 
 (1) In the cases of the salmon and the eel it is perfectly 
 true that the abdominal pore exactly corresponds with the 
 opening of the genital duct in other Osseous fishes, but the 
 absence of genital ducts in these cases must rather be viewed, 
 as Vogt and Pappehheim (loc. cit.) have already insisted, as a 
 case of degeneration than of a primitive condition. The pre- 
 sence of genital ducts in the near allies of the Salmonidae, and 
 even in the male salmon, are conclusive proofs of this. If we 
 admit that the presence of an abdominal pore in Salmonidae is 
 merely a result of degeneration, it obviously cannot be used as 
 an argument for the complementary nature of abdominal pores 
 and generative ducts. 
 
 (2) Hyrtl (Denkschriften dcr k. Akad. Wien, Vol I.) states 
 that in Mormyrus oxyrynchus there is a pair of abdominal 
 pores in addition to true generative ducts. If his statements 
 are correct, we have a strong argument against the generative 
 ducts of Osseous fishes being related to abdominal pores. For 
 though this is the solitary instance of the presence of both a 
 genital opening and abdominal pores known to me in Osseous 
 fishes, yet we have no right to assume that the abdominal pores 
 of Mormyrus are not equivalent to those of Ganoids and Se- 
 lachians. It must be admitted, with Gegenbaur, that embry- 
 ology alone can elucidate the meaning of the genital ducts of 
 Osseous fishes. 
 
 In Lepidosteus, as was before mentioned, the generative 
 ducts, though continuous with the investment of the genera- 
 tive bodies, unite with the ureters, and in this differ from the 
 generative ducts of Osseous fishes. The relation, indeed, of the 
 
 which the seminal tubules open. I could find at the posterior end of the testis no 
 central cavity which could be distinguished from the cavity of this duct. 
 
THE URIXOGEXITAL ORGANS OF VERTEBRATES. 155 
 
 generative ducts of Lepidosteus to the urinary ducts is very 
 similar to that existing in other Ganoid fishes ; and this, 
 coupled with the fact that Lepidosteus possesses a pair of 
 abdominal pores on each side of the anus 1 , makes it most proba- 
 ble that its generative ducts are true Mullerian ducts. 
 
 In the Amphibians the urinary system is again more primi- 
 tive than in the Selachians. . 
 
 The segmental duct of the kidneys is formed 2 by an elon- 
 gated fold arising from the outer wall of the body-cavity, in 
 the same position as in Selachians. This fold becomes con- 
 stricted into a canal, closed except at its anterior end, which 
 remains open to the body-cavity. This anterior end dilates, 
 and grows out into two horns, and at the same time its opening 
 into the body-cavity becomes partly constricted, and so divided 
 into three separate orifices, one for each horn and a central 
 one between the two. The horns become convoluted, blood 
 channels appearing between their convolutions, and a special 
 coil of vessels is formed arising from the aorta and projecting 
 into the body-cavity near the openings of the convolutions. 
 These formations together constitute the glandular portion 3 of 
 the original anterior segmental tube or segmental duct of the 
 kidneys. I have already pointed out the similarity which this 
 organ exhibits to the head-kidneys of Cyclostome fishes in its 
 mode of formation, especially with reference to the division of 
 the primitive opening. The lower end of the segmental duct 
 unites with a horn of the cloaca. 
 
 After the formation of the gland just described the remainder 
 of the kidney is formed. 
 
 1 This is mentioned by Muller (Ganoid fishes, Berlin Akad. 1844), Hyrtl (loc. fit.), 
 and Giinther (loc. cit.), and through the courtesy of Dr Giinther I have had an oppor- 
 tunity of confirming the fact of the presence of the abdominal pores on two specimens 
 of Lepidosteus in the British Museum. 
 
 2 My account of the development of these parts in Amphibians is derived for the 
 most part from Gotte, Die Entwickdungsgeschichte dcr Unkc. 
 
 3 It is called Kopfniere (head-kidney), or Urniere (primitive kidney), by German 
 authors. Leydig correctly looks upon it as together with the permanent kidney con- 
 stituting the Urniere of Amphibians. The term Urniere is one which lias arisen in 
 my opinion from a misconception ; but certainly the Kopfniere has no greater right t<> 
 the appellation thau the remainder of the kidney. 
 
156 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 This arises in the same way as in Selachians. A series of 
 involutions from the body-cavity are developed ; these soon form 
 convoluted tubes, which become branched and interlaced with 
 one another, and also unite with the primitive duct of the 
 kidneys. Owing to the branching and interlacing of the primi- 
 tive segmental tubes, the kidney is not divided into distinct 
 segments in the same way as with the Selachians. The mode 
 of development of these segmental tubes was discovered by 
 Gotte. Their openings are ciliated, and, as Spengel (loc, cit.) and 
 Meyer (loc. '/.) have independently discovered, persist in most 
 adult Amphibians. As both these investigators have pointed 
 out, the segmental openings are in the adult kidneys of most 
 Amphibians far more numerous than the vertebral segments to 
 which they appertain. This is due to secondary changes, and is 
 not to be looked upon as the primitive state of things. At this 
 stage the Amphibian kidneys are nearly in the same condition 
 as the Selachian, in the stage represented in Fig. 2. In both 
 there is the segmental duct of the kidneys, which is open in 
 front, communicates with the cloaca behind, and receives the 
 whole secretion from the kidneys. The parallelism between the 
 two is closely adhered to in the subsequent modifications of the 
 Amphibian kidney, but the changes are not completed so far in 
 Amphibians as in Selachians. The segmental duct of the 
 Amphibian kidney becomes, as in Selachians, split into a Mul- 
 lerian duct or oviduct, and a Wolffian duct or duct for the 
 kidney. 
 
 The following points about this are noteworthy : 
 
 (1) The separation of the two ducts is never completed, so 
 that they are united together behind, and for a short distance, 
 blend and form a common duct ; the ducts of the two sides so 
 formed also unite before opening to the exterior. 
 
 (2) The separation of the two ducts does not occur in the 
 form of a simple splitting, as in Selachians. But the efferent 
 ductules from the kidney gradually alter their points of en- 
 trance into the primitive duct. Their points of entrance become 
 carried backwards further and further, and since this process 
 affects the anterior ducts proportionally more than the posterior, 
 the efferent ducts finally all meet and form a common duct 
 which unites with the Mullerian duct near its posterior ex- 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 157 
 
 tremity. This process is not always carried out with equal 
 completeness. In the tailless Amphibians, however, the process 
 is generally 1 completed, and the ureters (Wolffian ducts) are of 
 considerable length. . Bufo cinereus, in the male of which the 
 Mullerian ducts are very conspicuous, serves as an excellent 
 example of this. 
 
 In the Salamander (Salamandra maculosa), Figs. 6 and 7, 
 the process is carried out with greater completeness in the 
 female than in the male, and this is the general rule in Amphi- 
 bians. In the male Proteus, the embryonic condition would 
 seem to be retained almost in its completeness so that the 
 ducts of the kidney open directly and separately into the still 
 persisting primitive duct of the kidney. The upper end of 
 the duct nevertheless extends some distance beyond the end 
 of the kidney and opens into the abdominal cavity. In the 
 female Proteus, on the other hand, the separation into a Mulle- 
 rian duct and a ureter is quite complete. The Newt (Triton) 
 also serves as an excellent example of the formation of distinct 
 Mullerian and Wolffian ducts being much more complete in the 
 female than the male. In the female Newt all the tubules 
 from the kidney open into a duct of some length which unites 
 with the Mullerian duct near its termination, but in the male 
 the anterior segrnental tubes, including those which, as will be 
 afterwards seen, serve as vasa efferentia of the testis, enter the 
 Mullerian duct directly, while the posterior unite as in the 
 female into a common duct before joining the Mullerian duct. 
 For further details as to the variations exhibited in the Amphi- 
 bians, the reader is referred to Leydig, Anat. Untersuclinng, 
 Fischen u. Reptilien. Ditto, Lehrbuch der Histologie, Menschen 
 u. TJtiere. Von Wittich, Siebold n. Kolliker, Zeitschriff, Vol. 
 IV. p. 125. 
 
 The different conditions of completeness of the Wolffian 
 ducts observable amongst the Amphibians are instructive in 
 reference to the manner of development of the Wolffian duct 
 in Selachians. The mode of division in the Selachians of the 
 segmental duct of the kidney into a Mullerian and Wolffian 
 
 1 In Bombinator igneus, Von Wittich stated that the embryonic condition was 
 retained. Leydig, Anatom. d. Amphib. u. Kcptilien, shewed that this is not the case, 
 but that in the male the Mullerian duct is very small, though distinct. 
 
158 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 duct is probably to be looked upon as an embryonic abbre- 
 viation of the process by which these two ducts are formed in 
 Amphibians. The fact that this separation into Miillerian and 
 Wolffian ducts proceeds further in the females of most Amphi- 
 bians than in the males, strikingly shews that it is the oviductal 
 function of the Miillerian duct which is the indirect cause of its 
 separation from the Wolffian duct. The Miillerian duct formed 
 in the way described persists almost invariably in both sexes, 
 and in the male sometimes functions as a sperm reservoir ; 
 e.g. Bufo cinereus. In the embryo it carries at its upper end 
 the glandular mass described above (Kopfniere), but this gene- 
 rally atrophies, though remnants of it persist in the males of 
 some species (e.g. Salamandra). Its anterior end opens, in most 
 cases by a single opening, into the perivisceral cavity in both 
 sexes, and is usually ciliated. As the female reaches maturity, 
 the oviduct dilates very much ; but it remains thin and incon- 
 spicuous in the male. 
 
 The only other developmental change of importance is the 
 connection of the testes with the kidneys. This probably 
 occurs in the same manner as in Selachians, viz. from the 
 junction of the open ends of the segmental tubes with the 
 follicles of the testes. In any case the vessels which carry off 
 the semen constitute part of the kidney, and the efferent 
 duct of the testis is also that of the kidney. The vasa effe- 
 rentia from the testis either pass through one or two nearly 
 isolated anterior portions of the kidney (Proteus, Triton) or 
 else no such special portion of the kidney becomes separated 
 from the rest, and the vasa efferentia enter the general body 
 of the kidney. 
 
 In the male Amphibian, then, the urinogenital system con- 
 sists of the following parts (Fig. 6) : 
 
 (1) Rudimentary Miillerian ducts, opening anteriorly into 
 the body-cavity, which sometimes carry aborted Kopfiiicren. 
 
 (2) The partially or completely formed Wolffian ducts 
 (ureters) which also serve as the ducts for the testes. 
 
 (3) The kidneys, parts of which also serve as the vasa 
 efferentia, and whose secretion, together with the testicular 
 products, is carried off by the Wolffian ducts. 
 
THE URINOGKNITAL ORGANS OF VERTEBRATES. 
 
 159 
 
 (4) The united lower parts of Wolffian and Miillerian ducts 
 which are really the lower unsplit part of the segmental ducts of 
 the kidneys. 
 
 m.tl 
 
 V / 
 
 FIG. 6. DIAGRAM OF THE URINOGENITAL ORGANS OF A MALE SALAMANDER. 
 (Copied from Ley dig's Histologic des Menschen u. der Thiere.} 
 
 md. Miiller's duct (rudimentary); y. remnant of the secretory portion of the 
 segmental duct Kopfniere ; Wd. Wolffian duct ; a less complete structure in the 
 male than in the female ; st. segmental tubes or kidney. The openings of these into 
 the body-cavity are not inserted in the figure ; t. testis. Its efferent ducts form part 
 of the kidney. 
 
 In the female, there are (Fig. 7) 
 
 (1) The Miillerian ducts which function as the oviducts. 
 
 (2) The Wolffian ducts. 
 
 (3) 
 
 (4) 
 male. 
 
 The kidneys. 
 
 The united Miillerian and Wolffian ducts as in the 
 
 m.d 
 
 DIAGRAM OF THE URINOGENITAL ORGANS OF A FEMALE SALAMANDER. 
 (Copied from Ley Jig's Histologie des Menschen u. der Thiere} 
 
 Aid. Miiller's duct or oviduct ; IVd. Wolffian duct or the duct of the kidneys ; 
 st. segmental tubes or kidney. The openings of these into the body-cavity are not 
 inserted in the figure ; o. ovary. 
 
 The urinogenital organs of the adult Amphibians agree in 
 almost all essential particulars with those of Selachians. The 
 
l6o THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 ova are carried off in both by a specialized oviduct. The 
 Wolffian duct, or ureter, is found both in Selachians and Am- 
 phibians, and the relations of the testis to it are the same in 
 both, the vasa efferentia of the testes having in both the same 
 anatomical peculiarities. 
 
 The following points are the main ones in which Selachians 
 and Amphibians differ as to the anatomy of the urinogenital 
 organs ; and in all but one of these, the organs of the Amphi- 
 bian exhibit a less differentiated condition than do those of the 
 Selachian. 
 
 (1) A glandular portion (Kopfniere) belonging to the first 
 segmental organ (segmental duct of the kidneys) is found in all 
 embryo Amphibians, but usually disappears, or only leaves a 
 remnant in the adult. It has not yet been found in any Se- 
 lachian. 
 
 (2) The division of the primitive duct of the kidney into 
 the Miillerian duct and the Wolffian duct is not completed so far 
 in Amphibians as Selachians, and in the former the two ducts 
 are confluent at their lower ends. 
 
 (3) The permanent kidney exhibits in Amphibians no 
 distinction into two glands (foreshadowing the Wolffian bodies 
 and true kidneys of higher vertebrates), as it does in the Se- 
 lachians. 
 
 (4) The Miillerian duct persists in its entirety in male Am- 
 phibians, but only its upper end remains in male Selachians. 
 
 (5) The openings of the segmental tubes into the body- 
 cavity correspond in number with the vertebral segments in 
 most Selachians, but are far more numerous than these in 
 Amphibians. This is the chief point in which the Amphibian 
 kidney is more differentiated than the Selachian. 
 
 The modifications in development which the urinogenital 
 system has suffered in higher vertebrates (Sauropsida and 
 Mammalia) are very considerable ; nevertheless it appears to 
 me to be possible with fair certainty to trace out the rela- 
 tionship of its various parts in them to those found in the 
 Ichthyopsida. The development of urinogenital organs has 
 been far more fully worked out for the bird than for any other 
 member of the amniotic vertebrates ; but, as far as we know, 
 
THE URTNOGENITAL ORGANS OF VERTEBRA IKS. l6l 
 
 there are no essential variations except in the later periods 
 of development throughout the division. These later varia- 
 tions, concerning for the most part the external apertures of 
 the various ducts, are so well known and have been so fully 
 described as to require no notice here. The development of 
 these parts in the bird will therefore serve as the most conve- 
 nient basis for comparison. 
 
 In the bird the development of these parts begins by the 
 appearance of a column of cells on the upper surface of the 
 intermediate cell-mass (Fig. 8, W.cT). As in Selachians, the in- 
 termediate cell-mass is a group of cells between the outer edge 
 of the protovertebrae and the upper end of the body cavity. 
 The column of cells thus formed is the commencement of the 
 duct of the Wolffian body. Its development is strikingly similar 
 to that of the segmental duct of the kidney in Selachians. I 
 shall attempt when I have given an account of the development 
 of the Miillerian duct to speak of the relations between the 
 Selachian duct and that of the bird. 
 
 Romiti (ArcJiiv f. Micr. Anat. Vol. X.) has recently stated 
 that the Wolffian duct developes as an involution from the 
 body cavity. The fact that the specimens drawn by Romiti 
 to support this view are too old to determine such a point, and 
 the inspection of a number of specimens made by my friend 
 Mr Adam Sedgwick of Trinity College, who, at my request, 
 has been examining the urinogenital organs of the fowl, have 
 led me to the conclusion that Romiti is in error in differing 
 from his predecessors as to the development of the Wolffian 
 duct. The solid string of cells to form the Wolffian duct lies 
 at first close to the epiblast, but, by the alteration in shape which 
 the protovertebrae undergo and the general growth of cells 
 around it, becomes gradually carried downwards till it lies close 
 to the germinal epithelium which lines the body cavity. While 
 undergoing this change of position it also acquires a lumen, 
 but ends blindly both in front and behind. Towards the end 
 of the fourth day the Wolffian duct opens into a horn of 
 the cloaca. The cells adjoining its inner border commence, 
 as it passes down on the third day, to undergo histological 
 changes, which, by the fourth day, result in the formation of a 
 B. II 
 
162 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 FIG. 8. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO 
 FOWL OF 45 h. To SHEW THE MODE OF FORMATION OF THE WOLFFIAN 
 DUCT. 
 
 A. epiblast ; B. mesoblast ; C. hypoblast ; M.c. medullary canal ; Pv. Pro- 
 tovertebrae ; W.d. Wolffian duct ; So. Somatopleure ; Sp. Splanchnopleure ; //. 
 pleuroperitoneal cavity ; ch. note-chord ; ao. dorsal aorta ; v. blood-vessels. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 163 
 
 series of ducts and Malpighian tufts which form the mass of the 
 Wolffian body 1 . 
 
 The Mullerian duct arises in the form of an involution, 
 whether at first solid or hollow, of the germinal epithelium, 
 and, as I am satisfied, quite independently of the Wolffian 
 duct. It is important to notice that its posterior end soon 
 unites with the Wolffian duct, from which however it not long 
 after becomes separated and opens independently into the 
 cloaca. The upper end remains permanently open to the body 
 cavity, and is situated nearly opposite the extreme front end of 
 the Wolffian body. 
 
 Between the 8oth and looth hour of incubaticn the ducts 
 of the permanent kidneys begin to make their appearance. 
 Near its posterior extremity each Wolffian duct becomes ex- 
 panded, and from the dorsal side of this portion a diverticulum 
 is constricted off, the blind end of which points forwards. This 
 is the duct of the permanent kidneys, and around its end the 
 kidneys are found. It is usually stated that the tubules of the 
 permanent kidneys arise as outgrowths from the duct, but this 
 requires to be worked over again. 
 
 The condition of the urinogenital system in birds im- 
 mediately after the formation of the permanent kidneys is 
 strikingly similar to its permanent condition in adult Sela- 
 chians. There is the Mullerian duct in both opening in front 
 into the body cavity and behind into the cloaca. In both 
 the kidneys consist of two parts an anterior and posterior 
 which have been called respectively Wolffian bodies and perma- 
 nent kidneys in birds and Leydig's glands and the kidneys 
 in Selachians. 
 
 The duct of the permanent kidney, which at first opens into 
 that of the Wolffian body, subsequently becomes further split 
 off from the Wolffian duct, and opens independently into the 
 cloaca. 
 
 1 This account of the origin of the Wolffian body differs from that given by Wal- 
 deyer, and by Dr Foster and myself (Elements of Embryology, Foster and Balfour), but 
 I have been led to alter my view from an inspection of Mr Sedgwick's preparations, 
 and I hope to shew that theoretical considerations lead to the expectation that the 
 Wolffian body would develop independently of the duct. 
 
 II 2 
 
164 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 The subsequent changes of these parts are different in the 
 two sexes. 
 
 In the female the Miillerian ducts 1 persist and become the 
 oviducts. Their anterior ends remain open to the body cavity. 
 The changes in their lower ends in the various orders of Sau- 
 ropsida and Mammalia are too well known to require repetition 
 here. The Wolffian body and duct atrophy : there are left 
 however in many cases slight remnants of the anterior extre- 
 mity of the body forming the parovarium of the bird, and also 
 frequently remnants of the posterior portion of the gland as 
 well as of the duct.. The permanent kidney and its duct remain 
 unaltered. 
 
 In the male the Miillerian duct becomes almost completely 
 obliterated. The Wolffian duct persists and forms the vas 
 deferens, and the anterior so-called sexual portion of the 
 Wolffian body also persists in an altered form. Its tubules 
 unite with the seminiferous tubules, and also form the epi- 
 didymis. Unimportant remnants of the posterior part of the 
 Wolffian body also persist, but are without function. In 
 both sexes the so-called permanent kidneys form the sole por- 
 tion of the primitive uriniferous system which persists in the 
 adult. 
 
 In considering the relations between the modes of develop- 
 ment of the urinogenital organs of the bird and of the Se- 
 lachians, the first important point to notice is, that whereas in 
 the Selachians the segmental duct of the kidneys is first de- 
 veloped and subsequently becomes split into the Mullerian and 
 Wolffian ducts ; in the bird these two ducts develope inde- 
 pendently. This difference in development would be accurately 
 described by saying that in birds the segmental duct of the kid- 
 neys developes as in Selachians, but that the Mullerian duct 
 developes independently of it. 
 
 Since in Selachians the Wolffian duct is equivalent to the 
 segmental duct of the kidneys with the Mullerian removed from 
 it, when in birds the Mullerian duct developes independently of 
 the segmental kidney duct, the latter becomes the same as the 
 Wolffian duct. 
 
 1 The right oviduct atrophies in birds, and the left alone persists in the adult. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 165 
 
 The second mode of stating the difference in development in 
 the two cases represents the embryological facts of the bird far 
 better than the other method. 
 
 It explains why the Wolffian duct appears earlier than the 
 Miillcrian and not at the same time, as one might expect ac- 
 cording to the other way of stating the case. If the Wolffian 
 duct is equivalent to the segmental duct of Selachians, it must 
 necessarily be the first duct to develope ; and not impro- 
 bably the development of the Miillerian duct would in birds 
 be expected to occur at the time corresponding to that at 
 which the primitive duct in Selachians became split into two 
 ducts. 
 
 It probably also explains the similarity in the mode of de- 
 velopment of the Wolffian duct in birds and the primitive duct 
 of the kidneys in Selachians. 
 
 This way of stating the case is also in accordance with 
 theoretical conclusions. As the egg-bearing function of the 
 Miillerian duct became more and more confirmed we might ex- 
 pect that the adult condition would impress itself more and 
 more upon the embryonic development, till finally the Miil- 
 lerian duct ceased to be at any period connected with the 
 kidneys, and the history of its origin ceased to be traceable in 
 its development. This seems to have actually occurred in the 
 higher vertebrates, so that the only persisting connection be- 
 tween the Miillerian duct and the urinary system is the brief but 
 important junction of the two at their lower ends on the sixth 
 or seventh day. This junction justly surprised Waldeyer (Eier- 
 stock u. Ei, p. 129), but receives a complete and satisfactory 
 explanation on the hypothesis given above. 
 
 The original development of the segmental tubes is in the 
 bird solely retained in the tubules of the Wolffian body arising 
 independently of the Wolffian duct, and I have hitherto failed 
 to find that there is a distinct division of the Wolffian bodies 
 into segments corresponding with the vertebral segments. 
 
 I have compared the permanent kidneys to the lower por- 
 tion of the kidneys of Selachians. The identity of the ana- 
 tomical condition of the adult Selachian and embryonic bird 
 which has been already pointed out speaks strongly in favour 
 of this view ; and when we further consider that the duct of 
 
166 THE URINOGENITAL ORGANS OF VERTEBRATES. 
 
 the permanent kidneys is developed in nearly the same way 
 as the supposed homologous duct in Selachians, the suggested 
 identity gains further support. The only difficulty is the fact 
 that in Selachians the tubules of the part of the kidneys under 
 comparison develope as segmental involutions in point of time 
 anteriorly to their duct, while in birds they develope in a manner 
 not hitherto certainly made out but apparently in point of time 
 posteriorly to their duct. But when the immense modifications 
 in development which the whole of the gland of the excretory 
 organ has undergone in the bird are considered, I do not think 
 that the fact I have mentioned can be brought forward as a 
 serious diffiulty. 
 
 The further points of comparison between the Selachian and 
 the bird are very simple. The Miillerian duct in its later 
 stages behaves in the higher vertebrates precisely as in the 
 lower. It becomes in fact the oviduct in the female and 
 atrophies in the male. The behaviour of the Wolffian duct is 
 also exactly that of the duct which I have called the Wolffian 
 duct in Ichthyopsida, and in the tubules of the Wolffian body 
 uniting with the tubuli seminiferi we have represented the 
 junction of the segmental tubes with the testis in Selachians 
 and Amphibians. It is probably this junction of two inde- 
 pendent organs which led Waldeyer to the erroneous view that 
 the tubuli seminiferi were developed from the tubules of the 
 Wolffian body. 
 
 With the bird I conclude the history of the origin of the 
 urinogenital system of vertebrates. I have attempted, and 
 I hope succeeded, in tracing out by the aid of comparative 
 anatomy and embryology the steps by which a series of inde- 
 pendent and simple segmental organs like those of Annelids 
 have become converted into the complicated series of glands 
 and ducts which constitute the urinogenital system of the 
 higher vertebrates. There are no doubt some points which 
 require further elucidation amongst the Ganoid and Osseous 
 fishes. The most important points which appear to me still 
 to need further research, both embryological and anatomi- 
 cal, are the abdominal pores of fishes, the generative ducts of 
 Ganoids, especially Lepidosteus, and the generative ducts of 
 Osseous fishes. 
 
THE URINOGENITAL ORGANS OF VERTEBRATES. 167 
 
 The only further point which requires discussion is the em- 
 bryonic layer from which these organs are derived. 
 
 I have shewn beyond a doubt (loc. cit) that in Selachians 
 these organs are formed from the mesoblast The unanimous 
 testimony of all the recent investigators of Amphibians leads to 
 the same conclusion. In birds, on the other hand, various in- 
 vestigators have attempted to prove that these organs are 
 derived from the epiblast. The proof they give is the fol- 
 lowing : the epiblast and mesoblast appear fused in the region 
 of the axis cord. From this some investigators have been led 
 to the conclusion that the whole of the' mesoblast is derived 
 from the upper of the two primitive embryonic layers. To 
 these it may be replied that, even granting their view to be 
 correct, it is no proof of the derivation of the urinogenital 
 organs from the epiblast, since it is not till the complete for- 
 mation of the three layers that any one of them can be said to 
 exist. Others look upon the fusion of the two layers as a proof 
 of the passage of cells from the epiblast into the mesoblast. 
 An assumption in itself, which however is followed by the further 
 assumption that it is from these epiblast cells that the urino- 
 genital system is derived ! Whatever may have been the primi- 
 tive origin of the system, its mesoblastic origin in vertebrates 
 cannot in my opinion be denied. 
 
 Kowalewsky (Embryo. Stud, an Vermen it. Arthropoda, Mem. 
 Akad. St Petersbourg, 1871) finds that the segmental tubes of 
 Annelids develope from the mesoblast. We must therefore look 
 upon the mesoblastic origin of the excretory system as having 
 an antiquity greater even than that of vertebrates. 
 
VIII. ON THE DEVELOPMENT OF THE SPINAL NERVES IN 
 ELASMOBRANCH FISHES \ 
 
 . 
 
 With Plates 22 and 23. 
 
 IN the course of an inquiry into the development of Elasmo- 
 branch Fishes, my attention has recently been specially directed 
 to the first appearance and early stages of the spinal nerves, 
 and I have been led to results which differ so materially from 
 those of former investigators, that I venture at once to lay 
 them before the Society. I have employed in my investiga- 
 tions embryos of Scyllium canicula, Scyllium stcllare, Pristiurus, 
 and Torpedo. The embryos of the latter animal, especially 
 those hardened in osmic acid, have proved by far the most 
 favourable for my purpose, though, as will be seen from the 
 sequel, I have been able to confirm the majority of my conclu- 
 sions on embryos of all the above-mentioned genera. 
 
 A great part of my work was done at the Zoological Station 
 founded by Dr Dohrn at Naples ; and I have to thank both 
 Dr Dohrn and Dr Eisig for the uniformly obliging manner 
 in which they have met my requirements for investigation. I 
 have more recently been able to fill up a number of lacunae in 
 my observations by the study of embryos bred in the Brighton 
 Aquarium ; for these I am indebted to the liberality of Mr Lee 
 and the directors of that institution. 
 
 The first appearance of the Spinal Nerves in Pristiitrns. 
 
 In a Pristiurus-embryo, at the time when two visceral 
 clefts become visible from the exterior (though there are as yet 
 
 1 [From the Philosophical Transactions of the Royal Society of London, Vol. 
 CLXVI. Pt. i. Received October 5, Read December 16, 1875.] 
 
DEVELOPMENT OF THE SPINAL NERVES, &C. 169 
 
 no openings from without into the throat), a transverse section 
 through the dorsal region exhibits the following features (PI. 
 22, fig. A) : 
 
 The external epiblast is formed of a single row of flattened 
 elongated cells. Vertically above the neural canal the cells of 
 this layer are more columnar, and form the rudiment of the 
 primitively continuous dorsal fin. 
 
 The neural canal (nc) is elliptical in section, and its walls 
 are composed of oval cells two or three deep. The wall at the 
 two sides is slightly thicker than at the ventral and dorsal ends, 
 and the cells at the two ends are also smaller than elsewhere. 
 A typical cell from the side walls of the canal is about y^ inch 
 in its longest diameter. The outlines of the cells are for the 
 most part distinctly marked in the specimens hardened in either 
 chromic or picric acid, but more difficult to see in those pre- 
 pared with osmic acid ; their protoplasm is clear, and in the 
 interior of each is an oval nucleus very large in proportion to 
 the size of its cell. The long diameter of a typical nucleus 
 is about ^ m inch, or about two-thirds of that of the cell. 
 
 The nuclei are granular, and very often contain several espe- 
 cially large and deeply stained granules ; in other cases only 
 one such is present, which may then be called a nucleolus. 
 
 In sections there may be seen round -the exterior of the 
 neural tube a distinct hyaline membrane : this becomes stained 
 of a brown colour with osmic acid, and purple or red with 
 haematoxylin or carmine respectively. Whether it is to be 
 looked upon as a distinct membrane differentiated from the 
 outermost portion of the protoplasm of the cells, or as a layer 
 of albumen coagulated by the reagents applied, I am unable 
 to decide for certain. It makes its appearance at a very early 
 period, long before that now being considered ; and similar 
 membranes are present around other organs as well as the neu- 
 ral tube. The membrane is at this stage perfectly continuous 
 round the whole exterior of the neural tube as well on tJie dorsal 
 surface as on tJie ventral. 
 
 The section figured, whose features I am describing, belongs 
 to the middle of the dorsal region. Anteriorly to this point the 
 spinal cord becomes more elliptical in section, and the spinal 
 canal more lanceolate ; posteriorly, on the other hand, the spinal 
 
I/O DEVELOPMENT OF THE SPINAL NERVES 
 
 canal and tube become more nearly circular in section. Im- 
 mediately beneath the neural tube is situated the notochord (cJi). 
 It exhibits at this stage a central area rich in protoplasm, and a 
 peripheral layer very poor in protoplasm ; externally it is in- 
 vested by a distinct cuticular membrane. 
 
 Beneath the notochord is a peculiar rod of cells, constricted 
 from the top of the alimentary canal 1 . On each side and below 
 this are the two aortae, just commencing to be formed, and 
 ventral to these is the alimentary canal. 
 
 On each side of the body two muscle-plates are situated ; 
 their upper ends reach about one-third of the way up the sides 
 of the neural tube. The two layers which together constitute 
 the muscle-plates are at this stage perfectly continuous with the 
 somatic and splanchnic layers of the mesoblast, and the space 
 between the two layers is continuous with the body cavity. 
 In addition to the muscle-plates and their ventral continuations, 
 there are no other mesoblast-cells to be seen. The absence of 
 all mesoblastic cells dorsal to the superior extremities of the 
 muscles is deserving of special notice. 
 
 Very shortly after this period and, as a rule, before a third 
 visceral cleft has become visible, the first traces of the spinal 
 nerves make their appearance. 
 
 First Stage. The spinal nerves do not appear at the same 
 time along the whole length of the spinal canal, but are formed 
 first of all in the neck and subsequently at successive points 
 posterior to this. 
 
 Their mode of formation will be most easily understood by 
 referring to PL 22, figs. B I, B II, Bill, which are representa- 
 tions of three sections taken from the same embryo. B I is 
 from the region of the heart ; B II belongs to a part of the 
 body posterior to this, and B III to a still posterior region. 
 
 In most points the sections scarcely differ from PI. 22, fig. A, 
 which, indeed, might very well be a posterior section of the 
 embryo to which these three sections belong. 
 
 The chief point, in addition to the formation of the spinal 
 nerves, which shews the greater age of the embryo from which 
 the sections were taken is the complete formation of the aortas 
 
 1 Vide Balfour, " Preliminary account of the Development of Elasinobranch 
 Fishes," Quart. Journ. of Microsc. Scn-nce, Oct. 1874, p. 33. [This edition, p. 96.] 
 
IN ELASMOBRANCH FISHES. 
 
 The upper ends of the muscle-plates have grovyn no further 
 round the neural canal than in fig. A, and no scattered meso- 
 blastic connective-tissue cells are visible. 
 
 In fig. A the dorsal surface of the neural canal was as com- 
 pletely rounded off as the ventral surface ; but in fig. B III this 
 has ceased to be the case. The cells at the dorsal surface of 
 the neural canal have become rounder and smaller and begun 
 to proliferate, and the uniform outline of the neural canal has 
 here become broken (fig. B III, pr). The peculiar membrane 
 completely surrounding the canal in fig. A now terminates 
 just below the point where the proliferation of cells is taking 
 place. 
 
 The prominence of cells which springs in this way from the 
 top of the neural canal is the commencing rudiment of a pair 
 of spinal nerves. In fig. B II, a section anterior to fig. B III, 
 this formation has advanced much further (fig. Bll,/r). From 
 the extreme top of the neural canal there have now grown out 
 two club-shaped masses of cells, one on each side ; they are 
 perfectly continuous with the cells which form the extreme top 
 of the neural canal, and necessarily also are in contact with 
 each other dorsally. Each grows outwards in contact with the 
 walls of the neural canal ; but, except at the point where they 
 take their origin, they are not continuous with its walls, and are 
 perfectly well separated by a sharp line from them. 
 
 In fig. B I, though the club-shaped processes still retain their 
 attachment to the summit of the neural canal, they have become 
 much longer and more conspicuous. 
 
 Specimens hardened in both chromic acid (PI. 22, fig. C) and 
 picric acid give similar appearances as to the formation of these 
 bodies. 
 
 In those hardened in osmic acid, though the mutual relations 
 of the masses of cells are very clear, yet it is difficult to dis- 
 tinguish the outlines of the individual cells. 
 
 In the chromic acid specimens (fig. C) the cells of these 
 rudiments appear rounded, and each of them contains a large 
 nucleus. 
 
 I have been unable to prepare longitudinal sections of this 
 stage, either horizontal or vertical, to shew satisfactorily the 
 extreme summit of the spinal cord ; but I would call attention 
 
DEVELOPMENT OF THE SPINAL NERVES 
 
 to the fact that the cells forming the proximal portion of the 
 outgrowth are seen in every transverse section at this stage, 
 and therefore exist the whole way along, whereas the distal 
 portion is seen only in every third or fourth section, accord- 
 ing to the thickness of the sections. It may be concluded 
 from this that there appears a continuous outgrowth from the 
 spinal canal, from which discontinuous processes grow out. 
 
 In specimens of a very much later period (PI. 23, fig. L) 
 the proximal portions of the outgrowth are unquestionably 
 continuous with each other, though their actual junctions with 
 the spinal cord are very limited in extent. The fact of this 
 continuity at a later period is strongly in favour of the view 
 that the posterior branches of the spinal nerves arise from the 
 first as a continuous outgrowth of the spinal cord, from which 
 a series of distal processes take their origin. I have, however, 
 failed to demonstrate this point absolutely. The processes, 
 which we may call the nerve-rudiments, are, as appears from 
 the later stages, equal in number to the muscle-plates. 
 
 It may be pointed out, as must have been gathered from 
 the description above, that the nerve-rudiments have at this 
 stage but one point of attachment to the spinal cord, and that 
 this one corresponds with the dorsal or posterior root of the 
 adult nerve. 
 
 The rudiments are, in fact, those of the posterior root only. 
 
 The next or second stage in the formation of these struc- 
 tures to which I would call attention occurs at about the time 
 when three to five visceral clefts are present. The disappear- 
 ance from the notochord in the anterior extremity of the body 
 of a special central area rich in protoplasm serves as an excellent 
 guide to the commencement of this epoch. 
 
 Its investigation is beset with far greater difficulties than 
 the previous one. This is owing partly to the fact that a 
 number of connective-tissue cells, which are only with great 
 difficulty to be distinguished from the cells which compose the 
 spinal nerves, make their appearance around the latter, and 
 partly to the fact that the attachment of the spinal nerves to 
 the neural canal becomes much smaller, and therefore more dif- 
 ficult to study. 
 
 Fortunately, however, in Torpedo these peculiar features 
 
IX ELASMOBRANCH FISHES. 
 
 are not present to nearly the same extent as in Pristiurus and 
 Scyllimn. 
 
 The connective-tissue cells, though they appear earlier in 
 Torpedo than in the two other genera, are much less densely 
 packed, and the large attachment of the nerves to the- neural 
 canal is retained for a longer period. 
 
 Under these circumstances I consider it better, before pro- 
 ceeding with this stage, to give a description of the occurrences 
 in Torpedo, and after that to return to the history of the nerves 
 in the genera Pristinrus and Scylliiun. 
 
 TJie development of the Spinal Nerves in Torpedo. 
 
 The youngest Torpedo-embryo in which I have found traces of 
 the spinal nerves belongs to the earliest part of what I called 
 the second stage. 
 
 The segmental duct 1 is just appearing, but the cells of the 
 notochord have not become completely vacuolated. The rudi- 
 ments of the spinal nerves extend half of the way towards the 
 ventral side of the spinal cord ; they grow out in a most 
 distinct manner from the dorsal surface of the spinal cord 
 (PI. 22, fig. D a, pr) ; but the nerve-rudiments of the two sides 
 are no longer continuous with each other at the dorsal median 
 line, as in the earlier Pristiurus-embryos. The cells forming 
 the proximal portion of the rudiment have the same elongated 
 form as the cells of the spinal cord, but the remaining cells are 
 more circular. 
 
 From the summit of the muscle-plates (mp] an outgrowth of 
 connective tissue has made its appearance (c], which eventually 
 fills up the space between the dorsal surface of the cord and the 
 external epiblast. There is not the slightest difficulty in distin- 
 guishing the connective-tissue cells from the nerve- rudiment. I 
 believe that in this embryo the origin of the nerves from the 
 neural canal was a continuous one, though naturally the peripheral 
 ends of the nerve-rudiments were separate from each other. 
 
 The most interesting feature of the stage is the commencing 
 formation of the anterior roots. Each of these arises (PI. 22, 
 
 1 Vide Balfour, " Origin and History of Urinogenital Organs of Vertebrates," 
 Journal of Anatomy and Physiology, Oct. 1875. [This edition, No. vn.] 
 
1/4 DEVELOPMENT OF THE SPINAL NERVES 
 
 fig. D a, ar) as a small but distinct outgrowth from the epiblast 
 of the spinal cord, near the ventral corner of which it appears as 
 a conical projection. Even from the very first it has an indis- 
 tinct form of termination and a fibrous appearance, while the 
 protoplasm of which it is composed becomes very attenuated 
 towards its termination. 
 
 The points of origin of the anterior roots from the spinal 
 cord are separated from each other by considerable intervals. 
 In this fact, and also in the nerves of the two sides never 
 being united with each other in the ventral median line, the 
 anterior roots exhibit a marked contrast to the posterior. 
 
 There exists, then, in Torpedo-embryos by the end of this 
 stage distinct rudiments of both the anterior and posterior 
 roots of the spinal nerves. These rudiments are at first quite 
 independent of and disconnected with each other, and both 
 take their rise as outgrowths of the epiblast of the neural 
 canal. 
 
 The next Torpedo-embryo (PL 22, fig. D b), though taken 
 from the same female, is somewhat older than the one last 
 described. The cells of the notochord are considerably vacuo- 
 lated ; but the segmental duct is still without a lumen. The 
 posterior nerve-rudiments are elongated, pear-shaped bodies of 
 considerable size, and, growing in a ventral direction, have 
 reached a point nearly opposite the base of the neural canal. 
 They still remain attached to the top of the neural canal, 
 though the connexion has in each case become a pedicle so 
 narrow that it can only be observed with great difficulty. 
 
 It is fairly certain that by this stage each posterior nerve- 
 rudiment has its own separate and independent junction with 
 the spinal cord ; their dorsal extremities are nevertheless pro- 
 bably connected with each other by a continuous commissure. 
 
 The cells composing the rudiments are still round, and 
 have, in fact, undergone no important modifications since the 
 last stage. 
 
 The important feature of the section figured (fig. Db), and 
 one which it shares with the other sections of the same embryo, 
 is the appearance of connective-tissue cells around the nerve- 
 rudiment These cells arise from two sources ; one of these 
 is supplied by the vertebral rudiments, which at the end of 
 
IN ELASMOBRANCH FISHES. 175 
 
 the last stage (PI. 22, fig. C, vr) become split off from the 
 inner layer of the muscle-plates. The vertebral rudiments have 
 in fact commenced to grow up on each side of the neural canal, 
 in order to form the mass of cells out of which the neural arches 
 are subsequently developed. 
 
 The dorsal extremities of the muscle-plates form the second 
 source of these connective-tissue cells. These latter cells lie 
 dorsal and external to the nerve-rudiments. 
 
 The presence of this connective tissue, in addition to the 
 nerve-rudiments, removes the possibility of erroneous interpre- 
 tations in the previous stages of the Pristiurus-zmbryo. 
 
 It might be urged that the two masses which I have called 
 nerve-rudiments are nothing else than mesoblastic connective 
 tissue commencing to develope around the neural canal, and 
 that the appearance of attachment to the neural canal which 
 they present is due to bad preparation or imperfect observation. 
 The sections of both this and the last Torpedo-embryo which 
 I have been describing clearly prove that this is not the case. 
 We have, in fact, in the same sections the developing connective 
 tissue as well as the nerve-rudiments, and at a time when the 
 latter still retains its primitive attachment to the neural canal. 
 The anterior root (fig. D b, ar) is still a distinct conical promi- 
 nence, but somewhat larger than in the previously described 
 embryo ; it is composed of several cells, and the cells of the 
 spinal cord in its neighbourhood converge towards its point 
 of origin. 
 
 In a Torpedo-embryo (PI. 22, fig. D c) somewhat older 
 than the one last described, though again derived from the 
 oviduct of the same female, both the anterior and the pos- 
 terior rudiments have made considerable steps in develop- 
 ment. 
 
 In sections taken from the hinder part of the body I found 
 that the posterior rudiments nearly agreed in size with those 
 in fig. D b. 
 
 It is, however, still less easy than there to trace the junc- 
 tion of the posterior rudiments with the spinal cord, and the 
 upper ends of the rudiments of the two sides do not nearly 
 meet. 
 
 In a considerable series of sections I failed to find any case 
 
DEVELOPMENT OF THE SPINAL NERVES 
 
 in which I could be absolutely certain that a junction between 
 the nerve and the spinal cord was effected ; and it is possible 
 that in course of the change of position which this junction 
 undergoes there may be for a short period a break of continuity 
 between the nerve and the cord. This, however, I do not think 
 probable. But if it takes place at all, it takes place before the 
 nerve becomes functionally active, and so cannot be looked upon 
 as possessing any physiological significance. 
 
 The rudiment of the posterior nerve in the hinder portion of 
 the body is still approximately homogeneous, and no distinction 
 of parts can be found in it. 
 
 In the same region of the body the anterior rudiment retains 
 nearly the same condition as in the previous stage, though it 
 has somewhat increased in size. 
 
 In the sections taken from the anterior part of the same 
 embryo the posterior rudiment has both grown in size and also 
 commenced to undergo histological changes by which it has 
 become divided into a root, a ganglion, and a nerve. 
 
 The root (fig. D c, pr) consists of small round cells which 
 lie close to the spinal cord, and ends dorsally in a rounded 
 extremity. 
 
 The ganglion (g) consists of larger and more elongated cells, 
 and forms an oval mass enclosed on the outside by the down- 
 ward continuation of the root, having its inner side nearly in 
 contact with the spinal cord. 
 
 From its ventral end is continued the nerve, which is of con- 
 siderable length, and has a course approximately parallel to 
 that of the muscle-plate. It forms a continuation of the root 
 rather than of the ganglion. 
 
 Further details in reference to the histology of the nerve- 
 rudiment at this stage are given later in this paper, in the 
 description of Pristiurus-embryos, of which I have a more com- 
 plete series of sections than of the Torpedo-embryos. 
 
 When compared with the nerve-rudiment in the posterior 
 part of the same embryo, the nerve-rudiment last described is, 
 in the first place, considerably larger, and has secondly under- 
 gone changes, so that it is possible to recognize in it parts 
 which can be histologically distinguished as nerve and ganglion. 
 
 The developmental changes which have taken place in the 
 
IN ELASMOBRANCH FISHES. 177 
 
 anterior root are not less important than those in the posterior. 
 The anterior root now forms a very conspicuous cellular promi- 
 nence growing out from the ventral corner of the spinal cord 
 (fig. D c, ar\ It has a straight course from the spinal cord 
 to the muscle-plate, and there shews a tendency to turn down- 
 wards at an open angle : this, however, is not represented in the 
 specimen figured. The cells of which it is composed each con- 
 tain a large oval nucleus, and are not unlike the cells which 
 form the posterior rudiment. The anterior and posterior nerves 
 are still quite unconnected with each other ; and in those sec- 
 tions in which the anterior root is present the posterior root 
 of the same side is either completely absent or only a small 
 part is to be seen. The cells of the spinal cord exhibit a 
 slight tendency to converge towards the origin of the anterior 
 nerve- root. 
 
 In the spinal cord itself the epithelium of the central canal 
 is commencing to become distinguished from the grey matter, 
 but no trace of the white matter is visible. 
 
 I have succeeded iij making longitudinal vertical sections of 
 this stage, which prove that the ends of the posterior roots 
 adjoining the junction with the cord are all connected with each 
 other (PI. 22, fig. Dd). 
 
 If the figure representing a transverse section of the em- 
 bryo (fig. D c) be examined, or better still the figure of a section 
 of the slightly older 6V/////7;//-embryo (PI. 23, fig. H I or I I), 
 the posterior root will be seen to end dorsally in a rounded 
 extremity, and the junction with the spinal cord to be effected, 
 not by the extremity of the nerve, but by a part of it at some 
 little distance from this. 
 
 It is from these upper ends of the rudiments beyond the 
 junction. with the spinal cord that I believe the commissures to 
 spring which connect together the posterior roots. 
 
 My sections shewing this for the stage under consideration 
 are not quite as satisfactory as is desirable ; nevertheless they 
 are sufficiently good to remove all doubt as to the presence of 
 these commissures. 
 
 A figure of one of these sections is represented (PI. 22, fig. 
 D d). In this figure pr points to the posterior roots and x to 
 the commissures uniting them. 
 
 B. 12 
 
1/8 DEVELOPMENT OF THE SPINAL NERVES 
 
 In a stage somewhat subsequent to this I have succeeded in 
 making longitudinal sections, which exhibit these junctions with 
 a clearness which leaves nothing to be desired. 
 
 It is there effected (PI. 23, fig. L) in each case by a proto- 
 plasmic commissure with imbedded nuclei 1 . Near its dorsal 
 extremity each posterior root dilates, and from the dilated por- 
 tion is given off on each side the commissure uniting it with the 
 adjoining roots. 
 
 Considering the clearness of this formation in this embryo, 
 as well as in the embryo belonging to the stage under descrip- 
 tion, there cannot be much doubt that at the first formation 
 of the posterior rudiments a continuous outgrowth arises from 
 the spinal cord, and that only at a later period do the junctions 
 of the roots with the cord become separated and distinct for 
 each nerve. 
 
 I now return to the more complete series of Pristiurus- 
 embryos, the development of whose spinal nerves I have been 
 able to observe. 
 
 Second Stage of the Spinal Nerves in Pristiums. 
 
 In the youngest of these (PI. 22, fig. E) the notochord has 
 undergone but very slight changes, but the segmental duct has 
 made its appearance, and is as much developed as in the Torpedo- 
 embryo from which fig. D b was taken. 
 
 (The embryo from which fig. E a was derived had three 
 visceral clefts.) 
 
 There have not as yet appeared any connective-tissue cells 
 dorsal to the top of the muscle-plates, so that the posterior 
 nerve-rudiments are still quite free and distinct. 
 
 The cells composing them are smaller than the cells of the 
 neural canal ; they are round and nucleated ; and, indeed, in 
 their histological constitution the nerve-rudiments exhibit no 
 important deviations from the previous stage, and they have 
 hardly increased in size. In their mode of attachment to the 
 neural tube an important change has, however, already com- 
 menced to be visible. 
 
 In the previous stage the two nerve-rudiments met above the 
 
 1 This commissure is not satisfactorily represented in the figure. Vide Explana- 
 tion of Plate 2. 
 
IN ELASMOBRANCH FISHES. 
 
 summit of the spinal cord and were broadly attached to it 
 there; now their points of attachment have glided a short dis- 
 tance down the sides of the spinal cord 1 . 
 
 The two nerve-rudiments have therefore ceased to meet 
 above the summit of the canal ; and in addition to this they 
 appear in section to narrow very much before becoming united 
 with its walls, so that their junctions with these appear in a 
 transverse section to be effected by at most one or two cells, and 
 are, comparatively speaking, very difficult to observe. 
 
 In an embryo but slightly older than that represented in 
 Fig. E a the first rudiment of the anterior root becomes visi- 
 ble. This appears, precisely as in Torpedo, in the form of a 
 small projection from the ventral corner of the spinal cord 
 (fig. E b, <7/-). 
 
 The second step in this stage (PI. 22, fig. F) is comparable, 
 as far as the connective-tissue is concerned, with the section of 
 Torpedo (PI. 22, fig. D d). The notochord (the histological 
 details of whose structure are not inserted in this figure) is 
 rather more developed, and the segmental duct, as was the case 
 with the corresponding Torpedo-embryo, has become hollow at 
 its anterior extremity. 
 
 The embryo from which the section was taken possessed five 
 visceral clefts, but no trace of external gills. 
 
 In the section represented, though from a posterior part of 
 the body, the dorsal nerve-rudiments have become considerably 
 larger than in the last embryo ; they now extend beyond the 
 base of the neural canal. They are surrounded to a great ex- 
 tent by mesoblastic tissue, which, as in the case of the Torpedo, 
 takes its origin from two sources, (i) from the commencing 
 vertebral bodies, (2) from the summits of the muscle-plates. 
 
 It is in many cases very difficult, especially with chromic- 
 acid specimens, to determine with certainty the limits of the 
 rudiments of the posterior root. 
 
 1 [May 18, 1876. Observations I have recently made upon the development of 
 the cranial nerves incline me to adopt an explanation of the change which takes place 
 in the point of attachment of the spinal nerves to the cord differing from that enun- 
 ciated in the text. I look upon this change as being apparent rather than real, and 
 as due to a growth of the roof of the neural canal in the median dorsal line, which 
 tends to separate the roots of the two sides more and more, and cause them to assume 
 a more ventral position.] 
 
l8o DEVELOPMENT OF THE SPINAL NERVES 
 
 In the best specimens a distinct bordering line can be seen, 
 and it is, as a rule, possible to state the characters by which 
 the cells of the nerve-rudiments and vertebral bodies differ. The 
 more important of these are the following: (i) The cells of 
 the nerve-rudiment are distinctly smaller than those of the 
 vertebral rudiment ; (2) the cells of the nerve-rudiment are 
 elongated, and have their long axis arranged parallel to the long 
 axis of the nerve-rudiment, while the cells surrounding them are 
 much more nearly circular. 
 
 The cells of the nerve-rudiment measure about -^-^ x ^^ to 
 unnr x W<nr mcn > those of the vertebral rudiment y^ X TI| L^ inch. 
 The greater difficulty experienced in distinguishing the nerve- 
 rudiment from the connective-tissue in Pristiurus than in 
 Torpedo arises from the fact that the connective-tissue is much 
 looser and less condensed in the latter than in the former. 
 
 The connective-tissue cells which have grown out from the 
 muscle-plates form a continuous arch over the dorsal surface of 
 the neural tube (vide PI. 22, fig. F) : and in some specimens 
 it is difficult to see whether the arch is formed by the rudiment 
 of the posterior root or by connective-tissue. It is, however, 
 quite easy with the best specimens to satisfy one's self that it is 
 from the connective-tissue, and not the nerve-rudiment, that the 
 dorsal investment of the neural canal is derived. 
 
 As in the previous case, the upper ends of each pair of 
 posterior nerve-rudiments are quite separate from one another, 
 and appear in sections to be united by a very narrow root 
 to the walls of the neural canal at the position indicated in 
 fig. F 1 . 
 
 The cells forming the nerve-rudiments have undergone slight 
 modifications ; they are for the most part more distinctly elon- 
 gated than in the earlier stage, and appear slightly smaller in 
 comparison with the cells of the neural canal. 
 
 They possess as yet no distinctive characters of nerve- 
 cells. They stain more deeply with osmic acid than the cells 
 around them, but with haematoxylin there is but a very slight 
 difference in intensity between their colouring and that of the 
 neighbouring connective-tissue cells. 
 
 The anterior roots have grown considerably in length, but 
 
 1 The artist has not been very successful in rendering this figure. 
 
TN ELASMOBRANCH FISHES. l8l 
 
 their observation is involved in the same difficulties with 
 chromic-acid specimens as that of the posterior rudiments. 
 
 There is a further difficulty in observing the anterior roots, 
 which arises from the commencing formation of white matter in 
 the cord. This is present in all the anterior sections of the 
 embryo from which fig. F is taken. When the white matter is 
 formed the cells constituting the junction of the anterior nerve- 
 root with the spinal cord undergo the same changes as the cells 
 which are being converted into the white matter of the cord, and 
 become converted .into nerve-fibres ; these do not stain with 
 haematoxylin, and thus an apparent space is left between the 
 nerve-root and the spinal cord. This space by careful examina- 
 tion may be seen to be filled up with fibres. In osmic acid 
 sections, although even in these the white matter is stained less 
 deeply than the other tissues, it is a matter of comparative ease 
 to observe the junction between the anterior nerve root and 
 the spinal cord. 
 
 I have been successful in preparing satisfactory longitudinal 
 sections of embryos somewhat older than that shewn in fig. F, 
 and they bring to light several important points in reference to 
 the development of the spinal nerves. Three of these sections 
 are represented in PI. 22, figs. G I, G 2, and G 3. 
 
 The sections are approximately horizontal and longitudinal. 
 G i is the most dorsal of the three ; it is not quite horizontal 
 though nearly longitudinal. The section passes exactly through 
 the point of attachment of the posterior roots to the walls of the 
 neural canal. 
 
 The posterior rudiments appear as slight prominences of 
 rounded cells projecting from the wall of the neural canal. 
 From transverse sections the attachment of the nerves to the 
 wall of the neural canal is proved to be very narrow, and from 
 these sections it appears to be of some length in the direction of 
 the long axis of the embryo. A combination of the sections 
 taken in the two directions leads to the conclusion that the nerves 
 at this stage thin out like a wedge before joining the spinal cord. 
 
 The independent junctions of the posterior rudiments with 
 the spinal cord at this stage are very clearly shewn, though the 
 rudiments are probably united with each other just dorsal to 
 their junction with the spinal cord. 
 
1 82 DEVELOPMENT OF THE SPINAL NERVES 
 
 The nerves correspond in number with the muscle-plates, 
 and each arises from the spinal cord, nearly opposite the middle 
 line of the corresponding muscle-plates (figs. G I and G 2). 
 
 Each nerve- rudiment is surrounded by connective -tissue 
 cells, and is separated from its neighbours by a considerable 
 interval. 
 
 At its origin each nerve-rudiment lies opposite the median 
 portion of a muscle-plate (figs. G I and G 2) ; but, owing to the 
 muscle-plate acquiring an oblique direction, at the level of the 
 dorsal surface of the notochord it appears in horizontal sections 
 more nearly opposite the interval between two muscle-plates 
 (figs. G 2 and G 3). 
 
 In horizontal sections I find masses of cells which make 
 their appearance on a level with the ventral surface of the 
 spinal cord. I believe I have in some sections successfully 
 traced these into the spinal cord, and I have little doubt that 
 they are the anterior roots of the spinal nerves ; they are op- 
 posite the median line of the muscle-plates, and do not appear 
 to join the posterior roots (vide fig. G 3, ar). 
 
 At the end of this period or second stage the main cha- 
 racters of the spinal nerves in Pristiurus are the following : 
 
 (1) The posterior nerve-rudiments form somewhat wedge- 
 shaped masses of tissue attached dorsally to the spinal cord. 
 
 (2) The cells of which they are composed are typical undif- 
 ferentiated embryonic cells, which can hardly be distinguished 
 from the connective-tissue cells around them. 
 
 (3) The nerves of each pair no longer meet above the 
 summit of the spinal canal, but are independently attached 
 to its sides. 
 
 (4) Their dorsal extremities are probably united by com- 
 missures. 
 
 (5) The anterior roots have appeared ; they form small 
 conical projections from the ventral corner of the spinal cord, 
 but have no connexion with the posterior rudiments. 
 
 The Third Stage of the Spinal Nerves in Pristiurus. 
 
 With the third stage the first distinct histological differen- 
 tiations of the nerve-rudiments commence. Owing to the 
 
IN ELASMOBRANCH FISHES. 183 
 
 changes both in the nerves themselves and in the connective- 
 tissue around them, which becomes less compact and its cells 
 stellate, the difficulty of distinguishing the nerves from the 
 surrounding cells vanishes ; and the difficulties of investigation 
 in the later stages are confined to the modes of attachment of 
 the nerves to the neural canal, and the histological changes 
 which take place in the rudiments themselves. 
 
 The stage may be considered to commence at the period 
 when the external gills first make their appearance as small 
 buds from the walls of the visceral clefts. Already, in the 
 earliest rudiments of the posterior root of this period now 
 figured, a number of distinct parts are visible (PI. 23, fig. H i). 
 
 Surrounding nearly the whole structure there is present a 
 delicate investment similar to that which I mentioned as sur- 
 rounding the neural canal and other organs ; it is quite struc- 
 tureless, but becomes coloured with all staining reagents. I 
 must again leave open the question whether it is to be looked 
 upon as a layer of coagulated protoplasm or as a more definite 
 structure. This investment completely surrounds the proxi- 
 mal portion of the posterior root, but vanishes near its distal 
 extremity. 
 
 The nerve-rudiment itself may be divided into three distinct 
 portions: (i) the proximal portion, in which is situated the 
 pedicle of attachment to the wall of the neural canal ; (2) an 
 enlarged portion, which may conveniently, from its future 
 fate, be called the ganglion ; (3) a distal portion beyond this. 
 The proximal portion presents a fairly uniform diameter, and 
 ends dorsally in a rounded expansion ; it is attached remark- 
 ably enough, not by its extremity, but by its side, to the spinal 
 cord. The dorsal extremities of the posterior nerves are there- 
 fore free ; as was before mentioned, they probably serve as the 
 starting-point of the longitudinal commissures between the 
 posterior roots. 
 
 The spinal cord at this stage is still made up of fairly uni- 
 form cells, which do not differ in any important particulars from 
 the cells which composed it during the last stage. The outer 
 portion of the most peripheral layer of cells has already begun to 
 be converted into the white matter. 
 
 The delicate investment spoken of before still surrounds the 
 
184 DEVELOPMENT OF THE SPINAL NERVES 
 
 whole spinal cord, except at the points of junction of the cord 
 with the nerve-rudiments. Externally to this investment, and 
 separated from it for the most part by a considerable interval, a 
 mesoblastic sheath (PL 23, fig. H I, i) for the spinal cord is 
 beginning to be formed. 
 
 The attachment of the nerve-rudiments to the spinal cord, on 
 account of its smallness, it still very difficult to observe. In 
 many specimens where the nerve is visible a small prominence 
 may be seen rising up from the spinal cord at a point cor- 
 responding to x (PI. 23, fig. H l). It is, however, rare to see 
 this prominence and the nerve continuous with each other : 
 as a rule they are separated by a slight space, and frequently 
 one of the cells of the mesoblastic investment of the spinal cord 
 is interposed between the two. In some especially favourable 
 specimens, similar to the one figured, there can be seen a dis- 
 tinct cellular prominence (fig. H I, x) from the spinal cord, 
 which becomes continuous with a small prominence on the 
 lateral border of the nerve-rudiment near its free extremity. 
 The absence of a junction between the two in a majority of 
 sections is only what might be expected, considering how minute 
 the junction is. 
 
 Owing to the presence of the commissure connecting the 
 posterior roots, some part of a nerve is present in every section. 
 
 The proximal extremity of the nerve-rudiment itself is com- 
 posed of cells, which, by their smaller size and a more circular 
 form, are easily distinguished from cells forming the ganglionic 
 portion of the nerve. 
 
 The ganglionic portion of the nerve, by its externally swollen 
 configuration, is at once recognizable in all the sections in 
 which the nerve is complete. The delicate investment before 
 mentioned is continuous around it. The cells forming it are 
 larger and more elongated than the cells forming the upper por- 
 tion of the nerve-rudiment : each of them possesses a large and 
 distinct nucleus. 
 
 The remainder of the nerve rudiment forms the commence- 
 ment of the true nerve. It can in this stage be traced only for a 
 very small distance, and gradually fades away, in such a manner 
 that its absolute termination is very difficult to observe. 
 
 The connective-tissue cells which surround the nerve-rudi- 
 
IN ELASMOBRANCH FISHES. 185 
 
 msnt are far looser than in the last stage, and are commencing 
 to throw out processes and become branched. 
 
 The anterior root-nerve has grown very considerable since 
 the last stage. It projects from the same region of the cord as 
 before, but on approaching the muscle-plate takes a sudden 
 bend downwards (fig. H II, ar). 
 
 I have failed to prove that the anterior and posterior roots 
 are at this stage united. 
 
 Fourth Stage. 
 
 In an embryo but slightly more advanced than the one last 
 described, important steps have been made in the development 
 of the nerve-rudiment. The spinal cord itself now possesses a 
 covering of white matter; this is thickest at the ventral portion 
 of the cord, and extends to the region of the posterior root of 
 the spinal nerve. 
 
 The junction of the posterior root with the spinal cord is 
 easier to observe than in the last stage. 
 
 It is still effected by means of unaltered cells, though the 
 cells which form the projection from the cord to the nerve are 
 commencing to undergo changes similar to those of the cells 
 which are being converted into white matter. 
 
 In the rudiment of the posterior root itself there are still 
 three distinct parts, though their arrangement has undergone 
 some alteration and their distinctness has become more marked 
 (PL 23, fig. I I). 
 
 The root of the nerve (fig. 1 1, pr) consists, as before, of nearly 
 circular cells, each containing a nucleus, very large in propor- 
 tion to the size of the cell. The cells have a diameter of about 
 3oW f an mcn - This mass forms not only the junction 
 between the ganglion and the spinal canal, but is also con- 
 tinued into a layer investing the outer side of the ganglion and 
 continuous with the nerve beyond the ganglion. 
 
 The cells which compose the ganglion (fig. I I, sp. g] are 
 easily distinguished from those of the root. Each cell is elon- 
 gated with an oval nucleus, large in proportion to the cell ; and 
 its protoplasm appears to be continued into an angular, not 
 to say fibrous process, sometimes at one and more rarely at 
 
1 86 DEVELOPMENT OF THE SPINAL NERVES 
 
 both ends. The processes of the cells are at this stage very 
 difficult to observe : figs. la, I b, I c represent three cells pro- 
 vided with them and placed in the positions they occupied in 
 the ganglion. 
 
 The relatively very small amount of protoplasm in com- 
 parison to the nucleus is fairly represented in these figures, 
 though not in the drawing of the ganglion as a whole. In the 
 centre of each nucleus is a nucleolus. 
 
 Fig. I b, in which the process points towards the root of 
 the nerve, I regard as a commencing nerve-fibre : its more elon- 
 gated shape seems to imply this. In the next stage special 
 bundles of nerve-fibres become very conspicuous in the gan- 
 glion. The long diameter of an average ganglion-cell is about 
 Ysm f an inch- The whole ganglion forms an oval mass, well 
 separated both from the nerve-root and the nerve, and is not 
 markedly continuous with either. On its outer side lies the 
 downward process of the nerve-root before mentioned. 
 
 The nerve itself is still, as in the last case, composed of cells 
 which are larger and more elongated than either the cells of the 
 root or the ganglion. 
 
 The condition of the anterior root at this stage is hardly 
 altered from what it was ; it is composed of very small cells, 
 which with haematoxylin stain more deeply than any other cell 
 of the section. A figure of it is given in I II. 
 
 Horizontal longitudinal sections of this stage are both easy 
 to make and very instructive. On PI. 23, fig. K I is represented 
 a horizontal section through a plane near the dorsal surface 
 of the spinal cord : each posterior root is seen in this sec- 
 tion to lie nearly opposite the anterior extremity of a muscle- 
 plate. 
 
 In a more ventral plane (fig. K 11) this relation is altered, 
 and the posterior roots lie opposite the hinder parts of the 
 muscle-plates. 
 
 The nerves themselves are invested by the hyaline mem- 
 brane spoken of above ; and surrounding this again there is 
 present a delicate mesoblastic investment of spindle-shaped cells. 
 
 Longitudinal sections also throw light upon the constitu- 
 tion of the anterior nerve roots (vide fig. K II, ar). In the two 
 segments on the left-hand side in this figure the anterior roots 
 
IN ELASMOBRANCH FISHES. 1 87 
 
 are cut through as they are proceeding, in a more or less hori- 
 zontal course, from the spinal cord to the muscle-plates. 
 
 Where the section (which is not quite horizontal) passes 
 through the plane of the notochord, as on the right-hand side, 
 the anterior roots are cut transversely. Each root, jn fact, 
 changes its direction, and takes a downward course. 
 
 The anterior roots are situated nearly opposite the middle 
 of the muscle-plates : their section is much smaller than that 
 of the posterior roots, and with haematoxylin they stain more 
 deeply than any of the other cells in the preparation. 
 
 The anterior roots, so far as I have been able to observe, do 
 not at this stage unite with the posterior ; but on this point I do 
 not speak with any confidence. 
 
 The period now arrived at forms a convenient break in the 
 development of the spinal nerves ; and I hope to treat the 
 remainder of the subject, especially the changes in the ganglion, 
 the development of the ganglion-cells, and of the nerve-fibres, 
 in a subsequent paper. 
 
 I will only add that, not long after the stage last described, 
 the posterior root unites with the anterior root at a consider- 
 able distance below the cord : this is shewn in PI. 23, fig. L. 
 Still later the portion of the roqt between the ganglion and 
 the spinal cord becomes converted into nerve-fibres, and the 
 ganglion becomes still further removed from the cord, while at 
 the same time it appears distinctly divided into two parts. 
 
 As regards the development of the cranial nerves, I have 
 made a few observations, which, though confessedly incomplete, 
 I would desire to mention here, because, imperfect as they are, 
 they seem to shew that in Elasmobranch Fishes the cranial 
 nerves resemble the spinal nerves in arising as outgrowths from 
 the central nervous system. 
 
 I have given a figure of the development of a posterior root 
 of a cranial nerve in fig. M I. The section is taken from the 
 same embryo as figs. B I, B II, and Bill. 
 
 It passes through the anterior portion of a thickening of 
 the external epiblast, which eventually becomes involuted as 
 the auditory vesicle. 
 
 The posterior root of a nerve (VII) is seen growing out from 
 the summit of the hind brain in precisely the same manner that 
 
1 88 DEVELOPMENT OF THE SPINAL NERVES 
 
 the posterior roots of the spinal nerves grow out from the spinal 
 cord : it is the rudiment of the seventh or facial nerve. The 
 section behind this (fig. M ll), still in the region of the ear, 
 has no trace of a nerve, and thus serves to shew the early dis- 
 continuity of the posterior nerve-rudiments which arise from 
 the brain. 
 
 I have as yet failed to detect any cranial anterior roots like 
 those of the spinal nerves 1 . The similarity in development be- 
 tween the cranial and spinal nerves is especially interesting, as 
 forming an important addition to the evidence which at present 
 exists that the cranial nerves are only to be looked on as 
 spinal nerves, especially modified in connexion with the changes 
 which the anterior extremity of the body has undergone in 
 existing vertebrates. 
 
 My results may be summarized as follows : 
 
 Along the extreme dorsal summit of the spinal cord there 
 arises on each side a continuous outgrowth. 
 
 From each outgrowth processes corresponding in number 
 to the muscle-plates grow downwards. These are the posterior 
 nerve-rudiments. 
 
 The outgrowths, at first attached to the spinal cord through- 
 out their whole length, soon cease to be so, and remain in con- 
 nexion with it in certain spots only, which form the junctions 
 of the posterior roots with the spinal cord. 
 
 The original outgrowth on each side remains as a bridge, 
 uniting together the dorsal extremities of all the posterior rudi- 
 ments. The points of junction of the posterior roots with the 
 spinal cord are at first situated at the extreme dorsal summit of 
 the latter, but eventually travel down, and are finally placed on 
 the sides of the cord. 
 
 After these events the posterior nerve-rudiments grow 
 rapidly in size, and become differentiated into a root (by 
 which they are attached to the spinal canal), a ganglion, and 
 a nerve. 
 
 The anterior roots, like the posterior, are outgrowths from 
 the spinal cord ; but the outgrowths to form them are from the 
 
 1 [May 18, 1876. Subsequent observations have led me to the conclusion that no 
 anterior nerve-roots are to be found- in the brain.] 
 
IN ELASMOBRANCH FISHES. 189 
 
 first discontinuous, and the points from which they originally 
 spring remain as those by which they are permanently attached 
 to the spinal cord, and do not, as in the case of the posterior 
 roots, undergo a change of position. The anterior roots arise, 
 not vertically below, but opposite the intervals between the 
 posterior roots. 
 
 The anterior roots are at first quite separate from the pos- 
 terior roots ; but soon after the differentiation of the posterior 
 rudiment into a root, ganglion, and nerve, a junction is effected 
 between each posterior nerve and the corresponding anterior 
 root. The junction is from the first at some little distance from 
 the ganglion. 
 
 Investigators have hitherto described the spinal nerves as 
 formed from part of the mesoblast of the protovertebrae. His 
 alone, so far as I know, takes a different view. 
 
 His's 1 observations lead him to the conclusion that the pos- 
 terior roots are developed as ingrowths from the external epiblast 
 into the space between the protovertebrae and the neural canal. 
 These subsequently become constricted off, unite with the neural 
 canal and form spinal nerves. 
 
 These statements, which have not been since confirmed, 
 diverge nearly to the same extent from my own results as does 
 the ordinary account of the development of these parts. 
 
 Hensen (Virchow's Archiv, Vol. XXXI. 1864) also looks upon 
 the spinal nerves as developed from the epiblast, but not as a 
 direct result of his own observations 2 . 
 
 Without attempting, for the present at least, to explain this 
 divergence, I venture to think that the facts which I have 
 just described have distinct bearings upon one or two important 
 problems. 
 
 One point of general anatomy upon which they throw con- 
 siderable light is the primitive origin of nerves. 
 
 So long as it was admitted that the spinal and cerebral nerves 
 
 1 Erste Anlage des Wirbclthier-Leibes. 
 
 2 [May 1 8, 1876. Since the above was written Hensen has succeeded in shewing 
 that in mammals the rudiments of the posterior roots arise in a manner closely re- 
 sembling that described in the present paper ; and I have myself, within the last few 
 days, made observations which incline me to believe that the same holds good for the 
 chick. My observations are as yet very incomplete.] 
 
1 90 DEVELOPMENT OF THE SPINAL NERVES 
 
 developed in the embryo independently of the central nervous 
 system, their mode of origin always presented to my mind con- 
 siderable difficulties. 
 
 It never appeared clear how it was possible for a state of 
 things to have arisen in which the central nervous system, as 
 well as the peripheral terminations of nerves, whether motor 
 or sensory, were formed independently of each other, while 
 between them a third structure was developed which, growing 
 in both directions (towards the centre and towards the peri- 
 phery), ultimately brought the two into connexion. 
 
 That such a condition could be a primive one seemed 
 scarcely possible. 
 
 Still more remarkable did it appear, on the supposition that 
 the primitive mode of formation of these parts was represented 
 in the developmental history of vertebrates, that we should find 
 similar structural elements in the central and in the peripheral 
 nervous systems. 
 
 The central nervous system arises from the epiblast, and yet 
 contains precisely similar nerve-cells and nerve-fibres to the 
 peripheral nervous system, which, if derived, as is usually stated, 
 from the mesoblast, was necessarily supposed to have a com- 
 pletely different origin from the central nervous system. 
 
 Both of these difficulties are to a great extent removed 
 by the facts of the development of these parts in Elasmo- 
 branchs. 
 
 If it be admitted that the spinal roots develop as outgrowths 
 from the central nervous system in Elasmobranch Fishes, the 
 question arises, how far can it be supposed to be possible that in 
 other vertebrates the spinal roots and ganglia develop indepen- 
 dently of the spinal cord, and only subsequently become united 
 with it. 
 
 I have already insisted that this cannot be the primary con- 
 dition ; and though I am of opinion that the origin of the 
 nerves in higher vertebrates ought to be worked over again, yet 
 I do not think it impossible that, by a secondary adaptation, the 
 nerve-roots might develop in the mesoblast 1 . 
 
 1 [May 1 8, 1876. Hensen's observations, as well as those recently made by 
 myself on the chick, render it almost certain that the nerves in all Vertebrates spring 
 from the spinal cord.] 
 
IN ELASMOBRANCH FISHES. 19 1 
 
 The presence of longitudinal commissures connecting the 
 central ends of all the posterior roots is very peculiar. The 
 commissures may possibly be looked on as outlying portions 
 of the cord, rather than as parts of the nerves. 
 
 I have not up to this time followed their history beyond a 
 somewhat early period in embryonic life, and am therefore un- 
 acquainted with their fate in the adult. 
 
 As far as I am aware, no trace of similar structures has been 
 met with in other vertebrates. 
 
 The commissures have a very strong resemblance to those 
 by which in Elasmobranch Fishes the glossopharyngeal nerve 
 and the branches of the pneumogastric are united in an early 
 embryonic stage 1 . 
 
 I think it not impossible that the commissures in the two 
 cases represent the same structures. If this is the case, it would 
 seem that the junction of a number of nerves to form the pneu- 
 mogastric is not a secondary state, but the remnant of a primary 
 one, in which all the spinal nerves were united, as they embryo- 
 nically are in Elasmobranchs. 
 
 One point brought out in my investigations appears to me 
 to have bearings upon the origin of the central canal of the 
 Vertebrate nervous system, and in consequence upon the origin 
 of the Vertebrate group itself. 
 
 The point I allude to is the posterior nerve-rudiments 
 making their first appearance at the extreme dorsal summit of 
 the spinal cord. 
 
 The transverse section of the ventral nervous cord of an ordi- 
 nary segmented worm consists of two symmetrical halves placed 
 side by side. 
 
 If by a mechanical folding the two lateral halves of the 
 nervous cord became bent towards each other, while into the 
 groove formed between the two the external skin became pushed, 
 we should have an approximation to the Vertebrate spinal cord. 
 Such a folding might take place to give extra rigidity to the 
 body in the absence of a vertebral column. 
 
 If this folding were then completed in such a way that 
 the groove, lined by external skin and situated between the 
 
 1 Balfour, "A Preliminary Account of the Development of Elasmobranch Fishes," 
 Q. y. Micros. Sc. 1874, plate xv. fig. 14, r.g. [This edition, PI. 4, fig. 14, v.g.}. 
 
1 92 DEVELOPMENT OF THE SPINAL NERVES 
 
 two lateral columns of the nervous system, became converted 
 into a canal, above and below which the two columns of the 
 nervous system united, we should have in the transformed 
 nervous cord an organ strongly resembling the spinal cord of 
 Vertebrates. 
 
 This resemblance would even extend beyond mere external 
 form. Let the ventral nervous cord of the common earthworm, 
 Lumbricus agricola, be used for comparison 1 , a transverse sec- 
 tion of which is represented by Leydig 2 and Claparede. In this 
 we find that on the ventral surface (the Annelidan ventral 
 surface) of the nervous cord the ganglion-cells (grey matter) (/) 
 are situated, and on the dorsal side the nerve-fibres or white 
 matter (//). If the folding that I have supposed were to take 
 place, the grey and white matters would have very nearly the 
 relative situations which they have in the Vertebrate spinal cord. 
 
 The grey matter would be situated in the interior and 
 surround the epithelium of the central canal, and the white 
 matter would nearly surround the grey and form the anterior 
 white commissure. The nerves would then arise, not from the 
 sides of the nervous cord as in existing Vertebrates, but from 
 its extreme ventral summit. 
 
 One of the most striking features which I have brought to 
 light with reference to the development of the posterior roots, is 
 the fact of their growing out from the extreme dorsal summit of 
 the neural canal a position analogous to the ventral summit of 
 the Annelidan nervous cord. Thus the posterior roots of the 
 nerves in Elasmobranchs arise in the exact manner which 
 might have been anticipated were the spinal cord due to such a 
 folding as I have suggested. The argument from the nerves 
 becomes the stronger, from the great peculiarity in the position 
 of the outgrowth, a feature which would be most perplexing 
 without some such explanation as I have proposed. The central 
 epithelium of the neural canal according to this view represents 
 the external skin ; and its ciliation is to be explained as a rem- 
 nant of the ciliation of the external skin now found amongst 
 many of the lower Annelids. 
 
 1 The nervous cords of other Annelids resemble that of Lumbricus in the relations 
 of the ganglion-cells of the nerve-fibres. 
 
 2 Tafeln zur vcrgleichenden Anatomic, Taf. iii. fig. 8. 
 
IN ELASMOBRANCI1 FISHKS. 
 
 I have, however, employed the comparison of the Vertebrate 
 and Annelidan nervous cords, not so much to prove a genetic 
 relation between the two as to shew the a priori possibility of 
 the formation of a spinal canal and the a posteriori evidence we 
 have of the Vertebrate spinal canal having been formed_in_the 
 way indicated. 
 
 I have not made use of what is really the strongest argument 
 for my view, viz. that the embryonic mode of formation of the 
 spinal canal, by a folding in of the external epiblast, is the very 
 method by which I have supposed the spinal canal to have been 
 formed in the ancestors of Vertebrates. 
 
 My object has been to suggest a meaning for the peculiar 
 primitive position of the posterior roots, rather than to attempt 
 to explain in full the origin of the spinal canal. 
 
 EXPLANATION OF THE PLATES 1 . 
 
 PLATE it. 
 
 Fig. A. Section through the dorsal region of an embryo of Scyllium stellare, with 
 the rudiments of two visceral clefts. The section illustrates the general features at a 
 period anterior to the appearance of the posterior nerve-roots. 
 
 nc. neural canal, mp. muscle-plate, ch. notochord. x, subnotochordal rod. 
 ao. rudiment of dorsal aorta, so. somatopleure. sp. splanchnopleure. al. alimentary 
 tract. All the parts of the section except the spinal cord are drawn somewhat 
 diagrammatically. 
 
 Figs. B I, B II, B in. Three sections of a Pristiurus-embryo. B I is through 
 the heart, B II through the anterior part of the dorsal region, and B III through 
 a point slightly behind this. Drawn with a camera. (Zeiss CC ocul. i. ) 
 
 In B in there is visible a slight proliferation of cells from the dorsal summit of the 
 neural canal. 
 
 In B II this proliferation definitely constitutes two club-shaped masses of cells (pr), 
 both attached to the dorsal summit of the neural canal. The masses are the rudi- 
 ments of the posterior nerve-roots. 
 
 In B I the rudiments of the posterior roots are of considerable length. 
 
 1 The figures on these Plates give a fair general idea of the appearance presented 
 by the developing spinal nerves ; but the finer details of the original drawings have in 
 several cases become lost in the process of copying. 
 
 The figures which are tinted represent sections of embryos hardened in osmic 
 acid ; those without colour sections of embryos hardened in chromic acid. 
 
 B. I 3 
 
194 DEVELOPMENT OF THE SPINAL NERVES 
 
 pr. rudiment of posterior roots, nc. neural canal, mp. muscle-plate, ch. noto- 
 chord. x. subnotochordal rod. ao. dorsal aorta, so. somatopleure. sp. splanchno- 
 pleure. al. alimentary canal, ht. heart. 
 
 Fig. C. Section from a Pristiurus-embryo, slightly older than B. Camera. 
 (Zeiss CC ocul. 2.) The embryo from which this figure was taken was slightly 
 distorted in the process of removal from the blastoderm. 
 
 vr. rudiment of vertebral body. Other reference letters as in previous figures. 
 
 Fig. D a. Section through the dorsal region of a Torpedo-embryo with three 
 visceral clefts. (Zeiss CC ocul. 2.) The section shews the formation of the dorsal 
 nerve-rudiments (pr) and of a ventral anterior nerve-rudiment (ar), which at this early 
 stage is not distinctly cellular. 
 
 ar. rudiment of an anterior nerve-root, y. cells left behind on the separation of 
 the external skin from the spinal cord. c. connective-tissue cells springing from the 
 summit of the muscle-plates. Other reference letters as above. 
 
 Fig. D b. Section from dorsal region of a Torpedo -embryo somewhat older than 
 Da. Camera. (Zeiss CC ocul. 2.) The posterior nerve-rudiment is considerably 
 longer than in fig. D a, and its pedicle of attachment to the spinal cord is thinner. 
 The anterior nerve-rudiment, of which only the edge is present in the section, is 
 distinctly cellular. 
 
 m. mesoblast growing up from vertebral rudiment, sd. segmental duct. 
 
 Fig. D c. Section from a still older Torpedo-embryo. Camera. (Zeiss CC 
 ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (g) 
 on the posterior root has appeared. The rudiment of the posterior nerve is much 
 longer than before, and its junction with the spinal cord is difficult to detect. The 
 anterior root is now an elongated cellular structure. 
 
 g. ganglion. 
 
 Fig. D d. Longitudinal and vertical section through a Torpedo-embryo of the 
 same age as D c. 
 
 The section shews the commissures (x) uniting the posterior roots. 
 
 Fig. E a. Section of a Pristiurus-embryo belonging to the second stage. Camera. 
 (Zeiss CC ocul. 2.) The section shews the constriction of the pedicle which attaches 
 the posterior nerve-rudiments to the spinal cord. 
 
 pr. rudiment of posterior nerve-root, nc. neural canal, mp. muscle-plate, vr. 
 vertebral rudiment, sd. segmental duct. ch. notochord. so. somatopleure. sp. 
 splanchnopleure. ao. aorta, al. alimentary canal. 
 
 Fig. E b. Section of a Pristiurus-embryo slightly older than E a. Camera. 
 (Zeiss CC ocul. 2.) The section shews the formation of the anterior nerve-root (ar). 
 ar. rudiment of the anterior nerve-root. 
 
 Fig. F. Section of a Pristiurus-embryo with the rudiments of five visceral clefts. 
 Camera. (Zeiss CC ocul. 2.) 
 
 The rudiment of the posterior root is seen surrounded by connective-tissue, from 
 which it cannot easily be distinguished. The artist has not been very successful in 
 rendering this figure. 
 
IN KI.ASMoIiRANCH F1SHF.S. 195 
 
 Figs. G i, G i, 03. Three longitudinal and horizontal sections of an embryo some- 
 what older than F. The embryo from which these sections were taken was hardened 
 in osmic acid, but the sections have been represented without tinting. G i is most 
 dorsal of the three sections. Camera. (Zeiss CC ocul. i.) 
 
 nc. neural canal, sp.c. spinal cord. pr. rudiment of posterior root. ar. rudiment 
 of anterior root. nip. muscle- plate, t. connective-tissue cells, ch. notochord. 
 
 PLATE 23. 
 
 Fig. H I. Section through the dorsal region of a Pristiurus-embryo in which the 
 rudimentary external gills are present as very small knobs. Camera. (Zeiss CC 
 ocul. 2.) 
 
 The section shews the commencing differentiation of the posterior nerve-rudiment 
 into root (pr), ganglion (sfl.g), and nerve (), and also the attachment of the nerve- 
 root to the spinal cord (x). The variations in the size and shape of the cells in the 
 different parts of the nerve-rudiment are completely lost in the figure. 
 
 pr. posterior nerve-root, sp.g. ganglion of posterior root. n. nerve of posterior 
 root. x. attachment of posterior root to spinal cord. w. white matter of spinal cord. 
 i. mesoblastic investment to the spinal cord. 
 
 Fig. H II. Section through the same embryo as H I. (Zeiss CC ocul. i.) 
 The section contains an anterior root, which takes its origin at a point opposite 
 the interval between two posterior roots. 
 
 The white matter has not been very satisfactorily represented by the artist. 
 
 Figs. I i, I n. Two sections of a Fristiitrus-embryo somewhat older than H. 
 Camera. (Zeiss CC ocul. i.) 
 
 The connective-tissue cells are omitted. 
 
 Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the posterior 
 roots of the same embryo. 
 
 Figs. K i, K n. Two horizontal longitudinal sections through an embryo in 
 which the external gills have just appeared. K I is the most dorsal of the two 
 sections. Camera. (Zeiss CC ocul. i.) 
 
 The sections shew the relative positions of the anterior and posterior roots at 
 different levels. 
 
 pr. posterior nerve-rudiment, ar. anterior nerve- rudiment, sp.c. spinal cord. 
 n.c. neural canal, nip. muscle-plate, nip '. first-formed muscles. 
 
 Fig. L. Longitudinal and vertical section through the trunk of a Scyllium-emhryo 
 after the external gills have attained their full development. . Camera. (Zeiss CC 
 ocul. i.) 
 
 The embryo was hardened in a mixture of chromic acid and osmic acid. 
 
 The section shews the commissures which dorsally unite the posterior roots, and 
 also the junction of the anterior and posterior roots. The commissures are unfortu- 
 nately not represented in the figure with great accuracy ; their outlines are in nature 
 perfectly regular, and not, as in the figure, notched at the junctions of the cells 
 composing them. Their cells are apparently more or less completely fused, and 
 certainly not nearly so clearly marked as in the figure. The commissures stain very 
 deeply with the mixture of osmic and chromic acid, and form one of the most con- 
 
 132 
 
196 DEVELOPMENT OF THE SPINAL NERVES, &C. 
 
 spicuous features in successful longitudinal sections of embryos so hardened. In 
 sections hardened with chromic acid only they cannot be seen with the same facility. 
 
 sp. c. spinal cord. gr. grey matter, w. white matter, ar, anterior root. pr. 
 posterior root. x. commissure uniting the posterior roots. 
 
 Figs. Mi, M II. Two sections through the head of the same embryo as fig. B. 
 M I, the foremost of the two, passes through the anterior part of the thickening of 
 epiblast, which becomes involuted as the auditory vesicle. It contains the rudiment 
 of the seventh nerve, VII. Camera. (Zeiss CC ocul. 2.) 
 
 VII. rudiment of seventh nerve. au. thickening of external epiblast, which 
 becomes involuted as the auditory vesicle, n. c. neural canal, ch. notochord. pp. 
 body-cavity in the head. so. somatopleure. sp. splanchnopleure. al. throat ex- 
 hibiting an outgrowth to form the first visceral cleft. 
 
IX. ON THE SPINAL NERVES OF AMPHIOXUS I . 
 
 DURING a short visit to Naples in January last, I was enabled, 
 through the kindness of Dr Dohrn, to make some observations 
 on the spinal nerves of Amphioxus. These were commenced 
 solely with the view of confirming the statements of Stieda on 
 the anatomy of the spinal nerves, which, if correct, appeared to 
 me to be of interest in connection with the observations I had 
 made that, in Elasmobranchs, the anterior and posterior roots 
 arise alternately and not in the same vertical plane. I have 
 ^ been led to conclusions on many points entirely opposed to those 
 of Stieda, but, before recording these, I shall proceed briefly to 
 state his results, and to examine how far they have been cor- 
 roborated by subsequent observers. 
 
 Stieda 2 , from an examination of sections and isolated spinal 
 cords, has been led to the conclusion that, in Amphioxus, the 
 nerves of the opposite sides arise alternately, except in the most 
 anterior part of the body, where they arise opposite each other. 
 He also states that the nerves of the same side issue alter- 
 nately from the dorsal and ventral corners of the spinal cord. 
 He regards two of these roots (dorsal and ventral) on the same 
 side as together equivalent to a single spinal nerve of higher 
 vertebrates formed by the coalescence of a dorsal and ventral 
 root. 
 
 Langerhans 3 apparently agrees with Stieda as to the facts 
 about the alternation of dorsal and ventral roots, but differs 
 
 1 From the Journal of Anatomy and Physiology, Vol. X. 1876. 
 - Mem. Acad. Pctersbour^, Vol. XIX. 
 3 Archiv f, mikr. Anatomic, Vol. xii. 
 
198 THE SPINAL NERVES OF AMPHIOXUS. 
 
 from him as to the conclusions to be drawn from those facts. 
 He does not, for two reasons, believe that two nerves of Amphi- 
 oxus can be equivalent to a single nerve in higher vertebrates : 
 (i) Because he finds no connecting branch between two suc- 
 ceeding nerves, and no trace of an anastomosis. (2) Because 
 he finds that each nerve in Amphioxus supplies a complete 
 myotome, and he considers it inadmissible to regard the nerves, 
 which in Amphioxus together supply two myotomes, as equiva- 
 lent to those which in higher vertebrates supply a single myo- 
 tome only. 
 
 Although the agreement as to facts between Langerhans 
 and Stieda is apparently a complete one, yet a critical exami- 
 nation of the statements of these two authors proves that their 
 results, on one important point at least, are absolutely contra- 
 dictory. Stieda, PL III. fig. 19, represents a longitudinal and 
 horizontal section through the spinal cord which exhibits the 
 nerves arising alternately on the two sides, and represents each 
 myotome supplied by one nerve. In his explanation of the 
 figure he expressly states that the nerves of one plane only (i.e. 
 only those with dorsal or only those with ventral roots) are 
 represented ; so that if all the nerves which issue from the* 
 spinal cord had been represented double the number figured 
 must have been present. But since each myotome is sup- 
 plied by one nerve in the figure, if all the nerves present 
 were represented, each myotome would be supplied by two 
 nerves. 
 
 Since Langerhans most emphatically states that only one 
 nerve is present for each myotome, it necessarily follows that 
 he or Stieda has made an important error ; and it is not too 
 much to say that this error is more than sufficient to counter- 
 balance the value of Langerhans' evidence as a confirmation of 
 Stieda's statements. 
 
 I commenced my investigations by completely isolating 
 the nervous system of Amphioxus by maceration in nitric acid 
 according to the method recommended by Langerhans 1 . On 
 examining specimens so obtained it appeared that, for the 
 greater length of the cord, the nerves arose alternately on the 
 
 1 Lot. at. 
 
THE SPINAL NERVES OF AMPHIOXUS. IQ9 
 
 two sides, as was first stated by Owsjannikow, and subsequently 
 by Stieda and Langerhans ; but to my surprise not a trace 
 could be seen of a difference of level in the origin of the nerves 
 of the same side. 
 
 The more carefully the specimens were examined from all 
 points of view, the more certainly was the conclusion forced 
 upon me, that nerves issuing from the ventral corner of the 
 spinal cord, as described by Stieda, had no existence. 
 
 Not satisfied by this examination, I also tested the point by 
 means of sections. I carefully made transverse sections of a 
 successfully hardened Amphioxus, through the whole length of 
 the body. There was no difficulty in seeing the dorsal roots in 
 every third section or so, but not a trace of a ventral root was to 
 be seen. There can, I think, be no doubt, that, had ventral 
 roots been present, they must, in some cases at least, have been 
 visible in my sections. 
 
 In dealing with questions of this kind it is no doubt difficult 
 to prove a negative; but, since the two methods of investiga- 
 tion employed by me both lead to the same result, I am able to 
 state with considerable confidence that my observations lend no 
 support to the view that the alternate spinal nerves of Amphi- 
 oxus have their roots attached to the ventral corner of the 
 spinal cord. 
 
 How a mistake on this point arose it is not easy to say. 
 All who have worked with Amphioxus must be aware how diffi- 
 cult it is to conserve the animal in a satisfactory state for 
 making sections. The spinal cord, especially, is apt to be 
 distorted in shape, and one of its ventral corners is frequently 
 produced into a horn-like projection terminating in close con- 
 tact with the sheath. In such cases the connective tissue 
 fibres of the sheath frequently present the appearance of a 
 nerve-like prolongation of the cord ; and for such they might 
 be mistaken if the sections were examined in a superficial 
 manner. It is not, however, easy to believe that, with well 
 conserved specimens, a mistake could be made on this point 
 by so careful and able an investigator as Stieda, especially 
 considering that the histological structure of the spinal nerves 
 is very different from that of the fibrous prolongations of the 
 sheath of the spinal cord. 
 
2OO THE SPINAL NERVES OF AMPHIOXUS. 
 
 It only remains for me to suppose that the specimens which 
 Stieda had at his disposal, were so shrunk as to render the 
 origin of the nerves very difficult to determine. 
 
 The arrangement of the nerves of Amphioxus, according 
 to my own observations, is as follows. 
 
 The anterior end of the central nervous system presents 
 on its left and dorsal side a small pointed projection, into 
 which is prolonged a diverticulum from the dilated anterior ven- 
 tricle of the brain. This may perhaps be called the olfactory 
 nerve, though clearly of a different character to the other nerves. 
 It was first accurately described by Langerhans l . 
 
 Vertically below the olfactory nerve there arise two nerves, 
 which issue at the same level from the ventral side of the. 
 anterior extremity of the central nervous system. These form 
 the first pair of nerves, and are the only pair which arise from 
 the ventral portion of the cerebro-spinal cord. The two nerves, 
 which form the second pair, arise also opposite each other 
 but from the dorsal side of the cord. The first and second 
 pair of nerves have both been accurately drawn and described 
 by Langerhans : they, together with the olfactory nerve, can 
 easily be seen in nervous systems which have been isolated by 
 maceration. 
 
 In the case of the third pair of nerves, the nerve on the 
 right-hand side is situated not quite opposite but slightly be- 
 hind that on the left. The right nerve of the fourth pair is 
 situated still more behind the left, and, in the case of the 
 fifth pair, the nerve to the right is situated so far behind the 
 left nerve that it occupies a position half-way between the 
 left nerves of the fifth and sixth pairs. In all succeeding nerves 
 the same arrangement holds good, so that they exactly alternate 
 on two sides. 
 
 Such is the arrangement carefully determined by me from 
 one specimen. It is possible that it may not be absolutely con- 
 stant, but the following general statement almost certainly 
 holds good. 
 
 All the nerves of Amphioxus, except the first pair, have 
 their roots inserted in the dorsal part of the cord. In the case of 
 
 <. cil. 
 
THE SPINAL NERVES OF AMPHIOXUS. 2OI 
 
 the first two pairs the nerves of the two sides arise opposite 
 each other ; in the next few pairs, the nerves on the right-hand 
 side gradually shift backwards : the remaining nerves spring 
 alternately from the two sides of the cord. 
 
 For each myotome there is a single nerve, which enters, as 
 in the case of other fishes, the intermuscular septum. This 
 point may easily be determined by means of longitudinal 
 sections, or less easily from an examination of macerated 
 specimens. I agree with Langerhans in denying the existence 
 of ganglia on the roots of the nerves. 
 
X. 
 
 A MONOGRAPH 
 
 ON THE 
 
 DEVELOPMENT OF 
 ELASMOBRANCH FISHES. 
 
 PUBLISHED 1878. 
 
PREFACE. 
 
 THE present Monograph is a reprint of a series of papers 
 published in the Journal of Anatomy and Physiology during the 
 -years 1876, 1877 and 1878. The successive parts were struck 
 off as they appeared, so that the earlier pages of the work were 
 in print fully two years ago. I trust the reader will find in this 
 fact a sufficient excuse for a certain want of coherence, which is 
 I fear observable, as well as for the omission of references to 
 several recent publications. The first and second chapters 
 would not have appeared in their present form had I been 
 acquainted, at the time of writing them, with the researches 
 which have since been published, on the behaviour of the ger- 
 minal vesicle and on the division of nuclei. I may also call 
 attention to the valuable papers of Prof. His 1 on the formation 
 of the layers in Elasmobranchs, and of Prof. Kowalevsky 2 on 
 the development of Amphioxus, to both of which I would 
 certainly have referred, had it been possible for me to do so. 
 
 Professor His deals mainly with the subjects treated of in 
 Chapter III,, and gives a description very similar to my own of 
 the early stages of development. His interpretations of the 
 observed changes are, however, very different from those at 
 which I have arrived. Although this is not the place for a 
 discussion of Prof. His's views, I may perhaps state that, in 
 spite of the arguments he has brought forward in support of his 
 position, I am still inclined to maintain the accuracy of my 
 original account. The very striking paper on Amphioxus by 
 Kowalevsky (the substance of which I understand to have 
 been published in Russia at an earlier period) contains a con- 
 firmation of the views expressed in chapter VI. on the develop- 
 
 1 Zeitschrift f. Anat. u. Entwicklungsgeschichte, Bd. II. 
 
 2 Archivf. Micr. Anat. Bd. xm. 
 
206 PREFACE. 
 
 ment of the mesoblast, and must be regarded as affording a 
 conclusive demonstration, that in the case of Vertebrata the 
 mesoblast has primitively the form of a pair of diverticula from 
 the walls of the archenteron. 
 
 The present Memoir, while differing essentially in scope and 
 object from the two important treatises by Professors His 1 and 
 Gotte 2 , which have recently appeared in Germany, has this 
 much in common with them, that it deals monographically with 
 the development of a single type : but here the resemblance 
 ends. Both of these authors seek to establish, by a careful 
 investigation of the development of a single species, the general 
 plan of development of Vertebrates in general, if not of the 
 whole animal kingdom. Both reject the theory of descent, as 
 propounded by Mr Darwin, and offer completely fresh explana- 
 tions of the phenomena of Embryology. Accepting, as I do, 
 the principle of natural selection, I have had before me, in 
 writing the Monograph, no such ambitious aim as the establish- 
 ment of a completely new system of Morphology. My object 
 will have been fully attained if I have succeeded in adding a 
 few stones to the edifice, the foundations of which were laid by 
 Mr Darwin in his work on the Origin of Species. 
 
 I may perhaps call attention to one or two special points in 
 this work which seem to give promise of further results. The 
 chapter on the Development of the Spinal and Cranial Nerves 
 contains a modification of the previously accepted views on this 
 subject, which may perhaps lead to a more satisfactory con- 
 ception of the origin of nerves than has before been possible, 
 and a more accurate account of the origin of the muscle-plates 
 and vertebral column. The attempt to employ the embryo- 
 logical relations of the cephalic prolongations of the body-cavity, 
 and of the cranial nerves, in the solution of the difficult problems 
 of the Morphology of the head, may prove of use in the line of 
 study so successfully cultivated by our great English Anatomist, 
 Professor Huxley. Lastly, I venture to hope that my con- 
 clusions in reference to the relations of the sympathetic system 
 and the suprarenal body, and to the development of the meso- 
 
 1 Erste A nlage des \Virbclthierldbes. 
 
 2 Enhvicklungsgeschichte dcr Unkc. 
 
PREFACE. 207 
 
 blast, the notochord, the limbs, the heart, the venous system, 
 and the excretory organs, are not unworthy of the attention of 
 Morphologists. 
 
 The masterly manner in which the systematic position of 
 Elasmobranchs is discussed by Professor Gegenbaur, in the 
 introduction to his Monograph on the Cranial Skeleton of the 
 group, relieves me from the necessity of entering upon this 
 complicated question. It is sufficient for my purpose that the 
 Elasmobranch Fishes be regarded as forming one of the most 
 primitive groups among Vertebrates, a view which finds ample 
 confirmation in the importance of the results to which Prof. 
 Gegenbaur and his pupils have been led in this branch of their 
 investigations. 
 
 Though I trust that the necessary references to previous 
 contributions in the same department of enquiry have not been 
 omitted, the 'literature of the subject' will nevertheless be found 
 to occupy a far smaller share of space than is usual in works of 
 a similar character. This is an intentional protest on my part 
 against, what appears to me, the unreasonable amount of space 
 so frequently occupied in this way. The pages devoted to the 
 ' previous literature ' only weary the reader, who is not wise 
 enough to skip them, and involve a great and useless expen- 
 diture of time on the part of any writer, who is capable of some- 
 thing better than the compilation of abstracts. 
 
 In conclusion, my best thanks are due to Drs Dohrn and 
 Eisig for the uniformly kind manner in which they have for- 
 warded my researches both at the Zoological Station in Naples, 
 and after my return to England ; and also to Mr Henry Lee 
 and to the Manager and Directors of the Brighton Aquarium, 
 who have always been ready to respond to my numerous de- 
 mands on their liberality. 
 
 To my friend and former teacher Dr Michael Foster I 
 tender my sincerest thanks for the neverfailing advice and 
 assistance which he has given throughout the whole course of 
 the work. 
 
TABLE OF CONTENTS. 
 
 CHAPTER I. 
 
 THE RIPE OVARIAN OVUM, pp. 213 221. 
 
 Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its 
 membrane and absorption of its contents. Oellacher's observations on the germinal 
 vesicle. Gotte's observations. Kleinenberg's observations. General conclusions 
 on the fate of the germinal vesicle. Germinal disc. 
 
 CHAPTER II. 
 
 THE SEGMENTATION, pp. 222 245. 
 
 Appearance of impregnated germinal disc. Stage with two furrows. Stage 
 with twenty-one segments. Structure of the sides of the furrows. Later stages of 
 segmentation. Spindle-shaped nuclei. Their presence outside the blastoderm. 
 Knobbed nuclei. Division of nuclei. Conclusion of segmentation. Nuclei of the 
 yolk. Asymmetry of the segmented blastoderm. Comparison of Elasmobranch 
 segmentation with that of other meroblastic ova. Literature of Elasmobranch seg- 
 mentation. 
 
 CHAPTER III. 
 
 FORMATION OF THE LAYERS, pp. 246 285. 
 
 Division of blastoderm into two layers. Formation of segmentation cavity. 
 Disappearance of cells from floor of segmentation cavity. Nuclei of yolk and of 
 blastoderm. Formation of embryonic rim. Appearance of a layer of cells on the 
 floor of the segmentation cavity. Formation of mesoblast. Formation of medullary 
 groove. Disappearance of segmentation cavity. Comparison of segmentation cavity 
 of Elasmobranchs with that of other types. Alimentary cavity. Formation of 
 mesoblast in two lateral plates. Protoplasmic network of yolk. Summary. Nature 
 of meroblastic ova. Comparison of Elasmobranch development with that of other 
 types. Its relation to the Gastrula. Haeckel's views on vertebrate Gastrula. Their 
 untenable nature. Comparison of primitive streak with blastopore. Literature. 
 
 CHAPTER IV. 
 
 GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE 
 STAGES, pp. 286 297. 
 
 Description of Stages A Q. Enclosure of yolk by blastoderm. Relation of the 
 anus of Rusconi to the blastopore. 
 
 B. 14 
 
210 TABLE OF CONTENTS. 
 
 CHAPTER V. 
 
 STAGES B G, pp. 298 314. 
 
 General features of the epiblast. Original uniform constitution. Separation into 
 lateral and central portions. The medullary groove. Its conversion into the me- 
 dullary canal. The mesoblast. Its division into somatic and splanchnic layers. 
 Formation of protovertebre. The lateral plates. The caudal swellings. The 
 formation of the body-cavity in the head. The alimentary canal. Its primitive 
 constitution. The anus of Rusconi. Floor formed by yolk. Formation of cellular 
 floor from cells formed around nuclei of the yolk. Communication behind of neural 
 and alimentary canals. Its discovery by Kowalevsky. Its occurrence in other 
 instances. General features of the hypoblast. The notochord. Its formation as a 
 median thickening of the hypoblast. Possible interpretations to be put on this. 
 Its occurrence in other instances. 
 
 CHAPTER VI. 
 
 DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. 315 360. 
 
 Order of treatment. External epiblast. Characters of epiblast. Its late division 
 into horny and epidermic layers. Comparison of with Amphibian epiblast. The 
 unpaired fins. The paired fins. Their formation as lateral ridges of epiblast. 
 Hypothesis that the limbs are remnants of continuous lateral fins. Mesoblast. Con- 
 stitution of lateral plates of mesoblast. Their splanchnic and somatic layers. 
 Body-cavity constituting space between them. Their division into lateral and ver- 
 tebral plates. Continuation of body-cavity into vertebral plates. Protovertebrae. 
 Division into muscle-plates and vertebral bodies. Development of muscle-plates. 
 Disappearance of segmentation in tissue to form vertebral bodies. Body-cavity 
 and parietal plates. Primitive independent halves of body-cavity. Their ventral 
 fusion. Separation of anterior part of body-cavity as pericardial cavity. Com- 
 munication of pericardial and peritoneal cavities. Somatopleure and splanchnopleure. 
 Resume. General considerations on development of mesoblast. Probability of 
 lateral plates of mesoblast in Elasmobranchs representing alimentary diverticula. 
 Meaning of secondary segmentation of vertebral column. The urinogenital system. 
 Development of segmental duct and segmental tubes as solid bodies. Formation of a 
 lumen in them, and their opening into body-cavity. Comparison of segmental duct 
 and segmental tubes. Primitive ova. Their position. Their structure. The noto- 
 chord. The formation of its sheath. The changes in its cells. 
 
 CHAPTER VII. 
 
 GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE 
 CLOSE OF EMBRYONIC LIFE, pp. 361 377. 
 
 External epiblast. Division into separate layers. Placoid scales. Formation 
 of their enamel. Lateral line. Previous investigations. Distinctness of lateral line 
 and lateral nerve. Lateral nerve a branch of vagus. Lateral line a thickening of 
 epiblast. Its greater width behind. Its conversion into a canal by, its cells assuming 
 a tubular arrangement. The formation of its segmental apertures. Mucous canals 
 of the head. Their nerve-supply. Reasons for dissenting from Semper's and Gotte's 
 view of lateral nerve. Muscle-plates. Their growth. Conversion of both layers into 
 
TABLE OF CONTENTS. 211 
 
 muscles. Division into dorso-lateral and ventro-lateral sections. Derivation of limb- 
 muscles from muscle-plates. Vertebral column and notochord. Previous investi- 
 gations. Formation of arches. Formation of cartilaginous sheath of notochord and 
 membrana elastica externa. Differentiation of neural arches. Differentiation of 
 hrcmal arches. Segmentation of cartilaginous sheath of notochord. Vertebral and 
 intervertebral regions. Notochord. 
 
 CHAPTER VIII. 
 
 DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC 
 NERVOUS SYSTEM, pp. 378 396. 
 
 The spinal nerves. Formation of posterior roots. Later formation of anterior 
 roots. Development of commissure uniting posterior roots. Subsequent develop- 
 ment of posterior roots. Their change in position. Development of ganglion. 
 Further changes in anterior roots. Junction of anterior and posterior roots. Summary. 
 General considerations. Origin of nerves. Hypothesis explaining peripheral growth. 
 Hensen's views. Later investigations. Gotte. Calberla. Relations between 
 Annelidan and Vertebrate nervous systems. Spinal canal. Dr Dohrn's views. 
 Their difficulties. Hypothesis of dorsal coalescence of lateral nerve cords. Sympa- 
 thetic nervous system. Development of sympathetic ganglia on branches of spinal 
 nerves. Formation of sympathetic commissure. 
 
 CHAPTER IX. 
 
 DEVELOPMENT OF THE ORGANS IN THE HEAD, pp. 397 445. 
 
 DEVELOPMENT OF THE BRAIN, pp. 397 407. General history. Fore-brain. 
 Optic vesicles. Infundibulum. Pineal gland. Olfactory lobes. Lateral ventricles. 
 Mid-brain. Hind-brain. Cerebellum. Medulla. Previous investigations. Huxley. 
 Miklucho-Maclay. Wilder. ORGANS OF SENSE, pp. 407 412. Olfactory organ. 
 Olfactory pit. Schneiderian folds. Eye. General development. Hyaloid mem- 
 brane. Lens capsule. Processus falciformis. Auditory organs. Auditory pit. 
 Semicircular canals. MOUTH INVOLUTION and PITUITARY BODY, pp. 412 414. 
 Outgrowth of pituitary involution. Separation of pituitary sack. Junction with 
 infundibulum. DEVELOPMENT OF CRANIAL NERVES, pp. 414 428. Early devel- 
 opment of 5th, 7th, 8th, Qth and roth cranial nerves. Distribution of the nerves in the 
 adult. The fifth nerve. Its division into ophthalmic and mandibular branches. 
 Later formation of superior maxillary branch. Seventh and auditory nerves. Separa- 
 tion of single rudiment into seventh and auditory. Forking of seventh nerve over 
 hyomandibular cleft. Formation of anterior branch to form ramus opthalmicus super- 
 ficialis of adult. General view of morphology of branches of seventh nerve. Glosso- 
 pharyngeal and vagus nerves. General distribution at stage L. Their connection 
 by a commissure. Junction of the commissure with commissure connecting posterior 
 roots of spinal nerves. Absence of anterior roots. Hypoglossal nerve. MESOBLAST 
 OF HEAD, pp. 429 432. Body-cavity and my otomes of head. Continuation of body- 
 cavity into head. Its division into segments. Development of muscles from their 
 walls. General mesoblast of head. NOTOCHORD IN HEAD, p. 433. HYPOBLAST 
 OF THE HEAD, pp. 433434. The formation of the gill-slits. Layer from which 
 gills are derived. SEGMENTATION OF THE HEAD, pp. 434 440. Indication of 
 segmentation afforded by (i) cranial nerves, (2) visceral clefts, (3) head-cavities. 
 Comparison of results obtained. 
 
212 TABLE OF CONTENTS. 
 
 CHAPTER X. 
 
 THE ALIMENTARY CANAL, pp. 446 459. 
 
 The solid (esophagus. OZsophagus originally hollow. Becomes solid during 
 Stage K. The postanal section of the alimentary tract. Continuity of neural and 
 alimentary canals. Its discovery by Kowalevsky. The postanal section of gut. Its 
 history in Scyllium. Its disappearance. The cloaca and anus. The formation of the 
 cloaca. Its junction with segmental ducts. Abdominal pockets. Anus. The 
 thyroid body. Its formation in region of mandibular arch. It becomes solid. Pre- 
 vious investigations. The pancreas. Arises as diverticulum from dorsal side of 
 duodenum. Its further growth. Formation of duct. The liver. Arises as ventral 
 diverticulum of duodenum. Hepatic cylinders. Comparison with other types. The 
 subnotochordal rod. Its separation from dorsal wall of alimentary canal. The 
 section of it in the trunk. In the head. Its disappearance. Views as to its 
 meaning. 
 
 CHAPTER XL 
 
 THE VASCULAR SYSTEM AND VASCULAR GLANDS, pp. 460 478. 
 
 The heart. Its development. Comparison with other types. Meaning of 
 double formation of heart. The general circulation. The venous system. The 
 primitive condition of. Comparison of, with Amphioxus and Annelids. The cardinal 
 veins. Relations of caudal veiri. The circulation of the yolk-sack. Previous obser- 
 vations. Various stages. Difference of type in amniotic Vertebrates. The vascular 
 glands. Supra-renal and inter-renal bodies. Previous investigations. The supra- 
 renal bodies. Their structure in the adult. Their development from the sympathetic 
 ganglia. The inter-renal body. Its structure in the adult. Its independence of supra- 
 renal bodies. Its development. 
 
 CHAPTER XII. 
 
 THE ORGANS OF EXCRETION, pp. 479 520. 
 
 Previous investigations. Excretory organs and genital ducts in adult. In male. 
 Kidney and Wolffian body. Wolffian duct. Ureters. Cloaca. Seminal bladders. 
 Rudimentary oviduct. In female. Wolffian duct. Ureters. Cloaca. Segmental 
 openings. Glandular tubuli of kidney. Malpighian bodies. Accessory Malpighian 
 bodies. Relations of to segmental tubes. Vasa efferentia. Comparison of Scyllium 
 with other Elasmobranchs. Development of segmental tubes. Their junction with 
 segmental duct. Their division into four segments. Formation of Malpighian bodies. 
 Connection between successive segments. Morphological interest of. Development 
 of Miillerian and Wolffian ducts. In female General account. Formation of ovi- 
 duct as nearly solid cord. Hymen. In male Rudimentary Miillerian duct. 
 Comparison of development of Miillerian duct in Birds and Elasmobranchs. Own 
 researches. Urinal cloaca. Formation of Wolffian body and kidney proper. 
 General account. Details of formation of ureters. Vasa efferentia. Views of 
 Semper and Spengel. Difficulties of Semper's views. Unsatisfactory result of own 
 researches. General homologies. Resume, Postscript. 
 
CHAPTER I. 
 THE RIPE OVARIAN OVUM. 
 
 THE ripe ovum is nearly spherical, and, after the removal 
 of its capsule, is found to be unprovided with any form of pro- 
 tecting membrane. 
 
 My investigations on the histology of the ripe ovarian ovum 
 have been made with the ova of the Gray Skate (Raja batis) 
 only, and owing to a deficiency of material are somewhat im- 
 perfect. 
 
 The bulk of the ovum is composed of yolk spherules, 
 imbedded in a protoplasmic matrix. Dr Alexander Schultz 1 , 
 who has studied with great care the constitution of the yolk, 
 finds, near the centre of the ovum, a kernel of small yolk sphe- 
 rules, which is succeeded by a zone of spherules which gradually 
 increase in size as they approach the surface. But, near the 
 surface, he finds a layer in which they again diminish in size 
 and exhibit numerous transitional forms on the way to molecular 
 yolk-granules. These Dr Schultz regards as in a retrogressive 
 condition. 
 
 Another interesting feature about the yolk is the presence 
 in it of a protoplasmic network. Dr Schultz has completely 
 confirmed, and on some points enlarged, my previous observa- 
 tions on this subject' 2 . Dr Schultz's confirmation is the more 
 important, since he appears to be unacquainted with my pre- 
 vious investigations. In my paper (loc. cit.\ after giving a 
 description of the network I make the following statement as to 
 its distribution. 
 
 1 Archiv fur Micro. Anat. Vol. XI. 1875. 
 
 ' 2 Quart. Journ. Micro- Science, Oct. 1874. [This edition, No. V.] 
 
214 THE DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 "A specimen of this kind is represented in Plate 14, fig. 2, n. y, where 
 the meshes of the network are seen to be finer immediately around the 
 nuclei, and coarser in the intervals. The specimen further shews, in the 
 clearest manner, that this network is not divided into areas, each represent- 
 ing a cell and each containing a nucleus. I do not know to what extent this 
 network extends into the yolk. I have never yet seen the limits of it, though 
 it is very common to see the coarsest yolk-granules lying in its meshes. 
 Some of these are shewn in Plate 14, fig. 2,_y. k." [This edition, p. 65.] 
 
 Dr Schultz, by employing special methods of hardening and 
 cutting sections of the whole egg, has been able to shew that 
 this network extends, in the form of fine radial lines, from the 
 centre to the circumference ; and he rightly states, that it exhibits 
 no cell-like structures. I have detected this network extending 
 throughout the whole yolk in young eggs, but have failed to see 
 it with the distinctness which Dr Schultz attributes to it in the 
 ripe ovum. Since it is my intention to enter fully both into the 
 structure and meaning of this network in my account of a later 
 stage, I say no more about it here. 
 
 At one pole of the ripe ovum a slight examination demon- 
 strates the presence of a small circular spot, sharply distinguished 
 from the remainder of the yolk by its lighter colour. Around 
 this spot is an area which is also of a lighter colour than the 
 yolk, and the outer border of which gradually shades into the 
 normal tint of the yolk. If a section be made through this part 
 (vide PI. 6, fig. i) the circular spot will be found to be the 
 germinal vesicle, and the area around it a disc of yolk containing 
 smaller spherules than the surrounding parts. The germinal 
 vesicle possessed the same structure in both the ripe eggs 
 examined by me ; and, in both, it was situated quite on the 
 external surface of the yolk. 
 
 In one of my specimens it was flat above, but convex below ; 
 in the other and, on the whole, the better preserved of the two, 
 it had the somewhat quadrangular but rather irregular section 
 represented in PI. 6, fig. I. It consisted of a thickish membrane 
 and its primitive contents. The membrane surrounded the 
 upper part of the contents and exhibited numerous folds and 
 creases (vide fig. i). As it extended downwards it became 
 thinner, and completely disappeared at some little distance from 
 the lower end of the contents. These, therefore, rested below on 
 the yolk. At its circumference the membrane of the disc was 
 
THE RIPE OVARIAN OVUM. 21$ 
 
 produced into a kind of fold, forming a rim which rested on the 
 surface of the yolk. 
 
 In neither of my specimens is the cavity in the upper part 
 of the membrane filled by the contents ; and the upper part of 
 the membrane is so folded and creased that sections through 
 almost any portion of it pass through the folds. The regularity 
 of the surface of the yolk is not broken by the germinal vesicle, 
 and the yolk around exhibits not the slightest signs of displace- 
 ment. In the germinal vesicle figured the contents are some- 
 what irregular in shape ; but in my other specimen they form a 
 regular mass concave above and convex below. In both cases 
 they rest on the yolk, and the floor of the yolk is exactly moulded 
 to suit the surface of the contents of the germinal vesicle. The 
 contents have a granular aspect, but differ in constitution from 
 the surrounding yolk. Each germinal vesicle measured about 
 one-fiftieth of an inch in diameter. 
 
 It does not appear to me possible to suppose that the pecu- 
 liar appearances which I have drawn and described are to be 
 looked upon as artificial products either of the chromic acid, in 
 which the ova were hardened, or of the instrument with which 
 sections of them were made. It is hardly conceivable that 
 chromic acid could cause a rupture of the membrane and the 
 ejection of the contents of the vesicle. At the same time the 
 uniformity of the appearances in the different sections, the regu- 
 larity of the whole outline of the egg, and the absence of any 
 signs of disturbance in the yolk, render it impossible to believe 
 that the structures described are due to faults of manipulation 
 during or before the cutting of the sections. 
 
 We can only therefore conclude that they represent the real 
 state of the germinal vesicle at this period. No doubt they 
 alone do not supply a sufficient basis for any firm conclusions 
 as to the fate of the germinal vesicle. Still, if they cannot 
 sustain, they unquestionably support certain views. The natural 
 interpretation of them is that the membrane of the germinal 
 vesicle is in the act of commencing to atrophy, preparatory to 
 being extruded from the egg, while the contents of the germinal 
 vesicle are about to be absorbed. 
 
 'In favour of the extrusion of the membrane rather than its 
 absorption are the following features, 
 
2l6 THE DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 (i) The thickness of its upper surface. (2) The extension of 
 its edge over the yolk. (3) Its position external to the yolk. 
 
 In favour of the view that the contents will be left behind 
 and absorbed when the membrane is pushed out, are the follow- 
 ing features of my sections : 
 
 (i) The rupture of the membrane of the germinal vesicle on 
 its lower surface. (2) The position of the contents almost com- 
 pletely below the membrane of the vesicle and surrounded by yolk. 
 
 In connection with this subject, Oellacher's valuable observa- 
 tions upon the behaviour of the germinal vesicle in Osseous 
 Fishes and in Birds at once suggest themselves 1 . Oellacher 
 sums up his results upon the behaviour of the germinal vesicle in 
 Osseous Fishes in the following way (p. 12) : 
 
 " The germinal vesicle of the Trout's egg, at a period when the egg is 
 very nearly ripe, lies near the surface of the germinal disc which is aggre- 
 gated together in a hollow of the yolk After this a hole appears in the 
 
 membrane of the germinal vesicle, which opens into the space between the 
 egg-membrane and the germinal disc. The hole widens more and more, 
 and the membrane frees itself little by little from the contents of the 
 germinal vesicle, which remain behind in the form of a ball on the floor of 
 the cavity formed in this way. The cavity becomes flatter and flatter and 
 the contents are pushed up further and further from the germinal disc. 
 When the hollow, in which lie the contents of the original germinal vesicle, 
 
 completely vanishes, the covering membrane becomes inverted and the 
 
 membrane is spread out on the convex surface of the germinal disc as a 
 circular, investing structure. It is clear that by the removal of the membrane 
 the contents of the germinal vesicle become lost." 
 
 These very definite statements of Oellacher tell strongly 
 against my interpretation of the appearance presented by the 
 germinal vesicle of the ripe Skate's egg. Oellacher's account is 
 so precise, and his drawings so fully bear out his interpretations, 
 that it is very difficult to see where any error can have crept in. 
 
 On the other hand, with the exception of those which 
 Oellacher has made, there cannot be said to be any satisfactory 
 observations demonstrating the extrusion of the germinal vesicle 
 from the ovum. Oellacher has observed this definitely for the 
 Trout, but his observations upon the same point in the Bird 
 would quite as well bear the interpretation that the membrane 
 alone became pushed out, as that this occurred to the germinal 
 vesicle, contents and all. 
 
 1 Archiv fiir Micr. Anat. Vol. vm. p. i. 
 
RIPE OVARIAN OVUM. 2 1/ 
 
 While, then, there are on the one hand Oellacher's observa- 
 tions on a single animal, hitherto unconfirmed, there are on the 
 other very definite observations tending to shew that the ger- 
 minal vesicle has in many cases an altogether different fate. 
 Gotte 1 , not to mention other observers before him, has in the 
 case of Batrachian's eggs traced out with great precision the 
 gradual atrophy of the germinal vesicle, and its final absorption 
 into the matter of the ovum. 
 
 Gotte distinguishes three stages in the degeneration of the 
 germinal vesicle of Bombinator's egg. In the first stage the 
 germinal vesicle has begun to travel up towards the surface of 
 the egg. It retains nearly its primitive condition, but its contents 
 have become more opaque and have partly withdrawn themselves 
 from the thin membrane. The germinal spots are still circular, 
 but in some cases have increased in size. The most important 
 feature of this stage is the smaller size of the germinal vesicle than 
 that of the cavity of the yolk in which it lies, a condition which 
 appears to demonstrate the commencing atrophy of the vesicle. 
 
 In the next stage the cavity containing the germinal vesicle 
 has vanished without leaving a trace. The germinal vesicle 
 itself has assumed a lenslike form, and its borders are irregular 
 and pressed in here and there by yolk. Of the membrane of the 
 germinal vesicle, and of the germinal spots, only scanty remnants 
 are to be seen, many of which lie in the immediately adjoining 
 yolk. 
 
 In the last stage no further trace of a distinct germinal 
 vesicle is present. In its place is a mass of very finely granular 
 matter, which is without a distinct border and graduates into 
 the surrounding yolk and is to be looked on as a remnant of the 
 germinal vesicle. 
 
 This careful investigation of Gotte proves beyond a doubt 
 that in Batrachians neither the membrane, nor the contents of 
 the germinal vesicle, are extruded from the egg. 
 
 In Mammalia, Van Beneden 2 finds that the germinal vesicle 
 becomes invisible, though he does not consider that it absolutely 
 ceases to exist. He has not traced the steps of the process with 
 the same care as Gotte, but it is difficult to believe that an 
 
 1 Entwicklungsgcschichte der Unke. 
 
 J Rechcrches sur la Composition et la Signification de FCEuf. 
 B. I 5 
 
2l8 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 extrusion of the vesicle in the way described by Oellacher would 
 have escaped his notice. 
 
 Passing from Vertebrates to Invertebrates, we find that 
 almost every careful investigator has observed the disappear- 
 ance, apparent or otherwise, of the germinal vesicle, but that 
 very few have watched with care the steps of the process. 
 
 The so-called Richtungskorper has been supposed to be the 
 extruded remnant of the germinal vesicle. This view has been 
 especially adopted and supported by Oellacher (loc. cit.), and 
 Flemming 1 . 
 
 The latter author regards the constant presence of this body, 
 and the facility with which it can be stained, as proofs of its 
 connection with the germinal vesicle, which has, however, accord- 
 ing to his observations, disappeared before the appearance of the 
 Richtungskorper. 
 
 Kleinenberg 2 , to whom we are indebted for the most precise 
 observations we possess on the disappearance of the germinal 
 vesicle, gives the following account of it, pp. 41 and 42. 
 
 "We left the germinal vesicle as a vesicle with a distinct doubly con- 
 toured membrane, and equally distributed granular contents, in which the 
 
 germinal spot had appeared The germinal vesicle reaches o - o6mm. in 
 
 diameter, and at the same time its contents undergo a separation. The 
 greater part withdraws itself from the membrane and collects as a dense 
 mass around the germinal spot, while closely adjoining the membrane there 
 remains only a very thin but unbroken lining of the plasmoid material. The 
 intermediate space is filled with a clear fluid, but the layer which lines the 
 membrane retains its connection with the mass around the germinal vesicle 
 by means of numerous fine threads which traverse the space filled with fluid. 
 
 At about the time when the formation of the pseudocells in the egg is 
 
 completed the germinal spot undergoes a retrogressive metamorphosis, it 
 loses its circular outline and it now appears as if coagulated ; then it breaks 
 up into small fragments, and I am fairly confident that these become 
 
 dissolved. The germinal vesicle becomes, on the egg assuming a 
 
 spherical form, drawn into an eccentric position towards the pole of the egg 
 directed outwards, where it lies close to the surface and only covered by a 
 very thin layer of plasma. In this situation its degeneration now begins, 
 and ends in its complete disappearance. The granular contents become 
 more and more fluid ; at the same time part of them pass out through the 
 membrane. This, which so far was firmly stretched, next collapses to a 
 somewhat egg-like sac, whose wall is thickened and in places folded. 
 
 1 " Studien in der Entwicklungsgeschichte der Najaden," Silz. d. k. Akad. Wien, 
 Bd. LXXI. 1875. 2 Hydra. Leipzig, 1872. 
 
RIPE OVARIAN OVUM. 219 
 
 "The inner mass which up to this time has remained compact now 
 breaks up into separate highly refractive bodies, of spherical or angular 
 form and of very different sizes ; between them, here and there, are scattered 
 
 drops of a fluid fat I am very much inclined to regard the solid bodies 
 
 in question as fat or as that peculiar modification of albuminoid bodies 
 which we recognise as the certain forerunner of the formation of fat in so 
 many pathologically altered tissues ; and therefore to refer the disappearance 
 of the germinal vesicle to a fatty degeneration. On one occasion I believe 
 that I observed an opening in the membrane at this stage ; if this is a 
 normal condition it would be possible to believe that its solid contents 
 passed out and were taken up in the surrounding plasma. What becomes 
 of the membrane I am unable to say ; in any case the germinal vesicle has 
 vanished to the very last trace before impregnation occurs." 
 
 Kleinenberg clearly finds that the germinal vesicle disappears 
 completely before the appearance of the Richtungskorper, in 
 which he states a pseudocell or yolk-sphere is usually found. 
 
 The connection between the Richtungskorper and the germi- 
 nal vesicle is not a result of strict observation, and there can be 
 no question that the evidence in the case of invertebrates tends 
 to prove that the germinal vesicle in no case disappears owing 
 to its extrusion from the egg, but that if part of it is extruded 
 from the egg as Richtungskorper this occurs when its constituents 
 can no longer be distinguished from the remainder of the yolk. 
 This is clearly the case in Hydra, where, as stated above, one of 
 the pseudocells or yolk-spheres is usually found imbedded in 
 the Richtungskorper. 
 
 My observations on the Skate tend to shew that, in its case, 
 the membrane of the germinal vesicle is extruded from the egg, 
 though they do not certainly prove this. That conclusion is 
 however supported by the observations of Schenk 1 . He found 
 in the impregnated, but not yet segmented, germinal disc a 
 cavity which, as he suggests, might well have been occupied by 
 the germinal vesicle. It is not unreasonable to suppose that 
 the membrane, being composed of formed matter and able only 
 to take a passive share in vital functions, could, without thereby 
 influencing the constitution of the ovum, be ejected. 
 
 If we suppose, and this is not contradicted by observation, 
 that the Richtungskorper is either only the metamorphosed 
 membrane of the germinal vesicle with parts of the yolk, or part 
 of the yolk alone, and assume that in Oellacher's observations 
 
 1 " Die Eier von Raja quadrimaculata," Si'/z. d?r k. Akad. Wien, Bel. LXVIII. 
 
 152 
 
220 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 only, the membrane and not the contents were extruded from 
 the egg, it would be possible to frame a consistent account of 
 the behaviour of the germinal vesicle throughout the animal 
 kingdom, which may be stated in the following way. 
 
 The germinal vesicle usually before, but sometimes imme- 
 diately after impregnation undergoes atrophy and its contents 
 become indistinguishable from the remainder of the egg. In 
 those cases in which its membrane is very thick and resistent, 
 e.g. Osseous and Elasmobranch Fishes, Birds, etc., this may be 
 incapable of complete resorption, and be extruded bodily from 
 the egg. In the case of most ova, it is completely absorbed, 
 though at a subsequent period it may be extruded from the egg 
 as the Richtungskorper. In all cases the contents of the 
 germinal vesicle remain in the ovum. 
 
 In some cases the germinal vesicle is stated to persist and to 
 undergo division during the process of segmentation ; but the 
 observations on this point stand in need of confirmation. 
 
 My investigations shew that the germinal vesicle atrophies in 
 the Skate before impregnation, and in this respect accord with 
 very many recent observations. Of these the following may be 
 mentioned. 
 
 (i) Oellacher (Bird, Osseous Fish). (2) Gotte (Bombinator 
 igneus). (3) Kupffer (Ascidia canina). (4) Strasburger 
 (Phallusia mamillata). (5) Kleinenberg (Hydra). (6) Metsch- 
 nikoff (Geryonia, Polyzenia leucostyla, Epibulia aurantiaca, and 
 other Hydrozoa). 
 
 This list is sufficient to shew that the disappearance of the 
 germinal vesicle before impregnation is very common, and I am 
 unacquainted with any observations tending to shew that its 
 disappearance is due to impregnation. 
 
 In some cases, e.g. Asterocanthion 1 , the germinal vesicle 
 vanishes after the spermatozoa have begun to surround the egg; 
 but I do not know that its disappearance in these cases has 
 been shewn to be due to impregnation. To do so it would be 
 necessary to prove that in ripe eggs let loose from the ovary, but 
 not fertilized, the germinal vesicle did not undergo the same 
 changes as in the case of fertilized eggs; and this, as far as I 
 
 1 Agassiz, Embryology of the Star-Fish. 
 
RIPE OVARIAN OVUM. 221 
 
 know, has not been done. After the disappearance of the 
 germinal vesicle, and before the first act of division, a fresh 
 nucleus frequently appears [ vide Auerbach (Ascaris nigro- 
 venosa), Fol (Geryonia), Kupffer (Ascidia canina), Strasburger 
 (Phallusia mamillata), Flemming (Anodon), Gotte (Bombinator 
 igneus)], which is generally stated to vanish before the appear- 
 ance of the first furrow ; but in some cases (Kupffer and Gotte, 
 and as studied with especial care, Strasburger) it is stated to 
 divide. Upon the second nucleus, or upon its relation to the 
 germinal vesicle, I have no observations ; but it appears to me 
 of great importance to determine whether this fresh nucleus 
 arises absolutely de novo, or is formed out of the matter of the 
 germinal vesicle. 
 
 The germinal vesicle is situated in a bed of finely divided 
 yolk-particles. These graduate insensibly into the coarser yolk- 
 spherules around them, though the band of passage between the 
 coarse and the finer yolk-particles is rather narrow. The mass 
 of fine yolk-granules may be called the germinal disc. It is 
 not to be looked upon as diverging in any essential particular 
 from the remainder of the yolk, for the difference between the two 
 is one of degree only. It contains in fact a larger bulk of active 
 protoplasm, as compared with yolk-granules, than does the 
 remainder of the ovum. The existence of this agreement in 
 kind has been already strongly insisted on in my preliminary 
 paper ; and Schultz (loc. cit.) has arrived at an entirely similar 
 conclusion, from his own independent observations. 
 
 One interesting feature about the germinal disc at this period 
 is its size. 
 
 My observations upon it have been made with the eggs of 
 the Skate (Raja) alone ; but I think that it is not probable that 
 its size in the Skate is greater than in Scyllium or Pristiurus. 
 If its size is the same in all these genera, then the germinal 
 disc of the unimpregnated ovum is very much greater than that 
 portion of the ovum which undergoes segmentation, and which 
 is usually spoken of as the germinal disc in impregnated ova. 
 
 I have no further observation on the ripe ovarian ovum ; and 
 my next observations concern an ovum in which two furrows 
 have already appeared. 
 
CHAPTER II. 
 THE SEGMENTATION. 
 
 I HAVE not been fortunate enough to obtain an absolutely 
 complete series of eggs during segmentation. 
 
 In the cases of Pristiurus and Scyllium only have I had any 
 considerable number of eggs in this condition, though one or 
 two eggs of Raja in which the process was not completed have 
 come into my hands. 
 
 In the youngest impregnated Pristiurus eggs, which I have 
 obtained, the germinal disc was already divided into four seg- 
 ments. 
 
 The external appearance of the blastoderm, which remains 
 nearly constant during segmentation, has been already well 
 described by Leydig 1 . 
 
 The yolk has a pale greenish tinge which, on exposure to the 
 air, acquires a yellower hue. The true germinal disc appears as 
 a circular spot of a bright orange colour, and is, according to 
 Leydig's measurements, i|m. in diameter. Its colour renders it 
 very conspicuous, a feature which is further increased by its 
 being surrounded by a narrow dark line (PI. 6, fig. 2), the indica- 
 tion of a shallow groove. Surrounding this line is a concentric 
 space which is lighter in colour than the remainder of the yolk, 
 but whose outer border passes by insensible gradations into the 
 yolk. As was mentioned in my preliminary paper (loc. '/.), and 
 as Leydig (loc. Y.) had before noticed, the germinal disc is 
 always situated at the pole of the yolk which is near the rounded 
 end of the Pristiurus egg. It occupies a corresponding position 
 in the eggs of both species of Scyllium (stellare and canicula) 
 near the narrower end of the egg to which the shorter pair of 
 strings is attached. The germinal disc in the youngest egg 
 
 1 Rochen mid Haie. 
 
SEGMENTATION. 223 
 
 examined, exhibited two furrows which crossed each other at 
 right angles in the centre of the disc, but neither of which 
 reached its edge. These furrows accordingly divided the disc 
 into four segments, completely separated from each other at the 
 centre of the disc, but united near its circumference. 
 
 I made sections, though not very satisfactorily, of this 
 germinal disc. The sections shewed that the disc was composed 
 of a protoplasmic basis, in which were imbedded innumerable 
 minute spherical yolk-globules so closely packed as to constitute 
 nearly the whole mass of the germinal disc. 
 
 In passing from the coarsest yolk-spheres to the fine spherules 
 of the germinal disc, three bands of different-sized yolk-particles 
 have to be traversed. These bands graduate into one another 
 and are without sharp lines of demarcation. The outer of the 
 three is composed of the largest-sized yolk-spherules which 
 constitute the greater part of the ovum. The middle band forms 
 a concentric layer around the germinal disc, and is composed of 
 yolk-spheres considerably smaller than those outside it.. Where 
 it cuts the surface it forms the zone of lighter colour im- 
 mediately surrounding the germinal disc. The innermost band 
 is formed by the germinal disc itself and is composed of sphe- 
 rules of the smallest size. These features are shewn in PL 6, 
 fig. 6, which is the section of a germinal disc with twenty-one 
 segments ; in it however the outermost band of spherules is not 
 present. 
 
 From this description it is clear, as has already been men- 
 tioned in the description of the ripe unimpregnated ovum, that 
 the germinal disc is not to be looked upon as a body entirely 
 distinct from the remainder of the ovum, but merely as a part 
 of the ovum in which the protoplasm is more concentrated and 
 the yolk-spherules smaller than elsewhere. Sections shew that 
 the furrows visible on the surface end below, as indeed they do 
 on the surface, before they reach the external limit of the finely 
 granular matter of the germinal disc. There are therefore at 
 this stage no distinct segments : the otherwise intact germinal 
 disc is merely grooved by two furrows. 
 
 I failed to observe any nuclei in the germinal disc just 
 described, but it by no means follows that they were not 
 present. 
 
224 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 In the next youngest of the eggs 1 examined the germinal 
 disc was already divided into twenty-one segments. When 
 viewed from the surface (PI. 6, fig. 3), the segments appeared 
 divided into two distinct groups an inner group of eleven 
 smaller segments, and an outer group of segments surrounding 
 the former. The segments of both the inner and the outer 
 group were very irregular in shape and varied considerably in 
 size. The amount of irregularity is far from constant and many 
 germinal discs are more regular than the one figured. 
 
 In this case the situation of the germinal disc and its relations 
 to the yolk were precisely the same as in the earlier stage. 
 
 In sections of this germinal disc (PI. 6, fig. 6), the groove 
 which separates it from the yolk is well marked on one side, but 
 hardly visible at the other extremity of the section. 
 
 Passing from the external features of this stage to those 
 which are displayed by sections, the striking point to be noticed 
 is the persisting continuity of the segments, marked out on the 
 surface, with the floor of the germinal disc. 
 
 The furrows which are visible on the surface merely form a 
 pattern, but do not isolate a series of distinct segments. They 
 do not even extend to the limit of the finely granular matter of 
 the germinal disc. 
 
 The section represented, PL 6, fig. 6, bears out the statements 
 about the segments as seen on the surface. There are three 
 smaller segments in the middle of the section, and two larger 
 at the two ends. These latter are continuous with the coarser 
 yolk-spheres surrounding the germinal disc and are not separated 
 from them by a segmentation furrow. 
 
 In a slightly older embryo than the one figured I met with 
 a few completely isolated segments at the surface. These 
 segments were formed by the apparent bifurcation of furrows 
 as they neared the surface of the germinal disc. The segments 
 thus produced are triangular in form. They probably owe 
 their origin to the meeting of two oblique furrows. The last- 
 formed of these furrows apparently ceases to be prolonged 
 after meeting the first-formed furrow. I have not in any case 
 
 1 The germinal disc figured was from the egg of a Scyllium stellare and not 
 Pristiurus, but I have also sections of a Pristiurus egg of the same age, which do 
 not differ materially from the Scyllium sections. 
 
SEGMENTATION. 22 5 
 
 observed an example of two furrows crossing one another at 
 this stage. 
 
 The furrows themselves for the most part are by no means 
 simple slits with parallel sides. They exhibit a beaded structure, 
 shewn imperfectly in PI. 6, fig. 6, but better in PL 6, fig. 6 a, 
 which is executed on a larger scale. They present intervals 
 of dilatations where the protoplasms of the segments on the 
 two sides of the furrow are widely separated, alternating with 
 intervals where the protoplasms of the two segments are almost 
 in contact and are only separated from one another by a very 
 narrow space. 
 
 A closer study of the germinal disc at this period shews that 
 the cavities which cause the beaded structure of the furrows are 
 not only present along the lines of the furrows but are also 
 found scattered generally through the germinal disc, though far 
 more thickly in the neighbourhood of the furrows. Their ap- 
 pearance is that of vacuoles, and with these they are probably 
 to be compared. There can be little question that in the living 
 germinal disc they are filled with fluid. In some cases, they 
 are collected in very large numbers in the region of a furrow. 
 Such a case as this is shewn in PL 6, fig. 6 b. In numerous 
 other cases they occur, roughly speaking, alternately on each 
 side of a furrow. Some furrows, though not many, are entirely 
 destitute of these structures. The character of their distribution 
 renders it impossible to overlook the fact that these vacuole-like 
 bodies have important relations with the formation of the seg- 
 mentation furrows. 
 
 Lining the two sides of the segmentation furrows there is 
 present in sections a layer which stains deeply with colouring 
 re-agents; and the surface of the blastoderm is stained in the 
 same manner. In neither case is it permissible to suppose that 
 any membrane-like structure is present. In many cases a 
 similar very delicate, but deeply-stained line, invests the vacuo- 
 lar cavities, but the fluid filling these remains. quite unstained. 
 When distinct segments are formed, each of these is surrounded 
 by a similarly stained line. 
 
 The yolk-spherules are so numerous, and render even the 
 thinnest section so opaque, that I have failed to make satis- 
 factory observations on the behaviour of the nucleus. I find 
 
226 DEVELOPMENT OF ELASMOBRANCH FISHES.' 
 
 nuclei in many of the segments, though it is very difficult even 
 to see them, and only in very favourable specimens can their 
 structure be studied. In some cases, two of them lie one on 
 each side of a furrow; and in one case at the extreme end of a 
 furrow I could see two peculiar aggregations of yolk-spherules 
 united by a band through which the furrow, had it been con- 
 tinued, would have passed. The connection (if any exists) be- 
 tween this appearance and the formation of the fresh nuclei 
 in the segments, I have been unable to elucidate. 
 
 The peculiar appearances attending the formation of fresh 
 nuclei in connection with cell-division, which have recently 
 been described by so many observers, have hitherto escaped my 
 observation at this stage of the segmentation, though I shall 
 describe them in a later stage. A nucleus of this stage is 
 shewn on PI. 6, fig. 6 c. It is lobate in form and is divided by 
 lines into areas in each of which a deeply-stained granule is 
 situated. 
 
 The succeeding stages of segmentation present from the 
 surface no fresh features of great interest. The somewhat 
 irregular (PL 6, figs. 4 and 5) circular line, which divides the 
 peripheral larger from the central smaller segments, remains for 
 a long time conspicuous. It appears to be the representative of 
 the horizontal furrow which, in the Batrachian ovum, separates 
 the smaller pigmented spheres from the larger spheres of the 
 lower pole of the egg. 
 
 As the segments become smaller and smaller, the distinction 
 between the peripheral and the central segments becomes less 
 and less marked; but it has not disappeared by the time that 
 the segments become too small to be seen with the simple 
 lens. When the spheres become smaller than in the germinal 
 disc represented on PL 6, fig. 5, the features of segmentation 
 can be more easily and more satisfactorily studied by means of 
 sections. 
 
 To the features presented in sections, both of the latter and 
 of the earlier blastoderms, I now return. A section of one of 
 the earlier germinal discs, of about the age of the one represented 
 on PL 6, fig. 4, is shewn in PL 6, fig. 7. 
 
 It is clear at a glance that we are now dealing with true seg- 
 ments completely circumscribed on all sides. The peripheral 
 
SEGMENTATION. 22J 
 
 segments are, as a rule, larger than the more central ones, though 
 in this respect there is considerable irregularity. The segments 
 are becoming smaller by repeated division; but, in addition to 
 this mode of increase, there is now going on outside the 
 germinal disc a segmentation of the yolk, by which fresh seg- 
 ments are being formed from the yolk and added to those which 
 already exist in the germinal disc. One or two such segments 
 are seen in the act of being formed (PI. 6, fig. 7 /); and it is to 
 be noticed that the furrows which will eventually mark out the 
 segments, do so at first in a partial manner only, and do not 
 circumscribe the whole circumference of the segment in the act 
 of being formed. These fresh furrows are thus repetitions on a 
 small scale of the earliest segmentation furrows. 
 
 It deserves to be noticed that the portion of the germinal 
 disc which has already undergone segmentation, is still sur- 
 rounded by a broad band of small-sized yolk-spherules. It 
 appears to me probable that owing to changes taking place in 
 the spherules of the yolk, which result in the formation of fresh 
 spherules of a small size, this band undergoes a continuous 
 renovation. 
 
 The uppermost row of segmentation spheres is now com- 
 mencing to be distinguished from the remainder as a separate 
 layer which becomes progressively more distinct as segmenta- 
 tion proceeds. 
 
 The largest segments in this section measure about the 
 Ti^th of an inch in diameter, and the smallest about ^hjth of 
 an inch. 
 
 The nuclei at this stage present points of rather a special in- 
 terest. In the first place, though visible in many, and certainly 
 present in all the segments 1 , they are not confined to these: 
 they are also to be seen, in small numbers, in the band of 
 fine spherules which surrounds the already segmented part of 
 the germinal disc. Those found outside the germinal disc are 
 not confined to the spots where fresh segments are appearing, 
 
 1 In the figure of this stage, I have inserted nuclei in all the segments. In the 
 section from which the figure was taken, nuclei were not to be seen in many of the 
 segments, but I have not a question that they were present in all of them. The 
 difficulty of seeing them is, in part, clue to the yolk-spherules and in part to the 
 thinness of the section as compared witli the diameter of a segmentation sphere. 
 
228 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 but are also to be seen in places where there are no traces of 
 fresh segments. 
 
 This fact, especially when taken in connection with the for- 
 mation of fresh segments outside the germinal disc and with 
 other facts which I shall mention hereafter, is of great morpho- 
 logical interest as bearing upon the nature and homologies of 
 the food-yolk. It also throws light upon the behaviour and 
 mode of increase of the nuclei. All the nuclei, both those of the 
 segments and those of the yolk, have the peculiar structure I* 
 described in the last stage. 
 
 In specimens of this stage I have been able to observe 
 certain points which have an important bearing upon the be- 
 haviour of the nucleus during cell-division. 
 
 Three figures, illustrating the behaviour of the nucleus, as I 
 have seen it in sections of blastoderms hardened in chromic acid, 
 are shewn in PL 6, figs. 7 a, 7 b and 7 c. 
 
 In the place of the nucleus is to be seen a sharply defined 
 figure (Fig. 7 a) stained in the same way as the nucleus or more 
 deeply. It has the shape of two cones placed base to base. 
 From the apex of each cone there diverge towards the base a 
 series of excessively fine striae. At the junction between the 
 two cones is an irregular linear series of small deeply stained 
 granules which form an apparent break between the two. The 
 line of this break is continued very indistinctly beyond the edge 
 of the figure on each side. 
 
 From the apex of each cone there diverge outwards into the 
 protoplasm of the cell a series of indistinct markings. They are 
 rendered obscure by the presence of yolk-spherules, which 
 completely surround the body just described, but which are not 
 arranged with any reference to these markings. These latter 
 striae, diverging from the apex of the cone, are more distinctly 
 seen when the apex points to the observer (Fig. 7 b), than when 
 a side of the cone is in view. 
 
 The striae diverging outwards from the apices of the cones 
 must be carefully distinguished from the striae of the cones 
 themselves. The cones are bodies quite as distinctly differ- 
 entiated from the protoplasm of the cell as nuclei, while the 
 striae which diverge from their apices are merely structures in 
 the general protoplasm of the cell. 
 
SEGMENTATION. 229 
 
 In some cells, which contain these bodies, no trace of a com- 
 mencing line of division is visible. In other cases (Fig. 7 c\ 
 such a line of division does appear and passes through the 
 junction of the two cones. In one case of this kind I fancied 
 I could see (and have represented) a coloured circular body in 
 each cone. I do not feel any confidence that these two bodies 
 are constantly present; and even where visible they are very 
 indistinct. 
 
 Instead of an ordinary nucleus a very indistinctly marked 
 vesicular body sometimes appears in a segment; but whether 
 it is to be looked on as a nucleus not satisfactorily stained, or as 
 a nucleus in the act of being formed, I cannot decide. 
 
 With reference to the situation of the cone-like bodies I have 
 described I have made an observation which appears to me to 
 be of some interest. I find that bodies of this kind are found in 
 the yolk completely outside the germinal disc. I have made this 
 observation, in at least two cases which admitted of no doubt 
 (vide Fig. 7 nx'\ 
 
 We have therefore the remarkable fact, that whatever 
 connection these bodies may have with cell-division, they can 
 occur in cases where this is altogether out of the question and 
 where an increase in the number of nuclei can be their only 
 product. 
 
 These are the main facts which I have been able to de- 
 termine with reference to the nuclei of this stage; but it will 
 conduce to clearness if I now finish what I have to say upon 
 this subject. 
 
 At a still later stage of segmentation the same peculiar 
 bodies are to be seen as during the stage just described, but 
 they are rarer; and, in addition to them, other bodies are to be 
 seen of a character intermediate between ordinary nuclei and 
 the former bodies. 
 
 Three such are represented in PI. 6, figs. 8 a, 8 b, 8 c. In all 
 of these there can be traced out the two cones, which are how- 
 ever very irregular. The striation of the cones is still present, 
 but is not nearly so clear as it was in the earlier stage. 
 
 In addition to this, there are numerous deeply stained 
 granules scattered about the two figures which resemble exactly 
 the granules of typical nuclei. 
 
230 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 All these bodies occupy the place of an ordinary nucleus, 
 they stain like an ordinary nucleus and are as sharply defined 
 as an ordinary nucleus. 
 
 There is present around some of these, especially those 
 situated in the yolk, the network of lines of the yolk de- 
 scribed by me in a preliminary paper 1 , and I feel satisfied that 
 there is in some cases an actual connection between the net- 
 work and the nuclei. This network I shall describe more fully 
 hereafter. 
 
 Further points about these figures and the nuclei of this 
 stage I should like to have been able to observe more com- 
 pletely than I have done, but they are so small that with the 
 highest powers I possess (Zeiss, Immersion No. 2 = T y n.) their 
 complete and satisfactory investigation is not possible. 
 
 Most of the true nuclei of the cells of the germinal disc are 
 regularly rounded; those however of the yolk are frequently 
 irregular in shape and often provided with knob-like processes. 
 The gradations are so complete between typical nuclei and 
 bodies like that shewn (PI. 6, fig. 8 c) that it is impossible to 
 refuse the name of nucleus to the latter. 
 
 In many cases two nuclei are present in one cell. 
 
 In later stages knob-like nuclei of various sizes are scattered 
 in very great numbers in the yolk around the blastoderm (vide 
 PI. 7). In some cases it appears to me that several of these 
 are in close juxta-position, as if they had been produced by the 
 division of one primitive nucleus. I do not feel absolutely 
 confident that this is the case, owing to the fact that in the 
 investigation of a knobbed body there is great difficulty in 
 ascertaining that the knobs, which appear separate in one plane, 
 are not in reality united in another. 
 
 I have, in spite of careful search, hitherto failed to find 
 amongst these later nuclei cone-like figures, similar to those I 
 found in the yolk during segmentation. This is the more re- 
 markable since in the early stages of segmentation, when very 
 few nuclei are present in the yolk, the cone-like figures are not 
 uncommon ; whereas, in the latter stages of development when 
 the nuclei of the yolk are very common and obviously increas- 
 ing rapidly, such figures are not to be met with. 
 
 1 Loc. dt. 
 
SEGMENTATION. 231 
 
 In no case have I been able to see a distinct membrane 
 round any of the nuclei. 
 
 I have hitherto attempted to describe the appearances 
 bearing on the behaviour of the nuclei in as objective a manner 
 as possible. 
 
 My observations are not as complete as could be desired ; 
 but, taken in conjunction with those of other investigators, they 
 appear to me to point towards certain definite conclusions with 
 reference to the behaviour of the nucleus in cell-division. 
 
 The most important of these conclusions may be stated as 
 follows. In the act of cell-division the nuclei of the resulting 
 cells are formed from the nucleus of the primitive c'ell. 
 
 This may occur ; 
 
 (1) By the complete solution of the old nucleus within the 
 protoplasm of the mother cell and the subsequent reaggregation 
 of its matter to form the nuclei of the freshly formed daughter 
 cells, 
 
 (2) By the simple division of the nucleus, 
 
 (3) Or by a process intermediate between these two where 
 part of the old nucleus passes into the general protoplasm and 
 part remains always distinguishable and divides ; the fresh 
 nucleus being in this case formed from the divided parts as well 
 as from the dissolved parts of the old nucleus. 
 
 Included in this third process it is permissible to suppose 
 that we may have a series of all possible gradations between 
 the extreme processes I and 2. If it be admitted, and the 
 evidence we have is certainly in favour of it, that in some 
 cases, both in animal and vegetable cells, the nucleus itself 
 divides during cell division, and in others the nucleus com- 
 pletely vanishes during the cell-division, it is more reasonable 
 to suspect the existence of some connection between the two 
 processes, than to suppose that they are entirely different in 
 kind. Such a connection is given by the hypothesis I have just 
 proposed. 
 
 The evidence for this view, derived both from my own 
 observations and those of other investigators, may be put as 
 follows. 
 
 The absolute division of the nucleus has been stated to 
 occur in animal cells, but the number of instances where the 
 
232 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 evidence is quite conclusive are not very numerous. Recently 
 F. E. Schultze 1 appears to have observed it in the case of an 
 Amoeba in an altogether satisfactory manner. The instance is 
 quoted by Flemming 2 . Schultze saw. the nucleus assume a 
 dumb-bell shape, divide, and the two halves collect themselves 
 together. The whole process occupied a minute and a half and 
 was shortly followed by the division of the Amceba, which occu- 
 pied eight minutes. Amongst vegetable cells the division of the 
 nucleus seems to be still rarer than with animal cells. Sachs 3 
 admits the division of the nucleus in the case of the paren- 
 chyma cells of certain Dicotyledons (Sambucus, Helianthus, 
 Lysimachia, Polygonum, Silene) on the authority of Hanstein. 
 
 The division of the nucleus during cell-division, though 
 seemingly not very common, must therefore be considered as 
 a thoroughly well authenticated occurrence. 
 
 The frequent disappearance of the nucleus during cell-division 
 is now so thoroughly recognised, both for animal and vegetable 
 cells, as to require no further mention. 
 
 In many cases the partial or complete disappearance of the 
 nucleus is accompanied by the formation of two peculiar star- 
 like figures. Appearances of the kind have been described by 
 Fol 4 , Flemming 5 , Auerbach 6 and possibly also Oellacher 7 as well 
 as other observers. 
 
 These figures 8 are possibly due to the streaming out of the 
 
 1 Archivf. Micr. Anat. xi. p. 592. 
 
 2 "EntwicklungsgeschictederNajaden,"LXXl.Bd.der5zV2.^r.^<ra</. Wim, 1875. 
 
 3 Text-Book of Botany, English trans, p. 19. 
 
 4 "Entw. d. Geryonideneies." Jenaische Zcitschrift, Bd. VII. 
 6 Loc. cit. 
 
 6 Organologische Studien, Zweites Heft. 
 
 7 "Beitrage z. Entwicklungsgeschichte der Knochenfischen." Zeit. fiir Wiss. 
 Zoologie. Bd. xxn. 1872. 
 
 8 The memoirs of Auerbach and Strasburger (Zellbildung a. Zelltheilung) have 
 unfortunately come into my hands too late for me to take advantage of them. Especi- 
 ally in the magnificent monograph of Strasburger I find drawings precisely resembling 
 those from my specimens already in the hands of the engraver. Strasburger comes to 
 the conclusion from his investigations that the modified nucleus always divides and 
 never vanishes as is usually stated. If his views on this point are correct part of the 
 hypothesis I have suggested above is rendered unnecessary. The striae of the proto- 
 plasm, which in accordance with Auerbach's view I have considered as being due to a 
 streaming out of the matter of the nucleus, he regards as resulting from a polarity of 
 the particles in the cell and the attraction of the nucleus. My own investigations 
 
SKCMKNTATION. 233 
 
 protoplasm of the nucleus into that of the cell 1 . The appear- 
 ance of striation may on this hypothesis be explained as due 
 to the presence of granules in the protoplasm. When the 
 streaming out of the protoplasm of a nucleus into that of a cell 
 takes place, any large granule which cannot be moved fey the 
 stream will leave behind it a slack area where there is no move- 
 ment of the fluid. Any granules which are carried into this 
 area will remain there, and by the continuation of a process 
 of this kind a row of granules may be formed, and a series of 
 such rows would produce an appearance of striation. In many 
 cases, e.g. Anodon, vide Flemming", even the larger yolk- 
 spherules are arranged in this fashion. 
 
 On the supposition that the striation of these figures is 
 due to the outflow from the nucleus, the appearances presented 
 in Elasmobranchs admit of the following explanation. 
 
 The central body consisting of two cones (figs. 7 a, 7 c) is 
 almost without question the remnant of the primitive nucleus. 
 This is shewn by its occupying the same position as the primitive 
 nucleus, staining in the same way, and by there being a series 
 of insensible gradations between it and a typical nucleus. The 
 contents must be supposed to be streaming out from the two 
 apices of the cones, as appears from the striae in the body con- 
 verging on each side towards the apex, and then diverging 
 again from it. In my specimens the yolk-spherules are not 
 arranged with any reference to the radiating striation. 
 
 It is very likely that in the cases of the disappearance of the 
 nucleus, its protoplasm streams out in two directions, towards 
 the two parts of the cell which will eventually become separated 
 from each other ; and probably, after the division, the matter of 
 the old nucleus is again collected to form two fresh nuclei. 
 
 In some cases of cell-division a remnant of the old nucleus is 
 stated to be visible after the fresh nuclei have appeared. These 
 cases, of which I have not seen full accounts, are perhaps 
 analogous to what occasionally happens with the germinal 
 
 though, as far as they go, quite in accordance with those of Strasburger, do not supply 
 any grounds for deciding on the meaning of these strire ; and in some respects they 
 support Strasburger's views against those of other observers, since they demonstrate 
 that in Elasmobranchs the modified nucleus does actually divide. 
 
 1 This is the view which has been taken by Auerbach (Organologische Studien). 
 
 Loc. fit. 
 
 H. 1 6 
 
234 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 vesicle of an ovum. The whole of the contents of the germinal 
 vesicle become at its disappearance mingled with the proto- 
 plasm of the ovum, but the resistant membrane remains and 
 is eventually ejected from the egg, vide p. 215 et seq. If the 
 remnant of the old nucleus in the cases described is nothing 
 more than its membrane, no difficulty is offered to the view 
 that the constituents of the old nucleus may help to form the 
 new ones. 
 
 In many cases the total bulk of the new nuclei is greater 
 than that of the old one ; in such instances part of the proto- 
 plasm of the cell necessarily has a share in forming the new 
 nuclei. 
 
 Although, in instances where the nucleus vanishes, an abso- 
 lute demonstration of the formation of the fresh nuclei from the 
 matter of the old one is not possible ; yet, if cases of the division 
 of the old nucleus to form the new ones be admitted to exist, 
 the derivation in the first process of the fresh nuclei from the 
 old ones must be postulated in order to maintain a continuity 
 between the two processes of formation ; and, as I have attempted 
 to shew, all the circumstantial evidence is in favour of it. 
 
 Admitting the existence of the two extreme processes of nu- 
 clear formation, I wish to shew that my results in Elasmobranchs 
 tend to demonstrate the existence of intermediate steps between 
 them. The first figures I described of two opposed cones, appear 
 to me almost certainly to represent nuclei in the act of dissolu- 
 tion ; but though a portion of the nucleus may stream out into 
 the yolk, I think it impossible that the whole of it does 1 . 
 
 I described these bodies in two states. An earlier one, in 
 which the two cones were separated by an irregular row of 
 deeply stained granules ; and a later one in which a furrow had 
 already appeared dividing the cones as well as the cell. In 
 neither of these conditions could I see any signs of the body 
 vanishing completely. It was as clearly defined and as deeply 
 stained as an ordinary nucleus, and in its later condition the 
 signs of the streaming out of material from its pointed extremi- 
 ties were less marked than in the earlier stage. 
 
 1 After Strasburger's observation it must be considered very doubtful whether the 
 streaming out of the contents of the nucleus, in the manner implied in the text, really 
 takes place. 
 
All these facts, to my mind, point to the view that these 
 cone-like bodies do not disappear, but form the basis for the new 
 nuclei. Possibly the body visible in each cone in the later 
 stage, was the commencement of this new nucleus. Gotte 1 has 
 figured structures somewhat similar to these bodies, but I~haTdly 
 understand either his figure or his account sufficiently clearly 
 to be able to pronounce upon the identity of the two. In case 
 they are identical, Gotte gives a very different explanation of 
 them from my own 2 . 
 
 A second of my results, which points to a series of inter- 
 mediate steps between division and solution of the nucleus, is 
 the distribution in time of the peculiar cone-like bodies. These 
 are present in fair abundance at an early period of segmentation, 
 when there are but few nuclei either in the blastoderm or the 
 yolk. But at later periods, when there are both more nuclei, 
 especially in the yolk, and they are also increasing in numbers 
 more rapidly than before, no bodies of this kind are to be seen. 
 This fact becomes the more striking from the lobate appearance 
 of the later nuclei of the yolk, an appearance which exactly 
 suits the hypothesis of the rapid budding off of fresh nuclei. 
 
 The observations of R. Hertwig 3 on the gemmation of Podo- 
 plirya geuimipara, support my interpretation of the knobbed 
 condition of the nuclei. Hertwig finds (p. 47) that 
 
 The horse-shoe shaped nucleus grows out into numerous anastomosing 
 projections. Over the free ends of the projections little knobs appear on 
 the surface of the body, into which the lengthening ends of the processes of 
 the nucleus grow up. Here they bend themselves into a horse-shoe form. 
 The newly-formed nucleus then separates from the original nucleus, and 
 afterwards the bud containing it from the body. 
 
 From the peculiar arrangement of the net-work of lines of 
 the yolk around these knobbed nuclei, it is reasonable to con- 
 clude that interchange of material between the protoplasm of 
 
 1 Entwickelungsgeschite dcr Unke, PI. i. fig. 18. 
 
 2 As I before mentioned, Strasburger (Zellbildung u. Zelltheilung) has represented 
 bodies precisely similar to those I have described, which appear during the seg- 
 mentation in the egg of Phalltisia mammillata as well as similar figures observed by 
 Butschli in eggs of Cucitllanus elegans and Blatla Gcrmanica, The figures in this 
 monograph are the only ones I have seen, which are identical with my own. 
 
 3 Morphologischcs Jahrbiich, Bd. i. pp. 46, 47. 
 
 1 6 2 
 
236 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the yolk and the nuclei is still taking place, even during the 
 later periods. 
 
 These facts about the distribution in time of the cone-like 
 bodies afford a strong presumptive evidence of a change in the 
 manner of nuclear increase. 
 
 The last argument I propose urging on this head is derived 
 from the bodies (PL 6, fig. 8 a, b, c) which I have described as 
 intermediate between the true cone-like bodies and typical 
 nuclei. They appear to afford evidence of less and less of the 
 matter of the nucleus streaming out into the yolk and of a large 
 proportion of it becoming divided. 
 
 The conclusion to be derived from all these facts is that 
 for Elasmobranchs in the earlier stages of segmentation, and 
 during the formation of fresh segments, a partial solution of 
 the old nucleus takes place, but all its constituents serve for 
 the reconstruction of the fresh nuclei. 
 
 In later periods of development a still smaller part of the 
 nucleus becomes dissolved, and the rest divides ; but the two 
 fresh nuclei are still derived from the two sources. After the 
 close of segmentation the fresh nuclei are formed by a simple 
 division of the older ones. 
 
 The appearance of the cone-like bodies in the yolk outside 
 the germinal disc is a point of some interest. It demonstrates 
 in a conclusive manner that whatever influence (if any) the 
 nucleus may have in ordinary cases of cell division, yet it may 
 undergo changes of a precisely similar character to those which 
 it experiences during cell division, without exerting any influence 
 on the surrounding protoplasm 1 . If the lobate nuclei are also 
 nuclei undergoing division, we have in the egg of an Elasmo- 
 branch examples of all the known forms of nuclear increase 
 unaccompanied by cell division. 
 
 The next stage in the segmentation does not present so 
 many features of interest as the last one. The segments are 
 
 1 Strasburger's (loc. cit.) arguments about the influence of the nucleus in cell 
 division are not to my mind conclusive ; though not without importance. It is 
 difficult to reconcile his views with the facts of cell division observable during the 
 Elasmobranch segmentation ; but even if their truth be admitted they do not bring us 
 much nearer to a satisfactory understanding of cell division, unless accompanied (and 
 at present they are not so) by a rational explanation of the forces which produce the 
 division of the nucleus. 
 
SEGMENTATION. 237 
 
 now so small, as to be barely visible from the surface with a 
 simple lens. A section of an embryo of this stage is repre- 
 sented in PI. 6, fig. 8. The section, which is drawn on the 
 same scale as the section belonging to the last stage, serves 
 to shew the relative size of the segments in the two cases. 
 
 The epiblast is now more distinct than it was. The seg- 
 ments composing it are markedly smaller than the remainder 
 of the cells of the germinal disc, but possess nuclei of an abso- 
 lutely larger size than do the other cells. They are irregular 
 in shape, with a slight tendency to be columnar. An average 
 segment of this layer measures about ^^ inch. 
 
 The cells of the lower layer are more polygonal than those 
 of the epiblast, and are decidedly larger. An average specimen 
 of the larger cells of the lower layer measures about -^^ in. in 
 diameter, and is therefore considerably smaller than one of the 
 smallest cells of the last stage. The formation of fresh segments 
 from the yolk still continues with fair rapidity, but nearly comes 
 to an end shortly after this. 
 
 Of the nuclei of the lower layer cells, there is not much 
 to add to what has already been said. Not infrequently two 
 nuclei may be observed in a single cell. 
 
 The nuclei in the yolk which surrounds the germinal disc are 
 more numerous than in the earlier periods, and are now to be 
 met with in fair numbers in every section (fig. 8 '). 
 
 These are the main features which characterise the present 
 stage, they are in all essential points similar to those of the 
 last stage, and the two germinal discs hardly differ except in 
 the size of the segments of which they are composed. 
 
 In the last stage which I consider as belonging to the seg- 
 mentation, the cells of the whole blastoderm have become 
 smaller (PI. 6, fig. 9). 
 
 The epiblast (ep} now consists of a very marked layer of 
 columnar cells. It is, as far as I have been able to observe, 
 never more than one cell deep. The cells of the lower layer 
 are of an approximately uniform size, though a few of those at 
 the circumference of the blastoderm considerably exceed the 
 remainder in the bulk. 
 
 There are two fresh features of importance in germinal discs 
 of this age. 
 
238 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Instead of being but indistinctly separated from the sur- 
 rounding yolk, the blastoderm has now very clearly defined 
 limits. 
 
 This is an especially marked feature of preparations made 
 with osmic acid. In these there may frequently be seen a 
 deeply stained doubly contoured line, which forms the limit of 
 the yolk, where it surrounds the germinal disc. Lines of this 
 kind are often to be seen on the surface of the yolk, or even of 
 the blastoderm, but are probably to be regarded as products of 
 reagents, rather than as organised structures. The outline of 
 the germinal disc is well rounded, though it is occasionally 
 broken, from the presence of a larger cell in the act of being 
 formed from the yolk. 
 
 It is not probable that any great importance is to be at- 
 tached to the comparative distinctness of the outline of the 
 germinal disc at this stage, which is in a great measure due 
 to a cessation in the formation of fresh cells in the surrounding 
 yolk, and in part to the small and comparatively uniform size of 
 the cells of the germinal disc. 
 
 The formation of fresh cells from the yolk nearly comes to 
 an end during this period, but it still continues on a small scale. 
 
 The number of the nuclei around the germinal disc has 
 increased. 
 
 Another feature of interest which first becomes apparent 
 during this stage is the asymmetry of the germinal disc. If a 
 section were made through the germinal disc, as it lay in situ in 
 the egg capsule, parallel or nearly so to the long axis of the 
 capsule, one end of the section would be found to be much 
 thicker than the other. There would in fact be a far larger 
 collection of cells at one extremity of the germinal disc than at 
 the other. The end at which this collection of cells is formed 
 points towards the end of the egg capsule opposite to that near 
 which the yolk is situated. This collection of cells is the first 
 trace of the embryo ; and with its appearance the segmentation 
 may be supposed to terminate. 
 
 The section I have represented, though not quite parallel 
 to the long axis of the egg, is sufficiently nearly so to shew 
 the greater mass of cells at the embryonic end of the germinal 
 disc. 
 
SEGMENTATION. 239 
 
 This very early appearance of a distinction in the germinal 
 disc between the extremity at which the embryo appears and 
 the non-embryonic part of the disc, besides its inherent interest, 
 has a further importance from the fact that in Osseous Fishes 
 a similar occurrence takes place. Oellacher 1 and Gotte 2 both 
 agree as to the very early period at which a thickening of one 
 extremity of the blastoderm in Osseous Fishes is formed, which 
 serves to indicate the position at which the embryo will appear. 
 There are many details of development in which Osseous Fish 
 and Elasmobranchs agree, which, although if taken individually 
 are without any great importance, yet serve to shew how long 
 even insignificant features in development may be retained. 
 
 The segmentation of the Elasmobranch egg presents in most 
 of its features great regularity, and exhibits in its mode of 
 occurrence the closest resemblance to that in other meroblastic 
 vertebrate ova. 
 
 There is, nevertheless, one point with reference to which a 
 slight irregularity may be observed. In almost all eggs seg- 
 mentation commences by, what for convenience may be called, 
 a vertical furrow which is followed by a second vertical furrow 
 at right angles to the first. The third furrow however is a 
 horizontal one, and cuts the other two at right angles. This 
 method of segmentation must be looked on as the normal one, 
 in almost all the important groups of the animal kingdom, both 
 for the so-called holoblastic and meroblastic eggs, and the 
 gradations intermediate between the two. The Frog amongst 
 vertebrates exhibits a most typical instance of this form of 
 segmentation. 
 
 In Elasmobranchs the first two furrows are formed in a per- 
 fectly normal manner, but though I have not observed the 
 actual formation of the next furrow, yet from the later stages, 
 which I have observed, I conclude that it is parallel to one of 
 the first formed furrows ; and it is fairly certain that, not till a 
 considerably later period, is a furrow homologous with the hori- 
 zontal furrow of the Batrachian egg formed. This furrow 
 appears to be represented in the Elasmobranch segmentation 
 
 1 Zeitschrift fiir Wiss. Zotlogie, Bd. xxm. 1873. 
 
 2 Archivfur MUr. An at. Bd. IX. 1873. 
 
240 DEVELOPMENT OF ELASMOBKANCH FISHES. 
 
 by the irregular circumscription of a body of central smaller 
 spheres from a ring of peripheral larger ones (vide PI. 6, figs. 
 3, 4 and 5). 
 
 In the Bird the representative of the horizontal furrow 
 appears relatively much earlier. It is formed when there are 
 eight segments marked out on the surface of the germinal disc 1 . 
 From Oellacher's 2 account of the segmentation in the fowl 3 it 
 seems certain, as might be anticipated, that this furrow is nearly 
 parallel to the surface of the disc, so that it cuts the earlier 
 formed vertical furrows and causes the segments of the germinal 
 disc to be completely circumscribed below as well as at the 
 surface. In the Elasmobranch egg this is not the case ; so that, 
 even after the smaller central segments have become separated 
 from the outer ring of larger ones, none of the segments of the 
 disc are completely circumscribed, and only appear to be so in 
 surface views (vide PI. 6, fig. 6). Segmentation in the Elasmo- 
 branch egg differs in the following particulars from that in the 
 Bird's egg : 
 
 (1) The equivalent of the horizontal furrow of the Batrachian 
 egg appears much later than in the Bird. 
 
 (2) When it has appeared it travels inwards much more 
 slowly. 
 
 As a result of these differences, the segments of the germinal 
 disc of the Birds' eggs are much earlier circumscribed on all 
 sides than those of the Elasmobranch egg. 
 
 As might be expected, the segmentation of the Elasmobranch 
 egg resembles in many points that of Osseous Fishes (vide 
 Oellacher 4 and Klein 8 ). It may be noticed, that with Osseous 
 as with Elasmobranch Fishes, the furrow corresponding with the 
 horizontal furrow of the Amphibian's egg does not appear at 
 as early a period as is normal. The third furrow of an Osseous 
 Fish egg is parallel to one of the first formed pair. 
 
 In Oellacher's 6 figures, PI. 23, figs. 19 21, peculiar headings 
 
 1 Vide Elements of Embryology, p. 23. 
 
 2 Strieker's Studien, 1869, Pt. i, PI. II. fig. 4. 
 
 3 Unfortunately Professor Oellacher gives no account of the surface appearance of 
 the germinal discs of which he describes the sections. It is therefore uncertain to 
 what period his sections belong. 
 
 4 Z-eitschrift fur Wiss. Zool. Bd. xxn. -1871. 
 
 5 Mo nthly Microscopical Journal, .March, 1872. K Loc. at. 
 
SEGMENTATION. 24! 
 
 of the sides of the earlier formed furrows are distinctly shewn. 
 No mention of these is made in the text, but they are un- 
 questionably similar to those I have described in the Elasmo- 
 branch furrows. In the case of Elasmobranchs I pointed out 
 that not only were the sides of the furrow beaded, but that 
 there appeared in the protoplasm, close to the furrows, peculiar 
 vacuole-like cavities, precisely similar to the cavities which 
 were the cause of the beadings of the furrows. 
 
 The presence of these seems to shew that the molecular 
 cohesion of the protoplasm becomes, as compared with other 
 parts, much diminished in the region where a furrow is about 
 to appear, so that before the protoplasm finally gives way along 
 a particular line to form a furrow, its cohesion is broken at 
 numerous points in this region, and thus a series of vacuole- 
 like spaces is formed. 
 
 If this is the true explanation of the formation of these 
 spaces, their presence gives considerable support to the views 
 of Dr Kleinenberg upon the causes of segmentation, so clearly 
 and precisely stated in his monograph upon Hydra ; and is 
 opposed to any view which regards the forces which come into 
 play during segmentation as resident in the nucleus. 
 
 I have not observed the peculiar threads of protoplasm which 
 Oellacher 1 describes as crossing the commencing segmentation 
 furrows. I have also failed to discover any signs of a concentra- 
 tion of the yolk-spherules, round one or two centres, in the 
 segmentation spheres, similar to that observed by Oellacher 
 in the segmenting eggs of Osseous Fish. The appearances 
 observed by him are probably connected with the behaviour of 
 the nucleus during segmentation, and are related to the curious 
 bodies I have already described. 
 
 With reference to the nuclei which Oellacher 2 has described 
 as occurring in the eggs of Osseous Fish during segmentation, 
 there can, I think, be little doubt that they are identical with 
 the peculiar nuclei in the Elasmobranch eggs. 
 
 He 3 says : 
 
 In an unsegmented germ there occurred at a certain point in the section 
 
 a small aggregation of round bodies. I do not feel satisfied whether 
 
 these aggregations represent one or more nuclei. 
 
 1 Loc. cit. - Liu. tit. '* Loc. cit. pp. 410, 411, &c. 
 
242 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Fig. 29 shews such aggregation ; by focusing at its optical section eleven 
 unequally large rounded bodies measuring from 0^004 0*009 mm. may be 
 distinguished. They lay as if in a multilocular gap in the germ mass, 
 which however they did not quite fill. In each of these bodies there appeared 
 another but far smaller body. These aggregations were distinguished from 
 the germ by an especially beautiful intense violet gold chloride colouration 
 of their elements. The smaller elements contained in the larger were still 
 more intensely coloured than the larger. 
 
 He further states that these aggregations equal the segments 
 in number, and that the small bodies within the elements are 
 not always to be seen with the same distinctness. 
 
 Oellacher's description as well as his figures of these bodies 
 leaves no doubt in my mind that they are exactly similar bodies 
 to those which I have already spoken of as nuclei, and the 
 characteristic features of which I have shortly mentioned, and 
 shall describe more fully at a later stage. A moderately full 
 description of them is to be found in my preliminary paper 1 . 
 
 Their division into a series of separate areas each with a 
 deeply-stained body, as well as the staining of the whole of them, 
 exactly corresponds to what I have found. That each is a single 
 nucleus is quite certain, though their knobbed form might 
 occasionally lead to the view of their being divided. This 
 knobbed condition, observed by Oellacher as well as myself, 
 certainly supports the view, that they are in the act of budding 
 off fresh nuclei. Oellacher conceives, that the areas into which 
 these nuclei are divided represent a series of separate bodies 
 this according to my observations is not the case. Nuclei of the 
 same form have already been described in Nephelis, and are 
 probably not very rare. They pass by insensible gradations into 
 ordinary nuclei with numerous granules. 
 
 One marked feature of the segmentation of the Elasmobranch 
 egg is the continuous advance of the process of segmentation 
 into the yolk and the assimilation of this into the germ by 
 the direct formation of fresh segments out of it. Into the 
 significance of this feature I intend to enter fully hereafter ; but 
 it is interesting to notice that Oellacher's descriptions point to 
 a similar feature in the segmentation of Osseous Fish. This 
 however consists chiefly in the formation of fresh segments 
 
 1 Loc. cit. p. 415. [This Edition, p. 64.] 
 
SEGMENTATION. 243 
 
 from the lower parts of the germinal disc which in Osseous Fish 
 is more distinctly marked off from the food-yolk than in Elasmo- 
 branchs. 
 
 I conclude my description of the segmentation by a short 
 account of what other investigators have written about its 
 features in these fishes. One of the earliest descriptions of 
 this process was given by Leydig 1 . To his description of the 
 germinal disc, I have already done full justice. 
 
 In the first stage of segmentation which he observed 20 30 
 segments were already visible on the surface. In each of these 
 he recognized a nucleus but no nucleolus. 
 
 He rightly states that the segments have no membrane, and 
 describes the yolk-spherules which fill them. 
 
 The next investigator is Gerbe*. I have unfortunately been 
 unable to refer to this elaborate paper, but I gather from an 
 abstract that M. Gerbe has given a careful description of the 
 external features of segmentation. 
 
 Schenk 3 has also made important investigations on the sub- 
 ject. He considers that the ovum is invested with a very 
 delicate membrane. This membrane I have failed to find a 
 trace of, and agree with Leydig 4 in denying its existence. 
 Schenk further found that after impregnation, but before seg- 
 mentation, the germinal disc divided itself into two layers, 
 an upper and a lower. Between the two a cavity made its 
 appearance which Schenk looks upon as the segmentation 
 cavity. Segmentation commences in the upper of the two 
 layers, but Schenk does not give a precise account of the fate 
 of the lower. I have had no opportunity of investigating the 
 impregnated ovum before the commencement of segmentation, 
 but my observations upon the early stages of this process render 
 it clear that no division of the germinal disc exists subsequently 
 
 1 Rochen u. Haie. It is here mentioned that Coste observed the segmentation in 
 these fishes. 
 
 3 "Recherches sur la segmentation des products adventifs de 1'ceuf des Plagios- 
 tomes et particulierement des Raies." Robin, jfoitrnal de r Anatomic et de la Phy- 
 siologie, p. 609, 1872. 
 
 3 "Die Eier von Raja quadrimaculata innerhalb der Eileiter." Silz. der k. Akad. 
 Wien. Vol. LXXlll. 1873. 
 
 4 Loc. cit. My denial of the existence of this membrane naturally applies only to 
 the egg after impregnation, and to the genera Scyllium and Prisliurus. 
 
244 . DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 to the commencement of segmentation, and that the cavity 
 discovered by Schenk can have no connection whatever with 
 the segmentation cavity. I am indeed inclined to look upon 
 this cavity as an artificial product. I have myself met with 
 somewhat similar appearances, after the completion of segmen- 
 tation, which were caused by the non-penetration of my harden- 
 ing reagent beyond a certain point. 
 
 Without attempting absolutely to explain the appearances 
 described by Professor Schenk, I think that his observations 
 ought to be repeated, either by himself or some other competent 
 observer. 
 
 Several further facts are recorded by Professor Schenk in 
 his interesting paper. He states that immediately after im- 
 pregnation, the germinal disc presents towards the yolk a 
 strongly convex surface, and that at a later period, but still be- 
 fore the commencement of segmentation, this becomes flattened 
 out. He has further detected amoeboid movements in the disc 
 at the same period. As to the changes of the germinal disc 
 during segmentation, his paper contains no facts of importance. 
 
 Next in point of time to the paper of Schenk, is my own 
 preliminary account of the development of the Elasmobranch 
 Fishes 1 . In this a large number of the facts here described in 
 full are briefly alluded to. 
 
 The last author who has investigated the segmentation in 
 Elasmobranchs, is Dr Alexander Schultz 2 . He merely states 
 that he has observed the segmentation, and confirms Professor 
 Schenk's statements about the amoeboid movements of the 
 germinal disc. 
 
 EXPLANATION OF PLATE 6. 
 
 Fig. i. Section through the germinal disc of a ripe ovarian ovum of the Skate. 
 gv. germinal vesicle. 
 
 Fig. 2. Surface-view of a germinal disc with two furrows. 
 
 Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmen- 
 tation. 
 
 1 Loc. cit. 
 
 2 "Die Embryonal Anlage der Selachier. Vorlaufige Mittheilung," Centralblatt f. 
 Med. IViss. No. 33, 1875. 
 
SEGMENTATION. 245 
 
 Fig. 6. Section through the germinal disc represented in fig 3. . nucleus ; x. edge 
 of germinal disc. The engraver has not accurately copied my original drawings in 
 respect to the structure of the segmentation furrows. 
 
 Figs. 6 a and 6b. Two furrows of the same germinal disc more highly magnified. 
 
 Fig. 6c. A nucleus from the same germinal disc highly magnified. 
 
 Fig. 7. Section through a germinal disc of the same age as that represented in 
 fig. 4. n. nucleus; nx. modified nucleus ; nx'. modified nucleus of the yolk ; /. furrow 
 appearing in the yolk around the germinal disc. 
 
 Figs. 7 a, fi>, ic. Three segments with modified nuclei from the same germinal 
 disc. 
 
 Fig. 8. Section through a somewhat older germinal disc. ep. epiblast ; '. nuclei 
 of yolk. 
 
 Figs. 8 a, 8i>, Sc. Modified nuclei from the yolk from the same germinal disc. 
 
 Fig. 8 d. Segment in the act of division from the same germinal disc. 
 
 Fig. 9. Section through a germinal disc in which the segmentation is completed. 
 It shews the larger collection of cells at the embryonic end of the germinal disc than 
 at the non-embryonic, ep. epiblast. 
 
CHAPTER III. 
 FORMATION OF THE LAYERS. 
 
 IN the last chapter the blastoderm was left as a solid lens- 
 shaped mass of cells, thicker at one end than at the other, 
 its uppermost row of cells forming a distinct layer. There 
 very soon appears in it a cavity, the well-known segmenta- 
 tion cavity, or cavity of von Baer, which arises as a small space 
 in the midst of the blastoderm, near its non-embryonic end 
 (PI. 7, % i). 
 
 This condition of the segmentation cavity, though already 1 
 described, has nevertheless been met with in one case only. 
 The circumstance of my having so rarely met with this con- 
 dition is the more striking because I have cut sections of a 
 considerable number of blastoderms in the hope of encountering 
 specimens similar to the one figured, and it can only be explained 
 on one of the two following hypotheses. Either the stage is 
 very transitory, and has therefore escaped my notice except 
 in the one instance ; or else the cavity present in this instance 
 is not the true segmentation cavity, but merely some abnormal 
 structure. That this latter explanation is a possible one, appears 
 from the fact that such cavities do at times occur in other parts 
 of the blastoderm. Dr Schultz 2 does not mention having found 
 any stage of this kind. 
 
 The position of the cavity in question, and its general ap- 
 pearance, incline me to the view that it is the segmentation 
 cavity 3 . If this is the true view of its nature the fact should be 
 
 1 Qy. Journal of Microsc. Science, Oct. 1874. [This Edition, No. V.] 
 
 2 Ctntr. f. Med. Wiss. No. 38, 1875. 
 
 3 Professor Bambeke (" Poissons Osseux," Mem. Acad. Belgique 1875) describes a 
 cavity in the blastoderm of Leuciscus rutilus, which he regards as the true seg- 
 mentation cavity, but not as identical with the segmentation cavity of Osseous Fishes, 
 
FORMATION OF THE LAYERS. 247 
 
 noted that at first its floor is formed by the lower layer cells 
 and not by the yolk, and that its roof is constituted by both the 
 lower layer cells and the epiblast cells. The relations of the 
 floor undergo considerable modifications in the course of de- 
 velopment. 
 
 The other features of the blastoderm at this stage are very 
 much those of the previous stage. 
 
 The embryonic swelling is very conspicuous. The cells of 
 the blastoderm are still disposed in two layers : an upper one 
 of slightly columnar cells one deep, which constitutes the epi- 
 blast, and a lower one consisting of the remaining cells of the 
 blastoderm. 
 
 An average cell of the lower layer has a diameter of about 
 T^U inch, but the cells at the periphery of the layer are in some 
 cases considerably larger than the more central ones. All the 
 cells of the blastoderm are still completely filled with yolk 
 spherules. In the yolk outside the peculiar nuclei, before spoken 
 of, are present in considerable numbers. They seem to have 
 been mistaken by Dr Schultz 1 for cells: there can however be 
 no question that they are true nuclei. 
 
 In the next stage the relations of the segmentation cavity 
 undergo important modifications. 
 
 The cells which form its floor disappear almost completely 
 from that position, and the floor becomes formed by the yolk. 
 
 The stage, during which the yolk serves as the floor of the 
 segmentation cavity, extends over a considerable period of time, 
 but during it I have been unable to detect any important change 
 in the constitution of the blastoderm. It no doubt gradually 
 extends over the yolk, but even this growth is not nearly so 
 rapid as in the succeeding stage. Although therefore the stage 
 I proceed to describe is of long continuance, a blastoderm at 
 the beginning of it exhibits, both in its external and in its 
 internal features, no important deviations from one at the 
 end of it. 
 
 Viewed from the surface (PI. 8, fig. A) the blastoderm 
 
 usually so called. Its relations are the same as those of my segmentation cavity at 
 this stage. This paper came into my hands at too late a period for me to be able to 
 do more than refer to it in this place. 
 1 Loc. cit. 
 
248 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 at this stage appears slightly oval, but the departure from the 
 circular form is not very considerable. The long axis of the 
 oval corresponds with what eventually becomes the long axis 
 of the embryo. From the yolk the blastoderm is still well 
 distinguished by its darker colour ; and it is surrounded by a 
 concentric ring of light-coloured yolk, the outer border of which 
 shades insensibly into the normal yolk. 
 
 At the embryonic portion of the blastoderm is a slight 
 swelling, clearly shewn in Plate 8, fig. A, which can easily 
 be detected in fresh and in hardened embryos. This swelling is 
 to be looked upon as a local exaggeration of a slightly raised 
 rim present around the whole circumference of the blastoderm. 
 The roof of the segmentation cavity (fig. A, s. c.} forms a second 
 swelling ; and in the fresh embryo this region appears of a 
 darker colour than other parts of the blastoderm. 
 
 It is difficult to determine the exact shape of the blasto- 
 derm, on account of the traction exercised upon it in opening 
 the egg ; and no reliance can be placed on the forms assumed 
 by hardened blastoderms. This remark also applies to the 
 sections of blastoderms of this stage. There can be no doubt 
 that the minor individual variations exhibited by almost every 
 specimen are produced in the course of manipulations while the 
 objects are fresh. These variations may affect even the relative 
 length of a particular region and certainly the curvature of it. 
 The roof of the segmentation cavity is especially apt to be 
 raised into a dome-like form. 
 
 The main internal feature of this stage is the disappearance 
 of the layer of cells which, during the first stage, formed the 
 floor of the segmentation cavity. This disappearance is never- 
 theless not absolute, and it is doubtful whether there is any 
 period in which the floor of the cavity is quite without cells. 
 
 Dr Schultz supposes 1 that the entire segmentation cavity 
 is, in the living animal, filled with a number of loose cells. 
 Though it is not in my power absolutely to deny this, the 
 point being one which cannot be satisfactorily investigated in 
 sections, yet no evidence has come under my notice which 
 would lead to the conclusion that more cells are present in the 
 segmentation cavity than are represented on PI. 13, fig. i, of 
 
 1 Loc. cit. 
 
FORMATION OF THE LAYERS. 249 
 
 my preliminary paper 1 , an illustration which is repeated on PL 
 7, fig. 2. 
 
 The number of cells on the floor of the cavity differs con- 
 siderably in different cases, but these cases come under the 
 category of individual variations, and are not to be looked upon 
 as indications of different states of development. 
 
 In many cases especially large cells are to be seen on the 
 floor of the cavity (PI. 7, fig. 2, b d\ In my preliminary paper 2 
 the view was expressed that these are probably cells formed 
 around the nuclei of the yolk. This view I am inclined to 
 abandon, and to substitute for it the suggestion made by Dr 
 Schultz, that they are remnants of the larger segmentation cells 
 which were to be seen in the previous stages. 
 
 Plate 7, figs. 2, 3, 4 (all sections of this stage) shew the 
 different appearances presented by the floor of the segmentation 
 cavity. In only one of these sections are there any large number 
 of cells upon the floor ; and in no case have cells been observed 
 imbedded in the yolk forming this floor, as described by Dr 
 Schultz 3 , but in all cases the cells simply rested upon it. 
 
 Passing from the segmentation cavity to the blastoderm 
 itself, the first feature to be noticed is the more decided differ- 
 entiation of the epiblast. This now forms a distinct layer 
 composed of a single row of columnar cells. These are slightly 
 more columnar in the region of the embryonic swelling than 
 elsewhere, and become less elongated at the edge of the blasto- 
 derm. In my specimens this layer was never more than one 
 cell deep, but Dr Schultz 4 states that, in the Elasmobranch 
 embryos investigated by him, the epiblast was composed of 
 more than a single row of cells. 
 
 Each epiblast cell is filled with yolk-spherules and contains 
 a nucleus. Very frequently the nuclei in the layer are arranged 
 in a regular row (vide PI. 7, fig. 3). In the later blastoderms of 
 this stage there is a tendency in the cells to assume a wedge-like 
 form with their thin ends pointing alternately in opposite 
 
 1 Loc. cit. 
 
 - Qy. Journal of Micros. Science, Oct. 1874. [This Edition, No. V.] 
 
 3 Loc. cit. Probably Dr Schultz, here as in other cases,"has mistaken nuclei for 
 cells. 
 
 4 Loc. cit. 
 
 B. 17 
 
250 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 directions. This arrangement is, however, by no means strictly 
 adhered to, and the regularity of it is exaggerated in Plate 
 
 7, fig- 4- 
 
 The nuclei of the epiblast cells have the same characters as 
 those of the lower layer cells to be presently described, but their 
 intimate structure can only be successfully studied in certain 
 exceptionally favourable sections. In most cases the yolk- 
 spherules around them render the finer details invisible. 
 
 There is at this stage no such obvious continuity as in the 
 succeeding stage between the epiblast and the lower layer cells ; 
 and this statement holds good more especially with the best 
 conserved specimens which have been hardened in osmic acid 
 (PI. 7, fig. 4). In these it is very easy to see that the epiblast 
 simply thins out at the edge of the blastoderm without exhibit- 
 ing the slightest tendency to become continuous with the lower 
 layer cells 1 . 
 
 The lower layer cells form a mass rather than a layer, 
 and constitute the whole of the blastoderm not included in the 
 epiblast. The shape of this mass in a longitudinal section may 
 be gathered from an examination of Plate 7, figs. 3 and 4. 
 
 It presents an especially thick portion forming the bulk of 
 the embryonic swelling, and frequently contains one or two 
 cavities, which from their constancy I regard as normal and not 
 as artificial products. , 
 
 In addition to the mass forming the embryonic swelling 
 there is seen in sections another mass of lower layer cells at 
 the opposite extremity of the blastoderm, connected with the 
 
 1 Prof. Haeckel ("Die Gastrula u. die Eifurchung d. Thiere," Jenaische Zeit- 
 schrift, Vol. IX.) has unfortunately copied a figure from my preliminary paper (loc. cit.) 
 (repeated now), which I had carefully avoided using for the purpose of describing the 
 formation of the layers on account of the epiblast cells in the original having been 
 much altered by the chromic acid, as a result of which the whole section gives a 
 somewhat erroneous impression of the condition of the blastoderm at this stage. I 
 take this opportunity of pointing out that the colouration employed by Professor 
 Haeckel to distinguish the layers in this section is not founded on my statements, 
 but is, on the contrary, in entire opposition to them. From the section as represented 
 by Professor Haeckel it might be gathered that I considered the lower layer cells to 
 be divided into two parts, one derived from the epiblast, while the other constituted 
 the hypoblast. Not only is no such division present at this period, but no part of the 
 lower layer cells, or the mesoblast cells into which they become converted, can in any 
 sense whatever be said to be derived from the epiblast. 
 
FORMATION OF THE LAYERS. 251 
 
 former by a bridge of cells, which constitutes the roof of the 
 segmentation cavity. The lower layer cells may thus be divided 
 into three distinct parts : 
 
 (1) The embryo swelling. 
 
 (2) The thick rim of cells round the edge of the remainder 
 of the blastoderm. 
 
 (3) The cells which form the roof of the segmentation 
 cavity. 
 
 These three parts form a continuous whole, but in addition 
 to these there exist the previously mentioned cells, which rest on 
 the floor of the segmentation cavity. 
 
 With the exception of these latter, the lower layer is com- 
 posed of cells having a fairly uniform size, and exhibits no trace 
 of a division into two layers. 
 
 The cells are for the most part irregularly polygonal from 
 mutual pressure ; and in their shape and arrangement, exhibit a 
 marked contrast to the epiblast cells. A few of the lower layer 
 cells, highly magnified, are represented in PI. 7, fig. 2 a. An 
 average cell measures about ^ to -^ of an inch, but some of 
 the larger ones on the floor attain to the -^ of an inch. 
 
 Owing to my having had the good fortune to prepare some 
 especially favourable specimens of this stage, it has been possible 
 for me to make accurate observations both upon the nuclei of 
 the cells of the blastoderm, and upon the nuclei of the yolk. 
 
 The nuclei of the blastoderm cells, both of the epiblast and 
 lower layer, have a uniform structure. Those of the lower layer 
 cells are about y^- of an inch in diameter. Roughly speaking 
 each consists of a spherical mass of clear protoplasm refracting 
 more highly than the protoplasm of its cell. The nucleus 
 appears in sections to be divided by deeply stained lines into a 
 number of separate areas, and in each of these a deeply stained 
 granule is placed. In some cases two or more of such granules 
 may be seen in a single area. The whole of the nucleus stains 
 with the colouring reagents more deeply than the protoplasm 
 of the cells; but this is especially the case with the granules 
 and lines. 
 
 Though usually spherical the nuclei not infrequently have a 
 somewhat lobate form. 
 
 Very similar to these nuclei are the nuclei of the yolk. 
 
 172 
 
252 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 One of the most important differences between the two is 
 that of size. The majority of the nuclei present in the yolk are 
 as large or larger than an ordinary blastoderm cell ; while many 
 of them reach a size very much greater than this. The examples 
 I have measured varied from -gfo to -^ of an inch in diameter. 
 
 Though they are divided, like the nuclei of the blastoderm, 
 with more or less distinctness into separate areas by a network 
 of lines, their greater size frequently causes them to present an 
 aspect somewhat different from the nuclei of the blastoderm. 
 They are moreover much less regular in outline than these, and 
 very many of them have lobate projections (PI. 7, figs. 2 a and 
 2c and 3), which vary from simple knobs to projections of such a 
 size as to cause the nucleus to present an appearance of com- 
 mencing constriction into halves. When there are several such 
 projections the nucleus acquires a peculiar knobbed figure. With 
 bodies of this form it becomes in many cases a matter of great 
 difficulty to decide whether or no a particular series of knobs, 
 which appear separate in one plane, are united in a lower plane, 
 whether, in fact, there is present a single knobbed nucleus or a 
 number of nuclei in close apposition. A nucleus in this con- 
 dition is represented in PL J, fig. 2 b. 
 
 The existence of a protoplasmic network in the yolk has 
 already been mentioned. This in favourable cases may be 
 observed to be in special connection with the nuclei just de- 
 scribed. Its meshes are finer in the vicinity of the nuclei, and 
 its fibres in some cases almost appear to start from them (PI. 7, 
 fig. 12). For reasons which I am unable to explain the nuclei 
 of the yolk and the surrounding meshwork present appearances 
 which differ greatly according to the reagent employed. In 
 most specimens hardened in osmic acid the protoplasm of the 
 nuclei is apparently prolonged in the surrounding meshwork 
 (PI. 7, fig. 12). In other specimens hardened in osmic acid 
 (PI. 7, fig. 11), and in all hardened in chromic acid (PI. 7, fig. 2a 
 and 2c), the appearances are far clearer than in the previous 
 case, and the protoplasmic meshwork merely surrounds the nuclei, 
 without shewing any signs of becoming continuous with them. 
 
 There is also around each nucleus a narrow space in which 
 the spherules of the yolk are either much smaller than else- 
 where or completely absent, vide PI. 7, fig. 2b. 
 
FORMATION OF THE LAYERS. 253 
 
 It has not been possible for me to satisfy myself as to the 
 exact meaning of the lines dividing these nuclei into a number 
 of distinct areas. My observations leave the question open as to 
 whether they are to be looked upon as lines of division, or as 
 protoplasmic lines such as have been described in nuclei by 
 Flemming 1 , Hertwig 2 and Van Beneden 8 . The latter view ap- 
 pears to me to be the more probable one. 
 
 Such are the chief structural features presented by these 
 nuclei, which are present during the whole of the earlier periods 
 of development and retain throughout the same appearance. 
 There can be little doubt that their knobbed condition implies 
 that they are undergoing a rapid division. The arguments 
 for this view I have already insisted on, and, in spite of the 
 observations of Dr Kleinenberg shewing that similar nu- 
 clei of Nephelis do not undergo division, the case for their 
 doing so in the Elasmobranch eggs is to my mind a very 
 strong one. 
 
 During this stage the distribution of these nuclei in the yolk 
 becomes somewhat altered from that in the earlier stages. 
 Although the nuclei are still scattered generally throughout the 
 finer yolk- matter around the blastoderm, yet they are especially 
 aggregated at one or two points. In the first place a special 
 collection of them may be noticed immediately below the floor 
 of the segmentation cavity. They here form a distinct row 
 or even layer. If the presence of this layer is coupled with the 
 fact that at this period cells are beginning to appear on the floor 
 of the segmentation cavity, a strong argument is obtained for 
 the supposition that around these nuclei cells are being produced, 
 which pass into the blastoderm to form the floor. Of the actual 
 formation of cells at this period I have not been able to obtain 
 any satisfactory example, so that it remains a matter of de- 
 duction rather than of direct observation. 
 
 Another special aggregation of nuclei is generally present 
 at the periphery of the blastoderm, and the same amount of 
 doubt hangs over the fate of these as over that of the previously 
 mentioned nuclei. 
 
 1 "Entwicklungsgeschichte der Najaden," Sitz. d. k. Akad. IVien, 1875. 
 
 2 Morphologische Jahrbuch, Vol. I. Heft 3. 
 
 3 " Developpement des Mammiferes," Bui. de r Acad.de Relgiqnc, \\.. No. 12, 1875. 
 
254 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The next stage is the most important in the whole history 
 of the formation of the layers. Not only does it serve to shew, 
 that the process by which the layers are formed in Elasmo- 
 branchs can easily be derived from a simple gastrula type like 
 that of Amphioxus, but it also serves as the key by which other 
 meroblastic types of development may be explained. At the 
 very commencement of this stage the embryonic swelling be- 
 comes more conspicuously visible than it was. It now projects 
 above the level of the yolk in the form of a rim. At one point, 
 which eventually forms the termination of the axis of the 
 embryo, this projection is at its greatest ; while on either side of 
 this it gradually diminishes and finally vanishes. This pro- 
 jection I propose calling, as in my preliminary paper 1 , the em- 
 bryonic rim. 
 
 The segmentation cavity can still be seen from the surface, 
 and a marked increase in the size of the blastoderm may be 
 noticed. During the stage last described, the growth was but 
 very slight ; hence the rather sudden and rapid growth which 
 now takes place becomes striking. 
 
 Longitudinal sections at this stage, as at the earlier stages, 
 are the most instructive. Such a section on the same scale as 
 PI. 7, fig. 4, is represented in PI. 7, fig. 5. It passes parallel 
 to the long axis of the embryo, through the point of greatest 
 development of the embryonic ring. 
 
 The three fresh features of the most striking kind are (i) 
 the complete envelopment of the segmentation cavity within the 
 lower layer cells, (2) the formation of the embryonic rim, (3) the 
 increase in distance between the posterior end of the blastoderm 
 and the segmentation cavity. The segmentation cavity has by 
 no means relatively increased in size. The roof has precisely its 
 earlier constitution, being composed of an internal lining of 
 lower layer cells and an external one of epiblast. The thin 
 lining of lower layer cells is, in the course of mounting the 
 sections, very apt to fall off ; but I am absolutely satisfied that 
 it is never absent. 
 
 The floor of the cavity has undergone an important change, 
 being now formed by a layer of cells instead of by the yolk. A 
 
 1 Qy. Journal Microsc. Science, Oct. 1874. [This Edition, No. V.] 
 
FORMATION OF THE LAYERS. 255 
 
 precisely similar but more partial change in the constitution of 
 the floor takes place in Osseous Fishes 1 . 
 
 The mode in which the floor is formed is a question of some 
 importance. The nuclei, which during the last stage formed 
 a row beneath it, probably, as previously pointed out, take some 
 share in its formation. An additional argument to those already 
 brought forward in favour of this view may be derived from 
 the fact that during this stage such a row of nuclei is no longer 
 present. 
 
 This argument may be stated as follows : 
 
 Before the floor of cells for the segmentation cavity is formed 
 a number of nuclei are present in a suitable situation to supply' 
 the cells for the floor ; as soon as the floor of cells makes its 
 appearance these nuclei are no longer to be seen. From this 
 it may be concluded that their disappearance arises from 
 their having become the nuclei of the cells which form the 
 floor. 
 
 It appears to me most probable that there is a growth in- 
 wards from the whole peripheral wall of the cavity, and that this 
 ingrowth, as well as the cells derived from the yolk, assist in 
 forming the floor of the cavity. In Osseous Fish there appears 
 to be no doubt that the floor is largely formed by an ingrowth of 
 this kind. 
 
 A great increase is observable in the distance between the 
 posterior end of the segmentation cavity and the edge of the 
 blastoderm. This is due to the rapid growth of the latter com- 
 bined with the stationary condition of the former. The growth 
 of the blastoderm at this period is not uniform, but is more rapid 
 in the non-embryonic than in the embryonic parts. 
 
 The main features of the epiblast remain the same as during 
 the last stages. It is still composed of a very distinct layer one 
 cell deep. Over the segmentation cavity, and over the whole 
 embryonic end of the blastoderm, the cells are very thin, 
 columnar, and, roughly speaking, wedge-shaped with the thin 
 ends pointing alternately in different directions. For this reason, 
 the nuclei form two rows ; but both the rows are situated near 
 the upper surface of the layer (vide PI. 7, fig. 5) Towards the 
 
 1 Gotte, "Der Keim d. Forelleneies, " Arch. f. Mikr. Anat. Vol. ix.; Haeckel, 
 "Die Gastrula u. die Eifurchung d. Thiere," Jenaische Zeitschrift, Bd. ix. 
 
256 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 posterior end of the blastoderm the cells are flatter and broader ; 
 and the layer terminates at the non-embryonic end of the 
 blastoderm without exhibiting the slightest tendency to become 
 continuous with the lower layer cells. At the embryonic end of 
 the blastoderm the relations of the epiblast and lower layer 
 cells are very different. At this part, throughout the whole 
 extent of the embryonic rim, the epiblast is reflected and be- 
 comes continuous with the lower layer cells. 
 
 The lower layer cells form, for the most part, a uniform 
 stratum in which no distinction into mesoblast and hypoblast 
 is to be seen. 
 
 Both the lower layer cells and the epiblast cells are still 
 filled with yolk-spherules. 
 
 The structures at the embryonic rim, and the changes which 
 are there taking place, unquestionably form the chief features of 
 interest at this stage. 
 
 The general relations of these parts are very fairly shewn 
 in PL 7, fig. 5, which represents a section passing through the 
 median line of the embryonic region. They are however more 
 accurately represented in PL 7, fig. 5#, taken from the same 
 embryo, but in a lateral part of the embryonic rim ; or in PL 7, 
 fig. 6, from a slightly older embryo. In all of these figures the 
 epiblast cells are reflected at the edge of the embryonic rim, and 
 become perfectly continuous with the hypoblast cells. A few of 
 the cells, immediately beyond the line of this reflection, precisely 
 resemble in character the typical epiblast cells ; but the remainder 
 exhibit a gradual transition into typical lower layer cells. Ad- 
 joining these transitional cells, or partly enclosed in the corner 
 formed between them and the epiblast, are a few unaltered lower 
 layer cells (m], which at this stage are not distinctly separated 
 from the transitional cells. The transitional cells form the com- 
 mencement of the hypoblast (hy) ; and the cells (m) between 
 them and the epiblast form the commencement of the mesoblast. 
 The gradual conversion of lower layer cells into columnar 
 hypoblast cells, is a very clear and observable phenomenon in 
 the best specimens. Where the embryonic rim projects most, a 
 larger number of cells have assumed a columnar form. Where 
 it projects less clearly, a smaller number have done so. But 
 in all cases there may be observed a series of gradations be- 
 
FORMATION OF THE LAYERS. 257 
 
 tween the columnar cells and the typical rounded lower layer 
 cells 1 . 
 
 In the last described embryo, although the embryonic rim 
 had attained to a considerable development, no trace of the 
 medullary groove had made its appearance. In an embryo in 
 the next stage of which I propose describing sections, this struc- 
 ture has become visible. 
 
 A surface view of a blastoderm of this age, with the embryo, 
 is represented on PI. 8, fig. B ; and I shall, for the sake of con- 
 venience, in future speak of embryos of this age as belonging to 
 period B. 
 
 The blastoderm is nearly circular. The embryonic rim is 
 represented by a darker shading at the edge. At one point 
 in this rim may be seen the embryo, consisting of a somewhat 
 raised area with an axial groove (mg). The head end of the 
 embryo is that which points towards the centre of the blasto- 
 derm, and its free peripheral extremity is at the edge of the 
 blastoderm. 
 
 A longitudinal section of an embryo of the same age as the 
 one figured 2 is represented on PL 7, fig. 7. The general growth 
 has been very considerable, though as before explained, it is 
 mainly confined to that part of the blastoderm where the 
 embryonic rim is absent. 
 
 A fresh feature of great importance is the complete dis- 
 appearance of the segmentation cavity, the place which was 
 previously occupied by it being now filled up by an irregular 
 network of cells. There can be little question that the oblite- 
 ration of the segmentation cavity is in part due to the entrance 
 into the blastoderm of fresh cells formed around the nuclei of the 
 yolk. The formation of these is now taking place with great 
 rapidity and can be very easily followed. 
 
 Since the segmentation cavity ceases to play any further 
 part in the history of the blastoderm, it will be well shortly to 
 review the main points in its history. 
 
 1 When writing my earlier paper I did not feel so confident about the mode of 
 formation of the hypoblast as I now do, and even doubted the possibility of determining 
 it from sections. The facts now brought forward are I hope sufficient to remove all 
 scepticism on this point. 
 
 a Owing to the small size of the plates this section has been drawn on a con- 
 siderably smaller scale than that represented in fig. 5. 
 
258 DEVELOPMENT OF ELASMOBRANCII FISHES. 
 
 Its earliest appearance is involved in some obscurity, though 
 it probably arises as a simple cavity in the midst of the lower 
 layer cells (PI. 7, fig. i). In its second phase the floor ceases 
 to be formed of lower layer cells, and the place of these is 
 taken by the yolk, on which however a few scattered cells 
 still remain (PI. 7, figs. 2, 3, 4). During the third period of 
 its history, a distinct cellular floor is again formed for it, so 
 that it comes a second time into the same relations with the 
 blastoderm as at its earliest appearance. The floor of cells 
 which it receives is in part due to a growth inwards from the 
 periphery of the blastoderm, and in part to the formation of 
 fresh cells from the yolk. Coincidently with the commencing 
 differentiation of hypoblast and mesoblast the segmentation 
 cavity grows smaller and vanishes. 
 
 One of the most important features of the segmentation 
 cavity in the Elasmobranchs which I have studied, is the fact 
 that throughout its whole existence its roof is formed of lower 
 layer cells. There is not the smallest question that the seg- 
 mentation cavity of these fishes is the homologue of that of 
 Amphioxus, Batrachians, etc., yet in the case of all of these 
 animals, the roof of the segmentation cavity is formed of epiblast 
 only. How comes it then to be formed of lower layer cells in 
 Elasmobranchii ? 
 
 To this question an answer was attempted in my paper, 
 "Upon the Early Stages of the Development of Vertebrates 1 ." 
 It was there pointed out, that as the food material in the ovum 
 increases, the bulk of the lower layer cells necessarily also in- 
 creases ; since these, as far as the blastoderm is concerned, are 
 the chief recipients of food material. This causes the lower layer 
 cells to encroach upon the segmentation cavity, and to close it 
 in not only on the sides, but also above ; from the same cause it 
 results that the lower layer cells assume, from the first, a position 
 around the spot where the future alimentary cavity will be 
 formed, and that this cavity becomes formed by a simple split in 
 the midst of the lower layer cells, and not by an involution. 
 
 All the most recent observations 2 on Osseous Fishes tend 
 
 1 Quart. Journ.of Microscop. Science, July, 1875. [This Edition, No. VI.] 
 
 2 Oellacher, Zeit. f. Wiss. Zoologie, Bd. xxin. Gotte, Archiv f. Mikr. Anat. 
 Vol. ix. Haeckel, loc. cit. 
 
FORMATION OF THE LAYERS. 259 
 
 to shew that in them, the roof of the segmentation cavity is 
 formed alone of epiblast ; but on account of the great difficulty 
 which is experienced in distinguishing the layers in the blasto- 
 derms of these animals, I still hesitate to accept as conclusive 
 the testimony on this point. 
 
 In the formation a second time of a cellular floor for the 
 segmentation cavity in the third stage, the Elasmobranch embryo 
 seems to resemble that of the Osseous Fish 1 . Upon this feature 
 great stress is laid both by Dr Gotte 2 and Prof. Haeckel 3 : but I 
 am unable to agree with the interpretation of it offered by them. 
 Both Dr Gotte and Prof. Haeckel regard the formation of this 
 floor as part of an involution to which the lower layer cells owe 
 their origin, and consider the involution an equivalent to the 
 alimentary involution of Batrachians, Amphioxus, &c. To this 
 question I hope to return, but it may be pointed out that my 
 observations prove that this view can only be true in a very 
 modified sense ; since the invagination by which hypoblast and 
 alimentary canal are formed in Amphioxus is represented in 
 Elasmobranchs by a structure quite separate from the ingrowth 
 of cells to form the floor of the segmentation cavity. 
 
 The eventual obliteration of the segmentation cavity by cells 
 derived from the yolk is to be regarded as an inherited remnant 
 of the involution by which this obliteration was primitively 
 effected. The passage upwards of cells from the yolk, may 
 possibly be a real survival of the tendency of the hypoblast cells 
 to grow inwards during the process of involution. 
 
 The last feature of the segmentation cavity which deserves 
 notice is its excentric position. It is from the first situated in 
 much closer proximity to the non-embryonic than to the embry- 
 onic end of the blastoderm. This peculiarity in position is also 
 characteristic of the segmentation cavity of Osseous Fishes, as is 
 shewn by the concordant observations of Oellacher 4 and Gotte 6 . 
 Its meaning becomes at once intelligible by referring to the 
 diagrams in my paper 6 on the Early Stages in the Development 
 of Vertebrates. It in fact arises from the asymmetrical character 
 
 1 This floor appears in most Osseous Fish to be only partially formed. Vide 
 Gotte, loc. at. 
 
 8 Loc. cit. s Loc. cit. * Loc. cit. 
 
 b Loc. cit. 6 Loc. cit. 
 
2<5<D DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of the primitive alimentary involution in all anamniotic verte- 
 brates with the exception of Amphioxus. 
 
 Leaving the segmentation cavity I pass on to the other 
 features of my sections. 
 
 There is still to be seen a considerable aggregation of cells 
 at the non-embryonic end of the blastoderm. The position of 
 this, and its relations with the portion of the blastoderm which 
 at an earlier period contained the segmentation cavity, indicate 
 that the growth of the blastoderm is not confined to its edge, 
 but that it proceeds at all points causing the peripheral parts to 
 glide over the yolk. 
 
 The main features of the cells of this blastoderm are the 
 same as they were in the one last described. In the non- 
 embryonic region the epiblast has thinned out, and is composed 
 of a single row of cells, which, in the succeeding stages, become 
 much flattened. 
 
 The lower layer cells over the greater part of their extent, 
 have not undergone any histological changes of importance. 
 Amongst them may frequently be seen a few exceptionally 
 large cells, which without doubt have been derived directly 
 from the yolk. 
 
 The embryonic rim is now a far more considerable structure 
 than it was. Vide PI. 7, fig. 7. Its elongation is mainly effected 
 by the continuous conversion of rounded lower layer cells into 
 columnar hypoblast cells at its central or anterior extremity. 
 
 This conversion of the lower layer cells into hypoblast cells 
 is still easy to follow, and in every section cells intermediate 
 between the two are to be seen. The nature of the changes 
 which are taking place requires for its elucidation transverse as 
 well as longitudinal sections. Transverse sections of a slightly 
 older embryo than B are represented on PI. 7, fig. 8 a, 8& 
 and 8^. 
 
 Of these sections a is the most peripheral or posterior, and c 
 the most central or anterior. By a combination of transverse 
 and longitudinal sections, and by an inspection qf a surface view, 
 it is rendered clear that, though the embryonic rim is a far more 
 considerable structure in the region of the embryo than else- 
 where (compare fig. 6 and fig. 7 and 7), yet that this gain in 
 size is not produced by an outgrowth of the embryo beyond 
 
FORMATION OF THE LAYERS. 26 1 
 
 the rest of the germ, but by the conversion of the lower layer 
 cells into hypoblast having been carried far further towards the 
 centre of the germ in the axial line than in the lateral regions of 
 the rim. 
 
 The most anterior of the series of transverse sections-(Pl. 7, 
 fig. 8<r) I have represented, is especially instructive with reference 
 to this point. Though the embryonic rim is cut through at 
 the sides of the section, yet in these parts the rim consists 
 of hardly more than a continuity between epiblast and lower 
 layer cells, and the lower layer cells shew no trace of a division 
 into mesoblast and hypoblast. In the axis of the embryo, how- 
 ever, the columnar hypoblast is quite distinct ; and on it a small 
 cap of mesoblast is seen on each side of the medullary groove. 
 Had the embryonic rim resulted from a projecting growth of the 
 blastoderm, such a condition could not have existed. It might 
 have been possible to find the hypoblast formed at the sides 
 of the section and not at the centre ; but the reverse, as in these 
 sections, could not have occurred. Indeed it is scarcely necessary 
 to have recourse to sections to prove that the growth of the 
 embryonic rim is towards the centre of the blastoderm. The 
 inspection of a surface view of a blastoderm at this period 
 demonstrates it beyond a doubt (PI. 8, fig. B). The embryo, 
 close to which the embryonic rim is alone largely developed, 
 does not project outwards beyond the edge of the germ, but 
 inwards towards its centre. 
 
 The space between the embryonic rim and the yolk (PL 7, 
 fig. 7 al.} is the alimentary cavity. The roof of this is therefore 
 primitively formed of hypoblast and the floor of yolk. The ex- 
 ternal opening of this space at the edge of the blastoderm is the 
 exact morphological homologue of the anus of Rusconi, or 
 blastopore of Amphioxus, the Amphibians, &c. The importance 
 of the mode of growth in the embryonic rim depends upon the 
 homology of the cavity between it and the yolk, with the alimen- 
 tary cavity of Amphioxus and Amphibians. Since this homology 
 exists, the direction of the growth of this cavity ought to be, 
 as it in fact is, the same as in Amphioxus, etc., viz. towards the 
 centre of the germ and original position of the segmentation 
 cavity. Thus though a true invagination is not present as in 
 the other cases, yet this is represented in Elasmobranchs by the 
 
262 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 continuous conversion of lower layer cells into hypoblast along a 
 line leading towards the centre of the blastoderm. 
 
 In the parts of the rim adjoining the embryo, the lower layer 
 cells, on becoming continuous with the epiblast cells, assume a 
 columnar form. At the sides of the rim this is not strictly the 
 case, and the lower layer cells retain their rounded form, though 
 quite continuous with the epiblast cells. One curious feature 
 of the layer of epiblast in these lateral parts of the rim is the 
 great thickness it acquires before being reflected and becoming 
 continuous with the hypoblast (PI. 7, fig. 8*:). In the vicinity 
 of the point of reflection there is often a rather large formation 
 of cells around the nuclei of the yolk. The cells formed here 
 no doubt pass into the blastoderm, and become converted into 
 columnar hypoblast cells. In some cases the formation of these 
 cells is very rapid, and they produce quite a projection on the 
 under side of the hypoblast. Such a case is represented .in 
 PI. 7, fig. 8b, n. al. The cells constituting this mass eventually 
 become converted into the lateral and ventral walls of the alimen- 
 tary canal. 
 
 The formation of the mesoblast has progressed rapidly. 
 While many of the lower layer cells become columnar and form 
 the hypoblast, others, between these and the epiblast, remain 
 spherical. The latter do not at once become separated as a 
 layer distinct from the hypoblast, and, at first, are only to be 
 distinguished from them through their different character, vide 
 Plate 7, figs. 6 and 7. They nevertheless constitute the com- 
 mencing mesoblast. 
 
 Thus much of the mode of formation of the mesoblast can 
 be easily made out in longitudinal sections, but transverse sec- 
 tions throw still further light upon it. 
 
 From these it may at once be seen that the mesoblast is 
 not formed in one continuous sheet, but as two lateral masses, 
 one on each side of the axial line of the embryo 1 . In my 
 
 1 Professor Lieberkiihn (Gesellschaft zu Marburg, Jan. 1876) finds in Mammalia a 
 bilateral arrangement of the mesoblast, which he compares with that described by me 
 in Elasmobranchs. In Mammalia, however, he finds the two masses of mesoblast 
 connected by a very thin layer of cells, and is apparently of opinion that a similar 
 thin layer exists in Elasmobranchs though overlooked by me. I can definitely state 
 that, whatever may be the condition of the mesoblast in Mammalia, in Elasmobranchs 
 at any rate no such layer exists. 
 
FORMATION OF THE LAYERS. 263 
 
 preliminary account 1 it was stated that this was a condition 
 of the mesoblast at a very early period, and that it was probably 
 its condition from the beginning. Sections are now in my 
 possession which satisfy me that, from the very first, the meso- 
 blast arises as two distinct lateral masses, one on each side~of the 
 axial line. 
 
 In the embryo from which the sections PL 7, fig. 8 a, 86, 
 8c were taken, the mesoblast had, in most parts, not yet become 
 separated from the hypoblast. It still formed with this a con- 
 tinuous layer, though the mesoblast cells were distinguishable by 
 their shape from the hypoblast. In only one section (&} was any 
 part of the mesoblast quite separated from the hypoblast. 
 
 In the hindermost part of the embryo the mesoblast is at its 
 maximum, and forms, on each side, a continuous sheet extending 
 from the median line to the periphery (fig. 8 a). The rounder 
 form of the mesoblast cells renders the line of junction between 
 the layer constituted by them and the hypoblast fairly distinct ; 
 but towards the periphery, where the hypoblast cells have the 
 same rounded form as the mesoblast, the fusion between the two 
 layers is nearly complete. 
 
 In an anterior section the mesoblast is only present as a cap 
 on both sides of the medullary groove, and as a mass of cells 
 at the periphery of the section (fig. 8b] ; but no continuous layer 
 of it is present In the foremost of the three sections (fig. 8^) 
 the mesoblast can scarcely be said to have become in any 
 way separated from the hypoblast except at the summit of the 
 medullary folds (m). 
 
 From these and similar sections it maybe certainly concluded, 
 that the mesoblast becomes first separated from the hypoblast 
 as a distinct layer in the posterior region of the embryo, and 
 only at a later period in the region of the head. 
 
 In an embryo but slightly more developed than B, the forma- 
 tion of the layer is quite completed in the region of the embryo. 
 To this embryo I now pass on. 
 
 In the non-embryonic parts of the blastoderm no fresh fea- 
 tures of interest have appeared. It still consists of two layers. 
 The epiblast is composed of flattened cells, and the lower layer 
 of a network of more rounded cells, elongated in a lateral 
 
 1 Loc. fit. 
 
264 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 direction. The growth of the blastoderm has continued to be 
 very rapid. 
 
 In the region of the embryo (PI. 7, fig. 9) more important 
 changes have occurred. The epiblast still remains as a single 
 row of columnar cells. The hypoblast is no longer fused with 
 the mesoblast, and forms a distinct dorsal wall for the alimentary 
 cavity. Though along the axis of the embryo the hypoblast is 
 composed of a single row of columnar cells, yet in the lateral 
 part of the embryo its cells are less columnar and are one or 
 two deep. 
 
 Owing to the manner in which the mesoblast became split 
 off from the hypoblast, a continuity is maintained between the 
 hypoblast and the lower layer cells of the blastoderm (PI. 7, 
 fig- 9)> while the two plates of mesoblast are isolated and dis- 
 connected from any other masses of cells. 
 
 The alimentary cavity is best studied in transverse sections. 
 (Vide PI. 7, fig. ioa, lob and loc, three sections from the same 
 embryo.) It is closed in above and at the sides by the hypoblast, 
 and below by the yolk. In its anterior part a floor is commencing 
 to be formed by a growth of cells from the walls of the two 
 sides. The cells for this growth are formed around the nuclei 
 of the yolk ; a feature which recalls the fact that in Amphibians 
 the ventral wall of the alimentary cavity is similarly formed in 
 part from the so-called yolk cells. 
 
 We left the mesoblast as two masses not completely sepa- 
 rated from the hypoblast. During this stage the separation 
 between the two becomes complete, and there are formed two 
 great lateral plates of mesoblast cells, one on each side of the 
 medullary groove. Each of these corresponds to a united 
 vertebral and lateral plate of the higher Vertebrates. The plates 
 are thickest in the middle and posterior regions (PI. 7, fig. ioa 
 and iob], but thin out and almost vanish in the region of the 
 head. The longitudinal section of this stage represented in PL 7, 
 fig. 9, passes through one of the lateral masses of mesoblast cells, 
 and shews very distinctly its complete independence of all the 
 other cells in the blastoderm. 
 
 From what has been stated with reference to the develop- 
 ment of the mesoblast, it is clear that in Elasmobranchs this 
 layer is derived from the same mass of cells as the hypoblast, 
 
FORMATION OF THE LAYERS. 265 
 
 and receives none of its elements from the epiblast. In connec- 
 tion with its development, as two independent lateral masses, 
 I may observe, as I have previously done 1 , that in this respect 
 it bears a close resemblance to mesoblast in Euaxes, as de- 
 scribed by Kowalevsky 2 . This resemblance is of some interest, 
 as bearing on a probable Annelid origin of Vertebrata. Kow- 
 alevsky has also shewn 3 that the mesoblast in Ascidians is 
 similarly formed as two independent masses, one on each side 
 of the middle line. 
 
 It ought, however, to be pointed out that a similar bilateral 
 origin of the mesoblast had been recently met with in Lym- 
 naeus by Carl Rabl 4 . A fact which somewhat diminishes the 
 genealogical value of this feature in the mesoblast in Elasmo- 
 branchs. 
 
 During the course of this stage the spherules of food-yolk 
 immediately beneath the embryo are used up very rapidly. As 
 a result of this the protoplasmic network, so often spoken of, 
 comes very plainly into view. Considerable areas may some- 
 times be seen without any yolk-spherule whatever. 
 
 On PI. 7, fig. ja, and figs. II and 12, I have attempted to 
 reproduce the various appearances presented by this network : 
 and these figures give a better idea of it than any description. 
 My observations tend to shew that it extends through the whole 
 yolk, and serves to hold it together. It has not been possible 
 for me to satisfy myself that it had any definite limits, but on 
 the other hand, in many parts all my efforts to demonstrate its 
 presence have failed. When the yolk-spherules are very thickly 
 packed, it is difficult to make out for certain whether it is present 
 or absent, and I have not succeeded in removing the yolk- 
 spherules from the network in cases of this kind. In medium- 
 sized ovarian eggs this network is very easily seen, and extends 
 through the whole yolk. Part of such an egg is shewn in PI. 7, 
 
 1 Quart. Journ. of Micro sc. Science, Oct., 1874. [This Edition, No. V.] 
 
 2 "Embryologische Studien an Wiirmern u. Arthropoden." Mtmoires de r Acad, 
 S. Pctersbourg. Vol. xiv. 1873. 
 
 3 Archivfur Mikr. Anat. Vol. vii. 
 
 4 Jenaische Zeitschrift, Vol. ix. 1875. A bilateral development of mesoblast, 
 according to Professor Haeckel (loc. cit.), occurs in some Osseous Fish. Hensen, 
 Zcit.fiir Anat. u. Ent-w. Vol. i., has recently described the mesoblast in Mammalia 
 as consisting of independent lateral masses. 
 
 B. 18 
 
266 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 fig. 14. In full-sized ovarian eggs, according to Schultz 1 , it 
 forms, as was mentioned in the first chapter, radiating striae, 
 extending from the centre to the periphery of the egg. When 
 examined with the highest powers, the lines of this network 
 appear to be composed of immeasurably small granules arranged 
 in a linear direction. These granules are more distinct in chromic 
 acid specimens than in those hardened in osmic acid, but are to 
 be seen in both. There can be little doubt that these granules 
 are imbedded in a thread or thin layer of protoplasm. 
 
 I have already (p. 252) touched upon the relation of this 
 network to the nuclei of the yolk 2 . 
 
 During the stages which have just been described specially 
 favourable views are frequently to be obtained of the formation 
 of cells in the yolk and their entrance into the blastoderm. 
 Two representations of these are given, in PI. 7, fig. 7 a, and 
 fig. 13. In both of these distinctly circumscribed cells are to be 
 seen in the yolk (c), and in all cases are situated near to the 
 typical nuclei of the yolk. The cells in the yolk have such a 
 relation to the surrounding parts, that it is quite certain that 
 their presence is not due to artificial manipulation, and in some 
 cases it is even difficult to decide whether or no a cell area is 
 circumscribed round a nucleus (PI. 7, fig. 13). Although it would 
 be possible for cells in the living state to pass from the blasto- 
 derm into the yolk, yet the view that they have done so in the 
 cases under consideration has not much to recommend it, if the 
 following facts be taken into consideration, (i) That the cells 
 
 1 Archivfiir Mikr. Anat. Vol. XI. 
 
 2 A protoplasmic network resembling in its essential features the one just de- 
 scribed has been noticed by many observers in other ova. Fol has figured and 
 described a network or sponge-like arrangement of the protoplasm in the eggs of 
 Geryonia. (JenaischeZeitschrift, Vol. vn.) Metschnikoff (Zeitschrift f. Wiss. Zoologie, 
 1874) nas demonstrated its presence in the ova of many Siphonophorire and Medusae. 
 Flemming (" Entwicklungsgeschichte der Najaden," Sitz. der k. Akad. Wien, 1875) has 
 found it in the ovarian ova of fresh-water mussels (Anodonta and Unio), but regards 
 it as due to the action of reagents, since he fails to find it in the fresh condition. 
 Amongst vertebrates it has been carefully described by Eimer (Archiv fur Mikr. 
 Anat., Vol. Vlll.) in the ovarian ova of Reptiles. Eimer moreover finds that it is 
 continuous with prolongations from cells of the epithelium of the follicle in which 
 the ovum is contained. According to him remnants of this network are to be met 
 with in the ripe ovum, but are no longer present in the ovum when taken from the 
 oviduct. 
 
FORMATION OF THE LAYERS. 267 
 
 in the yolk are frequently larger than those in the blastoderm. 
 (2) That there are present a very large number of nuclei in the 
 yolk which precisely resemble the nuclei of the cells under 
 discussion. (3) That in some cases (PI. 7, fig. 13) cells are seen 
 indistinctly circumscribed as if in the act of being formed.~ 
 
 Between the blastoderm and the yolk may frequently be 
 seen a membrane-like structure, which becomes stained with 
 haematoxylin, osmic acid etc. It appears to be a layer of 
 coagulated albumen and not a distinct membrane. 
 
 SUMMARY. 
 
 At the close of segmentation, the blastoderm forms a some- 
 what lens-shaped disc, thicker at one end than at the other ; the 
 thicker end being termed the embryonic end. 
 
 It is divided into two layers an upper one, the epiblast, 
 formed by a single row of columnar cells ; and a lower one, con- 
 sisting of the remaining cells of the blastoderm. 
 
 A cavity next appears in the lower layer cells, near the non- 
 embryonic end of the blastoderm, but the cells soon disappear 
 from the floor of this cavity which then comes to be constituted 
 by yolk alone. 
 
 The epiblast in the next stage is reflected for a small arc at 
 the embryonic end of the blastoderm, and becomes continuous 
 with the lower layer cells ; at the same time some of the lower 
 layer cells of the embryonic end of the blastoderm assume a 
 columnar form, and constitute the commencing hypoblast. The 
 portion of the blastoderm, where epiblast and hypoblast are 
 continuous, forms a projecting structure which I have called the 
 embryonic rim. This rim increases rapidly by growing inwards 
 more and more towards the centre of the blastoderm, through 
 the continuous conversion of lower layer cells into columnar 
 hypoblast. 
 
 While the embryonic rim is being formed, the segmentation 
 cavity undergoes important changes. In the first place, it receives 
 a floor of lower layer cells, partly from an ingrowth from the 
 two sides, and parti}- from the formation of cells around the 
 nuclei of the yolk. 
 
 1 8 2 
 
268 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Shortly after the floor of cells has appeared, the whole seg- 
 mentation cavity becomes obliterated. 
 
 When the embryonic rim has attained to some importance, 
 the position of the embryo becomes marked out by the appear- 
 ance of the medullary groove at its most projecting part. The 
 embryo extends from the edge of the blastoderm inwards to- 
 wards the centre. 
 
 At about the time of the formation of the medullary groove, 
 the mesoblast becomes definitely constituted. It arises as two 
 independent plates, one on each side of the medullary groove, 
 and is entirely derived from lower layer cells. 
 
 The two plates of mesoblast are at first unconnected with any 
 other cells of the blastoderm, and, on their formation, the hypo- 
 blast remains in connection with all the remaining lower layer 
 cells. Between the embryonic rim and the yolk is a cavity, 
 the primitive alimentary cavity. Its roof is formed of hypo- 
 blast, and its floor of yolk. Its external opening is homologous 
 with the anus of Rusconi, of Amphioxus and the Amphibians. 
 The ventral wall of the alimentary cavity is eventually derived 
 from cells formed in the yolk around the nuclei which are there 
 present. 
 
 Since the important researches of Gegenbaur 1 upon the 
 meroblastic vertebrate eggs, it has been generally admitted that 
 the ovum of every vertebrate, however complicated may be its 
 apparent constitution, is nevertheless to be regarded as a simple 
 cell. This view is, indeed, opposed by His 2 and to a very 
 modified extent by Waldeyer 3 , and has recently been attacked 
 from an entirely new standpoint by Gotte 4 ; but, to my mind, 
 the objections of these authors do not upset the well founded 
 conclusions of previous observations. 
 
 1 "Wirbelthiereier mit partieller Dottertheilung." Miiller's Arch. 1861. 
 ' 2 Erste Anlage des Wirbelthierleibes . 
 
 3 Eierstock u. Ei. 
 
 4 Entwicklungsgeschichte der Unke. The important researches of Gotte on the 
 development of the ovum, though meriting the most careful attention, do not admit of 
 discussion in this place. 
 
FORMATION OF THE LAYERS. 269 
 
 As soon as the fact is recognised that both meroblastic and 
 holoblastic eggs have the same fundamental constitution, the 
 admission follows, naturally, though not necessarily, that the 
 eggs belonging to these two classes differ solely in degree, not 
 only as regards their constitution, but also as regards the rrratvner 
 in which they become respectively converted into the embryo. 
 As might have been anticipated, this view has gained a wide 
 acceptance. 
 
 Amongst the observations, which have given a strong objective 
 support to this view, may be mentioned those of Professor 
 Lankester upon the development of Cephalopoda 1 , and of 
 Dr Gotte 2 upon the development of the Hen's egg. In Loligo 
 Professor Lankester shewed that there appeared, in the part 
 of the egg usually considered as food-yolk, a number of bodies, 
 which eventually developed a nucleus and became cells, and that 
 these cells entered into the blastoderm. These observations 
 demonstrate that in the eggs of Loligo the so-called food-yolk is 
 merely equivalent to a part of the egg which in other cases 
 undergoes segmentation. 
 
 The observations of Dr Gotte have a similar bearing. He 
 made out that in the eggs of the Hen no sharp line is to be 
 found separating the germinal disc from the yolk, and that, 
 independently of the normal segmentation, a number of cells 
 are derived from that part of the egg hitherto regarded as 
 exclusively food-yolk. This view of the nature of the food-yolk 
 was also advanced in my preliminary account of the develop- 
 ment of Elasmobranchs 3 , and it is now my intention to put 
 forward the positive evidence in favour of this view, which is 
 supplied from a knowledge of the phenomena of the develop- 
 ment of the Elasmobranch ovum ; and then to discuss how far 
 the facts of the growth of the blastoderm in Elasmobranchs 
 accord with the view that their large food-yolk is exactly 
 equivalent to part of the ovum, which in Amphibians undergoes 
 segmentation, rather than some fresh addition, which has no 
 equivalent in the Amphibian or other holoblastic ovum. 
 
 Taking for granted that the ripe ovum is a single cell, the 
 
 1 Annals and Magaz. of Natural History, Vol. xi. 1873, p. 81. 
 
 2 Archivf. Mikr. Anat. Vol. X. 
 
 3 Quart. Journ. of Micr. Science, Oct. 1874. 
 
270 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 question arises whether in the case of meroblastic ova the cell 
 is not constituted of two parts completely separated from one 
 another. 
 
 Is the meroblastic ovum, before or after impregnation, com- 
 posed of a germinal disc in which all the protoplasm of the cell 
 is aggregated, and of a food-yolk in which no protoplasm is 
 present ? or is the protoplasm present throughout, being simply 
 more concentrated at the germinal pole than elsewhere ? If the 
 former alternative is accepted, we must suppose that the mass of 
 food-yolk is a something added which is not present in holoblas- 
 tic ova. If the latter alternative is accepted, it may then be 
 maintained that holoblastic and meroblastic ova are constituted 
 in the same way and differ only in the proportions of their con- 
 stituents. 
 
 My own observations in conjunction with the specially inte- 
 resting observations of Dr Schultz 1 justify the view which regards 
 the protoplasm as present throughout the whole ovum, and not 
 confined to the germinal disc. Our observations shew that a 
 fine protoplasmic network, with ramifications extending through- 
 out the whole yolk, is present both before and after impregna- 
 tion. 
 
 The presence of this network is, in itself, only sufficient to 
 prove that the yolk may be equivalent to part of a holoblastic 
 ovum ; to demonstrate that it is so requires something more, and 
 this link in the chain of evidence is supplied by the nuclei of the 
 yolk, which have been so often referred to. 
 
 These nuclei arise independently in the yolk, and become 
 the nuclei of cells which enter the germ and the bodies of which 
 are derived from the protoplasm of the yolk. Not only so, but 
 the cells formed around these nuclei play the same part in the 
 development of Elasmobranchs as do the largest so-called yolk 
 cells in the development of Amphibians. Like the homologous 
 cells in Amphibians, they mainly serve to form the ventral wall 
 of the alimentary canal and the blood-corpuscles. The identity 
 in the fate of the so-called yolk cells of Amphibians with the cells 
 derived from the yolk in Elasmobranchs, must be considered 
 as a proof of the homology of the yolk cells in the first case 
 
 1 Archivf. Mikr. Anat. Vol. xxi. 
 
FORMATION OF THE LAYERS. 2/1 
 
 with the yolk in the second ; the difference between the yolk in 
 the two cases arising from the fact that in the Elasmobranch 
 ovum the yolk-spherules bear a larger proportion to the proto- 
 plasm than they do in the Amphibian ovum. As I have suggested 
 elsewhere 1 , the segmentation or non-segmentation of a particular 
 part of the ovum depends solely upon the proportion borne by 
 the protoplasm to the yolk particles ; so that, when the latter 
 exceed the former in a certain fixed proportion, segmentation 
 is no longer possible ; and, as this limit is approached, seg- 
 mentation becomes slower, and the resulting segments larger 
 and larger. 
 
 The question how far the facts in the developmental history 
 of the various vertebrate blastoderms accord with the view of 
 the nature of the yolk just propounded is one of considerable 
 interest. An answer to it has already been attempted from a 
 general point of view in my paper 2 entitled ' The Comparison of 
 the early stages of development in Vertebrates'; but the subject 
 may be conveniently treated here in a special manner for 
 Elasmobranch embryos. 
 
 In the wood-cut, fig. I A, B, C 3 , are represented three dia- 
 grammatic longitudinal sections of an Elasmobranch embryo. 
 A nearly corresponds with the longitudinal section represented 
 on PL 7, fig. 4, and B with PL 7, fig. 7. In PL 7, fig. 7, the 
 segmentation cavity has however completely disappeared, while 
 it is still represented as present in the diagram of the same 
 period. If these diagrams, or better still, the wood-cuts fig. 
 2 A, B, C (which only differ from those of the Elasmobranch fish 
 in the smaller amount of food-yolk), be compared with the 
 corresponding ones of Bombinator, fig. 3 A, B, C, they will 
 be found to be in fundamental agreement with them. First let 
 fig. i A, or fig. 2 A, or PL 7, fig. 4, be compared with fig. 3 A. 
 In all there is present a segmentation cavity situated not centrally 
 but near the surface of the egg. The roof of the cavity is thin in 
 all, being composed in the Amphibian of epiblast alone, and in 
 
 1 "Comparison," &c., Quart. Journ. Micr. Science, July, 1875. [This Edition, 
 No. VI.] 
 
 - Loc. cit. 
 
 '' This figure, together with figs. 2 and 3, are reproduced from my paper upon the 
 comparison of the early stages of development in vertebrates. 
 
2/2 
 
 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the Elasmobranch of epiblast and lower layer cells. The floor of 
 the cavity is, in all, formed of so-called yolk (vide PL 7, fig. 4), 
 which in all forms the main mass of the egg. In the Amphibian 
 the yolk is segmented, and, though it is not segmented in the 
 Elasmobranch, it contains in compensation the nuclei so often 
 mentioned. In all, the sides of the segmentation cavity are 
 formed by lower layer cells. In the Amphibian the sides are 
 
 FIG. i. 
 
 Diagrammatic longitudinal sections of an Elnsmobranch embryo. 
 
 Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower 
 layer cells and hypoblast with simple shading. 
 
 ep. epiblast. m. mesoblast. al. alimentary cavity, sg. segmentation cavity. 
 . nc. neural canal, ch. notochord. x. point where epiblast and hypoblast become 
 continuous at the posterior end of the embryo, n. nuclei of yolk. 
 
 A. Section of young blastoderm, with segmentation cavity in the middle of the 
 lower layer cells. 
 
 B. Older blastoderm with embryo in which hypoblast and mesoblast are dis- 
 tinctly formed, and in which the alimentary slit has appeared. The segmentation 
 cavity is still represented as being present, though by this stage it has in reality 
 disappeared. 
 
 C. Older blastoderm with embryo in which neural canal has become formed, and 
 is continuous posteriorly with alimentary canal. The notochord, though shaded like 
 mesoblast, belongs properly to the hypoblast. 
 
FORMATION OF THE LAYERS. 
 
 273 
 
 FIG. 2. 
 
 Diagrammatic longitudinal sections of embryo, which develops in the same manner as 
 the Elasmobranch embryo, but in which 'the ovum contains far less food-yolk 
 than is the case with the Elasmobranch ovum. 
 
 Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower 
 layer cells and hypoblast with simple shading. 
 
 ep. epiblast. m. mesoblast. hy. hypoblast. sg. segmentation cavity. al. 
 alimentary cavity. nc. neural canal, hf. head-fold. . nuclei of the yolk. 
 The stages A , B and C are the same as in figure . 
 
274 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 FIG. 3. 
 
 Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with modi- 
 fications from Gotte. 
 
 Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower 
 layer cells and hypoblast with simple shading. 
 
 ep, epiblast. /./. lower layer cells, y. smaller lower layer cells at the sides of 
 the segmentation cavity. m. mesoblast. hy. hypoblast. al. alimentary cavity. 
 sg. segmentation cavity. nc. neural cavity, yk. yolk-cells. 
 
 A is the youngest stage in which the alimentary involution has not yet appeared. 
 x is the point from which the involution will start to form the dorsal wall of the 
 alimentary tract. The line on each side of the segmentation cavity, which separates 
 the smaller lower layer cells from the epiblast cells, is not present in Gotte's original 
 figure. The two shadings employed in the diagram render it necessary to have some 
 line, but at this stage it is in reality not possible to assert which cells belong to the 
 epiblast and which to the lower layer. 
 
 B. In this stage the alimentary cavity has become formed, but the segmentation 
 cavity is not yet obliterated. 
 
 x. point where epiblast and hypoblast become continuous. 
 
 C. The neural canal is already formed, and communicates posteriorly with the 
 alimentary. 
 
 x. point where epiblast and hypoblast become continuous. 
 
FORMATION OF THE LAYERS. 275 
 
 enclosed by smaller cells (in the diagram) which correspond 
 exactly in function and position with the lower layer cells of the 
 Elasmobranch blastoderm. 
 
 The relation of the yolk to the blastoderm in theJElasmo- 
 branch embryo at this stage of development very well suits the 
 view of its homology with the large cells of the Amphibian 
 ovum. The only essential difference between the two ova 
 arises from the roof of the segmentation cavity being in the 
 Elasmobranch embryo formed of lower layer cells, which are 
 absent in the Amphibian embryo. This difference no doubt 
 depends upon the greater quantity of yolk particles present in 
 the Elasmobranch ovum. These increase the bulk of the lower 
 layer cells, which are thus compelled to creep up the sides of 
 the segmentation cavity till they close it in above. 
 
 In the next stage for the Elasmobranch, fig. I and 2 B and 
 PI. 7, fig. 7, and for the Amphibian, fig. 3 B, the agreement 
 between the two types is again very close. In both for a small 
 portion (x) of the edge of the blastoderm the epiblast and hypo- 
 blast become continuous, while at all other parts the epiblast, 
 accompanied by lower layer cells, grows round the yolk or round 
 the large cells which correspond to it. The yolk cells of the 
 Amphibian ovum form a comparatively small mass, and are 
 therefore rapidly enveloped ; while in the case of the Elasmo- 
 branch ovum, owing to the greater mass of the yolk, the same 
 process occupies a long period. In both ova the portion of 
 the blastoderm, where epiblast and hypoblast become continuous, 
 forms the dorsal lip of an opening the anus of Rusconi which 
 leads into the alimentary cavity. This cavity has the same 
 relation in both ova. It is lined dorsally by lower layer cells, 
 and ventrally by yolk or what corresponds with yolk ; the 
 ventral epithelium of the alimentary canal being in both cases 
 eventually supplied by the yolk cells. 
 
 As in the earlier stage, so in the present one, the anatomical 
 relations of the yolk to the blastoderm in the one case (Elasmo- 
 branch) are nearly identical with those of the yolk cells to the 
 blastoderm in the other (Amphibian). The mam features in 
 which'the two embryos differ, during the stage under considera- 
 tion, arise from the same cause as the solitary point of differ- 
 ence during the preceding stage. 
 
2/6 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 In Amphibians, the alimentary cavity is formed coincidently 
 with a true ingrowth of cells from the point where epiblast and 
 hypoblast become continuous, and from this ingrowth the dorsal 
 wall of the alimentary cavity is formed. The same ingrowth 
 causes the obliteration of the segmentation cavity. 
 
 In the Elasmobranchs, owing to the larger bulk of the lower 
 layer cells caused by the food-yolk, these have been compelled 
 to arrange themselves in their final position during segmenta- 
 tion, and no room is left for a true invagination ; but instead 
 of this there is formed a simple split between the blastoderm 
 and the yolk. The homology of this with the primitive invagi- 
 nation is nevertheless proved by the survival of a number of 
 features belonging to the ancestral condition in which a true 
 invagination was present Amongst the more important of 
 these are the following: (i) The continuity of epiblast and 
 hypoblast at the dorsal lip of the anus of Rusconi. (2) The 
 continuous conversion of indifferent lower layer cells into hypo- 
 blast, which gradually extends backwards towards the segmenta- 
 tion cavity, and exactly represents the course of the invagination 
 whereby in Amphibians the dorsal wall of the alimentary cavity 
 is formed. (3) The obliteration of the segmentation cavity 
 during the period when the pseudo-invagination is occurring. 
 
 The asymmetry of the gastrula or pseudo-gastrula in Cyclo- 
 stomes, Amphibians, Elasmobranchs and, I believe, Osseous 
 Fishes, is to be explained by the form of the vertebrate body. 
 In Amphioxus, where the small amount of food-yolk present is 
 distributed uniformly, there is no reason why the invagination 
 and resulting gastrula should not be symmetrical. In other 
 vertebrates, where more food-yolk is present, the shape and 
 structure of the body render it necessary for the food-yolk to 
 be stored away on the ventral side of the alimentary canal. 
 This, combined with the unsymmetrical position of the anus, 
 which primitively corresponds in position with the blastopore 
 or anus of Rusconi, causes the asymmetry of the gastrula invagi- 
 nation, since it is not possible for the part of the ovum which 
 will become the ventral wall of the alimentary canal, and 
 which is loaded with food-yolk, to be invaginated in the* same 
 fashion as the dorsal wall. From the asymmetry, so caused, 
 follow a large number of features in vertebrate development, 
 
FORMATION OF THE LAYERS. 277 
 
 which have been worked out in some detail in my paper already 
 quoted 1 . 
 
 Prof. Haeckel, in a paper recently published 2 , appears to 
 imply that because I do not find absolute invagination in 
 Elasmobranchs, I therefore look upon Elasmobranchs~as~ mili- 
 tating against his Gastraea theory. I cannot help thinking that 
 Prof. Haeckel must have somewhat misunderstood my meaning. 
 The importance of the Gastraea theory has always appeared to 
 me to consist not in the fact that an actual ingrowth of certain 
 cells occurs an ingrowth which might have many different 
 meanings 3 but in the fact that the types of early development 
 of all animals can be easily derived from that of the typical 
 gastrula. I am perfectly in accordance with Professor Haeckel 
 in regarding the type of Elasmobranch development to be a 
 simple derivative from that of the gastrula, although believing it 
 to be without any true ingrowth or invagination of cells. 
 
 Professor Haeckel 4 in the paper just referred to published 
 his view upon the mutual relationships of the various vertebrate 
 blastoderms. In this paper, which appeared but shortly after 
 my own 5 on the same subject, he has put forward views which 
 differ from mine in several important details. Some of these 
 bear upon the nature of food-yolk ; and it appears to me that 
 Professor Haeckel's scheme of development is incompatible with 
 the view that the food-yolk in meroblastic eggs is the homologue 
 of part of the hypoblast of the holoblastic eggs. 
 
 The following is Professor Haeckel's own statement of the 
 scheme or type, which he regards as characteristic of mero- 
 blastic eggs, pp. 98 and 99. 
 
 Jetzt folgt der hochst wichtige und interessante Vorgang, den ich als 
 Einstiilpung der Blastula auffasse und der zur Bildung der Gastrula 
 fiihrt (Fig. 63, 64) 6 . Es schlagt sich niimlich der verdickte Saum der Keim- 
 scheibe, der " Randwulst " oder das Properistom, nach innen um und eine 
 diinne Zellenschicht wachst als directe Fortsetzung desselben, wie ein immer 
 
 1 Quart. Journ. of Micr. Science, July, 1875. [This Edition, No. VI.] 
 
 2 " Die Gastrula u. Eifurchung d. Thiere," Jenaische Zeitschrift, Vol. IX. 
 
 3 For instance, in Crustaceans it does not in some cases appear certain whether 
 an invagination is the typical gastrula invagination, or only an invagination by which, 
 at a period subsequent to the gastrula invagination, the hind gut is frequently formed. 
 
 4 IM. fit. 5 Loc. clt. 
 
 6 The references in this quotation are to the figures in the original. 
 
2/8 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 enger werdendes Diaphragma, in die Keimhohle hinein. Diese Zellen- 
 schicht ist das entstehende Entoderm (Fig. 64 z, 74 z). Die Zellen, welche 
 dieselbe zusammensetzen und aus dem innern Theile des Randwulstes her- 
 vorwachsen, sind viel grosser aber flacher als die Zellen der Keimhohlen- 
 decke und zeigen ein dunkleres grobkorniges Protoplasma. Auf dem Boden 
 der Keimhohle, d. h. also auf der Eiweisskugel des Nahrungsdotters, liegen 
 sie unmittelbar auf und riicken hier durch centripetale Wanderung 
 gegen dessen Mitte vor, bis sie dieselbe zuletzt erreichen und nunmehr eine 
 zusammenhangende einschichtige Zellenlage auf dem ganzen Keimhohlen- 
 boden bilden. Diese ist die erste vollstandige Anlage des Darmblatts, 
 Entoderms oder " Hypoblasts", und von nun an konnen wir, im Gegen- 
 satz dazu den gesammten iibrigen Theil des Blastoderms, namlich die 
 mehrschichtige Wand der Keimhohlendecke als Hautblatt, Exoderm 
 oder " Epiblast " bezeichnen. Der verdickte Randwulst (Fig. 64 w, 74 w], 
 in welchem beide primare Keimblatter in einander iibergehen, besteht in 
 seinem oberen und ausseren Theile aus Exodermzellen, in seinem unteren 
 und inneren Theile aus Entodermzellen. 
 
 In diesem Stadium entspricht unser Fischkeim einer Amphiblastula, 
 welche mitten in der Invagination begrififen ist, und bei welcher die 
 entstehende Urdarmhohle eine grosse Dotterkugel aufgenommen hat. Die 
 Invagination wird nunmehr dadurch vervollstandigt und die Gastrula- 
 bildung dadurch abgeschlossen, dass die Keimhohle verschwindet. Das 
 wachsende Entoderm, dem die Dotterkugel innig anhangt, wolbt sich in 
 die letztere hinein und nahert sich so dem Exoderm. Die klare Fliissigkeit 
 in der Keimhohle wird resorbirt und schliesslich legt sich die obere convexe 
 Flache des Entoderms an die untere concave des Exoderms eng an : die 
 Gastrula des discoblastischen Eies oder die "Discogastrula" ist fertig 
 (Fig; 65, 76 ; Meridiandurchschnitt Fig. 66, 75). 
 
 Die Discogastrula unsers Knochenfisches in diesem Stadium der vollen 
 Ausbildung stellt nunmehr eine kreisrunde Kappe dar, welche wie ein 
 gefiittertes Miitzchen fast die ganze obere Hemisphere der hyalinen Dot- 
 terkugel eng anliegend bedeckt (Fig. 65). Der Ueberzug des Miitzchens 
 entspricht dem Exoderm (*>), sein Futter dem Entoderm (z). Ersteres 
 besteht aus drei Schichten von kleineren Zellen, letzteres aus einer einzigen 
 Schicht von grosseren Zellen. Die Exodermzellen (Fig. 77) messen 0,006 
 0,009 Mm., und haben ein klares, sehr feinkorniges Protoplasma. Die 
 Entodermzellen (Fig. 78) messen 0,02 0,03 Mm. und ihr Protoplasma ist 
 mehr grobkornig und triiber. Letztere bilden auch den grossten Theil des 
 Randwulstes, den wir nunmehr als Urmundrand der Gastrula, als 
 " Properistoma " oder auch als " RuscbNl'schen After" bezeichnen kon- 
 nen. Der letztere umfasst die Dotterkugel, welche die ganze Urdarm- 
 hohle ausfullt und weit aus der dadurch verstopften Urmund-Oeffnung 
 vorragt. 
 
 My objections to the view so lucidly explained in the passage 
 just quoted, fall under two heads. 
 
FORMATION OF THE LAYERS. 279 
 
 (1) That the facts of development of the meroblastic eggs 
 of vertebrates, are not in accordance with the views here 
 advanced. 
 
 (2) That even if these views be accepted as representing the 
 actual facts of development, the explanation offered of Ihese 
 facts would not be satisfactory. 
 
 Professor Haeckel's views are absolutely incompatible with 
 the facts of Elasmobranch development, if my investigations are 
 correct. 
 
 The grounds of the incompatibility may be summed up under 
 the following heads : 
 
 (1) In Elasmobranchs the hypoblast cells occupy, even 
 before the close of segmentation, the position which, on Pro- 
 fessor Haeckel's view, they ought only eventually to take up 
 after being involuted from the whole periphery of the blasto- 
 derm. 
 
 (2) There is no sign at any period of an invagination of the 
 periphery of the blastoderm, and the only structure (the embryonic 
 rim) which could be mistaken for such an invagination is confined 
 to a very limited arc. 
 
 (3) The growth of cells to form the floor of the segmenta- 
 tion cavity, which ought to be part of this general invagination 
 from the periphery, is mainly due to a formation of cells from 
 the yolk. 
 
 It is this ingrowth of cells for the floor of the segmentation 
 cavity which, I am inclined to think, Professor Haeckel has 
 mistaken for a general invagination in the Osseous Fish he has 
 investigated. 
 
 (4) Professor Haeckel fails to give an account of the asym- 
 metry of the blastoderm ; an asymmetry which is unquestion- 
 ably also present in the blastoderm of most Osseous Fishes, 
 though not noticed by Professor Haeckel in the investigations 
 recorded in his paper. 
 
 The facts of development of Osseous Fishes, upon which Pro- 
 fessor Haeckel rests his views, are too much disputed, for their 
 
280 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 discussion in this place to be profitable 1 . The eggs of Osseous 
 Fishes appear to me unsatisfactory objects for the study of this 
 question, partly on account of all the cells of the blastoderm 
 being so much alike, that it is a very difficult matter to dis- 
 tinguish between the various layers, and, partly, because there 
 can be little question that the eggs of existing Osseous Fishes 
 are very much modified, through having lost a great part of the 
 food-yolk possessed by the eggs of their ancestors 2 . This dis- 
 appearance of the food-yolk must, without doubt, have produced 
 important changes in development, which would be especially 
 marked in a pelagic egg, like that investigated by Professor 
 Haeckel. 
 
 The Avian egg has been a still more disputed object than 
 even the egg of the Osseous Fishes. The results of my own 
 investigations on this subject do not accord with those of Dr 
 Gotte, or the views of Professor Haeckel 3 . 
 
 Apart from disputed points of development, it appears to me 
 that a comparative account of the development of the meroblastic 
 
 1 A short statement by Kowalevsky on this subject in a note to his account of the 
 development of Ascidians, would seem to indicate that the type of development of 
 Osseous Fishes is precisely the same as that of Elasmobranchs. Kowalevsky says, 
 Arch. f. Mikr, Anat. Vol. vn. p. 114, note 5, "According to my observations on 
 Osseous Fishes the germinal wall consists of two layers, an upper and lower, which 
 are continuous with one another at the border. From the upper one develops skin 
 and nervous system, from the lower hypoblast and mesoblast. " This statement, 
 which leaves unanswered a number of important questions, is too short to serve as a 
 basis for supporting my views, but so far as it goes its agreement with the facts of 
 Elasmobranch development is undoubtedly striking. 
 
 2 The eggs of the Osseous Fishes have, I believe, undergone changes of the same 
 character, but not to the same extent, as those of Mammalia, which, according to 
 the views expressed both by Professor Haeckel and myself, are degenerated from an 
 ovum with a large food-yolk. The grounds on which I regard the eggs of Osseous 
 Fishes as having undergone an analogous change, are too foreign to the subject to be 
 stated here. 
 
 3 I find myself unable without figures to understand Dr Rauber's (Centralblatt 
 fur Med. Wiss. 1874, No. 50; 1875, Nos. 4 and 17) views with sufficient precision 
 
 to accord to them either my assent or dissent. It is quite in accordance with the view 
 propounded in my paper (loc. cit.) to regard, with Dr Rauber and Professor Haeckel, 
 the tfnckened edge of the blastoderm as the homologue of the lip of the blastopore 
 in Amphioxus; though an invagination, in the manner imagined by Professor Haeckel, 
 is no necessary consequence of this view. If Dr Rauber regards the whole egg of the 
 bird as the homologue of that of Amphioxus, and the inclosure of the yolk by the 
 blastoderm as the equivalent to the process of invagination in Amphioxus, then his 
 views are practically in accordance with my own. 
 
FORMATION OF THE LAYERS. 28 1 
 
 vertebrate ova ought to take into consideration the essential differ- 
 ences which exist between the Avian and Piscian blastoderms, 
 in that the embryo is situated in the centre of the blastoderm in 
 the first case and at the edge in the second 1 . 
 
 This difference entails important modifications in develop- 
 ment, and must necessarily affect the particular points under 
 discussion. As a result of the different positions of the embryo 
 in the two cases, there is present in Elasmobranchs and Osseous 
 Fishes a true anus of Rusconi, or primitive opening into the 
 alimentary canal, which is absent in Birds. Yet in neither 
 Elasmobranchs 2 nor Osseous Fishes does the anus of Rusconi 
 correspond in position with the point where the final closing in 
 of the yolk takes place, but in them this point corresponds 
 rather with the blastopore of Birds 3 . 
 
 Owing also to the respective situations of the embryo in the 
 
 1 I have suggested in a previous paper ("Comparison," &c., Quart. Journal of 
 Micr. Science, July, 1875) that the position occupied by the embryo of Birds at the 
 centre, and not at the periphery, of the blastoderm may be due to an abbreviation of 
 the process by which the Elasmobranch embryos cease to be situated at the edge of 
 the blastoderm (vide p. 296 and PI. 9, fig. i, 2). Assuming this to be the real expla- 
 nation of the position of the embryo in Birds, I feel inclined to repeat a speculation 
 which I made some time ago with reference to the primitive streak in Birds (Quart. 
 Journ. of Micr. Science, 1873, p. 280). In Birds there is, as is well known, a struc- 
 ture called the primitive streak, which has been shewn by the observations of Dursy, 
 corroborated by my observations (loc. cit.), to be situated behind the medullary groove, 
 and to take no part in the formations of the embryo. I further shewed that the 
 peculiar fusion of epiblast and mesoblast, called by His the axis cord, was confined 
 to this structure and did not occur in other parts of the blastoderm. Nearly similar 
 results have been recently arrived at by Hensen with reference to the primitive streak 
 in Mammals. The position of the primitive streak immediately behind the embryo 
 suggests the speculation that it may represent the line along which the edges of the 
 blastoderm coalesced, so as to give to the embryo the central position which it has 
 in the blastoderms of Birds and Mammals, and that the peculiar fusion of epiblast 
 and mesoblast at this point may represent the primitive continuity of epiblast and 
 lower layer cells at the dorsal lip of the anus of Rusconi in Elasmobranchs. 1 
 put this speculation forward as a mere suggestion, in the hope of elucidating the 
 peculiar structure of the primitive streak, which not improbably may be found to be 
 the keystone to the nature of the blastoderm of the higher vertebrates. 
 
 3 Vide p. 296 and Plate 9, fig. r and 2, and Self, "Comparison," &c., loc. cit. 
 
 3 The relation of the anus of Rusconi and blastopore in Elasmobranchs was fully 
 explained in the paper above quoted. It was there clearly shewn that neither the 
 one nor the other exactly corresponds with the blastopore of Amphioxus, but that the 
 two together do so. Professor Haeckel states that in the Osseous Fish investigated 
 by him the anus of Rusconi and the blastopore coincide. This is not the case in the 
 Salmon. 
 
 B. IQ 
 
282 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 blastoderm, the alimentary and neural canals communicate 
 posteriorly in Elasmobranchs and Osseous Fishes, but not in 
 Birds. Of all these points Professor Haeckel makes no mention. 
 
 The support of his views which Prof. Haeckel attempts to 
 gain from Gotte's researches in Mammalia is completely cut 
 away by the recent discoveries of Van Beneden 1 and Hensen 2 . 
 
 It thus appears that Professor Haeckel's views but ill accord 
 with the facts of vertebrate development ; but even if they were 
 to do so completely it would not in my opinion be easy to give a 
 rational explanation of them. 
 
 Professor Haeckel states that no sharp and fast line can be 
 drawn between the types of ' unequal ' and ' discoidal ' segmenta- 
 tion 3 . In the cases of unequal segmentation he admits, as is 
 certainly the case, that the larger yolk cells (hypoblast) are 
 simply enclosed by a growth of the epiblast around them ; which 
 is to be looked on as a modification of the typical gastrula inva- 
 gination, necessitated by the large size of the yolk cells (vide 
 Professor Haeckel's paper, Taf. II. fig. 30). In these instances 
 there is no commencement of an ingrowth in the manner supposed 
 for meroblastic ova. 
 
 When the food-yolk becomes more bulky, and the hypoblast 
 does not completely segment, it is not easy to understand why 
 an ingrowth, which had no existence in the former case, should 
 occur ; nor where it is to come from. Such an ingrowth as is 
 supposed to exist by Professor Haeckel would, in fact, break 
 the continuity of development between meroblastic -and holo- 
 blastic ova, and thus destroy one of the most important results 
 of the Gastraea theory. 
 
 It is quite easy to suppose, as I have done, that in the cases 
 of discoidal segmentation, the hypoblast (including the yolk) 
 becomes enclosed by the epiblast in precisely the same manner 
 as in the cases of unequal segmentation. 
 
 But even if Professor Haeckel supposes that in the unseg- 
 mented food-yolk a fresh element is added to the ovum, it 
 
 1 " Developpement Embryonnaire des Mammiferes, " Bulletin de I 'A cad. r. d. 
 Belgique, 1875. 
 
 2 Loc. cit. 
 
 3 For an explanation of these terms, vide Prof. Haeckel's original paper or the 
 abstract in Quart. Journ. of Micr. Science for January, 1876. 
 
FORMATION OF THE LAYERS. 283 
 
 remains quite unintelligible to me how an ingrowth of cells from 
 a circumferential line, to form a layer which had no previous 
 existence, can be equivalent to, or derived from, the imagination 
 of a layer, which exists before the process of invagination begins, 
 and which remains continuous throughout it. 
 
 If Professor Haeckel's views should eventually turn out to be 
 in accordance with the facts of vertebrate development, it will, in 
 my opinion, be very difficult to reduce them into conformity with 
 the Gastraea theory. 
 
 Although some space has been devoted to an attempt to 
 refute the views of Professor Haeckel on this question, I wish 
 it to be clearly understood that my disagreement from his 
 opinions concerns matters of detail only, and that I quite accept 
 the Gastrasa theory in its general bearings. 
 
 Observations upon the formation of the layers in Elasmo- 
 branchs have hitherto been very few in number. Those published 
 in my preliminary account of these fishes are, I believe, the 
 earliest 1 . 
 
 Since then there has been published a short notice on the 
 subject by Dr Alex. Schultz 2 . His observations in the main 
 accord with my own. He apparently speaks of the nuclei of 
 the yolk as cells, and also of the epiblast being more than one 
 cell deep. In Torpedo alone, amongst the genera investigated 
 by me, is the layer of epiblast, at about the age of the last 
 described embryo, composed of more than a single row of cells. 
 
 1 I omit all reference to a paper published in Russian by Prof. Kowalevsky. Being 
 unable to translate it, and the illustrations being too meagre to be in themselves of 
 much assistance, it has not been possible for me to make any use of it. 
 
 2 Centralblatt f. Med. Wiss. No. 33, 1875. 
 
 IQ 2 
 
284 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 EXPLANATION OF PLATE 7. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 c. Cells formed in the yolk around the nuclei of the yolk. ep. Epiblast. er. Em- 
 bryonic ring. es. Embryo swelling, hy. Hypoblast. //. Lower layer cells, ly. Line 
 separating the yolk from the blastoderm, m. Mesoblast. mg. Medullary groove. 
 ;/'. Nuclei of yolk. na. Cells to form ventral wall of alimentary canal which have 
 been derived from the yolk, n al. Cells formed around the nuclei of the yolk which 
 have entered the hypoblast. sc. Segmentation cavity, vp. Combined lateral and 
 vertebral plate of mesoblast. 
 
 Fig. r. Longitudinal section of a blastoderm at the first appearance of the seg- 
 mentation cavity. 
 
 Fig. i. Longitudinal section through a blastoderm after the layer of cells has 
 disappeared from the floor of the segmentation cavity, bd. Large cell resting on the 
 yolk, probably remaining over from the later periods of segmentation. Magnified 60 
 diameters. (Hardened in chromic acid.) 
 
 The section is intended to illustrate the fact that the nuclei form a layer in the yolk 
 under the floor of the segmentation cavity. The roof of the segmentation cavity is 
 broken. 
 
 Fig. i a. Portion of same blastoderm highly magnified, to shew the characters of 
 the nuclei of the yolk n' and the nuclei in the cells of the blastoderm. 
 
 Fig. 2 b. Large knobbed nucleus from the same blastoderm, very highly magnified. 
 
 Fig. 2 c. Nucleus of yolk from the same blastoderm. 
 
 Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2. (Hardened in 
 chromic acid.) 
 
 Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 
 45 diameters. (Hardened in osmic acid.) 
 
 It illustrates (i) the characters of the epiblast ; (2) the embryonic swelling; (3) 
 the segmentation cavity. 
 
 Fig. 5. Longitudinal section through a blastoderm at the time of the first appear- 
 ance of the embryonic rim, and before the formation of the medullary groove. 
 Magnified 45 diameters. 
 
 Fig. 5 a. Section through the periphery of the embryonic rim of the blastoderm 
 of which fig. 5 represents a section. 
 
 Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger 
 than that represented on PI. 8, fig. B. 
 
 Fig. 7. Section through the most projecting portion of the embryonic rim of a 
 blastoderm of the same age as that represented on PI. 8, fig. B. The section is drawn 
 on a very considerably smaller scale than that on fig. 5. It is intended to illustrate 
 the growth of the embryonic rim and the disappearance of the segmentation cavity. 
 
 Fig. 7 a. Section through peripheral portion of the embryonic rim of the same 
 blastoderm, highly magnified. It specially illustrates the formation of a cell (c} 
 around a nucleus in the yolk. The nuclei of the blastoderm have been inaccurately 
 rendered by the artist. 
 
FORMATION OF THE LAYERS. 285 
 
 Figs. 8 a, Sl>, 8r. Three sections of the same embryo. Inserted mainly to illus- 
 trate the formation of the mesoblast as two independent lateral masses of cells ; only 
 half of each section is represented. 8 a is the most posterior of the three sections. 
 In it the mesoblast forms a large mass on each side, imperfectly separated from the 
 hypoblast. In 8 l>, from the anterior part of the embryo, the main mass oLmegpblast 
 is far smaller, and only forms a cap to the hypoblast at the highest point of the 
 medullary fold. In 8 c a cap of mesoblast is present, similar to that in 8 b, though 
 much smaller. The sections of these embryos were somewhat oblique, and it has 
 unfortunately happened that while in 8 a one side is represented, in 8^ and 8c the 
 other side is figured, had it not been for this the sections 8 b and 8 c would have been 
 considerably longer than 8 a. 
 
 Fig. 9. Longitudinal section of an embryo belonging to a slightly later stage 
 than B. 
 
 This section passes through one of the medullary folds. It illustrates the continuity 
 of the hypoblast with the remaining lower layer cells of the blastoderm. 
 
 Figs, loa, lob, loc. Three sections of the same embryo belonging to a stage 
 slightly later than B, PL 8. The space between the mesoblast and the hypoblast 
 has been made considerably too great in the figures of the three sections. 
 
 ic-tf. The most posterior of the three sections. It shews the posterior flatness 
 of the medullary groove and the two isolated vertebral plates. 
 
 lob. This section is taken from the anterior part of the same embryo and 
 shews the deep medullary groove and the commencing formation of the ventral wall 
 of the alimentary canal from the nuclei of the yolk. 
 
 \oc shews the disappearance of the medullary groove and the thinning out of 
 the mesoblast plates in the region of the head. 
 
 Fig. ii. Small portion of the blastoderm and the subjacent yolk of an embryo at 
 the time of the first appearance of the medullary groove x 300. It shews two large 
 nuclei of the yolk () and the protoplasmic network in the yolk between them ; the 
 network is seen to be closer round the nuclei than in the intervening space. There 
 are no areas representing cells around the nuclei. 
 
 Fig. ii. Nucleus of the yolk in connection with the protoplasmic network 
 hardened in osmic acid. 
 
 Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the forma- 
 tion of cells around the nuclei of the yolk. 
 
 Fig. 14. Section through part of a young Scyllium egg, about t V tn of an inch in 
 diameter. 
 
 /. Protoplasmic network in yolk. zp. Zona pellucida. eh. Structureless 
 chorion. fcp. Follicular epithelium, x. Structureless membrane external to this. 
 
CHAPTER IV. 
 
 THE GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO 
 AT SUCCESSIVE STAGES. 
 
 No complete series of figures, representing the various stages 
 in development of an Elasmobranch Embryo, has hitherto been 
 published. With the view of supplying this deficiency Plate 
 8 has been inserted. The embryos represented in this Plate 
 form a fairly complete series, but do not all belong to a single 
 species. Figs. A, B, C, D, E, F, H, I represent embryos of 
 Pristiurus; G being an embryo of Torpedo. The remaining 
 figures, excepting K, which is a Pristiurus embryo, are embryos 
 of Scyllium canicula. The embryos A I were very accurately 
 drawn from nature by my sister, Miss A. B. Balfour. Un- 
 fortunately the exceptional beauty and clearness of the originals 
 is all but lost in the lithographs. To facilitate future description, 
 letters will be employed in the remainder of these pages to 
 signify that an embryo being described is of the same age 
 as the embryo on this Plate to which the letter used refers. 
 Thus an embryo of the same age as L will be spoken of here- 
 after as belonging to stage L. 
 
 A. 
 
 This figure represents a hardened blastoderm at a stage 
 when the embryo-swelling (e. s.} has become obvious, but before 
 the appearance of the medullary groove. The position of the 
 segmentation cavity is indicated by a slight swelling of the 
 blastoderm (s. c). The shape of the blastoderm, in hardened 
 specimens, is not to be relied upon, owing to the traction which 
 the blastoderm undergoes during the process of removing the 
 yolk from the egg-shell. 
 
 B. 
 
 B is the view of a fresh blastoderm. The projecting part 
 of this, already mentioned as the 'embryonic rim', is indicated 
 
GENERAL FEATURES. 287 
 
 by the shading. At the middle of the embryonic rim is to be 
 seen the rudiment of the embryo (m. g.). It consists of an 
 area of the blastoderm, circumscribed on its two sides and at 
 one end, by a slight fold, and whose other end formsjpart of 
 the edge of the blastoderm. The end of the embryo which 
 points towards the centre of the blastoderm is the head end, 
 and that which forms part of the edge of the blastoderm is 
 the tail end. To retain the nomenclature usually adopted 
 in treating of the development of the Bird, the fold at the 
 anterior end of the embryo may be called the liead fold, and 
 those at the sides the side folds. There is in Elasmobranchs 
 no tail fold, owing to the position of the embryo at the peri- 
 phery of the blastoderm, and it is by the meeting of the three 
 above-mentioned folds only, that the embryo becomes pinched 
 off from the remainder of the blastoderm. Along the median 
 line of the embryo is a shallow groove (m.g.}, the well-known 
 medullary groove of vertebrate embryology. It flattens out 
 both anteriorly and posteriorly, and is deepest in the middle 
 part of its course. 
 
 C. 
 
 This embryo resembles in most of its features the embryo 
 last described. It is, however, considerably larger, and the head- 
 fold and side-folds have become more pronounced structures. 
 The medullary groove is far deeper than in the earlier stage, and 
 widens out anteriorly. This anterior widening is the first indica- 
 tion of a distinction between the brain and the remainder of the 
 central nervous system, a distinction which arises long before 
 the closure of the medullary canal. 
 
 D. 
 
 This embryo is far larger than the one last described, but 
 the increase in length does not cause it to project beyond the 
 edge of the blastoderm, but has been due to a growth inwards 
 towards the centre of the blastoderm. The head is now indicated 
 by an anterior enlargement, and the embryo also widens out 
 posteriorly. The posterior widening (t. s.} is formed by a pair of 
 rounded prominences, one on each side of the middle line. These 
 are very conspicuous organs during the earlier stages of develop- 
 ment, and consist of two large aggregations of mesoblast cells. 
 
288 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 In accordance with the nomenclature adopted in my preliminary 
 paper 1 , they may be called * tail-swellings'. Between the cephalic 
 enlargements and the tail-swellings is situated the rudimentary 
 trunk of the embryo. It is more completely pinched off from 
 the blastoderm than in the last described embryo. The 
 medullary groove is of a fairly uniform size throughout the 
 trunk of the embryo, but flattens out and vanishes completely 
 in the region of the head. The blastoderm in Pristiurus and 
 Scyllium grows very rapidly, and has by this stage attained 
 a very considerable size ;. but in Torpedo its growth is very 
 slow. 
 
 E and F. 
 
 These two embryos may be considered together, for, although 
 they differ in appearance, yet they are of an almost identical 
 age; and the differences between the two are purely external. 
 E appears to be a little abnormal in not having the cephalic 
 region so distinctly marked off from the trunk as is usual. The 
 head is proportionally larger than in the last stage, and the tail- 
 swellings remain as conspicuous as before. The folding off from 
 the blastoderm has progressed rapidly, and the head and tail are 
 quite separated from it. The medullary groove has become 
 closed posteriorly in both embryos, but the closing has extended 
 further forwards in F than in E. In F the medullary folds have 
 not only united posteriorly, but have very nearly effected a fresh 
 junction in the region of the neck. At this point a second 
 junction of the two medullary folds is in fact actually effected 
 before the posterior closing has extended forwards so far. The 
 later junction in the region of the neck corresponds in position 
 with the point, where in the Bird the medullary folds first unite. 
 No trace of a medullary groove is to be met with in the head, 
 which simply consists of a wide flattened plate. Between the 
 two tail-swellings surface views present the appearance of a 
 groove, but this appearance is deceptive, since in sections no 
 groove^ or at most a very slight one, is perceptible. 
 
 G. 
 
 During the preceding stages growth in the embryo is very 
 slow, and considerable intervals of time elapse before any 
 
 1 Quart. Jour it. Micr. Science, Oct. 1874. [This Edition, No. V.] 
 
GENERAL FEATURES. 289 
 
 perceptible changes are effected. This state of things now 
 becomes altered, and the future changes succeed each other 
 with far greater rapidity. One of the most important of these, 
 and one which first presents itself during this stage, is the dis- 
 appearance of the yolk-spherules from the embryonic cells, and 
 the consequently increased transparency of the embryo. As a 
 result of this, a number of organs, which in the earlier stages were 
 only to be investigated by means of sections, now become visible 
 in the living embryo. 
 
 The tail-swellings (t. s.) are still conspicuous objects at the 
 posterior extremity of the embryo. The folding off of the 
 embryo from the yolk has progressed to such an extent that it is 
 now quite possible to place the embryo on its side and examine 
 it from that point of view. 
 
 The embryo may be said to be attached to the yolk by a 
 distinct stalk or cord, which in the succeeding stages gradually 
 narrows and elongates, and is known as the umbilical cord (so. s.). 
 The medullary canal has now become completely closed, even in 
 the region of the brain, where during the last stage no trace of 
 a medullary groove had appeared. Slight constrictions, not 
 perceptible in views of the embryo as a transparent object, 
 mark off three vesicles in the brain. These vesicles are known 
 as the fore, mid, and hind brain. From the fore-brain there is 
 an outgrowth on each side, the first rudiment of the optic 
 vesicle (op.}. 
 
 The mesoblast on each side of the body is divided into a 
 series of segments, known as protovertebrae or muscle-plates, 
 the first of which lies a little behind the head. The mesoblast 
 of the tail has not as yet undergone this segmentation. There 
 are present in all seventeen segments. These first appeared at a 
 much earlier date, but were not visible owing to the opacity 
 of the embryo. 
 
 Another structure which became developed in even a younger 
 embryo than C is now for the first time visible in the living 
 embryo. This is the notochord : it extends from almost the 
 extreme posterior to the anterior end of the embryo. It lies 
 between the ventral wall of the spinal canal and the dorsal wall 
 of the intestine ; and round its posterior end these two walls 
 become continuous with each other (vide fig.). Anteriorly the 
 
2QO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 termination of the notochord cannot be seen, it can only be 
 traced into a mass of mesoblast at the base of the brain, which 
 there separates the epiblast from the hypoblast. The alimentary 
 canal (/.) is completely closed anteriorly and posteriorly, though 
 still widely open to the yolk-sac in the middle part of its course. 
 In the region of the head it exhibits on each side a slight bulging 
 outwards, the rudiment of the first visceral cleft. This is repre- 
 sented in the figure by two lines (i v.c^. The visceral clefts 
 at this stage consist of a pair of simple diverticula from the 
 alimentary canal, and there is no communication between the 
 throat and the exterior. 
 
 H. 
 
 The present embryo is far larger than the last, but it has not 
 been possible to represent this increase in size in the drawings. 
 Accompanying this increase in size, the folding off of the embryo 
 from the yolk has considerably progressed, and the stalk which 
 unites the embryo with the yolk is proportionately narrower and 
 longer than before. 
 
 The brain is now very distinctly divided into the three 
 lobes, whose rudiments appeared during the last stage. From 
 the foremost of these, the optic vesicles now present themselves 
 as well-marked lateral outgrowths, towards which there appears 
 a growing in, or involution, from the external skin (pp^) to form 
 the lens. The opening of this involution is represented by the 
 dark spot in the centre. 
 
 A fresh organ of sense, the auditory sac, now for the first 
 time becomes visible as a shallow pit in the external skin on 
 each side of the hind- brain (au. v.\ The epiblast which is 
 involuted to form this pit becomes much thickened, and thereby 
 the opacity, indicated in the figure, is produced. 
 
 The muscle-plates have greatly increased in number by the 
 formation of fresh segments in the tail. Thirty-eight of them 
 were present in the embryo figured. The mesoblast at the base 
 of the brain has increased in quantity, and there is still a certain 
 mass of unsegmented mesoblast which forms the tail-swellings. 
 The first rudiment of the heart becomes visible during this 
 stage as a cavity between the mesoblast of the splanchnopleure 
 and the hypoblast (/;/.). 
 
GENERAL FEATURES. 2QI 
 
 The fore and hind guts are now longer than they were. A 
 slight pushing in from the exterior to form the mouth has 
 appeared (/.), and an indication of the future position of the 
 anus is afforded by a slight diverticulum of the hind gut towards 
 the exterior some little distance from the posterior end" of the 
 embryo (an.}. The portion of the alimentary canal behind this 
 point, though at this stage large, and even dilated into a vesicle 
 at its posterior end (al. v.}, becomes eventually completely 
 atrophied. In the region of the throat the rudiment of a second 
 visceral cleft has appeared behind the first ; neither of them are 
 as yet open to the exterior. The number of visceral clefts 
 present in any given Pristiurus embryo affords a very easy and 
 simple way of determining its age. 
 
 I 
 
 A great increase in size is again to be noticed in the embryo, 
 but, as in the case of the last embryo, it has not been possible to 
 represent this in the figure. The stalk connecting the embryo 
 with the yolk has become narrower and more elongated, and 
 the tail region of the embryo proportionately far longer than in 
 the last stage. During this stage the first spontaneous move- 
 ments of the embryo take place, and consist in somewhat rapid 
 excursions of the embryo from side to side, produced by a 
 serpentine motion of the body. 
 
 The cranial flexure, which commenced in stage G, has now 
 become very evident, and the mid-brain 1 begins to project in the 
 same manner as in the embryo fowl on the third day, and will 
 soon form the anterior termination of the long axis of the 
 embryo. The fore-brain has increased in size and distinctness, 
 and the anterior part of it may now be looked on as the unpaired 
 rudiment of the cerebral hemispheres. 
 
 Further growths have taken place in the organs of sense, 
 especially in the eye, in which the involution for the lens has 
 made considerable progress. The number of the muscle-plates 
 has again increased, but there is still a region of unsegmented 
 
 1 The part of the brain which I have here called mid-brain, and which unquestion- 
 ably corresponds to the part called mid-brain in the embryos of higher vertebrates, 
 becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of 
 the third ventricle or thalamencephalon. I shall always speak of it as the mid-brain. 
 
2Q2 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 mesoblast in the tail. The thickened portions of mesoblast 
 which caused the tail-swellings are still to be seen and would 
 seem to act as the reserve from which is drawn the matter for 
 the rapid growth of the tail, which occurs soon after this. The 
 mass of the mesoblast at the base of the brain has again in- 
 creased. No fresh features of interest are to be seen in the 
 notochord. The heart is now much more conspicuous than 
 before, and its commencing flexure is very apparent. It now 
 beats actively. The hind gut especially is much longer than 
 in the last specimen ; and the point where the anus will appear 
 is very easily detected by the bulging out of the gut towards 
 the external skin at that point (an.}. The alimentary vesicle, 
 first observable during the last stage, is now a more conspicuous 
 organ (al. v.). Three visceral clefts, none of which are as yet 
 open to the exterior, may now be seen. 
 
 K. 
 
 The figures G, H, I are representations of living and trans- 
 parent embryos, but the remainder of the figures are drawings of 
 opaque embryos which were hardened in chromic acid. 
 
 The stalk connecting the embryo with the yolk is now, com- 
 paratively speaking, quite narrow, and is of sufficient length to 
 permit the embryo to execute considerable movements. 
 
 The tail has grown immensely, but is still dilated terminally. 
 This terminal dilatation is mainly due to the alimentary vesicle, 
 but the tract of gut connecting this with the gut in front of the 
 anus is now a solid rod of cells and very soon becomes com- 
 pletely atrophied. 
 
 The two pairs of limbs have appeared as elongated ridges 
 of epiblast. The anterior pair is situated just at the front 
 end of the umbilical stalk ; and the posterior pair, which is 
 the more conspicuous of the two, is situated some little distance 
 behind the stalk. 
 
 The cranial flexure has greatly increased, and the angle 
 between the long axis of the front part of the head and of the 
 body is less than a right angle. The conspicuous mid-brain 
 forms the anterior termination of the long axis of the body. 
 The thin roof of the fourth ventricle may in the figure be noticed 
 behind the mid-brain. The auditory sac is nearly closed and its 
 
GENERAL FEATURES. 293 
 
 opening is not shewn in the figure. In the eye the lens is 
 completely formed. 
 
 Owing to the opacity of the embryo, the muscle-plates are 
 only indistinctly indicated, and no other features of the meso- 
 blast are to be seen. 
 
 The mouth is now a deep pit, whose borders are almost 
 completely formed by the thickening in front of the first visceral 
 cleft, which may be called the first visceral arch or mandibular 
 arch. 
 
 Four visceral clefts are now visible, all of which are open 
 to the exterior, but in a transparent embryo one more, not open 
 to the exterior, would have been visible behind the last of these. 
 
 L. 
 
 This embryo is considerably older than the one last described, 
 but growth is not quite so rapid as might be gathered from the 
 fact that L is nearly twice as long as K, since the two embryos 
 belong to different genera ; and the Scyllium embryos, of which 
 L is an example, are larger than Pristiurus embryos. The 
 umbilical stalk is now quite a narrow elongated structure, whose 
 subsequent external changes are very unimportant, and consist 
 for the most part merely in an increase in its length. 
 
 The tail has again grown greatly in length, and its terminal 
 dilatation together with the alimentary vesicle contained in it, 
 have both completely vanished. A dorsal and ventral fin are 
 now clearly visible ; they are continuous throughout their whole 
 length. The limbs have grown and are more easily seen than in 
 the previous stage. 
 
 Great changes have been effected in the head, resulting in a 
 diminution of the cranial flexure. This diminution is never- 
 theless apparent rather than real, and is chiefly due to the rapid 
 growth of the rudiment of the cerebral hemispheres. The three 
 main divisions of the brain may still be clearly seen from the 
 surface. Posteriorly is situated the hind-brain, now consisting 
 of the medulla oblongata and cerebellum. At the anterior 
 part of the medulla is to be seen the thin roof of the fourth 
 ventricle, and anteriorly to this again the roof becomes thickened 
 to form the rudiment of the cerebellum. In front of the hind- 
 brain lies the mid-brain, the roof of which is formed by the 
 
294 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 optic lobes, which are still situated at the front end of the long 
 axis of the embryo. 
 
 Beyond the mid-brain is placed the fore-brain, whose growth 
 is rapidly rendering the cranial flexure imperceptible. 
 
 The rudiments of the nasal sacs are now clearly visible as a 
 pair of small pits. The pits are widely open to the exterior, 
 and are situated one on each side, near the front end of the 
 cerebral hemispheres. Five visceral clefts are open to the 
 exterior, and in them the external gills have commenced to 
 appear (L/). 
 
 The first cleft is no longer similar to the rest, but has com- 
 menced to be metamorphosed into the spiracle. 
 
 Accompanying the change in position of the first cleft, the 
 mandibular arch has begun to bend round and enclose the front 
 as well as the side of the mouth. By this change in the mandi- 
 bular arch the mouth becomes narrowed in an antero-posterior 
 direction. 
 
 M. 
 
 Of this embryo the head alone has been represented. Two 
 views of it are given, one (M) from the side and the other (M') 
 from the under surface. The growth of the front part of the 
 head has considerably diminished the prominence of the cranial 
 flexure. The full complement of visceral clefts is now present 
 six in all. But the first has already atrophied considerably, and 
 may easily be recognized as the spiracle. In Scyllium, there 
 are present at no period more than six visceral clefts. The first 
 visceral arch on each side has become bent still further round, 
 to form the front border of the mouth. The opening of the 
 mouth has in consequence become still more narrowed in an 
 antero-posterior direction. The width of the mouth in this 
 direction, serves for the present and for some of the subsequent 
 stages as a very convenient indication of age. 
 
 N. 
 
 The limbs, or paired fins, have now acquired the general 
 features and form which they possess in the adult. 
 
 The unpaired fins have now also become divided in a manner 
 not only characteristic of the Elasmobranchs but even of the 
 genus Scyllium. 
 
GENERAL FEATURES. 295 
 
 There is a tail fin, an anal fin and two dorsal fins, both the 
 latter being situated behind the posterior paired fins. 
 
 In the head may be noticed a continuation of the rapid 
 growth of the anterior part. 
 
 The mouth has become far more narrow and slit-like ; ~and 
 with many other of the organs of the period commences to 
 approach the form of the adult. 
 
 The present and the three preceding stages shew the gradual 
 changes by which the first visceral arch becomes converted into 
 the rudiments of the upper and of the lower jaw. The fact of 
 the conversion was first made known through the investigations 
 of Messrs Parker and Gegenbaur. 
 
 O. 
 
 In this stage the embryo is very rapidly approaching the 
 form of the adult. 
 
 This is especially noticeable in the fins, which project in a 
 manner quite characteristic of the adult fish. The mouth is slit- 
 like, and the openings of the nasal sacs no longer retain their 
 primitive circular outline. The external gills project from all 
 the gill-slits including the spiracle. 
 
 P. 
 
 The head is rapidly elongating by the growth of the snout, 
 and the divisions of the brain can no longer be seen with distinct- 
 ness from the exterior, and, with the exception of the head and 
 of the external gills, the embryo almost completely resembles 
 the adult. 
 
 Q- 
 
 The snout has grown to such an extent, that the head has 
 nearly acquired its adult shape. In the form of its mouth the 
 embryo now quite resembles the adult fish. 
 
 This part of the subject may be conveniently supplemented 
 by a short description of the manner in which the blastoderm 
 encloses the yolk. It has been already mentioned that the 
 growth of the blastoderm is not uniform. The part of it in the 
 immediate neighbourhood of the embryo remains comparatively 
 stationary, while the growth elsewhere is very rapid. From 
 
296 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 this it results that that part of the edge of the blastoderm 
 where the embryo is attached forms a bay in the otherwise 
 regular outline of the edge of the blastoderm. By the time 
 that one-half of the yolk is enclosed the bay is a very con- 
 spicuous feature (PI. 9, fig. i). In this figure bl. points to the 
 blastoderm, and yk. to the part of the yolk not yet enclosed by 
 the blastoderm. 
 
 Shortly subsequent to this the bay becomes obliterated by 
 its two sides coming together and coalescing, and the embryo 
 ceases to lie at the edge of the yolk. 
 
 This stage is represented on PI. 9, fig. 2. In this figure 
 there is only a small patch of yolk not yet enclosed (yk}, which 
 is situated at some little distance behind the embryo. Through- 
 out all this period the edge of the blastoderm has remained 
 thickened, a feature which persists till the complete investment 
 of the yolk, which takes place shortly after the stage last figured. 
 In this thickened edge a circular vein arises, which brings back 
 the blood from the yolk-sac to the embryo. The opening in the 
 blastoderm (PI. 9, fig. 2 yk.}, exposing the portion of the yolk 
 not yet enclosed, may be conveniently called the blastopore, 
 according to Professor Lankester's nomenclature. 
 
 The interesting feature which characterizes the blastopore 
 in Elasmobranchs is the fact of its not corresponding in position 
 with the opening of the anus of Rusconi. We thus have in 
 Elasmobranchs two structures, each of which corresponds in part 
 with the single structure in Amphioxus which may be called 
 either blastopore or anus of Rusconi, which yet do not in Elas- 
 mobranchs coincide in position. It is the blastopore of Elasmo- 
 branchs which has undergone a change of position, owing to the 
 unequal growth of the blastoderm ; while the anus of Rusconi 
 retains its normal situation. In Osseous Fishes the blastopore 
 undergoes a similar change of position. The possibility of a 
 change in position of this structure is peculiarly interesting, in 
 that it possibly serves to explain how the blastopore of different 
 animals corresponds in different cases with the anus or the 
 mouth, and has not always a fixed situation 1 . 
 
 1 For a fuller discussion of this question vide Self, " A comparison of the early 
 stages of development in vertebrates." Quart. Journ. of Micr. Science, ]u\y, 1875. 
 [This Edition, No. VI.] 
 
GENERAL FEATURES. 297 
 
 EXPLANATION OF PLATES 8 AND 9. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 a. Arteries of yolk sac (red), al. Alimentary cavity, alv. Alimentary vesicle 
 at the posterior end of the alimentary canal, an. Point where anus will appear. 
 au v. Auditory vesicle, bl. Blastoderm, ch. Notochord. es. Embryo-swelling, h. 
 Head. ht. Heart. ;. Mouth, mg. Medullary groove, mp. Muscle-plate or proto- 
 vertebra. op. Eye. s c. Segmentation cavity, so s. Somatic stalk, is. Tail-swelling. 
 v. Veins of yolk sac (blue), vc. Visceral cleft. I.vc. ist visceral cleft, x. Portion 
 of blastoderm outside the arterial circle in which no blood-vessels are present. 
 yk. Yolk. 
 
 PLATE 8. 
 
 Fig. A. Surface view of blastoderm of Pristiurus hardened in chromic acid. 
 
 Fig. B. Surface view of fresh blastoderm of Pristiurus. 
 
 Figs. C, D, E, and F. Pristiurus embryos hardened in chromic acid. 
 
 Fig. G. Torpedo embryo viewed as a transparent object. 
 
 Figs. H, I. Pristiurus embryos viewed as transparent objects. 
 
 Fig. K. Pristiurus embryo hardened in chromic acid. 
 
 The remainder of the figures are representations of embryos of Scyllium canicula 
 hardened in chromic acid. In every case, with the exception of the figures marked P 
 and Q, two representations of the same embryo are given ; one from the side and one 
 from the under surface. 
 
 PLATE 9. 
 
 Fig. i . Yolk of a Pristiurus egg with blastoderm and embryo. About two-thirds 
 of the yolk have been enveloped by the blastoderm. The embryo is still situated at 
 the edge of the blastoderm, but at the end of a bay in the outline of this. The thick- 
 ened edge of the blastoderm is indicated by a darker shading. Two arteries have 
 appeared. 
 
 Fig. 2. Yolk of an older Pristiurus egg. The yolk has become all but enveloped 
 by the blastoderm, and the embryo ceases to lie at the edge of the blastoderm, owing 
 to the coalescence of the two sides of the bay which existed in the earlier stage. The 
 circulation is now largely developed. It consists of an external arterial ring, and an 
 internal venous ring, the latter having been developed in the thickened edge of the 
 blastoderm. Outside the arterial ring no vessels are developed. 
 
 Fig. 3. The yolk has now become completely enveloped by the blastoderm. 
 The arterial ring has increased in size. The venous ring has vanished, owing to the 
 complete enclosure of the yolk by the blastoderm. The point where it existed is still 
 indicated (y) by the brush-like termination of the main venous trunk in a number of 
 small branches. 
 
 Fig. 4. Diagrammatic projection of the vascular system of the yolk sac of a 
 somewhat older embryo. 
 
 The arterial ring has grown much larger and the portion of the yolk where no 
 vessels exist is very small (x). The brush-like termination of the venous trunk is still 
 to be noticed. 
 
 The two main trunks (arterial and venous) in reality are in close contact as in 
 fig. 5, and enter the somatic stalk close together. 
 
 The letter a which points to the venous (blue) trunk should be v and not a. 
 
 Eig- 5. Circulation of the yolk sac of a still older embryo, in which the arterial 
 circle has ceased to exist, owing to the space outside it having become smaller and 
 smaller and finally vanished. 
 
 B. 20 
 
CHAPTER V. 
 STAGES B TO G. 
 
 THE present chapter deals with the history of the development 
 of the Elasmobranch embryo from the period when the medul- 
 lary groove first arises till that in which it becomes completely 
 closed, and converted into the medullary canal. The majority 
 of the observations recorded were made on Pristiurus embryos, 
 a few on embryos of Torpedo. Where nothing is said to the 
 contrary the statements made apply to the embryos of Pristiurus 
 only. 
 
 The general external features for this period have already 
 been given in sufficient detail in the last chapter ; and I proceed 
 at once to describe consecutively the history of the three layers. 
 
 General Features of the Epiblast. 
 
 At the commencement of this period, during the stage inter- 
 mediate between B and C, the epiblast is composed of a single 
 layer of cells. (PI. 10, fig. i.) 
 
 These are very much elongated in the region of the embryo, 
 but flattened in other parts of the blastoderm. Throughout they 
 contain numerous yolk-spherules. 
 
 In a Torpedo embryo of this age (as determined by the con- 
 dition of the notochord) the epiblast presents a very different 
 structure. It is composed of small spindle-shaped cells several 
 rows deep. The nuclei of these are very large in proportion 
 to the cells containing them, and the yolk-spherules are far 
 less numerous than in the cells of corresponding Pristiurus 
 embryos. 
 
 During stage C the condition of the epiblast does not un- 
 dergo any important change, with the exception of the layer 
 
STAGES B TO G. MEDULLARY GROOVE. 299 
 
 becoming much thickened, and its cells two or three deep in the 
 anterior parts of the embryo. (PI. 10, fig. 2.) 
 
 In the succeeding stages that part of the epiblast, which will 
 form the spinal cord, gradually becomes two or three cells deep. 
 This change is effected by a decrease in the length of the cells 
 as compared with the thickness of the layer. In the earlier 
 stages the cells are wedge-shaped with an alternate arrange- 
 ment, so that a decrement in the length of the cells at once 
 causes the epiblast to be composed of two rows of interlocking 
 cells. 
 
 The lateral parts of the epiblast which form the epidermis of 
 the embryo are modified in quite a different manner to the 
 nervous parts of the layer, becoming very much diminished in 
 thickness and composed of a single row of flattened cells. 
 (PL 10, fig. 3.) 
 
 Till the end of stage F, the epiblast cells and indeed all the 
 cells of the blastoderm retain their yolk-spherules, but the epi- 
 blast begins to lose them and consequently to become transparent 
 in stage G. 
 
 Medullary Groove. 
 
 During stage B the medullary groove is shallow posteriorly, 
 deeper in the middle part, and flattened out again at the extreme 
 anterior end of the embryo. (PI. 7, fig. 10 a, b, c.) 
 
 A similar condition obtains in the stage between B and C, 
 but the canal has now in part become deeper. Anteriorly no 
 trace of it is to be seen. In stage C it exhibits the same general 
 features. (PI. 10, fig. 2 a, 2 b, 2 c.) 
 
 By stage D we find important modifications of the canal. 
 
 It is still shallow behind and deep in the dorsal region, PI. 
 10, figs. 3^, 3*?, 3/; but the anterior flattened area in the last 
 stage has grown into a round flat plate which may be called the 
 cephalic plate, PI. 8, D and PI. IO, figs. 3 a, 3 b, 3 c. This plate 
 becomes converted into the brain. Its size and form give it 
 a peculiar appearance, but the most remarkable feature about it 
 is the ventral curvature of its edges. Its edges do not, as might 
 be expected, bend dorsalwards towards each other, but become 
 sharply bent in a ventral direction. This feature is for the first 
 
 20 2 
 
300 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 time apparent at this stage, but becomes more conspicuous 
 during the succeeding ones, and attains its maximum in stage F 
 (PL 10, fig. 5). in which it might almost be supposed that the 
 edges of the cephalic plate were about to grow downwards and 
 meet on the ventral side of the embryo. 
 
 In the stages subsequent to D the posterior part of the 
 canal deepens much more rapidly than the rest (vide PI. 10, 
 fig. 4, taken from the posterior end of an embryo but slightly 
 younger than F), and the medullary folds unite and convert 
 the posterior end of the medullary groove into a closed canal 
 (PI. 8, fig. F), while the groove is still widely open elsewhere 1 . 
 The medullary canal does not end blindly behind, but simply 
 forms a tube not closed at either extremity. The importance of 
 this fact will appear later. 
 
 In a stage but slightly subsequent to F nearly the whole of 
 the medullary canal becomes formed. This occurs in the usual 
 way by the junction and coalescence of the medullary folds. In 
 the course of the closing of the medullary groove the edges of 
 the cephalic plate lose their ventral curvature and become bent 
 up in the normal manner (vide PI. 10, fig. 6, a section taken 
 through the posterior part of the cephalic plate), and the en- 
 larged plate merely serves to enclose a dilated cephalic portion 
 of the medullary canal. The closing of the medullary canal 
 takes place earlier in the head and neck than in the back. The 
 anterior end of the canal becomes closed and does not remain 
 open like the posterior end. 
 
 Elasmobranch embryos resemble those of the Sturgeon 
 (Acipenser) and the Amphibians in the possession of a spatula- 
 like cephalic expansion : but so far as I am aware a ventral 
 flexure in the medullary plates of the head has not been ob- 
 served in other groups. 
 
 The medullary canal in Elasmobranchs is formed precisely on 
 the type so well recognised for all groups of vertebrates with 
 the exception of the Osseous Fishes. The only feature in any 
 respect peculiar to these fishes is the closing of their medullary 
 canal first commencing behind, and then at a second point in the 
 
 1 Vide Preliminary Account, etc. Q. Jl. Micros. Science, Oct. 1874, PI. 14, 8 a. 
 [This Edition, No. V. PI. 3, 8a.] This and the other section from the same embryo 
 (stage F) may be referred to. I have not thought it worth while repeating them here. 
 
STAGES B TO G. MESOBLAST. 301 
 
 cervical region. In those vertebrates in which the medullary 
 folds do not unite at approximately the same time throughout 
 their length, they appear usually to do so first in the region 
 of the neck. 
 
 Mesoblast. 
 
 The separation from the hypoblast of two lateral masses of 
 mesoblast has already been described. Till the close of stage C 
 the mesoblast retains its primitive bilateral condition unaltered. 
 Throughout the whole length of the embryo, with the exception 
 of the extreme front part, there are present two plates of rounded 
 mesoblast cells, one on each side of the medullary groove. These 
 plates are in very close contact with the hypoblast, and also 
 follow with fair accuracy the outline of the epiblast. This 
 relation of the mesoblast plates to the epiblast must not how- 
 ever be supposed to indicate that the medullary groove is due 
 to growth in the mesoblast : a view which is absolutely negatived 
 by the manner of formation of the medullary groove in the 
 head. Anteriorly the mesoblast plates thin out and completely 
 vanish. 
 
 In stage D, the plates of mesoblast in the trunk undergo 
 important changes. The cells composing them become arranged 
 in two layers (PI. 10, fig. 3), a splanchnic layer adjoining the 
 hypoblast (sp), and a somatic layer adjoining the epiblast 1 (so). 
 Although these two layers are distinctly formed, they do not 
 become separated at this stage in the region of the trunk, and 
 in the trunk no true body-cavity is formed. 
 
 By stage D the plates of mesoblast have ceased to be quite 
 isolated, and are connected with the lower layer cells of the 
 general blastoderm. 
 
 Moreover the lower layer cells outside the embryo now 
 exhibit distinct traces of a separation into two layers, one con- 
 tinuous with the hypoblast, the other with the mesoblast. Both 
 layers are composed of very flattened cells, and the mesoblast 
 layer is often more than one cell deep, and sometimes exhibits 
 a mesh-like arrangement of its elements. 
 
 1 I underestimated the distinctness of this formation in my earlier paper, loc. cit., 
 although I recognised the fact that the mesoblast cells became arranged in two 
 distinct layers. 
 
302 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Coincidentally with the appearance of a differentiation into 
 a somatic and splanchnic layer the mesoblast plates become 
 partially split by a series of transverse lines of division into pro- 
 tovertebrae. Only the proximal regions of the plates become 
 split in this way, while their peripheral parts remain quite intact. 
 As a result of this each plate becomes divided into a proximal 
 portion adjoining the medullary canal, which is divided into 
 protovertebra, and may be called the vertebral plate, and a 
 peripheral portion not so divided, which may be called the 
 lateral plate. These two parts are at this stage quite continuous 
 with each other ; and, as will be seen in the sequel, the body- 
 cavity originally extends uninterruptedly to the summit of the 
 vertebral plates. 
 
 By stage D at the least ten protovertebrae have appeared. 
 In Torpedo the mesoblast commences to be divided into two 
 layers much earlier than in Pristiurus ; and even before stage C 
 this division is more or less clearly marked. 
 
 In the head and tail the condition of the mesoblast is by no 
 means the same as in the body. 
 
 In the tail the plates of mesoblast become considerably 
 thickened and give rise to two projections, one on each side, 
 which have already been alluded to as caudal or tail-swellings ; 
 vide PI. 8, figs. D, F, and PI. 10, fig. 3/and fig. 4 ts. 
 
 These masses of mesoblast are neither divided into proto- 
 vertebrae, nor do they exhibit any trace of a commencing dif- 
 ferentiation into somatopleure and splanchnopleure. 
 
 In the head, so far as I have yet been able to observe, the 
 mesoblastic plates do not at this stage become divided into 
 protovertebrse. The other changes exhibited in the cephalic 
 region are of interest, mainly from the fact that here appears a 
 cavity in the mesoblast directly continuous with the body-cavity 
 (when that cavity becomes formed), but which appears at a 
 very much earlier date than the body-cavity. This cavity can 
 only be looked on in the light of a direct continuation of the 
 body or peritoneal cavity into the head. Theoretical considera- 
 tions with reference to it I propose reserving till I have described 
 the changes which it undergoes in the subsequent periods. 
 
 PI. 10, figs. 3 a, 3 b and 3 c exhibit very well the condition 
 of the mesoblast in the head at this period. In fig. 3 c, a section 
 
STAGES B TO G. ALIMENTARY CANAL. 303 
 
 taken through the back part of the head, the mesoblast plates 
 have nearly the same form as in the sections immediately 
 behind. The ventral continuation of the mesoblast formed by 
 the lateral plate has, however, become much thinner, and the 
 dorsal or vertebral portion has acquired a more triangular form 
 than in the sections through the trunk (figs. 3 d and 3 e). 
 
 In the section (fig. 3 U) in front of this the ventral portion of 
 the plate is no longer present, and only that part exists which 
 corresponds with the vertebral division of the primitive plate 
 of mesoblast. 
 
 In this a distinct cavity, forming part of the body-cavity, has 
 appeared. 
 
 In a still anterior section (fig. 3 a) no cavity is any longer 
 present in the mesoblast ; whilst in sections taken from the 
 foremost part of the head no mesoblast is to be seen (vide PL 10, 
 fig. 5, taken from the front part of the head of the embryo 
 represented in PL 8, fig. F). 
 
 A continuation of the body-cavity into the head has already 
 been described by Oellacher 1 for the Trout : but he believes that 
 the cavity in this part is solely related to the formation of the 
 pericardial space. 
 
 The condition of the mesoblast undergoes no important 
 change till the end of the period treated of in this chapter. The 
 masses of mesoblast which form the tail-swellings become more 
 conspicuous (PI. 10, fig. 4) ; and indeed their convexity is so 
 great that the space between them has the appearance of a 
 median groove, even after the closure of the neural canal in the 
 caudal region. 
 
 In embryos of stage G, which may be considered to belong 
 to the close of this period, eighteen protovertebrae are present 
 both in Pristiurus and Torpedo embryos. 
 
 The A limentary Canal. 
 
 The alimentary canal at the commencement of this period 
 (stage B) forms a space between the embryo and the yolk, 
 ending blindly in front, but opening posteriorly by a widish 
 slit-like aperture, which corresponds to the anus of Rusconi 
 (PI- 7, %. 7). 
 
 1 Zeitschrift f. iviss. Zoologie, 1873. 
 
304 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The cavity anteriorly has a more or less definite form, having 
 lateral walls, as well as a roof and floor (P.I. 7, figs. iQb and ioc\ 
 Posteriorly it is not nearly so definitely enclosed (PL 7, fig. io). 
 The ventral wall of the cavity is formed by yolk. But even in 
 stage B there are beginnings of a cellular ventral wall derived 
 from an ingrowth of cells from the two sides. 
 
 By stage C considerable progress has been made in the 
 formation of the alimentary canal. Posteriorly it is as flattened 
 and indefinite as during stage B (PI. 10, figs. 2b and 2c}. But 
 in the anterior part of the embryo the cavity becomes much 
 deeper and narrower, and a floor of cells begins to be formed for 
 it (PI. 10, fig. 2) ; and, finally, in front, it forms a definite space 
 completely closed in on all sides by cells (PL 10, fig. 2.0). Two 
 distinct processes are concerned in effecting these changes in the 
 condition of the alimentary cavity. One of these is a process of 
 folding off the embryo from the blastoderm. The other is a 
 simple growth of cells independent of any folding. To the first 
 of these processes the depth and narrowness of the alimentary 
 cavity is due ; the second is concerned in forming its ventral 
 wall. The combination of the two processes produces the peculiar 
 triangular section which characterises the anterior closed end of 
 the alimentary cavity at this stage. The process of the folding 
 off of the embryo from , the blastoderm resembles exactly the 
 similar process in the embryo bird. The fold by which the 
 constricting off of the embryo is effected is a perfectly continuous 
 one, but may be conveniently spoken of as composed of a head- 
 fold and two lateral folds. 
 
 Of far greater interest than the nature of these folds is the 
 formation of the ventral wall of the alimentary canal. This, as 
 has been said, is effected by a growth of cells from the two 
 sides to the middle line (PL 10, fig. 2). The cells for this 
 are however not derived from pre-existing hypoblast cells, but 
 are formed spontaneously around nuclei of the yolk. This fact 
 can be determined in a large number of sections, and is fairly 
 well shewn in PL 10, fig. 2 na. The cells are formed in the 
 yolk, as has been already mentioned, by a simple aggregation of 
 protoplasm around pre-existing nuclei. 
 
 The cells being described are in most cases formed close to 
 the pre-existing hypoblast cells, but often require to undergo a 
 
STAGES B TO G. ALIMENTARY CANAL. 305 
 
 considerable change of position before attaining their final 
 situation in the wall of the alimentary canal. 
 
 I have already alluded to this feature in the formation of the 
 ventral wall of the alimentary cavity. Its interest, as bearing on 
 the homology of the yolk, is considerable, owing to the fact that 
 the so-called yolk-cells of Amphibians play a similar part in 
 supplying the ventral epithelium of the alimentary cavity, as do 
 the cells derived from the yolk in Elasmobranchs. 
 
 The fact of this feature being common to the yolk-cells of 
 Amphibians and the yolk of Elasmobranchs, supplies a strong 
 argument in favour of the homology of the yolk-cells in the one 
 case with the yolk in the other 1 . 
 
 1 Nearly simultaneously with Chapter III. of the present monograph on the 
 Development of Elasmobranchs, which dealt in a fairly complete manner with the 
 genesis of cells outside the blastoderm, there appeared two important papers dealing 
 wilh the same subject for Teleostei. One of these, by Professor Bambeke, " Em- 
 bryologie des Poissons Osseux," Mem. Cour. Acad. Belgique, 1875, which appeared 
 some little time before my paper, and a second by Dr Klein, Quart, your, of Micr. 
 Sci. April, 1876. In both of these papers a development of nuclei and of cells is 
 described as occurring outside the blastoderm in a manner which accords fairly well 
 with my own observations. 
 
 The conclusions of both these investigators differ however from my own. They 
 regard the finely granular matter, in which the nuclei appear, as pertaining to the 
 blastoderm, and morphologically quite distinct from the yolk. From their observa- 
 tions we can clearly recognise that the material in which the nuclei appear is far more 
 sharply separated off from the yolk in Osseous Fish than in Elasmobranchs, and this 
 sharp separation forms the main argument for the view of these authors. Dr Klein 
 admits, however, that this granular matter (which he calls parablast) graduates into the 
 typical food-yolk, though he explains this by supposing that the parablast takes up 
 part of the yolk for the purpose of growth. 
 
 It is clear that the argument from a sharp separation of yolk and parablast cannot 
 have much importance, when it is admitted (i) that in Osseous Fish there is a 
 gradation between the two substances, while (2) in Elasmobranchs the one merges 
 slowly and insensibly into the other. 
 
 The only other argument used by these authors is stated by Dr Klein in the 
 following way. "The fact that the parablast has, at the outset, been forming one 
 unit with what represents the archiblast, and, while increasing has spread i.e. grown 
 over the yolk which underlies the segmentation-cavity, is, I think, the most absolute 
 proof that the yolk is as much different from the parablast as it is from the archiblast." 
 This argument to me merely demonstrates that certain of the nutritive elements of 
 the yolk become in the course of development converted into protoplasm, a pheno- 
 menon which must necessarily be supposed to take place on my own as well as on 
 Dr Klein's view of the nature of the yolk. My own views on the subject "have already 
 been fully stated. I regard the so-called yolk as composed of a larger or smaller 
 amount of food-material imbedded in protoplasm, and the meroblastic ovum as a body 
 
3O6 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The history of the alimentary canal during the remainder of 
 this period may be told briefly. 
 
 The folding off and closing of the alimentary canal in the 
 anterior part of the body proceeds rapidly, and by stage D not only 
 is a considerable tract of alimentary canal formed, but a great 
 part of the head is completely folded off from the yolk (PI. 10, 
 fig. 3). By stage F a still greater part is folded off. The 
 posterior part of the alimentary canal retains for a long period 
 its primitive condition. It is not until stage F that it begins to 
 be folded off behind. After the folding has once commenced it 
 proceeds with great rapidity, and before stage G the hinder part 
 of the alimentary canal becomes completely closed in. 
 
 The folding in of the gut is produced by two lateral folds, 
 and the gut is not closed posteriorly. 
 
 It may be remembered that the neural canal also remained 
 open behind. Thus both the neural and alimentary canals are 
 open behind ; and, since both of them extend to the posterior 
 
 constituted of the same essential parts as a holoblastic ovum, though divided into 
 regions which differ in the proportion of protoplasm they contain. I do not propose 
 to repeat the positive arguments used by me in favour of this view, but content 
 myself with alluding to the protoplasmic network found by Schultz and myself ex- 
 tending through the whole yolk, and to the similar network described by Bambeke 
 as being present in the eggs of Osseous Fish after deposition but before impregnation. 
 The existence of these networks is to me a conclusive proof of the correctness of my 
 views. I admit that in Teleostei the 'parablast' contains more protoplasm than the 
 homologous material in the Elasmobranch ovum, while it is probable that after 
 impregnation the true yolk of Teleostei contains little or no protoplasm ; but these 
 facts do not appear to me to militate against my views. 
 
 I agree with Prof. Bambeke in regarding the cells derived from the sub-germinal 
 matter as homologous with the so-called yolk-cells of the Amphibian embryo. 
 
 I have recently, in some of the later stages of development, met with very 
 peculiar nuclei of the yolk immediately beneath the blastoderm at some little 
 distance from the embryo, PI. ro, fig. 8. They were situated not in finely sub- 
 germinal matter, but amongst large yolk-spherules. They were very large, and 
 presented still more peculiar forms than those already described by me, being pro- 
 duced into numerous long filiform processes. The processes from the various nuclei 
 were sometimes united together, forming a regular network of nuclei quite unlike 
 anything that I have previously seen described. 
 
 The sub-germinal matter, in which the nuclei are usually formed, becomes during 
 the later stages of development far richer in protoplasm than during the earlier. It 
 continually arises at fresh points, and often attains to considerable dimensions, no 
 doubt by feeding on yolk-spherules. Its development appears to be determined by 
 the necessities of growth in the blastoderm or embryo. 
 
STAGES B TO G. ALIMENTARY CANAL. 307 
 
 end of the body, they meet there, their walls coalesce, and a 
 direct communication from the neural to the alimentary canal 
 is instituted. The process may be described in another way 
 by saying that the medullary folds are continuous round the 
 end of the tail with the lateral walls of the alimentary canal ; so 
 that, when the medullary folds unite to form a canal, this canal 
 becomes continuous with the alimentary canal, which is closed 
 in at the same time. In whatever way this arrangement 
 is produced, the result of it is that it becomes possible to 
 pass in a continuously closed passage along the neural canal 
 round the end of the tail and into the alimentary canal. A 
 longitudinal section shewing this feature is represented on PI. 
 10, fig. 7. 
 
 This communication between the neural and alimentary 
 canals, which is coupled, as will be seen in the sequel, with the 
 atrophy of a posterior segment of the alimentary canal, is a 
 feature of great interest which ought to throw considerable 
 light upon the meaning of the neural canal. So far as I know, 
 no suggestion as to the origin of it has yet been made. It 
 is by no means confined to Elasmobranchs, but is present in 
 all the vertebrates whose embryos are situated at the centre and 
 not at the periphery of the blastoderm. It has been described 
 by Goette 1 in Amphibians and by Kowalevsky, Owsjannikow 
 and Wagner 2 in the Sturgeon (Acipenser). The same arrange- 
 ment is also stated by Kowalevsky 8 to exist in Osseous Fishes 
 and Amphioxus. The same investigator has shewn that the 
 alimentary and neural canals communicate in larval Ascidians, 
 and we may feel almost sure that they do so in the Marsipo- 
 branchii. 
 
 The Reptilia, Aves, and Mammalia have usually been dis- 
 tinguished from other vertebrates by the possession of a well- 
 developed allantois and amnion. I think that we may further 
 say that the lower vertebrates, Pisces and Amphibia, are to be 
 distinguished from the three above-mentioned groups of higher 
 
 1 Eniwicklungsgeschichte der Unke. 
 
 - Melanges Biologiques de F Acadhnie Ptiersbourg, Tome vil. 
 
 3 Archiv. f. mikros. Anat. Vol. vn. p. 114. In the passage on this point 
 Kowalevsky states that in Elasmobranchs the neural and alimentary canals com- 
 municate. This I believe to be the first notice published of this peculiar arrangement. 
 
308 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 vertebrates, by the positive embryonic character that their neural 
 and alimentary canals at first communicate posteriorly. The 
 presence or absence of this arrangement depends on the different 
 positions of the embryo in the blastoderm. In Reptiles, Birds 
 and Mammals, the embryo occupies a central position in the 
 blastoderm, and not, as in Pisces and Amphibia, a peripheral 
 one at its edge. We can, in fact, only compare the blastoderm 
 of the Bird and the Elasmobranch, by supposing that in the 
 blastoderm of the Bird there has occurred an abbreviation of the 
 processes, by which the embryo Elasmobranch is eventually 
 placed in the centre of the blastoderm : as a result of this abbre- 
 viation the embryo Bird occupies from the first a central position 
 in the blastoderm 1 . 
 
 The peculiar relations of the blastoderm and embryo, and 
 the resulting relations of the neural and alimentary canal, 
 appear to me to be features of quite as great an importance 
 for classification as the presence or absence of an amnion and 
 allantois. 
 
 General Features of the Hypoblast. 
 
 There are but few points to be noticed with reference to the 
 histology of the hypoblast cells. The cells of the dorsal wall of 
 the alimentary cavity are columnar and form a single row. 
 Those derived from the yolk to form the ventral wall are at first 
 roundish, but subsequently assume a more columnar form. 
 
 1 Vide Note on p. 281, also p. 295, and PI. 9, Figs, i and 2, and Comparison, 
 &c., Qy. JL of Micros. Sci. July, 1875, p. 219. [This Edition, No. VI. p. 125.] 
 These passages give an account of the change of position of the Elasmobranch em- 
 bryo, and the Note on p. 281 con tains- a speculation about the nature of the primitive 
 streak with its contained primitive groove. I have suggested that the primitive streak 
 is probably to be regarded as a rudiment at the position where the edges of the blas- 
 toderm coalesced to give to the embryos of Birds and Mammals the central position 
 which they occupy. 
 
 If my hypothesis should turn out to be correct, various, now unintelligible, 
 features about the primitive streak would be explained : such as its position behind 
 the embryo, the fusion of the epiblast and mesoblast in it, the groove it contains, &c. 
 
 The possibility of the primitive streak representing the blastopore, as it in fact 
 does according to my hypothesis, ought also to throw light on E. Van Beneden's 
 recent researches on the development of the Mammalian ovum. 
 
 In order clearly to understand the view here expressed, the reader ought to refer to 
 the passages above quoted. 
 
STAGES B TO G. THE NOTOCHORD. 309 
 
 The Notochord. 
 
 One of the most interesting features in the Elasmobranch 
 development is the formation of the notochord from the 4iypo- 
 blast. All the steps in the process by which this takes place 
 can be followed with great ease and certainty. 
 
 Up to stage B the hypoblast is in contact with the epiblast 
 immediately below the medullary groove, but exhibits no trace 
 of a thickening or any other formation at that point. 
 
 Between stage B and C the notochord first arises. 
 
 In the hindermost sections of this stage the hypoblast retains 
 a perfectly normal structure and uniform thickness throughout. 
 In the next few sections (PI. 10, fig. I c, ch') a slight thickening is 
 to be observed in the hypoblast, immediately below the medul- 
 lary canal. The layer, which elsewhere is composed of a single 
 row of cells, here becomes two cells deep, but no sign of a 
 division into two layers exhibited. 
 
 In the next few sections the thickening of the hypoblast 
 becomes much more pronounced ; we have, in fact, a ridge 
 projecting from the hypoblast towards the epiblast (PI. 10, 
 fig. i b, ch'). 
 
 This ridge is pressed firmly against the epiblast, and causes 
 in it a slight indentation. The hypoblast in the region of the 
 ridge is formed of two layers of cells, the ridge being entirely 
 due to the uppermost of the two. 
 
 In sections in front of this a cylindrical rod, which can at 
 once be recognised as the notochord and is continuous with the 
 ridge just described, begins to be split off from the hypoblast. 
 It is difficult to say at what point the separation of this rod 
 from the hypoblast is completed, since all intermediate gradations 
 between complete separation and complete attachment are to be 
 seen. 
 
 Where the separation first appears, a fairly thick bridge of 
 hypoblast is left connecting the two lateral halves of the layer, 
 but anteriorly this bridge becomes excessively delicate and thin 
 (PI. 10, fig. i a), and in some cases is barely visible except with 
 high powers. 
 
 From the series of sections represented, it is clear that the 
 
3IO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 notochord commences to be separated from the hypoblast an- 
 teriorly, and that the separation gradually extends backwards. 
 
 The posterior extremity of the notochord remains for a long 
 time attached to the hypoblast ; and it is not till the end of the 
 period treated of in this chapter that it becomes completely free. 
 
 A sheath is formed around the notochord, very soon after its 
 formation, at a stage intermediate between stages C and D. 
 This sheath is very delicate, though it stains with both osmic 
 acid and hsematoxylin. I conclude from its subsequent history, 
 that it is to be regarded as a product of the cells of the noto- 
 chord, but at the same time it should be stated that it precisely 
 resembles membrane-like structures, which I have already 
 described as being probably artificial. 
 
 Towards the end of this period the cells of the notochord 
 become very much flattened vertically, and cause the well-known 
 stratified appearance which characterises the notochord in longi- 
 tudinal sections. In transverse sections the outlines of the cells 
 of the notochord appear rounded. 
 
 Throughout this period the notochord cells are filled with 
 yolk-spherules, and near its close small vacuoles make their 
 appearance in them. 
 
 An account of the development of the notochord, substantially 
 similar to that I have just given, appeared in my preliminary 
 paper ' on the development of the Elasmobranch fishes. 
 
 To the remarks which were there made, I have little to add. 
 There are two possible views, which can be held with reference 
 to the development of the notochord from the hypoblast. 
 
 We may suppose that this is the primitive mode of develop- 
 ment of the notochord, or we may suppose that the separation 
 of the notochord from the hypoblast is due to a secondary 
 process. 
 
 If the latter view is accepted, it will be necessary to maintain 
 that the mesoblast becomes separated from the hypoblast as 
 three separate masses, two lateral, and one median, and that 
 the latter becomes separated much later than the two former. 
 
 We have, I think, no right to assume the truth of this view 
 without further proof. The general admission of assumptions 
 of this kind is apt to lead to an injurious form of speculation, in 
 
 1 Loc. tit. 
 
STAGES B TO G. THE NOTOCHORD. 3 1 I 
 
 which every fact presenting a difficulty in the way of some 
 general theory is explained away by an arbitrary assumption, 
 while all the facts in favour of it are taken for granted. It is 
 however clear that no theory can ever be fairly tested so long as 
 logic of this kind is permitted. If, in the present instance, the 
 view is adopted that the notochord has in reality a mesoblastic 
 origin, it will be possible to apply the same view to every other 
 organ derived from the hypoblast, and to say that it is really 
 mesoblastic, but has become separated at rather a late period 
 from the hypoblast. 
 
 If, however, we provisionally reject this explanation, and 
 accept the other alternative, that the notochord is derived from 
 the hypoblast, we must be prepared to adopt one of two views 
 with reference to the development of the notochord in other 
 vertebrates. We must either suppose that the current state- 
 ments as to the development of the notochord in other vertebrates 
 are inaccurate, or that the notochord has only become secondarily 
 mesoblastic. 
 
 The second of these alternatives is open to the same ob- 
 jections as the view that the notochord has only apparently a 
 hypoblastic source in Elasmobranchs, and, provisionally at least, 
 the first of them ought to be accepted. The reasons for ac- 
 cepting this alternative fall under two heads. In the first place, 
 the existing accounts and figures of the development of the 
 notochord exhibit in almost all cases a deficiency of clearness 
 and precision. The exact stage necessary to complete the series 
 never appears. It cannot, therefore, at present be said that the 
 existing observations on the development of the notochord 
 afford a strong presumption against its hypoblastic origin. 
 
 In the second place, the remarkable investigations of Hensen 1 , 
 on the development of the notochord in Mammalia, render it 
 very probable that, in this group, the notochord is developed 
 from the hypoblast. 
 
 Hensen finds that in Mammalia, as in Elasmobranchs, the 
 mesoblast forms two independent lateral masses, one on each 
 side of the medullary canal. 
 
 After the commencing formation of the protovertebrse the 
 hypoblast becomes considerably thickened beneath the medul- 
 
 1 Zeitschrift f. Anat. u. Entwicklun&geschichte, Vol. I. p. 366. 
 
312 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 lary groove ; and, though he has not followed out all the steps of 
 the process by which this thickening is converted into the noto- 
 chord, yet his observations go very far towards proving that it 
 does become the notochord. 
 
 Against the observations of Hensen, there ought, however, to 
 be mentioned those of Lieberkuhn \ He believes that the two 
 lateral masses of mesoblast, described by Hensen (in an earlier 
 paper than the one quoted), are in reality united by a delicate 
 layer of cells, and that the notochord is formed from a thickening 
 of these. 
 
 Lieberkuhn gives no further statements or figures, and it is 
 clear that, even if there is present the delicate layer of meso- 
 blast, which he fancies he has detected, yet this cannot in any 
 way invalidate such a section as that represented on PI. X. fig. 
 40, of Hensen's paper. 
 
 In this figure of Hensen's, the hypoblast cells become dis- 
 tinctly more columnar, and the whole layer much thicker im- 
 mediately below the medullary canal than elsewhere, and this 
 independently of any possible layer of mesoblast. 
 
 It appears to me reasonable to conclude that Lieberkiihn's 
 statements do not seriously weaken the certainty of Hensen's 
 results. 
 
 In addition to the observations of Hensen's on Mammalia, 
 those of Kowalevsky and Kuppfer on Ascidians may fairly be 
 pointed to as favouring the hypoblastic origin of the notochord. 
 
 It is not too much to say that at the present moment the 
 balance of evidence is in favour of regarding the notochord as a 
 hypoblastic organ. 
 
 This conclusion is, no doubt, rather startling, and difficult to 
 understand. The only feature of the notochord in its favour is 
 the fact of its being unsegmented 2 . 
 
 Should it eventually turn out that the notochord is developed 
 in most vertebrates from the mesoblast, and only exceptionally 
 from the hypoblast, the further question will have to be settled 
 
 1 Sits, der Gesell. zu Marburg, Jan. 1876. 
 
 a In my earlier paper I suggested that the endostyle of Ascidians afforded an 
 instance of a supporting organ being derived from the hypoblast. This parallel does 
 not hold since the endostyle has been shewn to possess a secretory function. I 
 never intended (as has been imagined by Professor Todaro) to regard the endostyle 
 as the homologue of the notochord. 
 
STAGES B TO G. THE NOTOCHORD. 313 
 
 as to whether it is primitively a hypoblastic or a mesoblastic 
 organ ; but, from whatever layer it has its source, an excellent 
 example will be afforded of an organ changing from the layer in 
 which it was originally developed into another distinct layer. 
 
 EXPLANATION OF PLATE 10. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 a/. Alimentary canal, ch. Chorda dorsalis or notochord. ch'. Ridge of hypoblast, 
 which will become separated off as the notochord. ep. Epiblast. Ay. Hypoblast. 
 //. Coalesced lateral and vertebral plate of mesoblast. m g. Medullary groove. 
 . Nucleus of yolk. n a. Cells formed around the nuclei of the yolk to enter into the 
 ventral wall of the alimentary canal, n c. Neural or medullary canal. / v. Proto- 
 vertebra. so. Somatopleure. sp. Splanchnopleure. t s. Mesoblast of tail-swelling. 
 yk. Yolk-spherules. 
 
 Figs, i a, i b, i c. Three sections from the same embryo belonging to a stage 
 intermediate between B and C, of which fig. i a is the most anterior, x 96 diameters. 
 
 The sections illustrate (i) The different characters of the medullary groove in the 
 different regions of the embryo. (2) The structure of the coalesced lateral and verte- 
 bral plates. (3) The mode of formation of the notochord as a thickening of the 
 hypoblast (ch'), which eventually becomes separated from the hypoblast as an 
 elliptical rod (i a, ch). 
 
 Fig. 2. Section through the anterior part of an embryo belonging to stage C. 
 The section is mainly intended to illustrate the formation of the ventral wall of the 
 alimentary canal from cells formed around the nuclei of the yolk. It also shews the 
 shallowness of the medullary groove in the anterior part of the body. 
 
 Figs. 2 a, ib, ic. Three sections from the same embryo as fig. 2. Fig. 2 a is the 
 most anterior of the three sections and is taken through a point shortly in front of 
 fig. 2. The figures illustrate the general features of an embryo of stage C, more 
 especially the complete closing of the alimentary canal in front and the triangular 
 section which it there presents. 
 
 Fig. 3. Section through the posterior part of an embryo belonging to stage D. 
 x 86 diameters. 
 
 It shews the general features of the layers during the stage, more especially the 
 differentiation of somatic and splanchnic layers of the mesoblast. 
 
 Figs. 3 a, 3 b, 3 c, 3 d, 3 e, $f. Sections of the same embryo as fig. 3 ( x 60 dia- 
 meters). Fig. 3 belongs to part of the embryo intermediate between figs. 3^ and %f. 
 
 The sections shew the features of various parts of the embryo. Figs. 30, 3 b and 
 3 c belong to the head, and special attention should be paid to the presence of a cavity 
 in the mesoblast in 3 b and to the ventral curvature of the medullary folds. 
 
 Fig. 3 d belongs to the neck, fig. 3 e to the back, and fig. 3/to the tail. 
 
 Fig. 4. Section through the region of the tail at the commencement of stage F. 
 x 60 diameters. 
 
 The section shews the character of the tail-swellings and the commencing closure 
 of the medullary groove. 
 
 B. 21 
 
3 14 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Fig. 5. Transverse section through the anterior part of the head of an embryo 
 belonging to stage F ( x 60 diameters). It shews (i) the ventral curvature of the 
 medullary folds next the head. (2) The absence of mesoblast in the anterior part of 
 the head, hy points to the extreme front end of the alimentary canal. 
 
 Fig. 6. Section through the head of an embryo at a stage intermediate between F 
 and G. x 86 diameters. 
 
 It shews the manner in which the medullary folds of the head unite to form the 
 medullary canal. 
 
 Fig. 7. Longitudinal and vertical section through the tail of an embryo belonging 
 to stage G. 
 
 It shews the direct communication which exists between the neural and alimentary 
 canals. 
 
 The section is not quite parallel to the long axis of the embryo, so that the proto- 
 vertebrse are cut through in its anterior part, and the neural canal passes out of the 
 section anteriorly. 
 
 Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H. 
 
CHAPTER VI. 
 DEVELOPMENT OF THE TRUNK DURING STAGES G TO K. 
 
 BY the stage when the external gills have become conspicuous 
 objects, the rudiments of the greater number of the important 
 organs of the body are definitely established. 
 
 Owing to this fact the first appearance of the external gills 
 forms a very convenient break in the Elasmobranch develop- 
 ment ; and in the present chapter the history is carried on to the 
 period of this occurrence. 
 
 While the last chapter dealt for the most part with the 
 formation of the main organic systems from the three embryonic 
 layers, the present one has for its subject the gradual differentia- 
 tion of these systems into individual organs. In treating of the 
 development of the separate organs a divergence from the plan 
 of the last chapter becomes necessary, and the following arrange- 
 ment has been substituted for it. First of all an account is given 
 of the development of the external epiblast, which is followed 
 by a description of the organs derived from the mesoblast and of 
 the notochord. 
 
 External Epiblast. 
 
 During stages G to I the epiblast 1 is formed of a single layer 
 of flattened cells ; and in this, as in the earlier stages, it deserves 
 to be especially noticed that the epiblast is never more than one 
 cell deep, and is therefore incapable of presenting any differentia- 
 tion into nervous and epidermic layers. (PL 11, figs, i 5.) 
 
 1 Unless the contrary is stated, the facts recorded in this chapter apply only to 
 the genera Scyllium and Pristiurus. 
 
 21 2 
 
316 DEVELOPMENT OF EL AS MOB RANCH FISHES. 
 
 The cells which compose it are flattened and polygonal in 
 outline, but more or less spindle-shaped in section. They present 
 a strong contrast to the remaining embryonic cells of the body 
 in possessing a considerable quantity of clear protoplasm, which 
 in most other cells is almost entirely absent. Their granular 
 nucleus is rounded or oval, and typically contains a single 
 nucleolus. Frequently, however, two nucleoli are present, and 
 when this is the case an area free from granules is to be seen 
 around each nucleolus, and a dark line, which could probably 
 be resolved into granules by the use of a sufficiently high 
 magnifying power, divides the nucleus into two halves. These 
 appearances probably indicate that nuclei, in which two nucleoli 
 are present, are about to divide. 
 
 The epiblast cells vary in diameter from '022 to '026 Mm. 
 and their nuclei from '014 to '018 Mm. They present a fairly 
 uniform character over the greater part of the body. In Torpedo 
 they present nearly the same characters as in Pristiurus and 
 Scyllium, but are somewhat more columnar. (PI. n, fig. 7.) 
 
 Along the summit of the back from the end of the tail to 
 the level of the anus, or slightly beyond this, epiblast cells form 
 a fold the rudiment of the embryonically undivided dorsal fin 
 and the cells forming this, unlike the general epiblast cells, are 
 markedly columnar ; they nevertheless, here as elsewhere, form 
 but a single layer. (PI. 11, fig. 3 and 5 df.} Although at 
 this stage the dorsal fin is not continued as a fold anteriorly 
 to the level of the anus, yet a columnar thickening or ridge 
 of epiblast, extending along the median dorsal line nearly to 
 the level of the heart, forms a true morphological prolongation 
 of the fin. 
 
 On the ventral side of the tail is present a rudiment of the 
 ventral unpaired fin, which stops short of the level of the anus, 
 but, though less prominent, is otherwise quite similar to the 
 dorsal fin and continuous with it round the end of the tail. At 
 this stage the mesoblast has no share in forming either fin. 
 
 In many sections of the tail there may be seen on each side 
 two folds of skin, which are very regular, and strongly simulate 
 the rudimentary fins just described. The cells composing them 
 are, however, not columnar, and the folds themselves are* merely 
 artificial products due to shrinking. 
 
STAGES G TO K. THE EXTERNAL EPIBLAST. 317 
 
 At a stage slightly younger than K an important change 
 takes place in the epiblast. 
 
 From being composed of a single layer of cells it becomes 
 two cells deep. The two layers appear first of all anteriorly, and 
 subsequently in the remaining parts of the body. At first, both 
 layers are formed of flattened cells (PL 11, figs. 8, 9) ; but at a 
 stage slightly subsequent to that dealt with in the present 
 chapter, the cells of the inner of the two layers become columnar, 
 and thus are established the two strata always present in the 
 epidermis of adult vertebrates, viz. an outer layer of flattened 
 cells and an inner one of columnar cells 1 . 
 
 The history of the epiblast in Elasmobranchs is interesting, 
 from the light which it throws upon the meaning of the nervous 
 and epidermic layers into which the epiblast of Amphibians and 
 some other Vertebrates is divided. The Amphibians and 
 Elasmobranchs present the strongest contrast in the develop- 
 ment of their epiblast, and it is worth while shortly to review 
 and compare the history of the layer in the two groups. 
 
 In Amphibians the epiblast is from the first divided into an 
 outer stratum formed of a single row of flattened cells, and an 
 inner stratum composed of several rows of more rounded cells. 
 These two strata were called by Strieker the nervous and 
 epidermic layers, and these names have been very generally 
 adopted. 
 
 Both strata have a share in forming the general epiblast, and 
 though eventually they partially fuse together, there can be but 
 little doubt that the horny layer of the adult epiblast, where such 
 can be distinguished 2 , is derived from the epidermic layer of the 
 embryo, and the mucous layer of the epiblast from the embryonic 
 nervous layer. Both layers of the epiblast assist in the formation 
 of the cerebro-spinal nervous system, and there also at first fuse 
 together 3 , though the epidermic layer probably separates itself 
 again, as the central epithelium of the spinal canal. The lens 
 and auditory sac are derived exclusively from the nervous layer 
 
 1 The layers are known as epidermic (horny) and mucous layers by English writers, 
 and as Hornschicht and Schleimschicht by the Germans. For their existence in all 
 Vertebrates, vide Leydig Uebcr attgcmrine Bedeckungen der Amphibien, p. 20. Bonn, 
 
 .1876. 
 
 2 Vide Leydig, loc. at. 
 
 :t Vide Gotte, Entwicklungsgeschichte der Unke, 
 
3l8 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of the epidermis, while this layer also has the greater share in 
 forming the olfactory sac. 
 
 In Elasmobranchs the epiblast is at first uniformly composed 
 of a single row of cells. The part of the layer which will form 
 the central nervous system next becomes two or three cells deep, 
 but presents no distinction into two layers; the remaining 
 portions of the layer remain, as before, one cell deep. Although 
 the epiblast at first presents this simple structure, it eventually, 
 as we have seen, becomes divided throughout into two layers, 
 homologous with the two layers which arise so early in Amphi- 
 bians. The outer one of the two forms the horny layer of the 
 epidermis and the central epithelium of the neural canal. The 
 inner one, the mucous layer of the epidermis and the nervous 
 part of the brain and spinal cord. Both layers apparently enter 
 into the formation of the organs of sense. 
 
 While there is no great difficulty in determining the equiva- 
 lent parts of the epidermis in Elasmobranchs and Amphibians, 
 it still remains an open question in which of these groups the 
 epiblast retains its primitive condition. 
 
 Though it is not easy to bring conclusive proofs on the one 
 side or the other, the balance of argument appears to me to be 
 decidedly in favour of regarding the condition of the epiblast in 
 Elasmobranchs, and most other Vertebrates, as the primitive one, 
 and its condition in Amphibians as a secondary one, due to the 
 throwing back of the differentiation of their epiblast into two 
 layers to a very early period in their development. 
 
 In favour of this view are the following points: (i) That a 
 primitive division of the epiblast into two layers is unknown in 
 the animal kingdom, except amongst Amphibians and (?) Osseous 
 Fish. (2) That it appears more likely for a particular feature of 
 development to be thrown back to an earlier period, than for 
 such an important feature as a distinction between two primary 
 layers to be absolutely lost during an early period of develop- 
 ment, and then to re-appear again in later stages. 
 
 The fact of the epiblast of the neural canal being divided, 
 like the remainder of the layer, into nervous and epidermic 
 parts, cannot, I think, be used as an argument in favour of the 
 opposite view to that here maintained. 
 
 It seems probable that the central canal of the nervous 
 
STAGES G TO K. THE FINS. 319 
 
 system arose as an involution from the exterior, and therefore 
 that the epidermis lining it is in reality merely a part of the 
 external epidermis, and as such is naturally separated from the 
 true nervous structures adjacent to it 1 . 
 
 Leaving the general features of the external skin, I pass to the 
 special organs derived from it during the stage just anterior to K. 
 
 The unpaired Fins. The unpaired fins have grown consider- 
 ably, and the epiblast composing them becomes, like the remainder 
 of the layer, divided into two strata, both however composed of 
 more or less columnar cells. The ventral fin has now become 
 more prominent than the dorsal fin ; but the latter extends 
 forward as a fold quite to the anterior part of the body. 
 
 The paired Fins, Along each side of the body there appears 
 during this stage a thickened line of epiblast, which from the 
 first exhibits two special developments : one of these just in front 
 of the anus, and a second and better marked one opposite the 
 front end of the segmental duct. These two special thickenings 
 are the rudiments of the paired fins, which thus arise as special 
 developments of a continuous ridge on each side, precisely like 
 the ridges of epiblast which form the rudiments of the un- 
 paired fins. 
 
 Similar thickenings to those in Elasmobranchs are found at 
 the ends of the limbs in the embryos of both Birds and Mammals, 
 in the form of caps of columnar epiblast 2 . 
 
 The ridge, of which the limbs are special developments, is 
 situated on a level slightly ventral to that of the dorsal aorta, 
 and extends from just behind the head to the level of the anus. 
 It is not noticeable in surface views, but appears in sections 
 as a portion of the epiblast where the cells are more columnar 
 than elsewhere ; precisely resembling in this respect the forward 
 continuation of the dorsal fin. At the present stage the posterior 
 thickenings of this ridge which form the abdominal fins are so 
 slight as to be barely visible, and their real nature can only 
 be detected by a careful comparison between sections of this and 
 the succeeding stages. The rudiments of the anterior pair of 
 
 1 Vide Self, "Development of Spinal Nerves in Elasmobranchs." Phil. Transact. 
 1876. [This Edition, No. VIII.] 
 
 2 For Birds, vide Elements of Embryology, Foster and Balfour, pp. 144, 145, and 
 for Mammals, Kolliker, Entwicklungsgesehichte, p. 283. 
 
320 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 limbs are more visible than those of the posterior, though the 
 passage between them and the remainder of the ridges is most 
 gradual. Thus at first the rudiments of both the limbs are 
 nothing more than slight thickenings of the epiblast, where its 
 cells are more columnar than elsewhere. During stage K the 
 rudiments of both pairs of limbs, but especially of the anterior 
 pair, grow considerably, while at the same time the thickened 
 ridge of epiblast which connects them together rapidly disappears. 
 The thoracic limbs develop into an elongated projecting fold of 
 epiblast, in every way like the folds forming the unpaired fins ; 
 while at the same time the cells of the subjacent mesoblast 
 become closely packed, and form a slight projection, at the 
 summit of which the fold of the epiblast is situated (PI. n, fig. 
 9). The maximum projection of the thoracic fin is slightly 
 in advance of the front end of the segmental duct. The 
 abdominal fins do not, during stage K, develop quite so fast 
 as the thoracic, and at its close are merely elongated areas 
 where the epiblast is much thickened, and below which the 
 mesoblast is slightly condensed. In the succeeding stages 
 they develop into projecting folds of skin, precisely as do the 
 thoracic fins. 
 
 The features of the development of the limbs just described, 
 are especially well shewn in Torpedo ; in the embryos of which 
 the passage from the general linear thickening of epiblast into 
 the but slightly better marked thickening of the thoracic fin 
 is very gradual, and the fact of the limb being nothing else than 
 a special development of the linear lateral thickening is proved 
 in a most conclusive manner. 
 
 If the account just given of the development of the limbs is 
 an accurate record of what really takes place, it is not possible to 
 deny that some light is thrown by it upon the first origin of the 
 vertebrate limbs. The facts can only bear one interpretation, 
 viz.: that the limbs are the remnants of continuous lateral fins. 
 
 The unpaired dorsal fin develops as a continuous thickening, 
 which then grows up into a projecting fold of columnar cells. 
 The greater part of this eventually atrophies, but three separate 
 lobes are left which form the two dorsal fins and the upper lobe 
 of the caudal fin. 
 
 The development of the limbs is almost identically similar 
 
STAGES G TO K. THE PAIRED FINS. 321 
 
 to that of the dorsal fins. There appears a lateral linear thick- 
 ening of epiblast, which however does not, like the similar 
 thickening of the fins, grow into a distinct fold. Its develop- 
 ment becomes confined to two special points, at each of which 
 is formed a continuous elongated fold of columnar cells precisely 
 like the fold of skin forming the dorsal fins. These two folds 
 form the paired fins. If it be taken into consideration that the 
 continuous lateral fin, of which the rudiment appears in Elasmo- 
 branchs, does not exist in any adult Vertebrate, and also that a 
 continuous dorsal fin exists in many Fishes, the small differences 
 in development between the paired fins and the dorsal fins will 
 be seen to be exactly those which might have been anticipated 
 beforehand. Whereas the continuous dorsal fin, which often 
 persists in adult fishes, attains a considerable development before 
 vanishing, the originally continuous lateral one has only a very 
 ephemeral existence. 
 
 While the facts of development strongly favour a view which 
 would regard the limbs as remnants of a primitively continuous 
 lateral fin, there is nothing in the structure of the limbs of adult 
 Fishes which is opposed to this view. Externally they closely 
 resemble the unpaired fins, and both their position and nervous 
 supply appear clearly to indicate that they do not belong to one 
 special segment of the body. They appear rather to be connected 
 with a varying number of segments ; a fact which would receive 
 a simple explanation on the hypothesis here adopted 1 . 
 
 My researches throw no light on the nature of the skeletal 
 parts of the limb, but the suggestion which has been made by 
 Giinther* with reference to the limb of Ceratodus (the most 
 primitive known), that it is a modification of a series of parallel 
 rays, would very well suit the view here proposed. 
 
 Dr Dohrn 8 in speaking of the limbs, points out the difficulties 
 
 1 For the nervous supply in fishes, vide Stannius, Peripher. Nerv. System d. Fische. 
 In Osseous Fishes he states that the thoracic fin is supplied by branches from the first 
 three though sometimes from the first four spinal nerves. In Acipenser there are 
 branches from the first six nerves. In Spinax the limb is supplied by the rami 
 anteriores of the fourth and succeeding ten spinal nerves. In the Rays not only do 
 the sixteen anterior spinal nerves unite to supply the fin, but in all there are rami 
 anteriores from thirty spinal nerves which pass to the thoracic limb. 
 
 2 Philosophical Transactions, 1871. 
 
 8 Ursprung d. Wirbelthierc and Functionnvechsels. 
 
322 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 in the way of supposing that they can have originated de novo, 
 and not by the modification of some pre-existing organ, and 
 suggests that the limbs are modified gill-arches ; a view similar 
 to which has been hinted at by Professor Gegenbaur 1 . 
 
 Dr Dohrn has not as yet given the grounds for his determina- 
 tion, so that any judgment on his views is premature. 
 
 None of my observations on Elasmobranchs lends any sup- 
 port to these views ; but perhaps, while regarding the limbs as 
 the remains of a continuous fin, it might be permissible to 
 suppose that the pelvic and thoracic girdles are altered remnants 
 of the skeletal parts of some of the gill-arches which have 
 vanished in existing Vertebrates. 
 
 The absence of limbs in the Marsipobranchii and Amphioxus, 
 for reasons already insisted upon by Dr Dohrn 2 , cannot be used 
 as an argument against limbs having existed in still more 
 primitive Vertebrates. 
 
 Though it does not seem probable that a dorsal and ventral 
 fin can have existed contemporaneously with lateral fins (at 
 least not as continuous fins), yet, judging from such forms as 
 the Rays, there is no reason why small balancing dorsal and 
 caudal fins should not have co-existed with fully developed 
 lateral fins. 
 
 Mesoblast. G K. 
 
 The mesoblast in stage F forms two independent lateral 
 plates, each with a splanchnic and somatic layer, and divided, 
 as before explained, into a vertebral portion and a parietal 
 portion. At their peripheral edge these plates are continuous 
 with the general mesoblastic tissue of the non-embryonic part of 
 the blastoderm ; except in the free parts of the embryo, where 
 they are necessarily separated from the mesoblast of the yolk- 
 sac, and form completely independent lateral masses of cells. 
 
 During the stages G and H, the two layers of which the 
 mesoblast is composed cease to be in contact, and leave be- 
 tween them a space which constitutes the commencement of the 
 body-cavity (PI. 10, fig. i). From the very first this cavity is 
 more or less clearly divided into two distinct parts ; one of them 
 
 1 Grundriss d. Vergleichenden Anat. p. 494. 
 2 Loc. cit. 
 
STAGES G TO K. THE MESOBLAST. 323 
 
 in the vertebral portion of the plates of mesoblast, the other in 
 the parietal. The cavity in the parietal part of the plates alone 
 becomes the true body-cavity. It extends uninterruptedly 
 through the anterior parts of the embryo, but does not appear 
 in the caudal region, being there indicated only by the presence 
 of two layers in the mesoblast plates. Though fairly wide 
 below, it narrows dorsally before becoming continuous with the 
 cavity in the vertebral plates. The line of junction of the verte- 
 bral and parietal plates is a little ventral to the dorsal summit 
 of the alimentary canal (PL 10, fig. 5). Owing to the fact that 
 the vertebral plates are split up into a series of segments (proto- 
 vertebras), the section of the body-cavity they enclose is 
 necessarily also divided into a series of segments, one for each 
 protovertebra. 
 
 Thus the whole body-cavity consists of a continuous parietal 
 space which communicates by a series of apertures with a number 
 of separate cavities enclosed in the protovertebrae. The cavity 
 in each of the protovertebrae is formed of a narrowed dorsal and 
 a dilated ventral segment, the latter on the level of the dorsal 
 aorta (PI. n, fig. 5). Cavities are present in all the vertebral 
 plates with the exception of a few far back in the tail ; and 
 exist in part of the caudal region posterior to that in which a 
 cavity in the parietal plate is present. 
 
 Protovertebrce. Each protovertebra 1 or vertebral segment of 
 the mesoblast plate forms a flattened rectangular body, ventrally 
 continuous with the parietal plate of mesoblast. During stage 
 G the dorsal edge of the protovertebrae is throughout on about a 
 level with the ventral third of the spinal cord. Each vertebral 
 plate is composed of two layers, a somatic and a splanchnic, and 
 encloses the already-mentioned section of the body-cavity. The 
 cells of both layers of the plate are columnar, and each consists 
 of a very large nucleus, invested by a delicate layer of proto- 
 plasm. 
 
 Before the end of stage H the inner or splanchnic wall of the 
 protovertebra loses its simple constitution, owing to the middle 
 part of it, opposite the dorsal two-thirds of the notochord, under- 
 
 1 No attempt has been made to describe in detail the different appearances 
 presented by the protovertebrae in the various parts of the body, but in each stage a 
 protovertebra from the dorsal region is taken as typical. 
 
324 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 going peculiar changes. These changes are indicated in trans- 
 verse sections (PL 11, figs. 5 and 6 mp'\ by the cells in the part 
 we are speaking of acquiring a peculiar angular appearance, and 
 becoming one or two deep ; and the meaning of the changes is 
 at once shewn by longitudinal horizontal sections. These prove 
 (PI. 12, fig. 10) that the cells in this situation have become elong- 
 ated in a longitudinal direction, and, in fact, form typical spindle- 
 shaped embryonic muscle-cells, each with a large nucleus. Every 
 muscle-cell extends for the whole length of a protovertebra, and 
 in the present stage, or at any rate in stage I, acquires a peculiar 
 granulation, which clearly foreshadows transverse striation (PI. 
 12, figs. 1113). 
 
 Thus by stage H a small portion of the splanchnopleure 
 which forms the inner layer of each protovertebra, becomes 
 differentiated into a distinct band of longitudinal striated muscles; 
 these almost at once become functional, and produce the peculiar 
 serpentine movements of the embryo, spoken of in a previous 
 chapter, p. 291. 
 
 It may be well to say at once that these muscles form but a 
 very small part of the muscles which eventually appear ; which 
 latter are developed at a very much later period from the re- 
 maining cells of the protovertebrae. The band developed at this 
 stage appears to be a special formation, which has arisen through 
 the action of natural selection, to enable the embryo to meet its 
 respiratory requirements, by continually moving about, and so 
 subjecting its body to fresh oxydizing influences ; and as such 
 affords an interesting example of an important structure acquired 
 during and for embryonic life. 
 
 Though the cavities in the protovertebra are at first per 
 fectly continuous with the general body-cavity, of which indeed 
 they merely form a specialized part, yet by the close of stage H 
 they begin to be constricted off from the general body-cavity, 
 and this process is continued rapidly, and completed shortly 
 after stage I, and considerably before the commencement of 
 stage K (PI. n, figs. 6 and 8). While this is taking place, 
 part of the splanchnic layer of each protovertebra, immediately 
 below the muscle-band just described, begins to proliferate, and 
 produce a number of cells, which at once grow in between 
 the muscles and the notochord. These cells are very easily 
 
STAGES G TO K. THE PROTOVERTEBR^E. 325 
 
 seen both in transverse and longitudinal sections, and form the 
 commencing vertebral bodies (PI. u, fig. 6, and PI. 12, figs. IO 
 and 1 1 Vr}. 
 
 At first the vertebral bodies have the same segmentation as 
 the protovertebrae from which they sprang ; that is to say, they 
 form masses of embryonic cells separated from each other by 
 narrow slits, continuous with the slits separating the protoverte- 
 brae. They have therefore at their first appearance a segmentation 
 completely different from that which they eventually acquire 
 (PI. 12, fig. ii). 
 
 After the separation of the vertebral bodies from the proto- 
 vertebrae, the remaining parts of the protovertebrae may be 
 called muscle-plates ; since they become directly converted into 
 the whole voluntary muscular system of the trunk. At the time 
 when the cavity of the muscle-plates has become completely 
 separate from the body-cavity, the muscle-plates themselves 
 are oblong structures, with two walls enclosing the cavity just 
 mentioned, in which the original ventral dilatation is still visible. 
 The outer or somatic wall of the plates retains its previous simple 
 constitution. The splanchnic wall has however a somewhat 
 complicated structure. It is composed dorsally and ventrally of 
 a columnar epithelium, but in its middle portion of the muscle- 
 cells previously spoken of. Between these and the central cavity 
 of the plates the epithelium forming the remainder of the layer 
 commences to insert itself; so that between the first-formed 
 muscle and the cavity of the muscle-plate there appears a thin 
 layer of cells, not however continuous throughout. 
 
 At the end of the period K the muscle-plates have extended 
 dorsally two-thirds of the way up the sides of the spinal cord, 
 and ventrally to the level of the segmental duct. Their edges 
 are not straight, but are bent into an angular form, with the 
 apex pointing forwards. Vide PI. 12, fig. 17 nip. 
 
 Before the end of the period a number of connective-tissue 
 cells make their appearance, and extend upwards from the dorsal 
 summit of the muscle-plates around the top of the spinal cord. 
 These cells are at first rounded, but become typical branched 
 connective-tissue cells before the close of the period (PI. u, figs. 
 7 and 8). 
 
 Between stages I and K the bodies of the vertebrae rapidly 
 
326 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 increase in size and send prolongations downwards and inwards 
 to meet below the notochord. 
 
 These soon become indistinguishably fused with other cells 
 which appear in the area between the alimentary cavity and the 
 notochord, but probably serve alone to form the vertebral bodies, 
 while the cells adjoining them form the basis for the connective 
 tissue of the kidneys, &c. 
 
 The vertebral bodies also send prolongations dorsalwards 
 between the sides of the spinal cord and the muscle-plates. 
 These grow round till they meet above the spinal and enclose 
 the dorsal nerve-roots. They soon however become fused with 
 the dorsal prolongations from the muscle-plates, at least so far 
 as my methods of investigation enable me to determine ; but it 
 appears to me probable that they in reality remain distinct, and 
 become converted into the neural arches, while the connective- 
 tissue cells from the muscle-plates form the adjoining subcutaneous 
 and inter-muscular connective tissue. 
 
 All the cells of the vertebral rudiments become stellate and 
 form typical embryonic connective-tissue. The rudiments how- 
 ever still retain their primitive segmentation, corresponding with 
 that of the muscle-plates, and do not during this period acquire 
 their secondary segmentation. Their segmentation is however 
 less clear than it was at an earlier period, and in the dorsal 
 part of the vertebral rudiments is mainly indicated by the dorsal 
 nerve-roots, which always pass out in the interval between two 
 vertebral rudiments. Vide PI. 12, fig. 12 pr. 
 
 Intermediate Cell-mass. At about the period when the 
 muscle-plates become completely free, a fusion takes place be- 
 tween the somatopleure and splanchnopleure immediately above 
 the dorsal extremity of the true body-cavity (PL n, fig. 6). 
 The cells in the immediate neighbourhood of this fusion form 
 a special mass, which we may call the intermediate cell-mass 
 a name originally used by Waldeyer for the homologous cells 
 in the Chick. Out of it are developed the urino-genital organs 
 and the adjoining tissues. At first it forms little more than a 
 columnar epithelium, but by the close of the period is divided 
 into (i) An epithelium on the free surface ; from this are derived 
 the glandular parts of the kidneys and functional parts of the 
 genital glands ; and (2) a subjacent stroma which forms the 
 
STAGES G TO K. THE BODY-CAVITY. 327 
 
 basis for the connective-tissue and vascular parts of these 
 organs. 
 
 To the history of these parts a special section is devoted ; 
 and I now pass to the description of the mesoblast which lines 
 the body-cavity and forms the connective tissue of the body-wall, 
 and the muscular and connective tissue of the wall of the alimen- 
 tary canal. 
 
 Body-cavity and Parietal Plates. By the close of stage H, as 
 has been already mentioned, a cavity is formed between the 
 somatopleure and splanchnopleure in the anterior part of the 
 trunk, which rapidly widens during the succeeding stages. 
 Anteriorly, it invests the heart, which arises during stage G, 
 as a simple space between the ventral wall of the throat 
 and the splanchnopleure (PI. n, fig. 4). Posteriorly it ends 
 blindly. 
 
 This cavity forms in the region of the heart the rudiment of 
 the pericardial cavity. The remainder of the cavity forms the 
 true body-cavity. 
 
 Immediately behind the heart the alimentary canal is still 
 open to the yolk-sac, and here naturally the two lateral halves of 
 the body-cavity are separated from each other. In the tail of 
 the embryo no body-cavity has appeared by stage I, although 
 the parietal plates of mesoblast are distinctly divided into somatic 
 and splanchnic layers. In the caudal region the lateral plates of 
 mesoblast of the two sides do not unite ventrally, but are, on the 
 contrary, quite disconnected. Their ventral edge is moreover 
 much swollen (PI. 11, fig. i). At the caudal swelling the meso- 
 blast plates cease to be distinctly divided into somatopleure and 
 splanchnopleure, and more or less fuse with the hypoblast of the 
 caudal vesicle (PI. n, fig. 2). 
 
 Between stages I and K the body-cavity extends backwards 
 behind the point where the anus is about to appear, though it 
 never reaches quite to the extreme end of the tail. The backward 
 extension of the body-cavity, as is primitively the case every- 
 where, is formed of two independent lateral halves (PI. 1 1, fig. 90). 
 Anteriorly, opposite the hind end of the small intestine, these 
 two lateral halves unite ventrally to form a single cavity in which 
 hangs the small intestine (PI. n, fig. 8) suspended by a very 
 short mesentery. 
 
328 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The most important change which takes place in the body- 
 cavity during this period is the formation of a septum which 
 separates off a pericardial cavity from the true body-cavity. 
 
 Immediately in front of the liver the splanchnic and somatic 
 walls of the body come into very close contact, and I believe 
 unite over the greater part of their extent. The septum so 
 formed divides the original body-cavity into an anterior section 
 or pericardial cavity, and a posterior section or true body-cavity. 
 There is left, however, on each side dorsally a rather narrow 
 passage which serves to unite the pericardial cavity in front with 
 the true body-cavity behind. 
 
 In PI. n, fig. 8 a, there is seen on one side a section through 
 this passage, while on the other side the passage is seen to be 
 connected with the pericardial cavity. 
 
 It is not possible from transverse sections to determine for 
 certain whether the septum spoken of is complete. An exami- 
 nation of longitudinal horizontal sections from an embryo be- 
 longing to the close of the stage K has however satisfied me that 
 this septum, by that stage at any rate, is fully formed. 
 
 The two lateral passages spoken of above probably unite in 
 the adult to form the passage connecting the pericardial with the 
 peritoneal cavity, which, though provided with but a single orifice 
 into the pericardial cavity, divides into two limbs before opening 
 into the peritoneal cavity. 
 
 The body-cavity undergoes no further changes of importance 
 till the close of the period. 
 
 Somatopleure and Splanchnopleure. Both the somatic and 
 splanchnic walls of the body-cavity during stage I exhibit a 
 simple uniform character throughout their whole extent. They 
 are formed of columnar cells where they line the dorsal part 
 of the body-cavity, but ventrally of more rounded and irregular 
 cells (PI. 11, fig. 5). 
 
 In them may occasionally be seen aggregations of very 
 peculiar and large cells with numerous highly refracting spherules ; 
 the cells forming these are not unlike the primitive ova to be 
 described subsequently, but are probably large cells derived from 
 the yolk. 
 
 It is during the stage intermediate between I and K that the 
 first changes become visible which indicate a distinction between 
 
STAGES G TO K. THE MESOBLAST. 329 
 
 an epithelium (endothelium) lining the body-cavity and the 
 connective tissue adjoining this. 
 
 There are at first but very few connective-tissue cells between 
 the epithelium of the somatic layer of the mesoblast and the 
 epiblast, but a connection between them is established by peculiar 
 protoplasmic processes which pass from the one to the other 
 (PI. u, fig. 8). Towards the end of stage K, however, there 
 appears between the two a network of mesoblastic cells connect- 
 ing them together. In the rudimentary outgrowth to form the 
 limbs the mesoblast cells of the somatic layer are crowded in an 
 especially dense manner. 
 
 From the first the connective-tissue cells around the hypo- 
 blastic epithelium of the alimentary tract are fairly numerous 
 (PI. 1 1, fig. 8), and by the close of this period become concentric- 
 ally arranged round the intestinal epithelium, though not divided 
 into distinct layers. A special aggregation of them is present in 
 the hollow of the rudimentary spiral valve. 
 
 Behind the anal region the two layers of the mesoblast 
 (somatic and splanchnic) completely fuse during stage K, and 
 form a mass of stellate cells in which no distinction into two 
 layers can be detected (PI. u, figs, gc, Qd\ 
 
 The alimentary canal, which at first lies close below the aorta, 
 becomes during this period gradually carried further and further 
 from this, remaining however attached to the roof of the body- 
 cavity by a thin layer of the mesoblast of the splanchnopleure 
 formed of an epithelium on each side, and a few interposed 
 connective-tissue cells. This is the mesentery, which by the 
 close of stage K is of considerable length in the region of the 
 stomach, though shorter elsewhere. 
 
 The above account of the protovertebrae and body-cavity ap- 
 plies solely to the genera Pristiurus and Scyllium. The changes 
 of these parts in Torpedo only differ from those of Pristiurus in 
 unimportant though fairly noticeable details. Without entering 
 into any full description of these it may be pointed out that 
 both the true body-cavity and its continuations into the proto- 
 vertebrae appear later in Torpedo than in Pristiurus and Scyllium. 
 In some cases even the muscle-plates become definitely separated 
 and independent before the true body-cavity has appeared. As 
 B. 22 
 
33O DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 a result of this the primitive continuity of the body-cavity and 
 cavity of the muscle-plates becomes to a certain extent masked, 
 though its presence may easily be detected by the obvious 
 continuity which at first exists between the somatic and splanch- 
 nic layers of mesoblast and the two layers of the muscle-plate. 
 In the muscle-plate itself the chief point to be noticed is the fact 
 that the earlier formed bands of muscles (tnp'} arise very much 
 later, and are less conspicuous, in Torpedo than in the genera 
 first described. They are however present and functional. 
 
 ' The anatomical relations of the body-cavity itself are pre- 
 cisely the same in Torpedo as in Pristiurus and Scyllium, and 
 the pericardial cavity becomes separated from the peritoneal in 
 the same way in all the genera ; the two lateral canals connect- 
 ing the two cavities being also present in all the three genera. 
 The two independent parietal plates of mesoblast of the posterior 
 parts of the body have ventrally a swollen edge, as in Pristiurus, 
 and in this a cavity appears which forms a posterior continuation 
 of the true body-cavity. 
 
 Resumt. The primitive independent mesoblast plates of the 
 two sides of the body become divided into two layers, a somatic 
 and a splanchnic (Hautfaserblatt and Darmfaserblatt). At the 
 same time in the dorsal part of the mesoblast plate a series of 
 transverse splits appear which mark out the limits of the proto- 
 vertebrae and serve to distinguish a dorsal or vertebral part of the 
 plate from a ventral or parietal part. 
 
 Between the somatic and splanchnic layers of the mesoblast 
 plate a cavity arises which is continued quite to the summit 
 of the vertebral part of the plate. This is the primitive body- 
 cavity ; and at first the cavity is divided into two lateral and 
 independent halves. 
 
 The next change which takes place is the complete separa- 
 tion of the vertebral portion of the plate from the parietal ; 
 thereby the upper segmented part of the body-cavity becomes 
 isolated and separated from the lower and unsegmented part. 
 In connection with this change in the constitution of the body- 
 cavity there are formed a series of rectangular plates, each com- 
 posed of two layers, a somatic and a splanchnic, between which 
 is the cavity originally continuous with the body-cavity. The 
 splanchnic layer of the plates buds off cells to form the rudi- 
 
STAGES G TO K. RESUME. 33 l 
 
 ments of the vertebral bodies which are originally segmented 
 in the same planes as the protovertebrae. The plates themselves 
 remain as the muscle-plates and develop a special layer of 
 muscle (mp 1 ) in their splanchnic layer. 
 
 In the meantime the parietal plates of the two sides unite 
 ventrally throughout the intestinal and cardiac regions of the 
 body, and the two primitively isolated cavities contained in them 
 coalesce. Posteriorly however the plates do not unite ventrally, 
 and their contained cavities remain distinct. 
 
 At first the pericardial cavity is quite continuous with the 
 body-cavity; but by the close of the period included in the 
 present chapter it becomes separated from the body-cavity by a 
 septum in front of the liver, which is however pierced dorsally 
 by two narrow channels. 
 
 The parts derived from the two layers of the mesoblast (not 
 including special organs or the vascular system) are as follow : 
 
 From the somatic layer are formed 
 
 (1) A considerable part of the voluntary muscular system 
 
 of the body. 
 
 (2) The dermis. 
 
 (3) A large part of the intermuscular connective tissue. 
 
 (4) Part of the peritoneal epithelium. 
 
 From the splanchnic layer are formed 
 
 (1) A great part of the voluntary muscular system. 
 
 (2) Part of the intermuscular connective tissue (?). 
 
 (3) The axial skeleton. 
 
 (4) The muscular and connective-tissue wall of the 
 
 alimentary tract. 
 
 (5) A great part of the peritoneal epithelium. 
 
 General Considerations. In the history which has just been 
 given of the development of the mesoblast, there are several 
 points which appear to me to throw light upon the primitive 
 origin of that layer. Before entering into these it is however 
 necessary to institute a comparison between the history of the 
 mesoblast in Elasmobranchs and in other Vertebrates, in order 
 to distinguish as far as possible the primitive and the secondary 
 characters present in the various groups. 
 
 22 2 
 
332 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Though the Mammals are to be looked on as the most 
 differentiated group amongst the Vertebrates, yet in their 
 embryonic history they retain many very primitive features, 
 and, as has been recently shewn by Hensen 1 , present numerous 
 remarkable approximations to the Elasmobranchs. We find ac- 
 cordingly 2 that the primitive lateral plates of mesoblast undergo 
 nearly the same changes in these two groups. In Mammals 
 there is at first a continuous cavity extending through both 
 the parietal and vertebral portions of each plate, and dividing 
 the plates into a somatic and a splanchnic layer : this cavity is 
 the primitive body-cavity. The vertebral portion of each plate 
 with its contained cavity then becomes divided off from the 
 parietal. The later development of these parts is not accurately 
 known, but it seems that the outer portion of each vertebral 
 plate, composed of two layers (somatic and splanchnic) enclosing 
 between them a remnant of the primitive body-cavity, becomes 
 separated off as a muscle-plate. The remainder forms a vertebral 
 rudiment, &c. Thus the extension of the body-cavity into the 
 vertebral portion of the mesoblast, and the constriction of the 
 vertebral portion of the cavity from the remainder, are as 
 distinctive features of Mammals as they are of the Elasmo- 
 branchs. 
 
 In Birds 3 the horizontal splitting of the mesoblast into 
 somatic and splanchnic layers appears, as in Mammals, to extend 
 at first to the summit of the protovertebrae, but these bodies 
 become so early separated from the parietal plates that this 
 fact has usually been overlooked or denied ; but even on the 
 second day of incubation the outer layer of the protovertebrae is 
 continuous with the somatic layer of the lateral plates, and the 
 inner layer and kernel of the protovertebrae with the splanchnic 
 layer of the lateral plates 4 . After the isolation of the proto- 
 vertebrae the primitive position of the split which separated 
 their somatic and splanchnic layers becomes obscured, but when 
 
 1 Zeitschriftf. Anat. Ent-wicklungsgeschichte, Vol. I. 
 8 Hensen loc. cit. 
 
 3 For the history of protovertebrae and muscle-plates in Birds, vide Elements of 
 Embryology, Foster and Balfour. The statement there made that the horizontal 
 splitting of the mesoblast does not extend to the summit of the vertebral plate, must 
 however be regarded as doubtful. 
 
 4 Vide Elements of Embryology, p. 56. 
 
STAGES G TO K. GENERAL CONSIDERATIONS. 333 
 
 on the third day the muscle-plates are formed they are found to 
 be constituted of two layers, an inner and an outer, which enclose 
 between them a central cavity. This remarkable fact, which has 
 not received much attention, though noticeable in most figures, 
 receives a simple explanation as a surviving rudiment on Dar- . 
 winian principles. The central cavity of the muscle-plate is, in 
 fact, a remnant of the vertebral extension of the body-cavity, and 
 is the same cavity as that found in the muscle-plates of Elasmo- 
 branchs. The two layers of the muscle-plate also correspond 
 with the two layers present in Elasmobranchs, the one belonging 
 to the somatic, the other to the splanchnic layer of mesoblast. 
 The remainder of the protovertebrae internal to the muscle-plates 
 is very large in Birds, and is the equivalent of that portion of the 
 protovertebrae which in Elasmobranchs is split off to form the 
 vertebral bodies 1 (PI. 11, figs. 6, 7, 8, Vr\ Thus, though the 
 history of the development of the mesoblast is not precisely the 
 same for Birds as for Elasmobranchs, yet the differences between 
 the two groups are of such a character as to prove in a striking 
 manner that the Avian development is a derivation from a more 
 primary form, like that of the Elasmobranchs. 
 
 According to the statements of Bambeke and Gotte, the 
 Amphibians present rather remarkable peculiarities in the develop- 
 ment of their muscular system. Each side-plate of mesoblast is 
 divided into a somatic and a splanchnic layer, continuous 
 throughout the vertebral and parietal portions of the plate. The 
 vertebral portions (protovertebrae) of the plates soon become 
 separated from the parietal, and form an independent mass of 
 cells constituted of two layers, which were originally continuous 
 with the somatic and splanchnic layers of the parietal plates. 
 The outer or somatic layer of the vertebral plates is formed of a 
 single row of cells, but the inner or splanchnic layer is made up 
 of a central kernel of cells and an inner single layer. This 
 central kernel is the first portion of the vertebral body to undergo 
 
 1 Dr Gbtte, Enlwicklungsgeschichte der Unke, p. 534, gives a different account of 
 the development of the protovertebrse from that in the text. He states that the 
 muscle-plates do not give rise to the main dorso-lateral muscles, hut only to some 
 superficial ventral muscles, while the dorso-lateral muscles are according to him formed 
 from part of the kernel of the protovertebrae internal to the muscle-plates. The 
 account given in the text is the result of my own investigations, and accords precisely 
 with the recent statements of Professor Kolliker, Entiuicklungsgeschichte, 1876. 
 
334 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 any change, and it becomes converted into the main dorso-lateral 
 muscles of the body, which apparently correspond with the 
 muscles derived from the whole muscle-plate of the Elasmo- 
 branchs. From the inner layer of the splanchnic division there 
 are next formed the main internal ventral muscles, rectus 
 abdominis, &c., as well as the chief connective-tissue elements of 
 the parts surrounding the spinal cord. The outer layer of the 
 vertebral plates forms the dermis and subcutaneous connective 
 tissue, as well as some of the superficial muscles of the trunk 
 and the muscles of the limbs. 
 
 Dr Gotte appears to think that the vertebral plates in Am- 
 phibians present a perfectly normal development very similar 
 to that of other Vertebrates. The divergences between Am- 
 phibians and other Vertebrates appear, however, to myself, to be 
 very great, and although the very careful account given by Dr 
 Gotte is probably to be relied on, yet some further explanation 
 than he has offered of the development of these parts amongst 
 the Amphibians would seem to be required. 
 
 A primary stage in which the two layers of the vertebral 
 plates are continuous with the somatic and splanchnic layers 
 of the body-wall is equally characteristic of Amphibians, Elasmo- 
 branchs and Mammals. In the subsequent development, how- 
 ever, a great difference between the types becomes apparent, for 
 whereas in Elasmobranchs both layers of the vertebral plates 
 combine to form the muscle-plates, out of which the great dorso- 
 lateral muscles are formed, in Amphibians what appear to be 
 the equivalent muscles are derived from a few of the cells (the 
 kernel) of the inner layer of the vertebral plates only. The cells 
 which form the lateral muscles in Amphibians might be thought 
 to correspond in position with the cells which become, in Elas- 
 mobranchs, converted into the special early formed band of 
 muscles (m.p'.}, rather than, as their development seems to 
 indicate, with the whole Elasmobranch muscle-plates 1 . 
 
 1 The type of development of the muscle-plates of Amphibians would become 
 identical with that of Elasmobranchs if their first-formed mass of muscle corresponded 
 with the early-formed muscles of Elasmobranchs, and the remaining cells of both 
 layers of the protovertebrse became in the course of development converted into 
 muscle-cells indistinguishable from those formed at first. Is it possible that, owing 
 (o the distinctness of the first-formed mass of muscle, Dr Gotte can have overlooked 
 
DERIVATION OF THE MESOBLAST. 335 
 
 Osseous Fishes are stated to agree with Amphibians in the 
 development of their protovertebrae and muscular system 1 , but 
 further observations on this point are required. 
 
 Though the development of the general muscular system 
 and muscle-plates does not, according to existing statements, 
 take place on quite the same type throughout the Vertebrate 
 sub-kingdom, yet the comparison which has been instituted 
 between Elasmobranchs and other Vertebrates appears to prove 
 that there are one or two common features in their development, 
 which may be regarded as primitive, and as having been in- 
 herited from the ancestors of Vertebrates. These features are 
 (i) The extension of the body-cavity into the vertebral plates, 
 and subsequent enclosure of this cavity between the two layers 
 of the muscle-plates ; (2) The primitive division of the vertebral 
 plate into a somatic and a splanchnic layer, and the formation 
 of a large part of the voluntary muscular system out of the 
 splanchnic layer. 
 
 The ultimate derivation of the mesoblast forms one of the 
 numerous burning questions of modern embryology, and there 
 are advocates to be found for almost every one of the possible 
 views the question admits of. 
 
 All who accept the doctrine of descent are agreed that primi- 
 tively only two embryonic layers were present the epiblast 
 and the hypoblast and that the mesoblast subsequently ap- 
 peared as a distinct layer, after a certain complexity of organiza- 
 tion had been attained. 
 
 The general agreement stops, however, at this point, and 
 the greatest divergence of opinion exists with reference to all 
 further questions which bear on the development of the meso- 
 blast. There appear to be four possibilities as to the origin of 
 this layer. 
 
 It may be derived : 
 
 (1) entirely from the epiblast, 
 
 (2) partly from the epiblast, and partly from the hypoblast, 
 
 the fact that its subsequent growth is carried on at the expense of the adjacent cells 
 of the somatic layer ? 
 
 1 Ehrlich, "Ueber den peripher. Theil d. Urwirbel." Archiv f. Mic. Anat, 
 Vol. XI. 
 
336 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 (3) entirely from the hypoblast, 
 
 (4) or may have no fixed origin. 
 
 The fourth of these possibilities may for the present be 
 dismissed, since it can be only maintained should it turn out 
 that all the other views are erroneous. The first possibility is 
 supported by the case of the Ccelenterata, and we might almost 
 say by that of this group only 1 . 
 
 Amongst the Ccelenterata the mesoblast, when present, is 
 unquestionably a derivative of the epiblast, and when, as is 
 frequently the case, a distinct mesoblast is not present, the 
 muscle-cells form a specialized part of the epidermic cells. 
 
 The condition of the mesoblast in these lowly organized 
 animals is exactly what might d priori have been anticipated, 
 but the absence throughout the group of a true body-cavity, or 
 specially developed muscular system of the alimentary tract, 
 prevents the possibility of generalizing for other groups, from 
 the condition of the mesoblast in this one. 
 
 In those animals in which a body-cavity and muscular 
 alimentary tract are present, it would certainly appear reasonable 
 to expect the mesoblast to be derived from both the primitive 
 layers : the voluntary muscular system from epiblast, and the 
 splanchnic system from the hypoblast. This view has been 
 taken and strongly advocated by so distinguished an embry- 
 ologist as Professor Haeckel, and it must be admitted, that on 
 d priori grounds there is much to recommend it ; there are, 
 however, so far as I am aware of, comparatively few observed 
 facts in its favour. 
 
 Professor Haeckel's own objective arguments in support of 
 his view are as follows : 
 
 1 The most important other instances in addition to that of the Ccelenterata which 
 can be adduced in favour of the epiblastic origin of the mesoblast are the Bird and 
 Mammal, in which according to the recent observations of Hensen for the Mammal, 
 and Kblliker for the Mammal and Bird, the mesoblast is split off from the epiblast. 
 If the views I have elsewhere put forward about the meaning of the primitive groove 
 be accepted, the derivation of the mesoblast from the epiblast in these instances 
 would be apparent rather than real, and have no deep morphological significance for 
 the present question. 
 
 Other instances may be brought forward from various groups, but none of these 
 are sufficiently well confirmed to be of any value in the determination of the present 
 question, 
 
DERIVATION OF THE MESOBLAST. 337 
 
 (1) From the fact that some investigators derive the meso- 
 blast with absolute confidence from the hypoblast, while others 
 do so with equal confidence from the epiblast, he concludes that 
 it is really derived from both these layers. 
 
 (2) A second argument is founded on the supposed deriva- 
 tion of the mesoblast in Amphioxus from .both epiblast and 
 hypoblast. Kowalevsky's account (on which apparently Prof. 
 Haeckel's 1 statements are based) appears to me, however, too 
 vague, and his observations too imperfect, for much confidence 
 to be placed in his statements on this head. It does not indeed 
 appear to me that the formation of the layers in Amphioxus, 
 till better known, can be used as an argument for any special 
 view about this question. 
 
 (3) Professor Haeckel's own observations on the develop- 
 ment of Osseous fish form a third argument in support of his 
 views. These observations do not, however, accord with those 
 of the majority of investigators, and not having been made by 
 means of sections, require further confirmation before they can 
 be definitely accepted. 
 
 (4) A fourth argument rests on the fact that the various 
 embryonic layers fuse together to form the primitive streak or 
 axis-cord in higher vertebrates. This he thinks proves that the 
 mesoblast is derived from both the primitive layers. The primi- 
 tive streak has, however, according to my views, quite another 
 significance to that attributed to it by Professor Haeckel 2 ; but 
 in any case Professor Kolliker's researches, and on this point 
 my own observations accord with his, appear to me to prove 
 that the fusion which there takes place is only capable of being 
 used as an argument in favour of an epiblastic origin of the 
 mesoblast, and not of its derivation from both epiblast and 
 hypoblast. 
 
 The objective arguments in favour of Professor Haeckel's 
 views are not very conclusive, and he himself does not deny 
 that the mesoblast as a rule apparently arises as a single and 
 undivided mass from one of the two primary layers, and only 
 
 1 Vide Anthropogenie, p. 197. 
 
 * Vide Self, " Development of Elasmobranch Fishes," Journal of Anat. and Phys. 
 Vol. X. note on p. 682, and also Review of Professor Kolliker's " Entwicklungs- 
 geschichte des Menschen u. d. hoheren Thiere," Journal of Anat. and Phys. Vol. x. 
 
338 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 subsequently becomes split into somatic and splanchnic strata. 
 This original fusion and subsequent splitting of the mesoblast 
 is explained by him as a secondary condition, a possibility 
 which cannot by any means be thrown on one side. It seems 
 therefore worth while examining how far the history of the 
 somatic and splanchnic layers of the mesoblast in Elasmobranchs 
 and other Vertebrates accords with the supposition that they 
 were primitively split off from the epiblast and the hypoblast 
 respectively. 
 
 It is well to consider first of all what parts of the mesoblast 
 of the body might be expected to be derived from the somatic 
 and splanchnic layers on this view of their origin 1 . 
 
 From the somatic layer of the mesoblast there would no 
 doubt be formed the whole of the voluntary muscular system of 
 the body, the dermis, the subcutaneous connective tissue, and 
 the connective tissue between the muscles. It is probable also, 
 though this point is less certain, that the skeleton would be 
 derived from the somatic layer. From the splanchnic layer 
 would be formed the connective tissue and muscular layers of 
 the alimentary tract, and possibly also the vascular system. 
 
 Turning to the actual development of these parts, the dis- 
 crepancy between theory and fact becomes very remarkable. 
 From the somatic layer of the mesoblast, part of the voluntary 
 muscular system and the dermis is no doubt derived, but the 
 splanchnic layer supplies the material, not only for the muscular 
 wall of the digestive canal and the vascular system, but also for 
 the whole of the axial skeleton and a great part of the voluntary 
 muscular system of the body, including the first-formed muscles. 
 Though remarkable, it is nevertheless not inconceivable, that the 
 skeleton might be derived from the splanchnic mesoblast, but 
 
 1 Professor Haeckel speaks of the splitting of the mesoblast in Vertebrates into 
 a somatic and splanchnic layer as a secondary process (Gastrula u. Eifurchung d. 
 TAiere), but does not make it clear whether he regards this secondary splitting as 
 taking place along the old lines. It appears to me to be fairly certain that even if the 
 original unsplit condition of the mesoblast is to be regarded as a secondary condition, 
 yet that the splitting of this must take place along the old lines, otherwise a change in 
 the position of the body-cavity in the adult would have to be supposed an unlikely 
 change producing unnecessary complication. The succeeding argument is based on 
 the assumption that the unsplit condition is a secondary condition, but that the split 
 which eventually appears in this occurs along the old lines, separating the primitive 
 splanchnopleure from the primitive somatopleure. 
 
DERIVATION OF THE MESOBLAST. 339 
 
 it is very difficult to understand how there could be formed from 
 it a part of the voluntary muscular system of the body in- 
 distinguishably fused with part of the muscular system derived 
 from the somatopleure. No fact in my investigations comes 
 out more clearly than that a great part of the voluntary .mus- 
 cular system is formed from the splanchnic layer of the meso- 
 blast, yet this fact presents a most serious difficulty to the view 
 that the somatic and splanchnic layers of the mesoblast in 
 Vertebrates are respectively derived from the epiblast and 
 hypoblast. 
 
 In spite, therefore, of general a priori considerations of 
 a very convincing kind which tell in favour of the double origin 
 of the mesoblast, this view is supported by so few objective 
 facts, and there exists so powerful an array of facts against it, 
 that at present, at least, it seems impossible to maintain it. 
 The full strength of the facts against it will appear more fully 
 in a review of the present state of our knowledge as to the 
 development of the mesoblast in the different groups. 
 
 To this I now pass. 
 
 In a paper on the " Early stages of Development in Ver- 
 tebrates 1 " a short resume was given of the development of the 
 mesoblast throughout the animal kingdom, which it may be 
 worth while repeating here with a few additions. So far as we 
 know at present, the mesoblast is derived from the hypoblast in 
 the following groups : 
 
 Echinoderms (Hensen, Agassiz, Metschnikoff, Selenka, Gotte), 
 Nematodes (Blitschli), Sagitta (Kowalevsky, Biitschli), Lum- 
 bricus and probably other Annelids (Kowalevsky), Brachiopoda 
 (Kowalevsky), Crustaceans (Bobretzky), Insects (Kowalevsky, 
 Ulianin, Dohrn), Myriapods (Metschnikoff), Tunicates (Kowa- 
 levsky, Kuppfer), Petromyzon (Owsjanikoff), Osseous fishes 
 (Oellacher, Gotte, Kowalevsky), Elasmobranchs (Self), Amphi- 
 bians (Remak, Strieker, Gotte). 
 
 The list includes members from the greater number of the 
 groups of the animal kingdom ; the most striking omissions 
 being the Coelenterates, Mollusks, and the Amniotic Vertebrates. 
 The absence of the Ccelenterates has been already explained 
 and my grounds for regarding the Amniotic Vertebrates as 
 
 1 Quart. JL of Micros. Science, July, 1875. [This Edition, No. vi.J 
 
340 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 apparent rather than real exceptions have also been pointed 
 out. The Mollusks, however, remain as a large group, in which 
 we as yet know very little as to the formation of the mesoblast. 
 
 Dr Rabl 1 , who seems recently to have studied the develop- 
 ment of Lymnseus by means of sections, gives some figures 
 shewing the origin of the mesoblast ; they are, however, too 
 diagrammatic to be of much service in settling the present 
 question, and the memoirs of Professor Lankester 2 and Dr 
 Fol 3 are equally inconclusive for this purpose, for, though they 
 contain figures of elongated and branched mesoblast cells 
 passing from the epiblast to the hypoblast, no satisfactory 
 representations are given of the origin of these cells. I have 
 myself observed in embryos of Turbo or Trochus similar 
 elongated cells to those figured by Lankester and Fol, but was 
 unable clearly to determine whence they arose. The most 
 accurate observations which we have on this question are those 
 of Professor Bobretzky 4 . In Nassa he finds that the three 
 embryonic layers are all established during segmentation. The 
 outermost and smallest cells form the epiblast, somewhat larger 
 cells adjoining these the mesoblast, and the large yolk-cells the 
 hypoblast. These observations do not, however, demonstrate 
 from which of the primary layers the mesoblast is derived. 
 
 The evidence at present existing is clearly in favour of the 
 mesoblast being, in almost all groups of animals, developed 
 from the hypoblast. but strong as this evidence is, it has not its 
 full weight unless the actual manner in which the mesoblast is 
 in many groups derived from the hypoblast, is taken into Con- 
 sideration. The most important of these are the Echinoderms, 
 Brachiopods and Sagitta. 
 
 In the Echinoderms the mesoblast is in part formed by cells 
 budded ofif from the hypoblast, the remainder, however, arises as 
 one or more diverticula of the alimentary tract. From the separate 
 cells first budded off there are formed the cutis, part of the 
 connective tissue and the calcareous skeleton 5 . The diverticula 
 
 1 Jcnaische Zeitschrift, Vol. IX. 
 
 2 Quart. Jl. of Micros. Science, Vol. xxv. 1874, and Phil. Trans. 1875. 
 1 Archives de Zoologie, Vol. IV. 
 
 4 Archiv f. Micr. Anat. Vol. xm. 
 
 5 The recent researches of Selenka, Zeitschrift f. Wiss. Zoologie, Vol. xxvn. 1876, 
 demonstrate that in Echjnoderrns the muscles are derived from the cells first split off 
 
DERIVATION OF THE MESOBLAST. 34! 
 
 from the alimentary cavity form the water- vascular system and 
 the somatic and splanchnic layers of mesoblast. The cavity of 
 the diverticula after the separation of the water-vascular system, 
 forms the body-cavity. The outer lining layer of the cavity forms 
 the somatic layer of mesoblast and the voluntary muscles-; the 
 inner lining layer the splanchnic mesoblast which unites with the 
 epWtelium of the alimentary tract. Though this fundamental 
 arrangement would seem to be universal amongst Echinoderms, 
 considerable variations of it are exhibited in different groups. 
 
 There is one outgrowth from the alimentary tract in Sy- 
 napta; two in Echinoids, Asteroids and Ophiura; three in 
 Comatula, and four (?) in Amphiura. The cavity of the out- 
 growth usually forms the body-cavity, but sometimes in Ophiura 
 and Amphiura (Metschnikoff) the outgrowths are from the first 
 or soon become solid, and only secondarily acquire a cavity, 
 which is however homologous with the body-cavity of the other 
 groups. 
 
 In Sagitta 1 the formation of the mesoblast and the ali- 
 mentary tract takes place in nearly the same fashion as in the 
 Echinoderms. The simple invaginate alimentary cavity becomes 
 divided into three lobes, a central and two lateral. The two 
 lateral lobes are gradually more and more constricted off from 
 the central one, and become eventually quite separated from it ; 
 their cavities remain independent, and form in the adult the 
 body-cavity, divided by a mesentery into two distinct lateral 
 sections. The inner layer of each of the two lateral lobes forms 
 the mesoblast of the splanchnopleure, the outer layer tfie mesoblast 
 of the somatopleure. The central division of the primitive 
 gastraea cavity remains as the alimentary tract of the adult. 
 
 The remarkable observations of Kowalevsky* on the devel- 
 opment of the Brachiopoda have brought to light the unexpected 
 fact that in two genera at least (Argiope and Terebratula) the 
 mesoblast and body-cavity develope as paired constrictions from 
 
 from the hypoblast, and that the diverticula only form the water-vascular system and 
 the epithelial lining of the body-cavity. 
 
 1 Kowalevsky, " Wiirmer u. Arthropoden," Mem. Acad. Petersbourg, 1871. 
 
 * "Zur Entwicklungsgeschichte d. Brachiopoden ", Protokoll d. ersten Session der 
 Versammlung Russischer Naturforscher in Kasan, 1873. Published in Kaiserliche 
 Gesellschaft Moskau, 1874 (Russian). Abstracted in Hoffmann and Schwalbe, Jahres- 
 btricht f. 1873. 
 
342 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the alimentary tract in a manner almost identically the same as 
 in Sagitta. 
 
 It thus appears that, so far as can be determined from the 
 facts at our disposal, the mesoblast in almost all cases is derived 
 from the hypoblast, and in three widely separated groups it 
 arises as a pair of diverticula from the alimentary tract, each 
 diverticulum containing a cavity which eventually becomes the 
 body-cavity. I have elsewhere suggested 1 that the origin of 
 the mesoblast from alimentary diverticula is to be regarded as 
 primitive for all higher animals, and that the more general cases 
 in which the mesoblast becomes split off, as an undivided layer, 
 from the hypoblast, are in reality derivates from this. The 
 chief obstacle in the way of this view arises from the difficulty of 
 understanding how the whole voluntary muscular system can 
 have been derived at first from the alimentary tract. That part 
 of a voluntary system of muscles might be derived from the con- 
 tractile diverticula of the alimentary canal attached to the body- 
 wall is not difficult to understand, but it is not easy to believe 
 that the secondary system so formed could completely replace 
 the primitive muscular system, derived, as it must have been, 
 from the epiblast. In my paper above quoted will be found 
 various speculative suggestions for removing this difficulty, 
 which I do not repeat here. If it be granted, however, that 
 in Sagitta, Brachiopods, and Echinoderms we have genuine 
 examples of the formation of the whole mesoblast from ali- 
 mentary diverticula, it is easy to see how the formation of the 
 mesoblast in Vertebrates may be a secondary derivate from an 
 origin of this nature. 
 
 An attempt has been already made to shew that the meso- 
 blast in Elasmobranchs is formed in a very primitive fashion, 
 and for this reason the Elasmobranchs appear to be especially 
 adapted for determining whether any signs are exhibited of a 
 derivation of the mesoblast as paired diverticula of the ali- 
 mentary tract. There are, it appears to me, several such 
 features. In the first place, the mesoblast is split off from the 
 hypoblast not as a single mass but as a pair of distinct masses, 
 comparable with the paired diverticula already alluded to. 
 
 1 Comparison of Early Stages, Quart. Jl. Micros. Science, July, 1875. [This 
 Edition, No. vi.] 
 
DERIVATION OF THE MESOBLAST. 343 
 
 Secondly, the body-cavity when it appears in the mesoblast 
 plates, does not arise as a single cavity, but as a pair of cavities, 
 one for each plate of mesoblast, and these cavities remain 
 permanently distinct in some parts of the body, and nowhere 
 unite till a comparatively late period. Thirdly, the primitive 
 body-cavity of the embryo is not confined to the region in 
 which a body-cavity exists in the adult, but extends to the 
 summit of tlie mtiscle- plates, at first separating parts which 
 become completely fused in the adult to form the great lateral 
 muscles of the body. It is difficult to understand how the body- 
 cavity could have such an extension as this, on the supposition 
 that it represents a primitive split in the mesoblast between 
 the wall of the gut and the body-wall; but its extension to this 
 part is quite intelligible, on the supposition that it represents 
 the cavities of two diverticula of the alimentary tract, from 
 whose muscular walls the voluntary muscular system has been 
 derived. Lastly, I would point out that the derivation of part 
 of the muscular system from what appears as the splanchno- 
 pleure is quite intelligible on the assumed hypothesis, but, as 
 far as I see, on no other. 
 
 Such are the main features presented by the mesoblast in 
 Elasmobranchs, which favour the view of its having originally 
 formed the walls of the alimentary diverticula. Against this 
 view of its nature are the facts (i) of the mesoblast plates 
 being at first solid, and (2), as a consequence of this, of the body- 
 cavity never communicating with the alimentary canal. These 
 points, in view of our knowledge of embryological modifications, 
 cannot be regarded as great difficulties to my view. We have 
 many examples of organs, which, though in most cases arising 
 as involutions, yet appear in other cases as solid ingrowths. 
 Such examples are afforded by the optic vesicle, auditory 
 vesicle, and probably also by the central nervous system, of 
 Osseous Fish. In most Vertebrates these organs are formed as 
 hollow involutions from the exterior; in Osseous Fish, however, 
 as solid involutions, in which a cavity secondarily appears. 
 
 The segmental duct of Elasmobranchs or the Wolffian duct 
 (segmental duct) of Birds are cases of a similar kind, being 
 organs which must originally have been formed as hollow 
 involutions, but which now arise as solid bodies. 
 
344 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Only one more instance of this kind need be cited, taken 
 from the Echinoderms. 
 
 The body-cavity and the mesoblast investing it arise in the 
 case of most Echinoderms as hollow involutions of the alimentary 
 tract, but in some exceptional groups, Ophiura and Amphiura, 
 are stated to be solid at first and only subsequently to become 
 hollow. Should the accuracy of Metschnikoff's account of this 
 point be confirmed, an almost exact parallel to what has been 
 supposed by me to have occurred with the mesoblast in Elasmo- 
 branchs, and other groups, will be supplied. 
 
 The tendency of our present knowledge appears to be in 
 favour of regarding the body-cavity in Vertebrates as having 
 been primitively the cavity of alimentary diverticula, and the 
 mesoblast as having formed the walls of the diverticula. 
 
 This view, to say the least of it, suits the facts which we 
 know far better than any other theory which has been proposed, 
 and though no doubt the a priori difficulties in its way are very 
 great, yet it appears to me to be sufficiently strongly supported 
 to deserve the attention of investigators. In the meantime, 
 however, our knowledge of invertebrate embryology is so new 
 and imperfect that no certainty on a question like that which 
 has just been discussed can be obtained; and any generalizations 
 made at present are not unlikely to be upset by the discovery of 
 fresh facts. 
 
 The only other point in connection with the mesoblast 
 which I would call attention to is the formation of the vertebral 
 bodies. 
 
 My observations confirm those of Remak and Gegenbaur, 
 shewing that there is a primary segmentation of the vertebral 
 bodies corresponding to that of the muscle-plates, followed by a 
 secondary segmentation in which the central lines of the vertebral 
 bodies are opposite the partitions between the muscle-plates. 
 
 The explanation of these changes is not difficult to find. 
 The primary segmentation of the body is that of the muscle- 
 plates, which must have been present at a time when the 
 vertebral bodies had no existence. As soon however as the 
 notochordal sheath was required to be strong as well as flexible, 
 it necessarily became divided into a series of segments. 
 
 The conditions under which the lateral muscles can cause the 
 
THE URINOGENITAL SYSTEM. 345 
 
 flexure of the vertebral column are clearly that each muscle- 
 segment shall be capable of acting on two vertebrae; and this 
 condition can only be fulfilled when the muscle-segments are 
 opposite the intervals between the vertebrae. Owing to this 
 necessity, when the vertebral segments became formed^ thnir 
 centres corresponded, not with the centres of the muscle-plates, 
 but with the inter-muscular septa. 
 
 These considerations fully explain the secondary segmen- 
 tation of the vertebrae by which they become opposite the inter- 
 muscular septa. On the other hand, the primary segmentation 
 is clearly a remnant of the time when no vertebral bodies were 
 present, and has no greater morphological significance than the 
 fact that the cells to form the unsegmerited investment of the 
 notochord were derived from the segmented muscle-plates, and 
 only secondarily became fused into a continuous tube. 
 
 The Urinogenital System. 
 
 The first traces of the urinary system become visible at 
 about the time of the appearance of the third visceral cleft. At 
 about this period the somatopleure and splanchnopleure become 
 more or less fused together at the level of the dorsal aorta, and 
 thus, as has been already mentioned, each of the original plates 
 of mesoblast becomes divided into a vertebral plate and lateral 
 plate (PI. 11, fig. 6). The mass of cells resulting from this fusion 
 corresponds with Waldeyer's intermediate cell-mass in the Fowl. 
 
 At about the level of the fifth protovertebra the first trace of 
 the urinary system appears. 
 
 From the intermediate cell-mass a solid knob grows outwards 
 towards the epiblast (woodcut, fig. 4,/^). This knob consists at 
 first of 20 30 cells, which agree in character with the neigh- 
 bouring cells of the intermediate cell-mass, and are at this period 
 rounded. It is mainly, if not entirely, derived from the somatic 
 layer of the mesoblast 
 
 From this knob there grows backwards a solid rod of cells 
 which keeps in very close contact with the epiblast, and rapidly 
 diminishes in size towards its posterior extremity. Its hinder- 
 most part consists in section of at most one or two cells. It 
 keeps so close to the epiblast that it might be supposed to be 
 B. 23 
 
346 
 
 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 derived from that layer were it not for the sections shewing its 
 origin from the knob above mentioned. We have in this rod the 
 commencement of what I have elsewhere 1 called the segmental 
 duct. 
 
 FlG. 4. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. 
 
 spn 
 
 spn 
 
 The sections are to shew the development of the segmental duct (pd) or primitive 
 duct of the kidneys. In A (the anterior of the two sections) this appears as a solid 
 knob projecting towards the epiblast. In B is seen a section of the column which 
 has grown backwards from the knob in A. 
 
 spn. rudiment of a spinal nerve ; me. medullary canal ; ch. notochord ; X. string 
 of cells below the notochord ; mp. muscle-plate ; mp'. specially developed portion of 
 muscle-plate ; ao. dorsal aorta ; pd. segmental duct ; so. somatopleura ; sp. splanch- 
 nopleura ; pp. pleuroperitoneal or body-cavity ; ep. epiblast ; al. alimentary canal. 
 
 My observations shew that the segmental duct is developed 
 in the way just described in both Pristiurus and Torpedo. Its 
 origin in Pristiurus is shewn in the adjoining woodcut, and in 
 Torpedo in PI. 1 1, fig. 7, sd. 
 
 At a stage somewhat older than I, the condition of the 
 segmental duct has not very materially altered. It has in- 
 creased considerably in length, and the knob at its front end 
 is both absolutely smaller, and also consists of fewer cells than 
 before (PI. 1 1, fig. J,sd}. These cells have become more columnar, 
 and have begun to arrange themselves radially ; thus indicating 
 the early appearance of the lumen of the duct. The cells forming 
 the front part of the rod, as well as those of the knob, commence 
 to exhibit a columnar character, but in the hinder part of the 
 
 1 " Urinogenital Organs of Vertebrates," Journ. of Anat. and Phys. Vol. x. 
 [This Edition, No. vn.] 
 
THE URINOGEN1TAL SYSTEM. 347 
 
 rod the cells are still rounded. In no part of it has a lumen 
 appeared. 
 
 At this period also the knob, partly owing to the com- 
 mencing separation of the muscle-plate from the remainder of 
 the mesoblast, begins to pass inwards and approach the plcuro- 
 peritoneal cavity. 
 
 At the same stage the first not very distinct traces of the 
 remainder of the urinary system become developed. These 
 appear in the form of solid outgrowths from the intermediate 
 cell-mass just at the most dorsal part of the body-cavity. 
 
 The outgrowths correspond in numbers with the vertebral 
 segments, and are at first quite disconnected with the segmental 
 duct. At this stage they are only distinctly visible in the 
 first few segments behind the front end of the segmental duct. 
 A full description of them will come more conveniently in the 
 next stage. 
 
 By a stage somewhat earlier than K important changes have 
 taken place in the urinary system. 
 
 The segmental duct has acquired a lumen in its anterior 
 portion, which opens at its front end into the body-cavity. (PI. 
 u, fig. 9, sd.) The lumen is formed by the columnar cells 
 spoken of in the last stage, acquiring a radiating arrange- 
 ment round a central point, at which a small hole appears. 
 After the lumen has once become formed, it rapidly increases 
 in size. 
 
 The duct has also grown considerably in length, but its hind 
 extremity is still as thin, and lies as close to the epiblast, as at 
 first. The segmental involutions which commenced to be formed 
 in the last stage, have now appeared for every vertebral segment 
 along the whole length of the segmental duct, and even for two 
 or three segments behind this. 
 
 They are simple independent outgrowths arising from the 
 outer and uppermost angle of the body-cavity, and are at first 
 almost without a trace of a lumen, though their cells are arranged 
 as two layers. They grow in such a way as to encircle the 
 oviduct on its inner and upper side (PI. u, fig. 8 and PI. 12, fig. 
 \^b,st}. When the hindermost ones are formed, a slight trace 
 of a lumen is perhaps visible in the front ones. At a stage 
 slightly subsequent to this, in Scyllium canicula, I noticed 29 
 
 232 
 
348 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of them ; the first of them arising in the segment immediately 
 behind the front end of the oviduct (PI. 12, fig. 17, sf), and two 
 of them being formed in segments just posterior to the hinder 
 extremity of the oviduct. 
 
 PI. 12, figs. 16 and 1 8 represent two longitudinal sections 
 shewing the segmental nature of the involutions and their 
 relation to the segmental duct. 
 
 Many of the points which have been mentioned can be seen 
 by referring to PL 11 and 12. Anteriorly the segmental duct 
 opens into the pleuro-peritoneal cavity. In the sections behind 
 this there may be seen the segmental duct with a distinct lumen, 
 and also a pair of segmental involutions (PL 12, fig. 140), In 
 the still posterior sections the segmental duct would be quite 
 without a lumen, and would closely adjoin the epiblast. 
 
 It seems not out of place to point out that the modes of 
 the development of the segmental duct and of the segmental 
 involutions are strikingly similar. Both arise as solid involu- 
 tions, from homologous parts of the mesoblast. The segmental 
 duct arises in the vertebral segment immediately in front of 
 that in which the first segmental involution appears ; so that the 
 segmental duct appears to be equivalent to a single segmental 
 involution. 
 
 The next stage corresponds with the first appearance of the 
 external gills. The segmental duct now communicates by a 
 wide opening with the body-cavity (PL n, fig. 9, sd}. It pos- 
 sesses a lumen along its whole length up to the extreme hind 
 end (PI. n, fig. 9). It is, however, at this hinder extremity 
 that the most important change has taken place. This end has 
 grown downwards towards that part of the alimentary canal 
 which still lies behind the anus. This downgrowth is begin- 
 ning to shew distinct traces of a lumen, and will appear in the 
 next stage as one of the horns by which the segmental ducts 
 communicate with the cloaca (PL u, fig. 9^). All the anterior 
 segmental involutions have now acquired a lumen. But this 
 is still absent in the posterior ones (PL 11, fig. 9 a}. 
 
 Owing to the disappearance of the body-cavity in the region 
 behind the anus, the primitive involutions there remain as simple 
 masses of cells still disconnected with the segmental duct (PI. TI. 
 fig s - 9 ^> 9 ^ and 9 d). 
 
THE URINOGENITAL SYSTEM. 349 
 
 Primitive Ova. The true generative products make their 
 first appearance as tin& primitive ova between stages I and K. 
 
 In the sections of one of my embryos of this stage they are 
 especially well shewn, and the following description is taken 
 from those displayed in that embryo. 
 
 They are confined to the region which extends posteriorly 
 nearly to the end of the small intestine and anteriorly to the 
 abdominal opening of the segmental duct. 
 
 Their situation in this region is peculiar. There is no trace 
 of a distinct genital ridge, but the ova mainly lie in the dorsal 
 portion of the mesentery, and therefore in a part of the mesoblast 
 which distinctly belongs to the splanchnopleure (PL 12, fig. 140). 
 Some are situated external to the segmental involutions ; and 
 others again, though this is not common, in a part of the 
 mesoblast which distinctly belongs to the body-wall (PL 12, 
 fig. 140). 
 
 The portion of mesentery, in which the primitive ova are 
 most densely aggregated, corresponds to the future position of 
 the genital ridge, but the other positions occupied by ova are 
 quite outside this. Some ova are in fact situated on the outside 
 of the segmental duct and segmented tubes, and must therefore 
 effect a considerable migration before reaching their final positions 
 in the genital ridge on the inner side of the segmental duct 
 (PL 12, fig. 14 b\ 
 
 The condition of the tissue in which the ova appear may at 
 once be gathered from an examination of the figures given. 
 It consists of an irregular epithelium of cells partly belonging 
 to the somatopleure and partly to the splanchnopleure, but 
 passing uninterruptedly from one layer to the other. The cells 
 which compose it are irregular in shape, but frequently columnar 
 (PL 12, figs. 140 and 14 b}. 
 
 They are formed of a nucleus which stains deeply, invested 
 by a very delicate layer of protoplasm. At the junction of somato- 
 pleure and splanchnopleure they are more rounded than else- 
 where. Very few loose connective-tissue cells are present. The 
 cells just described vary from '008 Mm. to 'Oi Mm. in diameter. 
 
 The primitive ova are situated amongst them and stand out 
 with extraordinary clearness, to which justice is hardly done in 
 my figures. 
 
350 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The normal full-sized ova exhibit the following structure. 
 They consist of a mass of somewhat granular protoplasm of 
 irregular, but more or less rounded, form. Their size varies 
 from -016 '036 Mm. In their interior a nucleus is present, 
 which varies from "012 '016 Mm., but its size as a rule bears no 
 relation to the size of the containing cell. 
 
 This is illustrated by the subjoined list of measurements. 
 
 Size of Primitive ova in Size of nucleus of Primitive 
 
 degrees of micrometer scale ova in degrees of micrometer 
 
 with F. ocul 2. scale with F. ocul 2. 
 
 TO 8 
 
 13 8 
 
 13 * 
 
 H 7 
 
 i5 ' 7 
 
 13 7i- 
 
 ii 8 
 
 16 Si 
 
 12 7 
 
 10 7 
 
 iS 6 
 
 13 6 
 
 12 7 
 
 The numbers given refer to degrees on my micrometer scale. 
 
 Since it is the ratio alone which it is necessary to call attention 
 to, the numbers are not reduced to decimals of a millimeter. 
 Each degree of my scale is equal, however, with the object glass 
 employed, to '002 Mm. 
 
 This series brings out the result I have just mentioned with 
 great clearness. 
 
 In one case we find a cell has three times the diameter of 
 the nucleus 16:5^; in another case 10 : 8, the nucleus has 
 only a slightly smaller diameter than the cell. The irration- 
 ality of the ratio is fairly shewn in some of my figures, though 
 none of the largest cells with very small nuclei have been 
 represented. 
 
 The nuclei are granular, and stain fairly well with haema- 
 toxylin. They usually contain a single deeply stained nucleolus, 
 but in many cases, especially where large (and this independently 
 
THE URINOGENITAL SYSTEM. 351 
 
 of the size of the cell), they contain two nucleoli (PI. 12, figs. 14^ 
 and 14^), and are at times so lobed as to give an apparent 
 indication of commencing division. 
 
 A multi-nucleolar condition of the nuclei, like that figured 
 by Gotte 1 , does not appear till near the close of embryonic 
 life, and is then found equally in the large ova and in those not 
 larger than the ova which exist at this early date. 
 
 As regards the relation of the primitive ova to each other 
 and the neighbouring cells, there are a few points which deserve 
 attention. In the first place, the ova are, as a rule, collected in 
 masses at particular points, and not distributed uniformly (fig. 
 14 a). The masses in some cases appear as if they had resulted 
 from the division of one primitive ovum, but can hardly be 
 adduced as instances of a commencing coalescence ; since if the 
 ova thus aggregated were to coalesce, an ovum would be produced 
 of a very much greater size than any which is found during the 
 early stages. Though at this stage no indication is present of 
 such a coalescence of cells to form ova as is believed to take 
 place by Gotte, still the origin of the primitive ova is not quite 
 clear. One would naturally expect to find a great number of 
 cells intermediate between primitive ova and ordinary columnar 
 cells. Cells which may be intermediate are no doubt found, but 
 not nearly so frequently as might have been anticipated. One 
 or two cells are shewn in PI. 12, fig. 14 a, x, which are perhaps 
 of an intermediate character; but in most sections it is not 
 possible to satisfy oneself that any such intermediate cells are 
 present. 
 
 In one case what appeared to be an intermediate cell was 
 measured, and presented a diameter of '012 Mm. while its 
 nucleus was "008 Mm. Apart from certain features of the 
 nucleus, which at this stage are hardly very marked, the easiest 
 method of distinguishing a primitive ovum from an adjacent 
 cell is the presence of a large quantity of protoplasm around 
 the nucleus. The nucleus of one of the smallest primitive ova 
 is not larger than the nucleus of an ordinary cell (being about 
 008 Mm. in both). It is perhaps the similarity in the size of 
 the nuclei which renders it difficult at first to distinguish de- 
 veloping primitive ova from ordinary cells. Except with the 
 1 Entvricklungsgeschichte der Unke, PI. i, fig. 8. 
 
352 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 very thinnest sections a small extra quantity of protoplasm 
 around a nucleus might easily escape detection, and the de- 
 veloping cell might only become visible when it had attained to 
 the size of a small typical primitive ovum. 
 
 It deserves to be noticed that the nuclei even of some of the 
 largest primitive ova scarcely exceed the surrounding nuclei in 
 size. This appears to me to be an argument of some weight in 
 shewing that the great size of primitive ova is not due to the 
 fact of their having been formed by a coalescence of different 
 cells (in which case the nucleus would have increased in the same 
 proportion as the cell) ; but to an increase by a normal method 
 of growth in the protoplasm around the nucleus. 
 
 It appears to me to be a point of great importance certainly 
 to determine whether the primitive ova arise by a metamor- 
 phosis of adjoining cells, or may not be introduced from else- 
 where. In some of the lower animals, e.g. Hydrozoa, there is no 
 question that the ova are derived from the epiblast; we might 
 therefore expect to find that they had the same origin in Verte- 
 brates. Further than this, ova are frequently capable in a 
 young state of executing amoeboid movements, and accordingly 
 of migrating from one layer to another. In the Elasmobranchs 
 the primitive ova exhibit in a hardened state an irregular form 
 which might appear to indicate that they possess a power of 
 altering their shape, a view which is further supported by some 
 of them being at the present stage situated in a position very 
 different from that which they eventually occupy, and which 
 they can only reach by migration. If it could be shewn that 
 there were no intermediate stages between the primitive ova 
 and the adjoining cells (their migratory powers being admitted) 
 a strong presumption would be offered in favour of their having 
 migrated from elsewhere to their present position. In view of 
 this possibility I have made some special investigations, which 
 have however led to no very satisfactory results. There are to 
 be seen in the stages immediately preceding the present one, 
 numerous cells in a corresponding position to that of the 
 primitive ova, which might very well be intermediate between 
 the primitive ova and ordinary cells, but which offer no suffi- 
 ciently well marked features for a certain determination of their 
 true nature. 
 
THE URINOGENITAL SYSTEM. 353 
 
 In the particular embryo whose primitive ova have been 
 described these bodies were more conspicuous than in the 
 majority of cases, but at the same time they presented no 
 special or peculiar characters. 
 
 In a somewhat older embryo of Scyllium the cells amongst 
 which the primitive ova lay had become very distinctly dif- 
 ferentiated as an epithelium (the germinal epithelium of 
 Waldeyer) well separated by what might almost be called a 
 basement membrane from the adjoining connective-tissue cells. 
 Hardly any indication of a germinal ridge had appeared, but 
 the ova were more definitely confined than in previous embryos 
 to the restricted area which eventually forms this. The ova on 
 the average were somewhat smaller than in the previous cases. 
 
 In several embryos intermediate in age between the embryo 
 whose primitive ova were described at the commencement of 
 this section and the embryo last described, the primitive ova 
 presented some peculiarities, about the meaning of which I am 
 not quite clear, but which may perhaps throw some light on the 
 origin of these bodies. 
 
 O 
 
 Instead of the protoplasm around the nucleus being clear or 
 slightly granular, as in the cases just described, it was filled in 
 the most typical instances with numerous highly refracting 
 bodies resembling yolk-spherules. In osmic acid specimens (PL 
 12, fig. 15) these stain very darkly, and it is then as a rule very 
 difficult to see the nucleus; in specimens hardened in picric 
 acid and stained with ha^matoxylin these bodies are stained of a 
 deep purple colour, but the nucleus can in most cases be dis- 
 tinctly seen. In addition to the instances in which the proto- 
 plasm of the ova is quite filled with these bodies, there are 
 others in which they only occupy a small area adjoining the 
 nucleus (PI. 12, fig. 15 a), and finally some in which only one or 
 two of these bodies are present. The protoplasm of the 
 primitive ova appears in fact to present a series of gradations 
 between a state in which it is completely filled with highly 
 refracting spherules and one in which these are completely 
 absent. 
 
 This state of things naturally leads to the view that the 
 primitive ova, when they are first formed, are filled with these 
 spherules, which are probably yolk-spherules, but that they 
 
354 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 gradually lose them in the course of development. Against this 
 interpretation is the fact that the primitive ova in the younger 
 embryo first described are completely without these bodies; this 
 embryo however unquestionably presented an abnormally early 
 development of the ova; and I am satisfied that embryos present 
 considerable variations in this respect. 
 
 If the primitive ova are in reality in the first instance filled 
 with yolk-spherules, the question arises as to whether, consider- 
 ing that they are the only mesoblast cells filled at this period 
 with yolk-spherules, we must not suppose that they have 
 migrated from some peripheral part of the blastoderm into their 
 present position. To this question I can give no satisfactory 
 answer. Against a view which would regard the spherules in 
 the protoplasm as bodies which appear subsequently to the first 
 formation of the ova, is the fact that hitherto no instances in 
 which these spherules were present have been met with in the 
 late stages of development; and they seem therefore to be 
 confined to the first stages. 
 
 Notochord, 
 
 The changes undergone by the notochord during this period 
 present considerable differences according to the genus examined. 
 One type of development is characteristic of Scyllium and 
 Pristiurus ; a second type, of Torpedo. 
 
 My observations being far more complete for Scyllium and 
 Pristiurus than for Torpedo, it is to the two former genera only 
 that .the following account applies, unless the contrary is ex- 
 pressly stated. Only the development of the parts of the noto- 
 chord in the trunk are here dealt with ; the cephalic section of 
 the notochord is treated of in a subsequent section. 
 
 During stage G the notochord is composed of flattened cells 
 arranged vertically, rendering the histological characters of the 
 notochord difficult to determine in transverse sections. In longi- 
 tudinal sections, however, the form and arrangement of the cells 
 can be recognised with great ease. At the beginning of stage 
 G each cell is composed of a nucleus invested by granular pro- 
 toplasm frequently vacuolated and containing in suspension 
 numerous yolk-spherules. It is difficult to determine whether 
 
THE NOTOCHORD. 355 
 
 there is only one vacuole for each cell, or whether in some cases 
 there may not be more than one. 
 
 Round the exterior of the notochord there is present a 
 distinct though delicate cuticular sheath. 
 
 The vacuoles are at first small, but during stage G rapidly 
 increase in size, while at the same time the yolk-spherules 
 completely vanish from the notochord. 
 
 As a result of the rapid growth of the vacuoles, the nuclei, 
 surrounded in each case by a small amount of protoplasm, 
 become pushed to the centre of the notochord, the remainder of 
 the protoplasm being carried to the edge. The notochord thus 
 becomes composed during stages H and I (PI. n, fig. 4 6) of a 
 central area mainly formed of nuclei with a small quantity of 
 protoplasm around them, and of a thin peripheral layer of 
 protoplasm without nuclei, the widish space between the two 
 being filled with clear fluid. The exterior of the cells is 
 indurated, so that they may be said tobe invested by a mem- 
 brane 1 ; the cells themselves have a flattened form, and each ex- 
 tends from the edge to the centre of the notochord, the long axis 
 of each being rather greater than half the diameter of the cord. 
 
 The nuclei of the notochord are elliptical vesicles, consisting 
 of a membrane filled with granular contents, amongst which is 
 situated a distinct nucleolus. They stain deeply with haema- 
 toxylin. Their long diameter in Scyllium is about O'O2 Mm. 
 
 The diameter of the whole notochord in Pristiurus during 
 stage I is about OT Mm. in the region of the back, and about 
 o - o8 Mm. near the posterior end of the body. 
 
 Owing to the form of its constituent cells, the notochord 
 presents in transverse sections a dark central area surrounded 
 by a lighter peripheral one, but its true structure cannot be 
 unravelled without the assistance of longitudinal sections. In 
 these (PI. 12, fig. 10) the nuclei form an irregular double row in 
 the centre of the cord. Their outlines are very clear, but those 
 of the individual cells cannot for certain be made out. It is, 
 however, easy to see that the cells have a flattened and wedge- 
 shaped form, with the narrow ends overlapping and interlocking 
 at the centre of the notochord. 
 
 1 This membrane is better looked upon, as is done by Gegenbaur and Gotte f as 
 intercellular matter. 
 
356 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 By the close of stage I the cuticular sheath of the notochord 
 has greatly increased in thickness. 
 
 During the period intermediate between stages I and K the 
 notochord undergoes considerable transformations. Its cells 
 cease to be flattened, and become irregularly polygonal, and 
 appear but slightly more compressed in longitudinal sections 
 than in transverse ones. The vacuolation of the cells proceeds 
 rapidly, and there is left in each cell only a very thin layer of 
 protoplasm around the nucleus. Each cell, as in the earlier 
 stages, is bounded by a membrane-like wall. 
 
 Accompanying these general changes special alterations 
 take place in the distribution of the nuclei and the protoplasm. 
 The nuclei, accompanied by protoplasm, gradually leave the 
 centre and migrate towards the periphery of the notochord. At 
 the same time the protoplasm of the cells forms a special layer 
 in contact with the investing sheath. 
 
 The changes by which this takes place can easily be followed 
 in longitudinal sections. In PI. 12, fig. 11 the migration of the 
 nuclei has commenced. They are still, however, more or less 
 aggregated at the centre, and very little protoplasm is present 
 at the edges of the notochord. The cells, though more or less 
 irregularly polygonal, are still somewhat flattened. In PI. 12, 
 fig. 12 the notochord has made a further progress. The nuclei 
 now mainly lie at the side of the notochord, where they exist in 
 a somewhat shrivelled state, though still invested by a layer of 
 protoplasm. 
 
 A large portion of the protoplasm of the cord forms an 
 almost continuous layer in close contact with the sheath, which 
 is more distinctly visible in some cases than in others. 
 
 While the changes above described are taking place the 
 notochord increases in size. At the age of fig. n it is in the 
 anterior part of the body of Pristiurus about O'li Mm. At the 
 age of fig. 12 it is in the same species O'I2 Mm., while in Scyl- 
 lium stellare it reaches about O'lj Mm. 
 
 During stage K (PI. 1 1, fig. 8) the vacuolation of the cells of 
 the notochord becomes even more complete than during the 
 earlier stages, and in the central cells hardly any protoplasm 
 is present, though a starved nucleus surrounded by a little pro- 
 toplasm may be found in an occasional corner. 
 
THE NOTOCHORD. 357 
 
 The whole notochord becomes very delicate, and can with 
 great difficulty be conserved whole in transverse sections. 
 
 The layer of protoplasm which appeared during the last 
 stage on the inner side of the cuticular membrane of the noto- 
 chord becomes during the present stage a far thicker and more 
 definite structure. It forms a continuous layer with irregular 
 prominences on its inner surface ; and contains numerous nuclei. 
 The layer sometimes presents in transverse sections hardly any 
 indication of a division into a number of separate cells, but in 
 longitudinal sections this is generally very obvious. The cells 
 are directed very obliquely forwards, and consist of an oblong 
 nucleus invested by protoplasm. The layer formed by them 
 is very delicate and very easily destroyed. In one example its 
 thickness varied from '004 to "006 Mm., in another it reached 
 QI2 Mm. The thickness of the cuticular membrane is about 
 002 Mm. or rather less. 
 
 The diameter of a notochord in the anterior part of the 
 body of a Pristiurus embryo of this stage is about O'2i Mm. 
 Round the exterior of the notochord the mesoblast cells are 
 commencing to arrange themselves as a special sheath. 
 
 In Torpedo the notochord at first presents the same struc- 
 ture as in Pristiurus, i.e. it forms a cylindrical rod of flattened 
 cells. 
 
 The vacuolation of these cells does not however commence 
 till a relatively very much later period than in Pristiurus, and 
 also presents a very different character (PL 11, fig. 7). 
 
 The vacuoles are smaller, more numerous, and more rounded 
 than in the other genera, and there can be no question that in 
 many cases there is more than one vacuole in a cell. The most 
 striking point in which the notochord of Torpedo differs from 
 that of Pristiurus consists in the fact that in Torpedo there is 
 never any aggregation of the nuclei at the centre of the cord, 
 but the nuclei are always distributed uniformly through it. As 
 the vacuolation proceeds the differences between Torpedo and 
 the other genera become less and less marked. The vacuoles 
 become angular in form, and the cells of the cord cease to be 
 flattened, and become polygonal. 
 
 At my final stage for Torpedo (slightly younger than K) the 
 only important feature distinguishing the notochord from that 
 
358 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of Pristiurus, is the absence of any signs of nuclei or pro- 
 toplasm passing to the periphery. Around the exterior of the 
 cord there is early found in Torpedo a special investment of 
 mesoblastic cells. 
 
 EXPLANATION OF PLATES n AND 12. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 al. Alimentary tract, an. Point where anus will be formed, ao. Dorsal aorta. 
 ar. Rudiment of anterior root of spinal nerve, b. Anterior fin. c. Connective-tissue 
 cells, cav. Cardinal vein. ch. Notochord. df Dorsal fin. ep. Epiblast. ge. 
 Germinal epithelium, ht. Heart. /. Liver, mp. Muscle-plate, mp' . Early formed 
 band of muscles from the splanchnic layer of the muscle-plates, nc. Neural canal. 
 /. Protoplasm from yolk in the alimentary tract, pc. Pericardial cavity, po. Primi- 
 tive ovum. //. Body cavity, pr. Rudiment of posterior root of spinal nerve, sd. 
 Segmental duct. sh. Cuticular sheath of notochord. so. Somatic layer of mesoblast. 
 sp. Splanchnic layer of mesoblast. sp c. Spinal cord. sp. v. Spiral valve, sr. Inter- 
 renal body. st. Segmental tube. sv. Sinus venosus. ua. Umbilical artery, um. 
 Umbilical cord. uv. Umbilical vein. v. Splanchnic vein. v. Blood-vessel, vc. Visceral 
 cleft. Vr. Vertebral rudiment. W. White matter of spinal cord. x. Subnotochordal 
 rod (except in fig. 14 a), y. Passage connecting the neural and alimentary canals. 
 
 PLATE 11. 
 
 Fig. i . Section from the caudal region of a Pristiurus embryo belonging to stage 
 H. Zeiss C, ocul. i. Osmic acid specimen. 
 
 It shews (i) the constriction of the subnotpchordal rod (x} from the summit of the 
 alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the 
 ventral thickening of the parietal plate. 
 
 Fig. i a. Portion of alimentary wall of the same embryo, shewing the formation 
 of the subnotochord rod (x) . 
 
 Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to 
 stage H. Zeiss C, ocul. i. 
 
 It shews the bilobed condition of the alimentary vesicle and the fusion of the 
 mesoblast and hypoblast at the caudal vesicle. 
 
 Fig- 3 a- Sections from the caudal region of a Pristiurus embryo belonging to 
 stage H. Zeiss C, ocul. i. Picric acid specimen. 
 
 It shews the communication which exists posteriorly between the neural and 
 alimentary canals, and also by comparison with 3 b it exhibits the dilatation undergone 
 by the alimentary canal in the caudal vesicle. 
 
 Fig. 3^. Section from the caudal region of an embryo slightly younger than 30. 
 Zeiss C, ocul. i. Osmic acid specimen. 
 
PLATES II AND 12. 359 
 
 Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage 
 H. Zeiss C, ocul. i. Osmic acid specimen. 
 
 It shews the formation of the heart (fit) as a cavity between the splanchnopleure 
 and the wall of the throat. 
 
 Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging 
 to stage H. Zeiss C, ocul. i. Osmic acid specimen. 
 
 It shews the general features of an embryo of stage H, more especially the rela- 
 tions of the body-cavity in the parietal and vertebral portions of the lateral plate, and 
 the early-formed band of muscle (mp') in the splanchnic layer of the vertebral plate. 
 
 Fig. 6. Section from the cesophageal region of Scyllium embryo belonging to 
 stage I. Zeiss C, ocul. i. Chromic acid specimen. 
 
 It shews the formation of the rudiments of the posterior nerve-roots (pr) and of 
 the vertebral rudiments (Vr). 
 
 Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. 
 Zeiss C, ocul. i, reduced \. Osmic acid specimen. 
 
 It shews (i) the formation of the anterior and posterior nerve-roots. (2) The solid 
 knob from which the segmental duct (sd) originates. 
 
 Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a stage 
 intermediate between I and K. Zeiss C, ocul. i. Chromic acid specimen. 
 
 It illustrates the structure of the primitive ova, segmental tubes, notochord, etc. 
 
 Fig. 8 a. Section from the caudal region of an embryo of the same age as 8. 
 Zeiss A, ocul. i. 
 
 It shews (i) the solid oesophagus. (2) The narrow passage connecting the peri- 
 cardial (pc) and body cavities (//). 
 
 Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul. i . 
 Osmic acid specimen. 
 
 It shews the formation of the liver (/), the structure'of the anterior fins (/>), and the 
 anterior opening of the segmental duct into the body-cavity (sd). 
 
 Figs. 9 a, 96, gc, gd. Four sections through the anterior region of the same 
 embryo as 9. Osmic acid specimens. 
 
 The sections shew (i) the atrophy of the post-anal section of the alimentary tract 
 (gb, gc, gd). (2) The existence of the segmental tubes behind the anus (9^, gc, gd). 
 With reference to these it deserves to be noted that the segmental tubes behind the 
 anus are quite disconnected, as is proved by the fact that a tube is absent on one side 
 in gc but reappears in gd. (3) The downward prolongation of the segmental duct to 
 join the posterior or cloacal extremity of the alimentary tract (9 b). 
 
 PLATE 12. 
 
 Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of stage H. 
 Zeiss C, ocul. i. Reduced by \. Picric acid specimen. 
 
 It shews (i) the structure of the notochord ; (2) the appearance of the early formed 
 band of muscles (inp 1 ) in the splanchnic layer of the protovertebra. 
 
 Fig. ii. Longitudinal and horizontal sections of an embryo belonging to stage I. 
 Zeiss C, ocul. i. Chromic acid specimen. It illustrates the same points as the 
 previous section, but in addition shews the formation of the rudiments of the vertebral 
 bodies ( Vr) which are seen to have the same segmentation as the muscle-plates. 
 
360 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Fig. I2. 1 Longitudinal and horizontal section of an embryo belonging to the 
 stage intermediate between I and K. Zeiss C, ocul. i. Osmic acid specimen 
 illustrating the same points as the previous section. 
 
 Fig. 13. Longitudinal and horizontal section of an embryo belonging to stage K. 
 Zeiss C, ocul. i, and illustrating same points as previous section. 
 
 Figs. i4#, 14^, 14*:, \^d. Figures taken from preparations of an embryo of an 
 age intermediate between I and K, and illustrating the structure of the primitive ova. 
 Figs. 14 a and 14^ are portions of transverse sections. Zeiss C, ocul. 3 reduced \. 
 Figs. 14 c and 14 d are individual ova, shewing the lobate form of nucleus. Zeiss F, 
 ocul. 2. 
 
 Fig. 15. Osmic acid preparation of primitive ova belonging to stage K. Zeiss 
 immersion No. i, ocul. i. The protoplasm of the ova is seen to be nearly filled with 
 bodies resembling yolk-spherules : and one ovum is apparently undergoing division. 
 
 Fig. 1 5 a. Picric acid preparation shewing a primitive ovum partially filled with 
 bodies resembling yolk-spherules. 
 
 Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging to 
 stage K. Zeiss A, ocul. i. Picric acid preparation. The connective-tissue cells are 
 omitted. 
 
 The section shews that there is one segmental tube to each vertebral segment. 
 
 Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a trans- 
 parent object. 
 
 It shews the segmental duct and the segmental involutions two of which are seen 
 to belong to segments behind the end of the alimentary tract. 
 
 Fig. 1 8. Vertical longitudinal section of a Scyllium embryo "belonging to stage K. 
 Zeiss A, ocul. i. Hardened in a mixture of osmic and chromic acid. It shews 
 
 (1) the commissures connecting together the posterior roots of the spinal nerves ; 
 
 (2) the junction of the anterior and posterior roots 
 
 (3) the relations of the segmental ducts to the segmental involutions and the 
 
 alternation of calibre in the segmental tube ; 
 
 (4) the germinal epithelium lining the body-cavity. 
 
 1 The apparent structure in the sheath of the notochord in this and the succeeding figure is merely 
 the result of an attempt on the part of the engraver to represent the dark colour of the sheath in the 
 original figure. 
 
CHAPTER VII. 
 
 GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE H 
 TO THE CLOSE OF EMBRYONIC LIFE. 
 
 External Epiblast.. 
 
 THE change already alluded to in the previous chapter 
 (p. 317) by, which the external epiblast or epidermis becomes 
 divided into two layers, is completed before the close of stage L. 
 
 In the tail region at this stage three distinct strata may be 
 recognized in the epidermis, (i) An outer stratum of flattened 
 horny cells, which fuse together to form an almost continuous 
 membrane. (2) A middle stratum of irregular partly rounded 
 and partly flattened cells. (3) An internal stratum of columnar 
 cells, bounded towards the mesoblast by a distinct basement 
 membrane (PI. 13, fig. 8), unquestionably pertaining to the 
 epiblast. This layer is especially thickened in the terminal 
 parts of the paired fins (PI. 13, fig. i). The two former of these 
 strata together constitute the epidermic layer of the skin, and 
 the latter the mucous layer. 
 
 In the anterior parts of the body during stage L the skin 
 only presents two distinct strata, viz. an inner somewhat irregular 
 layer of rounded cells, the mucous layer, and an outer layer of 
 flattened cells (PL 13, fig. 8). 
 
 The remaining history of the external epiblast, consisting as 
 it does of a record of the gradual increase in thickness of the 
 epidermic strata, and a topographical description of its variations 
 in structure and thickness in different parts, is of no special 
 interest and need not detain us here. 
 
 In the late embryonic periods subsequent to stage Q the 
 layers of the skin cease to be so distinct as at an earlier period, 
 B. 24 
 
362 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 partly owing to the innermost layer becoming less columnar, 
 and partly to the presence of a large number of mucous cells, 
 which have by that stage made their appearance. 
 
 I have followed with some care the development of the 
 placoid scales, but my observations so completely accord with 
 those of Dr O. Hertwig 1 , that it is not necessary to record 
 them. The so-called enamel layer is a simple product of the 
 thickening and calcification of the basement membrane, and 
 since this membrane is derived from the mucous layer of the 
 epidermis, the enamel is clearly to be viewed as an epidermic 
 product. There is no indication of a gradual conversion of the 
 bases of the columnar cells forming the mucous layer of the 
 epidermis into enamel prisms, as is frequently stated to occur in 
 the formation of the enamel of the teeth in higher Vertebrates. 
 
 Lateral line. 
 
 The lateral line and the nervous structures appended to it 
 have been recently studied from an embryological point of view 
 by Gotte* in Amphibians and by Semper 3 in Elasmobranchs. 
 
 The most important morphological result which these two 
 distinguished investigators believe themselves to have arrived at 
 is the direct derivation of the lateral nerve from the ectoderm. 
 On this point there is a complete accord between them, and 
 Semper especially explains that it is extremely easy to establish 
 the fact. 
 
 As will appear from the sequel, I have not been so fortunate 
 as Semper in elucidating the origin of the lateral nerve, and my 
 observations bear an interpretation not in the least in accord- 
 ance with the views of my predecessors, though not perhaps 
 quite conclusive against them. 
 
 It must be premised that two distinct structures have to be 
 dealt with, viz. the lateral line formed of modified epidermis, and 
 the lateral nerve whose origin is in question. 
 
 The lateral line is the first of the two to make its appear- 
 ance, at a stage slightly subsequent to K, in the form of a 
 
 1 Jenaische Zeitschrift, Vol. vili. 
 
 2 Ent-wicklungsgeschichte d. Unke. 
 
 :t Urogenital-system d. Selachier. Semper's Arbeiten, Bd. II. 
 
THE LATERAL LINE. 363 
 
 linear thickening of the inner row of cells of the external epi- 
 blast, on each side, at the level of the notochord. 
 
 This thickening, in my youngest embryo in which it is found, 
 has but a very small longitudinal extension, being present 
 through about 10 thin sections in the last part of the head "and 
 first part of the trunk. The thickening, though short, is very 
 broad, measuring about O'28 Mm. in transverse section, and 
 presents no signs of a commencing differentiation of nervous 
 structures. The large intestinal branch of the vagus can be 
 seen in all the anterior sections in close proximity to this line, 
 and appears to me to give off to it posteriorly a small special 
 branch which can be traced through a few sections, vide PI. 1 3, 
 fig. 2 n.l. But this branch is not sufficiently well marked to 
 enable me to be certain of its real character. In any case the 
 posterior part of the lateral line is absolutely without any ad- 
 joining nervous structures or traces of stick. 
 
 The rudiment of the epidermic part of the lateral line is 
 formed of specially elongated cells of the mucous layer of the 
 epiblast, but around the bases of these certain rounder cells of a 
 somewhat curious appearance are intercalated. 
 
 There is between this and my next youngest embryo an 
 unfortunately large gap with reference to the lateral line, 
 although in almost every other respect the two embryos might 
 be regarded as belonging to the same stage. The lateral line 
 in the older embryo extends from the hind part of the head to a 
 point well behind the anus, and is accompanied by a nerve for 
 at least two-thirds of its length. 
 
 In the foremost section in which it appears the intestinal 
 branch of the vagus is situated not far from it, and may be seen 
 at intervals giving off branches to it. There is no sign that these 
 are otherwise than perfectly normal branches of the vagus. 
 Near the level of the last visceral cleft the intestinal branch of 
 the vagus gives off a fair-sized branch, which from the first 
 occupies a position close to the lateral line though well within 
 the mesoblast (PL 13, fig. 3^, ;/./). This branch is the lateral 
 nerve, and though somewhat larger, is otherwise much like the 
 nerve I fancied I could see originating from the intestinal branch 
 of the vagus during the previous stage. 
 
 It rapidly thins out posteriorly and also approaches closer 
 
 24 2 
 
364 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 and closer to the lateral line. At the front end of the trunk it 
 is quite in contact with it, and a short way behind this region 
 the cells of the lateral line arrange themselves in a gable-like 
 form, in the angle of which the nerve is situated (PI. 13, figs, ^b, 
 and 3^). In this position the nerve though small is still very 
 distinct in all good sections, and is formed of a rod of pro- 
 toplasm, with scattered nuclei, in which I could not detect a 
 distinct indication of cell-areas. The hinder part of the nerve 
 becomes continually smaller and smaller, without however pre- 
 senting any indication of becoming fused with the epiblast, and 
 eventually ceases to be visible some considerable distance in 
 front of the posterior end of the lateral line. 
 
 The lateral line itself presents some points of not incon- 
 siderable interest. In the first place, it is very narrow anteriorly 
 and throughout the greater part of its length, but widens out at 
 its hinder end, and is widest of all at its termination, which is 
 perfectly abrupt. The following measurements of it were taken 
 from an embryo belonging to stage L, which though not quite 
 my second youngest embryo is only slightly older. At its 
 hinder end it was O'i7 Mm. broad. At a point not far from this 
 it was O'OQ Mm. broad, and anteriorly it was 0^05 Mm. broad. 
 These measurements clearly shew that the lateral line is broadest 
 at what may be called its growing-point, a fact which explains its 
 extraordinary breadth in the anterior part of the body at my first 
 stage, viz. 0^28 Mm., a breadth which strangely contrasts with the 
 breadth, viz. 0^05 Mm., which it has in the same part of the body 
 at the present stage. 
 
 It still continues to form a linear area of modified epidermis, 
 and has no segmental characters. Anteriorly it is formed by the 
 cells of the mucous layer becoming more columnar (PI. 13 fig. 
 3). In its middle region the cells of the mucous layer in it are 
 still simply elongated, but, as has been said above, have a gable- 
 like arrangement, so as partially to enclose the nerve (PL 13, fig. 
 3^). Nearer the hind end of the trunk a space appears in it 
 between its columnar cells and the flattened cells of the outer- 
 most layer of the skin (PI. 13, fig. y], and this space becomes 
 posteriorly invested by a very definite layer of cells. The space 
 (Pi. 13, fig. 3<aO or lumen has a slit-like section, and is not 
 formed by the closing in of an originally open groove, but by 
 
THE LATERAL LINE. 365 
 
 the formation of a cavity in the midst of the cells of the lateral 
 line. Its walls are formed by a layer of columnar cells on the 
 inner side, and flattened cells on the outer side, both layers 
 however appearing to be derived from the mucous layer of the 
 epidermis. The outer layer of cells attains its greatest thickness 
 dorsally. 
 
 During stages M, N, O, the lateral nerve gradually passes 
 inwards into the connective tissue between the dorso-lateral and 
 the ventro-lateral muscles, and becomes even before the close of 
 stage N completely isolated from the lateral line. 
 
 The growth of the lateral line itself remains for some time 
 almost stationary; anteriorly the cells retain the gable-like 
 arrangement which characterised them at an earlier period, but 
 cease to enclose the nerve; posteriorly the line retains its original 
 more complicated constitution as a closed canal. In stage O 
 the cells of the anterior part of the line, as well as those of the 
 posterior, commence to assume a tubular arrangement, and the 
 lateral line takes the form of a canal. The tubular form is due 
 to a hollowing out of the lateral line itself and a rearrangement 
 of its cells. As the lateral line becomes converted into a canal 
 it recedes from the surface. 
 
 In stage P the first indication of segmental apertures to the 
 exterior make their appearance, vide PI. 13, fig. 4. The lateral 
 line forms a canal situated completely below the skin, but at 
 intervals (corresponding with segments) sends upwards and .out- 
 wards prolongations towards the exterior. These prolongations 
 do not during stage P acquire external openings. As is shewn 
 in my figure, a special area of the inner border of the canal of 
 the lateral line becomes distinguished by its structure from the 
 remainder. 
 
 No account of the lateral line would be complete without 
 some allusion to the similar sensory structures which have such 
 a wide distribution on the heads of Elasmobranchs; and this is 
 especially important in the present instance, owing to the light 
 thrown by a study of their development on the origin of the 
 nerves which supply the sense-organs of this class. The so- 
 called mucous canals of the head originate in the same way as 
 does the lateral line; they are products of the mucous layer of 
 the epidermis. They eventually form either canals with nume- 
 
366 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 rous openings to the exterior, or isolated tubes with terminal 
 ampulliform dilatations. 
 
 I have not definitely determined whether the canal-system 
 of the head arises in connection with the lateral line, or only 
 eventually becomes so connected. The important point to be 
 noticed is, that at first no nervous structures are to be seen in 
 connection with it. In stage O nerves for the 'mucous canals 
 make their appearance as delicate branches of the main stems. 
 These nerve-stems are very much ramified, and their branches 
 have, in a large number of instances, an obvious tendency 
 towards a particular sense-organ (PL 13, figs. 5 and 6). 
 
 I have not during stage O been able to detect a case of direct 
 continuity between the two. This is, however, established in the 
 succeeding stage P, in the case of the canals, and the facility 
 with which it may be observed would probably render the 
 embryo Elasmobranch a very favourable object for studying the 
 connection between nerves and terminal sense-organs. The 
 nerve (PL 13, fig. 7) dilates somewhat before uniting with the 
 sense-organ, and the protoplasm of the nerve and the sense- 
 organ become completely fused. The basement membrane of 
 the skin is not continuous across their point of junction, and 
 appears to unite with a delicate membrane-like structure, which 
 invests the termination of the nerve. The ampullae would seem 
 to receive their nervous supply somewhat later than the canals, 
 and the terminal swellings of the nerves supplying them are 
 larger than in the case of the canals, and the connection between 
 the ampullae and the nerves not so clear. In the case of the 
 head, there can for Elasmobranchs be hardly a question that the 
 nerves which supply the mucous canals grow centrifugally from 
 the original cranial nerve-stems, and do not originate in a peri- 
 pheral manner from the integument. 
 
 This is an important point to make certain of in settling any 
 doubtful features in the nervous supply of the lateral line. 
 Professor Semper 1 , with whom as dealing with Elasmobranchs 
 we are more directly concerned, makes the following statement: 
 "At the time when at the front end the lateral nerve has already 
 completely separated itself from the ectoderm, and is situated 
 amongst the muscles, it still lies in the middle of the body close 
 
 1 Loc. cit. p. 398. 
 
DERIVATION OF THE LATERAL NERVE. 367 
 
 to the ectoderm, and at the hind end of the body is not yet 
 completely segmented off (abgegliedert) from the ectoderm." 
 Although the last sentence of this quotation may seem to be 
 opposed to my statements, yet it appears to me probable that 
 Professor Semper has merely seen the lateral nerve partially 
 enclosed in the ectoderm. This position of the nerve no doubt 
 affords a presumption, but only a presumption, in favour of a direct 
 origin of the lateral nerve from the ectoderm ; but against this 
 interpretation of it are the following facts: 
 
 (1) That the front part of the lateral line is undoubtedly 
 supplied by branches which arise in the ordinary way from the 
 intestinal branch of the vagus ; and we should not expect to find 
 part of the lateral line supplied by nerves which originate in one 
 way, and the remainder supplied by a nerve having a completely 
 different and abnormal mode of origin. 
 
 (2) The growth of the lateral line is quite independent of 
 that of the lateral nerve: the latter arises subsequently to the 
 lateral line, and, so far as is shewn by the inconclusive observa- 
 tion of my earliest stage, as an offshoot from the intestinal 
 branch of the vagus; and though it grows along at first in close 
 contact with the lateral line, yet it never presents, so far as I 
 have seen, any indubitable indication of becoming split off from 
 this, or of fusing with it. 
 
 (3) The fact that the cranial representatives of the lateral 
 line are supplied with nerves which originate in the normal 
 way 1 , affords a strong argument in favour of the lateral line 
 receiving an ordinary nerve-supply. 
 
 Considering all these facts, I am led to the conclusion that 
 the lateral nerve in Elasmobranchs arises as a branch of the 
 vagus, and not as a direct product of the external epiblast. 
 
 An interesting feature about the lateral line and the similar 
 cephalic structures, is the fact of these being the only sense- 
 organs in Elasmobranchs which originate entirely from the 
 mucous layer of the epiblast. This, coupled with the well- 
 known facts about the Amphibian epiblast, and the fact that the 
 
 1 Gotte extends his statements about the lateral nerve to the nerves supplying the 
 mucous canals in the head ; but my observations appear to me, as far as Elasrno- 
 branchs are concerned, nearly conclusive against such a derivation of the nerves in the 
 
 head. 
 
368 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 mucous canals are the only sense-organs which originate subse- 
 quently to the distinct differentiation of the epiblast into mu- 
 cous and horny layers, goes far to prove 1 that the mucous layer 
 is to be regarded as the active layer of the epiblast, and that 
 after this has become differentiated, an organ formed from the 
 epiblast is always a product of it. 
 
 Muscle-plates. 
 
 The muscle-plates at the close of stage K were flattened 
 angular bodies with the apex directed forwards, their ventral 
 edge being opposite the segmental duct, and their dorsal edge 
 on a level with the middle of the spinal cord. They were com- 
 posed of two layers, formed for the most part of columnar cells, 
 but a small part of their splanchnic layer opposite the notochord 
 had already become differentiated into longitudinal muscles. 
 
 During stage L the growth of these plates is very rapid, and 
 their upper ends extend to the summit of the neural canal, and 
 their lower ones nearly meet in the median ventral line. The 
 original band of muscles (PI. 11, fig. 8 m. /), whose growth was 
 so slow during stages I and K, now increases with great rapidity, 
 and forms the nucleus of the whole voluntary muscular system. 
 It extends upwards and downwards by the continuous conver- 
 sion of fresh cells of the splanchnic layer into muscle-cells. At 
 the same time it grows rapidly in thickness, but it requires some 
 little patience and care to unravel the details of this growth; 
 and it will be necessary to enter on a slight digression as to 
 the relations of the muscle-plates to the surrounding connective 
 tissue. 
 
 As the muscle-plates grow dorsalwards and ventralwards 
 their ends dive into the general connective tissue, whose origin 
 has already been described (PI. 13, fig. i). At the same time 
 the connective-tissue cells, which by this process become situ- 
 ated between the ends of the muscle-plates and the skin, grow 
 upwards and downwards, and gradually form a complete layer 
 separating the muscle-plates from the skin. The cells forming 
 
 1 I believe that Gotte, amongst his very numerous valuable remarks in the 
 Enhvickhmgsgeschichte der Unke, has put forward a view similar to this, though I 
 cannot put my hand on the reference. 
 
THE MUSCLE-PLATES. 369 
 
 the ends of the muscle-plates retain ' unaltered their primitive 
 undifferentiated character, and the separation between them and 
 the surrounding connective-tissue cells is very marked. This 
 however ceases to be the case in the parts of the muscle-plates 
 on a level with the notochord and lower part of the mecluttary 
 canal; the thinnest sections and most careful examination are 
 needed to elucidate the changes taking place in this region. 
 The cells which form the somatic layer of the muscle-plates then 
 begin to elongate and become converted into muscle-cells, at the 
 same time that they are increasing in number to meet the rapid 
 demands upon them. One result of these changes is the loss of 
 the original clearness in the external boundary between the 
 muscle-plates and the adjoining connective-tissue cells, which is 
 only in exceptional cases to be seen so distinctly as it may be 
 in PI. 13, figs, i and 8. Longitudinal horizontal sections are 
 the most instructive for studying the growth of the muscles, but 
 transverse sections are also needed. The interpretation of the 
 transverse ones is however rendered difficult, both by rapid 
 alterations in the thickness of the connective-tissue layer between 
 the skin and the muscle-plates (shewn in PI. 13, fig. 8), and by 
 the angular shape of the muscle-plates themselves. 
 
 A careful study of both longitudinal and transverse sections 
 has enabled me to satisfy myself of the fact that the cells of the 
 somatic layer of the protovertebrae, equally with the cells of the 
 splanchnic layer, are converted into muscle-cells, and some of 
 these are represented in the act of undergoing this conversion in 
 PI. 13, fig. 8; but the difficulty of distinguishing the outline of 
 the somatic layer of the muscle-plates, at the time its cells 
 become converted into muscle-cells, renders it very difficult to 
 determine whether any cells of this layer join the surrounding 
 connective tissue. General considerations certainly lead me to 
 think that they do not; but my observations do not definitely 
 settle the point. 
 
 From these facts it is clear, as was briefly stated in the last 
 chapter, that both layers of the muscle-plate are concerned in 
 forming the great lateral muscle, though the splanchnic layer is 
 converted into muscles very much sooner than the somatic 1 . 
 
 1 The difference between Dr Gbtte's account of the development of the muscles 
 and my own consists mainly in my attributing to the somatic layer of the muscle-plates 
 
3/O DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The remainder of the history of the muscle-plates presents 
 no points of special interest. 
 
 Till the close of stage L, the muscle-plates are not distinctly 
 divided into dorsal and ventral segments, but this division, which 
 is so characteristic of the adult, commences to manifest itself 
 during stage M, and is quite completed in the succeeding stage. 
 It is effected by the appearance, nearly opposite the lateral line, 
 of a layer of connective tissue which divides the muscles on each 
 side into a dorso-lateral and ventro-lateral section. Even during 
 stage O the ends of the muscle-plates are formed of undiffer- 
 entiated columnar cells. The peculiar outlines of the inter- 
 muscular septa gradually appear during the later stages of 
 development, causing the well-known appearances of the mus- 
 cles in transverse sections, but require no special notice here. 
 
 With reference to the histological features of the develop- 
 ment of the muscle-fibres, I have not pushed my investigations 
 very far. The primitive cells present the ordinary division, well 
 known since Remak, into a striated portion and a non-striated 
 portion, and in the latter a nucleus is to be seen which soon 
 undergoes division and gives rise to several nuclei in the non- 
 striated part, while the striated part of each cell becomes divided 
 up into a number of fibrilla:. I have not however determined 
 what exact relation the original cells hold to the eventual 
 primitive bundles, or anything with reference to the development 
 of the sarcolemma. 
 
 The Muscles of the Limbs. These are formed during stage O 
 coincidently with the cartilaginous skeleton, in the form of two 
 bands of longitudinal fibres on the dorsal and ventral surfaces of 
 the limbs. Dr Kleinenberg first called my attention to the fact 
 that he had proved the limb-muscles in Lacerta to be derived 
 from the muscle-plates. This I at first believed did not hold 
 good for Elasmobranchs, but have since determined that it does 
 so. Between stages K and L the muscle-plates grow downwards 
 as far as the limbs and then turn outwards and grow into them 
 
 a share in the formation of the great lateral muscles, which he denies to it. In an 
 earlier section of this Monograph, pp. 333, 334, too much stress was unintentionally 
 laid on the divergence of our views ; a divergence which appears to have, in part at 
 least, arisen, not from our observations being opposed, but from Dr Gotte's having 
 taken the highly differentiated Bombinator as his type instead of the less differentiated 
 Elasmobranch, 
 
THE VERTEBRAL COLUMN. 371 
 
 (PL 1 8, fig. i). Small portions of several muscle-plates come in 
 this way to be situated in the limbs, and are very soon seg- 
 mented off from the remainder of the muscle-plates. The por- 
 tions of muscle-plates thus introduced into the limbs soon lose 
 their original distinctness, and can no longer be recognized in 
 stage L. There can however be but little doubt that they 
 supply the tissue for the muscles of the limbs. The muscle-plates 
 themselves after giving off these buds to the limbs grow down- 
 wards, and by stage L cease to shew any trace of what has 
 occurred (PL 13, fig. i). This fact, coupled with the late develop- 
 ment of the muscles of the limbs (stage O), caused me to fall 
 into my original error. 
 
 T/te Vertebral Column and Notochord. 
 
 In the previous chapter (p. 325) an account was given of the 
 origin of the tissue destined to form the vertebral bodies; it 
 merely remains to describe the changes undergone by this in 
 becoming converted into the permanent vertebrae. 
 
 This subject has already been dealt with by a considerable 
 number of anatomists, and my investigations coincide in the 
 main with the results of my predecessors. Especially the re- 
 searches of Gegenbaur 1 may be singled out as containing the 
 pith of the whole subject, and my results, while agreeing in all 
 but minor points with his, do not supplement them to any very 
 great extent. I cannot do more than confirm Gotte's 2 account 
 of the development of the haemal arches, and may add that 
 Cartier 3 has given a good account of the later development of 
 the centra. Under the circumstances it has not appeared to me 
 to be worth while recording with great detail my investigations ; 
 but I hope to be able to give a somewhat more complete history 
 of the whole subject than has appeared in any single previous 
 memoir. 
 
 At their first appearance the cells destined to form the per- 
 manent vertebrae present the same segmentation as the muscle- 
 
 1 Das Kopfskelet d. Selachier, p. 123. 
 
 3 Entivicklungsgeschichte d. Unke, pp. 433 4. 
 
 3 Zeitschrift f. Wiss. Anat. Bd. XXV., Supplement. 
 
372 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 plates. This segmentation soon disappears, and between stages 
 K and L the tissue of the vertebral column forms a continuous 
 investment of the notochord which cannot be distinguished from 
 the adjoining connective tissue. Immediately surrounding the 
 notochord a layer formed of a single row of cells may be ob- 
 served, which is not however very distinctly marked 1 . 
 
 During the stage L there appear four special concentrations 
 of mesoblastic tissue adjoining the notochord, two of them 
 dorsal and two of them ventral. They are not segmented, and 
 form four ridges seated on the sides of the notochord. They 
 are united with each other by a delicate layer of tissue, and 
 constitute the rudiments of the neural and haemal arches. In 
 longitudinal sections of stage L special concentrated wedge- 
 shaped masses of tissue are to be seen between the muscle- 
 plates, which must not be confused with these rudiments. 
 Immediately around the notochord the delicate investment of 
 cells previously mentioned, is still present. 
 
 The rudiments of the arches increase in size and distinct- 
 ness in the succeeding stages, and by stage N have unques- 
 tionably assumed the constitution of embryonic cartilage. In 
 the meantime there has appeared surrounding the sheath of 
 the notochord a well-marked layer of tissue which stains deeply 
 with haematoxylin, and with the highest power may be observed 
 to contain flattened nuclei. It is barely thicker than the ad- 
 joining sheath, but is nevertheless the rudiment of the vertebral 
 bodies. PI. 13, fig. 9, vb. Whence does this layer arise? To 
 this question I cannot give a quite satisfactory answer. It is 
 natural to conclude that it is derived from the previously existing 
 mesoblastic investment of the notochord, but in the case of the 
 vertebral column I have not been able to prove this. Observa- 
 tions on the base of the brain afford fairly conclusive evidence 
 that the homologous tissue present there has this origin. Gegen- 
 baur apparently answers the question of the origin of this layer 
 in the way suggested above, and gives a figure in support of his 
 conclusion (PI. XXII. fig. 3)*. 
 
 1 Vide pp. 356, 357. 
 
 * None of my specimens resembles this figure, and the layer when first formed is 
 in my embryos much thinner than represented by Gegenbaur, and the histological 
 structure of the embryonic cartilage is very different from that of the cartilage in the 
 
THE NEURAL AND H.EMAL ARCHES. 373 
 
 The layer of tissue which forms the vertebral bodies rapidly 
 increases in thickness, and very soon, at a somewhat earlier 
 period than represented in Gegenbaur's PI. XXII. fig. 4, a distinct 
 membrane (Kolliker's Membrana Elastica Externa) may easily 
 be recognized surrounding it and separating it from the adjoin- 
 ing tissue of the arches. Gegenbaur's figure gives an excellent 
 representation of the appearance of this layer at the period 
 under consideration. It is formed of a homogeneous basis 
 containing elongated concentrically arranged nuclei, and con- 
 stitutes a uniform unsegmented investment for the notochord 
 (vide PI. 13, fig. 10). 
 
 The neural and hsemal arches now either cease altogether 
 to be united with each other by a layer of embryonic cartilage, 
 or else the layer uniting them is so delicate that it cannot be 
 recognized as true cartilage. They have moreover by stage P 
 undergone a series of important changes. The tissue of the 
 neural arches does not any longer form a continuous sheet, but 
 is divided into (i) a series of arches encircling the spinal cord, 
 and 1.2) a basal portion resting on the cartilaginous sheath of 
 the notochord. There are two arches to each muscle-plate, one 
 continuous with the basal portion of the arch-tissue and forming 
 the true arch, which springs opposite the centre of a vertebral 
 body, and the second not so continuous, which forms what is 
 usually known as the intercalated piece. Between every pair 
 of true arches the two roots of a single spinal nerve pass out. 
 The anterior root passes out in front of an intercalated piece and 
 the posterior behind it 1 . 
 
 The basal portion of the arch-tissue likewise undergoes 
 differentiation into a vertebral part continuous with the true arch 
 and formed of hyaline cartilage, and an intervertebral segment 
 formed of a more fibrous tissue. 
 
 The haemal arches, like the neural arches, become divided 
 into a layer of tissue adjoining the cartilaginous sheath of the 
 notochord, and processes springing out from this opposite the 
 
 figures alluded to. Gotte's very valuable researches with reference to the origin of 
 this layer in Amphibians tend to confirm the view advocated in the text. 
 
 1 In the adult Scyllium it is well known that the posterior root pierces the inter- 
 calated cartilage and the anterior root the true neural arch. This however does not 
 seem to be the case in the embryo at stage 1'. 
 
374 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 centres of the vertebrae. These processes throughout the region 
 of the trunk in front of the anus pass into the space between the 
 dorsal and ventral muscles, and are to be regarded as rudiments 
 of ribs. The tissue with which they are continuous, which is 
 exactly equivalent to the tissue from which the neural arches 
 originate, is not truly a part of the rib. In the tail, behind the 
 anus and kidneys, the cardinal veins fuse to form an unpaired 
 caudal vein below the aorta, and in this part a fresh series of 
 processes originates on each side from the haemal tissue adjoining 
 the cartilaginous sheath of the notochord, and eventually, by the 
 junction of the processes of the two sides, a canal which contains 
 the aorta and caudal vein is formed below the notochord. These 
 processes for a few segments coexist with small ribs (vide PI. 13, 
 fig. 10), a fact which shews (i) that they cannot be regarded as 
 modified ribs, and (2) that the tissue from which they spring is 
 to be viewed as a kind of general basis for all the haemal pro- 
 cesses which may arise, and is not specially connected with any 
 one set of processes. 
 
 While these changes (all of which are effected during stage 
 P) are taking place in the arches, the tissue of the vertebral 
 bodies or cartilaginous investment of the notochord, though 
 much thicker than before, still remains as a continuous tube 
 whose wall exhibits no segmental differentiations. 
 
 It is in stage Q that these differentiations first appear in the 
 vertebral regions opposite the origin of the neural arches. The 
 outermost part of the cartilage at these points becomes hyaline 
 and almost undistinguishable in structure from the tissue of the 
 arches 1 . These patches of hyaline cartilage grow larger and cause 
 the vertebral parts of the column to constrict the notochord, 
 whilst the intervertebral parts remain more passive, but become 
 composed of cells with very ' little intercellular substance. 
 Coincidently also with these changes, part of the layer internal 
 to the hyaline cartilage becomes modified to form a somewhat 
 peculiar tissue, the intercellular substance of which does not 
 stain, and in which calcification eventually arises (PI. 13, fig. 11). 
 The innermost layer adjoining the notochord retains its primitive 
 
 1 A good representation of a longitudinal section at this stage is given by Cartier 
 (Zeitschrift f. Wiss. Zoologie, Bd. xxv., Supplement PI. iv. fig. i), who also gives a 
 fair description of the succeeding changes of the vertebral column. 
 
THE NOTOCHORD. 375 
 
 fibrous character, and is distinguishable as a separate layer through 
 both the vertebral and the intervertebral regions. As a result of 
 these changes a transverse section through the centre of the 
 vertebral regions now exhibits three successive rings (vide PL 
 13, fig. 1 1), an external ring of hyaline cartilage invested by "-the 
 membrana elastica externa " (m.el), followed by a ring of calcify- 
 ing cartilage, and internal to this a ring of fibrous cartilage, 
 which adjoins the now slightly constricted notochord. A trans- 
 verse section of an intervertebral region shews only a thick outer 
 and thin inner ring of fibrous cartilage, the latter in contact with 
 the sheath of the unconstricted notochord. 
 
 The constriction of the notochord proceeds till in the centre 
 of the vertebrae it merely forms a fibrous band. The tissue 
 internal to the calcifying cartilage then becomes hyaline, so that 
 there is formed in the centre of each vertebral body a ring of 
 hyaline cartilage immediately surrounding the fibrous band which 
 connects the two unconstricted segments of the notochord. The 
 intervertebral tissue becomes more and more fibrous. In Cartier's 
 paper before quoted there is a figure (fig. 3) which represents 
 the appearance presented by a longitudinal section of the verte- 
 bral column at this stage. 
 
 The relation of the vertebral bodies to the arches requires a 
 short notice. The vertebral hyaline cartilage becomes almost 
 precisely similar to the tissue of the arches, and the result is, 
 that were it not for the " membrana elastica externa " it would 
 be hardly possible to distinguish the limits of the two tissues. 
 This membrane however persists till the hyaline cartilage has 
 become a very thick layer (PI. 13, fig. 11), but I have failed 
 to detect it in the adult, so that I cannot there clearly dis- 
 tinguish the arches from the body of the vertebrae. From a 
 comparison however of the adult with the embryo, it is clear 
 that the arches at most form but a small part of what is usually 
 spoken of as the body of the vertebrae. 
 
 The changes in the notochord itself during the stages sub- 
 sequent to K are not of great importance. The central part 
 retains for some time its previous structure, being formed of 
 large vacuolated cells with an occasional triangular patch of 
 protoplasm containing the starved nucleus and invested by 
 indurated layers of protoplasm. These indurated layers are all 
 
3/6 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 fused, and are probably rightly regarded by Gegenbaur and 
 Gotte as representing a sparse intercellular matter. The external 
 protoplasmic layer of the notochord ceases shortly after stage K 
 to exhibit any traces of a division into separate cells, but forms 
 a continuous layer with irregular prominences and numerous 
 nuclei (PI. 13, fig. 9). In the stages subsequent to P further 
 changes take place in the notochord : the remains of the cells 
 become more scanty and the intercellular tissue assumes 1 a 
 radiating arrangement, giving to sections of the notochord the 
 appearance of a number of lines radiating from the centre to the 
 periphery (PI. 13, fig. n). 
 
 The sheath of the notochord at first grows in thickness, and 
 during stage L there is no difficulty in seeing in it the fine radial 
 markings already noticed by Miiller 1 and Gegenbaur 2 , and re- 
 garded by them as indicating pores. Closely investing the sheath 
 of the notochord there is to be seen a distinct membrane, which, 
 though as a rule closely adherent to the sheath, in some examples 
 separates itself from it. It is perhaps the membrane identified 
 by W. Miiller 3 (though not by Gegenbaur) as Kolliker's " mem- 
 brana elastica interna." After the formation of the cartilaginous 
 investment of the notochord, this membrane becomes more 
 difficult to see than in the earlier stage, though I still fancy that 
 I have been able to detect it. The sheath of notochord also 
 appears to me to become thinner, and its radial striation is 
 certainly less easy to detect 4 . 
 
 EXPLANATION OF PLATE 13. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 al. Alimentary tract, ao. Aorta, c. Connective tissue, ca v. Cardinal vein. 
 ch. Notochord. ep. Epiblast ha. Haemal arch. /. Liver. //. Lateral line, m c. 
 Mucous canal of the head. mel. Membrana elastica externa. nip. Muscle-plate. 
 mp'. Muscles of muscle-plate, na. Neural arch. nl. Nervus lateralis. rp. Rib 
 process, s d. Segmental duct. sh. Sheath of notochord. spc. Spinal cord, sp g. 
 Spinal ganglion, syg. Sympathetic ganglion, um. Ductus choledochus. v. Blood- 
 
 1 Jenaische Zeitschrift, Vol. VI. 2 Loc. cit. * Loc. cit. 
 
 4 Gegenbaur makes the reserve statement with reference to the sheath of the noto- 
 chord. For my own sections the statement in the text certainly holds good. For- 
 tunately the point is one of no importance. 
 
PLATE I 3 . 377 
 
 vessel, var. Vertebral arch. vb. Vertebral body, vcati. Caudal vein. vin. In- 
 testinal branch of the vagus, v op. Ramus ophthalmicus of the fifth nerve, x. Sub- 
 notochordal rod. 
 
 Fig. i. Section through the anterior part of an embryo of Scy ilium canicula 
 during stage L. 
 
 c. Peculiar large cells which are found at the dorsal part of the spinal cord. 
 Sympathetic ganglion shewn at syg. Zeiss A, ocul. i. 
 
 Fig. 2. Section through the lateral line at the time of its first formation. 
 
 The cells marked / were not sufficiently distinct to make it quite certain that 
 they really formed part of the lateral nerve. Zeiss B, ocul. 2. 
 
 Figs. 3, 3<$, ^c, 3</. Four sections of the lateral line from an embryo belonging 
 to stage L. 30 is the most anterior. In ^a the lateral nerve (/) is seen to lie in the 
 mesoblast at some little distance from the lateral line. In 3 b and 3 c it lies in 
 immediate contact with and partly enclosed by the modified epiblast cells of the 
 lateral line. In 3 d, the hindermost section, the lateral line is much larger than in 
 the other sections, but no trace is present of the lateral nerve. The sections were 
 taken from the following slides of my series of the embryo (the series commencing at 
 the tail end) 3^ (46), $c (64), 3 b (84), 30 (93). The figures all drawn on the same 
 scale, but 3 is not from the same side of the body as the other sections. 
 
 Fig. 4. Section through lateral line of an embryo of stage P at the point where 
 it is acquiring an opening to the exterior. The peculiar modified cells of its inner- 
 most part deserve to be noticed. Zeiss D, ocul. i. 
 
 Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus 
 growing towards them. Stage O. Zeiss A, ocul. 2. 
 
 Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus growing 
 towards them. Stage between O and P. Zeiss aa, ocul. i. 
 
 Fig- 7- Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. T.. 
 
 Fig. 8. Longitudinal and horizontal section through the muscle-plates and adjoin- 
 ing structures at a stage intermediate between L and M. The section is intended to 
 shew the gradual conversion of the cells of the somatic layer of muscle-plates into 
 muscles. 
 
 Fig. 9. Longitudinal section through the notochord and adjoining parts to shew 
 the first appearance of the cartilaginous notochordal sheath which forms the vertebral 
 centra. Stage N. 
 
 Fig. 10. Transverse section through the tail of an embryo of stage P to shew the 
 coexistence of the rib-process and haemal arches in the first few sections behind the 
 point where the latter appear. Zeiss C, ocul. i . 
 
 Fig. 1 1 . Transverse section through the centre of a caudal vertebra of an embryo 
 somewhat older than Q. It shews (i) the similarity between the arch-tissue and the 
 hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the 
 two by the membrana elastica externa 1 (met). It shews also the differentiation of 
 three layers in the vertebral centrum : vide p. 374. 
 
 1 The slight difference observable between these two tissues in the arrangement of their nuclei has 
 been much exaggerated by the engraver. 
 
 B. 25 
 
CHAPTER VIII. 
 
 DEVELOPMENT OF THE SPINAL NERVES AND OF THE 
 SYMPATHETIC NERVOUS SYSTEM. 
 
 The spinal nerves. 
 
 THE development of the spinal nerves has been already 
 treated by me at considerable length in a paper read before 
 the Royal Society in December, 1875*, and I have but little 
 fresh matter to add to the facts narrated in that paper. The 
 succeeding account, though fairly complete, is much less full 
 than the previous one in the Philosophical Transactions, but a 
 number of morphological considerations bearing on this sub- 
 ject are discussed. 
 
 The rudiments of the posterior roots make their appearance 
 considerably before those of the anterior roots. They arise 
 during stage I, as outgrowths from the spinal cord, at a time 
 when the muscle-plates do not extend beyond a third of the way 
 up the sides of the spinal cord, and in a part where no scattered 
 mesoblast-cells are present. They are formed first in the 
 anterior part of the body and successively in the posterior parts, 
 in the following way. At a point where a spinal nerve is about 
 to arise, the cells of the dorsal part of the cord begin to pro- 
 liferate, and the uniform outline of the cord becomes broken 
 (PI. 14, fig. 3). There is formed in this way a small prominence 
 of cells springing from the summit of the spinal cord, and con- 
 stituting a rudiment of a pair of posterior roots. In sections 
 anterior to the point where a nerve is about to appear, the nerve- 
 rudiments are always very distinctly formed. Such a section is 
 shewn in PI. 14, fig. 2, and the rudiments may there be seen 
 
 1 Phil. Trans. Vol. 166, p. 175. [This Edition, No. vni.] 
 
THE SPINAL NERVES. 379 
 
 as two club-shaped masses of cells, 'which have grown outwards 
 and downwards from the extreme dorsal summit of the neural 
 canal and in contact with its walls. The rudiments of the two 
 sides meet at their point of origin at the dorsal median line, 
 and are dorsally perfectly continuous with the walls -of the 
 canal. 
 
 It is a remarkable fact that rudiments of posterior roots 
 are to be seen in every section. This may be interpreted as 
 meaning that the rudiments are in very close contact with each 
 other, but more probably means, as I hope to shew in the sequel, 
 that there arises from the spinal cord a continuous outgrowth 
 from which discontinuous processes (the rudiments of posterior 
 roots) grow out. 
 
 After their first formation these rudiments grow rapidly 
 ventralwards in close contact with the spinal cord (vide PI. 14, 
 fig. i, and PI. u, figs. 6 and 7), but soon meet with and become 
 partially enclosed in the mesoblastic tissue (PI. u, fig. 7). The 
 similarity of the mesoblast and nerve-tissue in Scyllium and 
 Pristiurus embryos hardened in picric or chromic acid, render 
 the nerves in these genera, at the stage when they first become 
 enveloped in mesoblast, difficult objects to observe ; but no 
 similar difficulty is encountered in the case of Torpedo embryos. 
 
 While the rudiments of the posterior roots are still quite 
 short, those of the anterior roots make their first appearance. 
 Each of these (PI. 14, fig. 4 a. r.) arises as a very small but dis- 
 tinct conical outgrowth from a ventral corner of the spinal cord. 
 From the very first the rudiments of the anterior roots have an 
 indistinct form of peripheral termination and somewhat fibrous 
 appearance, while the protoplasm of which they are composed 
 becomes attenuated towards its end. The points of origin of 
 the anterior roots from the spinal cord are separated by con- 
 siderable intervals. In this fact, and also in the fact of the 
 nerves of the two sides never being united with each other in 
 the median line, the anterior roots exhibit a marked contrast to 
 the posterior. There are thus constituted, before the close of 
 stage I, the rudiments of both the anterior and posterior roots of 
 the spinal nerves. The rudiments of both of these take their 
 origin from the involuted epiblast of the neural canal, and the 
 two roots of each spinal nerve are at first quite unconnected 
 
 25 2 
 
380 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 with each other. It is scarcely necessary to state that the pairs 
 of roots correspond in number with the muscle-plates. 
 
 It is not my intention to enter with any detail into the 
 subsequent changes of the rudiments whose origin has been 
 described, but a few points especially connected with their early 
 development are sufficiently important to call for attention. 
 
 One feature of the posterior roots at their first formation 
 is the fact that they appear as processes of a continuous out- 
 growth of the spinal cord. This state of affairs is not of long 
 continuance, and before the close of stage I each posterior root 
 has a separate junction with the spinal cord. What then be- 
 comes of the originally continuous outgrowth ? It has not been 
 possible for me to trace the fate of this step by step ; but the 
 discovery that at a slightly later period (stage K) there is present 
 a continuous commissure independent of the spinal cord connect- 
 ing the dorsal and central extremities of all the spinal nerves, 
 renders it very probable that the original continuous outgrowth 
 becomes converted into this commissure. Like all the other 
 nervous structures, this commissure is far more easily seen in 
 embryos hardened in a mixture of osmic and chromic acids or 
 osmic acid, than in those hardened in picric acid. Its existence 
 must be regarded as one of the most remarkable results of my 
 researches upon the Elasmobranch nervous system. At stage K 
 it is fairly thick, though it becomes much thinner at a slightly 
 later period. Its condition during stage K is shewn in PI. 12, 
 fig. 1 8, com. What it has been possible for me to make out of its 
 eventual fate is mentioned subsequently 1 . 
 
 A second feature of the earliest condition of the posterior 
 roots is their attachment to the extreme dorsal summit of the 
 spinal cord a point of attachment very different from that 
 which they eventually acquire. Before the commencement of 
 stage K this state of things has become altered ; and the pos- 
 terior roots spring from the spinal cord in the position normal 
 for Vertebrates. 
 
 This apparent migration caused me at first great perplexity, 
 
 1 It is not by any means always possible to detect this commissure in transverse 
 sections. As I have suggested, in connection with a similar commissure connecting 
 the vagus branches, it perhaps easily falls out of the section, and is always so small 
 that the hole left would certainly be invisible. 
 
THE SPINAL NERVES. 381 
 
 and I do not feel quite satisfied that I have yet got completely 
 to the bottom of its meaning. The explanation which appears 
 to me most probable has suggested itself in the course of some 
 observations on the development of the thin roof of the fourth 
 ventricle. A growth of cells appears to take place in the median 
 dorsal line of the roof of the spinal cord. This growth tends to 
 divaricate the two lateral parts of the cord, which are originally 
 contiguous in the dorsal line, and causes therefore the posterior 
 roots, which at first spring from the dorsal summit, to assume 
 an apparent attachment to the side of the cord at some little 
 distance from the summit. If this is the true explanation of 
 the change of position which takes place, it must be regarded 
 as due rather to peculiar growths in the spinal cord, than to any 
 alteration in the absolute attachment of the nerves. 
 
 By stage K the rudiment of the posterior root has become 
 greatly elongated, and exhibits a division into three distinct 
 portions (PI. 14, fig. 6) : 
 
 (1) A proximal portion, in which is situated the pedicle of 
 attachment to the wall of the neural canal. 
 
 (2) An enlarged portion, which may conveniently from its 
 future fate be called the spinal ganglion. 
 
 (3) A distal portion beyond this. 
 
 The proximal portion presents a fairly uniform diameter, and 
 ends dorsally in a rounded expansion; it is attached, remarkably 
 enough, not by its extremity, but by its side, to tlie spinal cord. 
 The dorsal extremities of the posterior roots are therefore free. 
 It seems almost certain that the free dorsal extremities of these 
 roots serve as the starting points for the dorsal commissure 
 before mentioned, which connects the roots together. The 
 attachment of the posterior nerve-root to the spinal cord is, 
 on account of its small size, very difficult to observe. In 
 favourable specimens there may however be seen a distinct 
 cellular prominence from the spinal cord, which becomes con- 
 tinuous with a small prominence on the lateral border of the 
 nerve-root near its distal extremity. The proximal extremity 
 of the rudiment is composed of cells, which, by their small size 
 and circular form, are easily distinguished from those which 
 form the succeeding or ganglionic portion of the nerve. This 
 succeeding part has a swollen configuration, and is composed 
 
382 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of large elongated cells with oval nuclei. The remainder of the 
 rudiment forms the commencement of the true nerve. 
 
 The anterior root, which, at the close of stage I, formed a 
 small and inconspicuous prominence from the spinal cord, 
 grows rapidly during the succeeding stages, and soon forms an 
 elongated cellular structure with a wide attachment to the spinal 
 cord (PL 14, fig. 5). At first it passes obliquely and nearly 
 horizontally outwards, but, before reaching the muscle-plate of 
 its side, takes a bend downwards (PL 14, fig. 7). 
 
 I have not definitely made out when the anterior and pos- 
 terior roots unite, but this may easily be seen to take place 
 before the close of stage K (PL 12, fig. 18). 
 
 One feature of some interest with reference to the anterior 
 roots, is the fact that they arise not vertically below, but alter- 
 nately with the dorsal roots, a condition which persists in the 
 adult. 
 
 Although I have made some efforts to determine the even- 
 tual fate of the commissure uniting the dorsal roots, these have 
 not hitherto been crowned with success. It grows thinner and 
 thinner, becoming at the same time composed of fibrous pro- 
 toplasm with imbedded nuclei (PL 14, figs. 8 and 9). By stage 
 M it is so small as to be quite indistinguishable in transverse 
 sections ; and I have failed in stage P to recognize it at all. I 
 can only conclude that it gradually atrophies, and finally 
 vanishes without leaving a trace. Both its appearance and 
 history are very remarkable, and deserve the careful attention 
 of future investigators. 
 
 There can be little doubt that it is some sort of remnant of 
 an ancestral structure in the nervous system ; and it would 
 appear to indicate that the central nervous system must origi- 
 nally have been formed of a median and two lateral strands. 
 At the same time I very much doubt whether it can be brought 
 into relation with the three rows of ganglion-cells (a median and 
 two lateral) which are so frequently present on the ventral side 
 of annelidan nerve-cords. 
 
 My results may be summarised as follows: Along the ex- 
 treme dorsal summit of the spinal cord there arises on each 
 side a continuous outgrowth. From each outgrowth processes 
 corresponding in number to the muscle-plates grow downwards. 
 
THE SPINAL NERVES. 383 
 
 These are the rudiments of the posterior nerve-roots. The 
 outgrowths, though at first attached to the spinal cord through- 
 out their whole length, soon cease to be so, and remain in 
 connection with it at certain points only, which form the 
 primitive junctions of the posterior roots with the spinal cord. 
 The original outgrowth on each side remains as a bridge, 
 uniting together the dorsal extremities of all the posterior roots. 
 The posterior roots, though primitively attached to the dorsal 
 summit of the spinal cord, eventually come to arise from its 
 sides. The original homogeneous rudiments before the close of 
 stage K become differentiated into a root, a ganglion, and a 
 nerve. 
 
 The anterior roots, like the posterior, are outgrowths from 
 the spinal cord, but are united independently with it, and the 
 points from which they spring originally, remain as those by 
 which they are permanently attached. The anterior roots arise, 
 not vertically below, but in the intervals between the posterior 
 roots. They are at first quite separate from the posterior roots ; 
 but before the close of stage K a junction is effected between 
 each posterior root and the corresponding anterior root. The 
 anterior root joins the posterior at some little distance below its 
 ganglion. 
 
 The results here arrived at are nearly in direct opposition to 
 those of the majority of investigators, though in accordance, at 
 least so far as the posterior roots are concerned, with the 
 beautiful observations of Hensen 'on the Development of Mam- 
 malia 1 .' 
 
 Mr Marshall 2 has more recently published a paper on the 
 development of the nerves in Birds, in which he shews in a 
 most striking manner that the observations recorded here for 
 Elasmobranchs hold good for the posterior roots of Birds. The 
 similarity between his figures and my own is very noticeable. 
 A further discussion of the literature would be quite unprofit- 
 able, and I proceed at once to certain considerations suggested 
 by the above observations. 
 
 1 Zeit. f. Anat. u. Enhvicklungsgeschichtc, Vol. I. 
 
 - Journal of Anatomy and Physiology, Vol. xi. April, 18771 
 
384 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 General considerations. One point of general anatomy upon 
 which my observations throw considerable light, is the primitive 
 origin of nerves. So long as it was admitted that the spinal 
 and cerebral nerves developed in the embryo independently 
 of the central nervous system, their mode of origin always 
 presented to my mind considerable difficulties. It never ap- 
 peared clear how it was possible for a state of things to have 
 arisen in which the central nervous system as well as the 
 peripheral terminations of nerves, whether motor or sensory, 
 were formed independently of each other ; while between them 
 a third structure was developed, which, growing out either 
 towards the centre or towards the periphery, ultimately brought 
 the two into connection. That such a condition could be a 
 primitive one seemed scarcely possible. 
 
 Still more remarkable did it appear, on the supposition that 
 the primitive mode of formation of these parts was represented 
 in the developmental history of Vertebrates, that we should find 
 similar structural elements in the central and in the peripheral 
 nervous systems. The central nervous system arises from the 
 epiblast, and yet contains precisely similar nerve-cells and nerve- 
 fibres to the peripheral nervous system, which, when derived 
 from the mesoblast, was necessarily supposed to have an origin 
 completely different from that of the central nervous system. 
 Both of these difficulties are to a great extent removed by the 
 facts of the development of these parts in Elasmobranchs. 
 
 It is possible to suppose that in their primitive differentia- 
 tion contractile and sensory systems may, as in Hydra 1 , have 
 been developed from the protoplasm of even the same cell. 
 As the sensory and motor systems became more complicated, the 
 sensory portion of a cell would become separated by an in- 
 creasing interval from the muscular part of a cell, and the two 
 parts of a cell would only be connected by a long protoplasmic 
 process. When such a condition as that was reached, the 
 sensory portion of the cell would be called a ganglion-cell or 
 terminal sensory organ, the connecting process a nerve, and the 
 contractile portion of the cell a muscle-cell. When these organs 
 were in this condition, it might not impossibly happen for the 
 general developmental growth which tended to separate the 
 
 1 Kleinenberg Hydra. 
 
ORIGIN OF NERVES. 385 
 
 ganglion-cell and the muscle-cell to be so rapid as to render it 
 impossible for the growth of the connecting nerve to keep pace 
 with it, and that thus the process connecting the ganglion-cell 
 and the muscle-cell might become ruptured. Nevertheless the 
 tendency of the process to grow from the ganglion-celhtxrthe 
 muscle-cell, would remain, and when the rapid developmental 
 growth had ceased, the two would become united again by the 
 growth of the process which had previously been ruptured. It 
 will be seen that this hypothesis, which I have considered only 
 with reference to a single nerve and muscle-cell, might be 
 extended so as to apply to a complicated central nervous system 
 and peripheral nerves and muscles, and also could apply equally 
 as well to the sensory as to the motor terminations of a nerve. 
 In the case of the sensory termination, we should only have to 
 suppose that the centre nervous cell became more and more 
 separated by the general growth from the recipient terminal 
 sensory cell, and that during the general growth the connection 
 between the two was mechanically ruptured but restored again 
 on the termination of the more rapid growth. 
 
 As the descendants of the animal in which the rupture 
 occurred became progressively more complicated, the two ter- 
 minal cells must have become widely separated at a continually 
 earlier period, till finally they may have been separated at a 
 period of development when they were indistinguishable from 
 the surrounding embryonic cells ; and since the rupture would 
 also occur at this period, the primitive junction between the 
 nerve-centre and termination would escape detection. The object 
 of this hypothesis is to explain the facts, so far as they are known, 
 of the development of the nervous system in Vertebrates. 
 
 In Vertebrates we certainly appear to have an outgrowth 
 from the nervous system, which eventually becomes united 
 with the muscle or sensory terminal organs. The ingenious 
 hypothetical scheme of development of the nerves given by 
 Hensen ' would be far preferable to the one suggested if it could 
 be brought into conformity with the facts. There is, however, 
 at present no evidence for Hensen's view, as he himself admits, 
 but considering how little we know of the finer details of the 
 development of nerves, it seems not impossible that such 
 
 1 Virchow's Archiv, Vol. XXXI. 1864. 
 
386 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 evidence may be eventually forthcoming. The evidence from 
 my own observation is, so far as it goes, against it. At a time 
 anterior to the outgrowth of the spinal nerves, I have shewn 1 
 that the spinal cord is completely invested by a delicate hyaline 
 membrane. It is difficult to believe that this is pierced by a 
 number of fine processes, which completely escape detection, but 
 which must, nevertheless, be present on the hypothesis of Hensen. 
 
 The facts of the development of nerves in Vertebrates are 
 unquestionably still involved in considerable doubt. It may, 
 I think, be considered as certain, that in Elasmobranchs the 
 roots of the spinal and cranial nerves are outgrowths of the 
 central nervous system. How the final terminations of the 
 nerves are formed is, however, far from being settled. Gotte 2 , 
 whose account of the development of the spinal ganglia is com- 
 pletely in accordance with the ordinary views, yet states 3 that 
 the growth of the nerve fibres themselves is a centrifugal one 
 from the ganglia. My own investigations prove that the ganglia 
 have a centrifugal development, and also appear to demonstrate 
 that the nerves themselves near the ganglion have a similar 
 manner of growth. Moreover, the account given in the pre- 
 ceding chapter of the manner in which the nerves become con- 
 nected with the mucous canals of the head, goes far to prove 
 that the whole growth of the nerves is a centrifugal one. The 
 combination of all these converging observations tells strongly 
 in favour of this view. 
 
 On the other hand, Calberla 4 believes that in the tails of 
 larval Amphibians he has seen connective-tissue cells unite with 
 nerve-processes, and become converted into nerves, but he ad- 
 mits that he cannot definitely prove that the axis-cylinder has 
 not a centrifugal growth, while the connective-tissue cells merely 
 become converted into the sheath of the nerve. If Calberla's 
 view be adopted, that the nerves are developed directly out of a 
 chain of originally indifferent cells, each cell of the chain being 
 converted in turn into a section of the nerve, an altogether 
 different origin of nerves from that I have just suggested would 
 seem to be indicated. 
 
 1 Phil. Trans., 1876. [This Edition, No. vm.] 
 
 2 Entwicklungsgeschichte der Unke. 3 Loc. cit. p. 516. 
 4 Archiv fiir Micros. Anat. Vol. xi. 1875. 
 
VERTEBRATE AND ANNELIDAN NERVOUS SYSTEMS. 387 
 
 The obvious difficulty, already alluded to, of understanding 
 how it is, according to the generally accepted mode of develop- 
 ment of the spinal nerves, that precisely similar nerve-cells and 
 nerves should arise in structures which have such different 
 origins as the central nervous system and the spinal nerves, is 
 completely removed if my statements on the development of the 
 nerves in Elasmobranch represent the truth. 
 
 One point brought out in my investigations appears to me 
 to have bearings upon the origin of the central canal of the 
 vertebrate nervous system, and in consequence upon the origin 
 of the vertebrate nervous system itself. This point is, that the 
 posterior nerve-rudiments make their first appearance at the 
 extreme dorsal summit of the spinal cord. The transverse 
 section of the ventral nervous cord of an ordinary segmented 
 Annelid consists of two symmetrical halves placed side by side. 
 If by a mechanical folding the two lateral halves of the nervous 
 cord became bent towards each other, while into the groove 
 between the two the external skin became pushed, we should 
 have an approximation to the vertebrate nervous system. Such 
 a folding as this might take place to give extra rigidity to the 
 body in the absence of a vertebral column. 
 
 If this folding were then completed in such a way that the 
 groove, lined by external skin and situated between the two 
 lateral columns of the nervous system, became converted into 
 a canal, above and below which the two columns of the nervous 
 system united, we should have in the transformed nervous cord 
 an organ strongly resembling the spinal cord of Vertebrates. 
 
 It is well known that the nerve-cells are always situated on 
 the ventral side of the abdominal nerve-cord of Annelids, either 
 as a continuous layer, or in the form of two, or more usually, 
 three bands. The dorsal side of the cord is composed of nerve- 
 fibres or white matter. If the folding I have supposed were to 
 take place in the Annelid nervous cord, the grey and white 
 matters would have very nearly the same relative situations as 
 they have in the Vertebrate spinal cord. The grey matter would 
 be situated in the interior and line the central canal, and the 
 white matter would nearly surround the grey. The nerves 
 would then arise, not from the sides of the nervous cord as in 
 existing Annelids, but from its extreme ventral summit One 
 
388 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 of the most striking features which I have brought to light with 
 reference to the development of the posterior roots, is the fact 
 of their growing out from the extreme dorsal summit of the 
 neural canal, a position analogous to the ventral summit of the 
 Annelidan nervous cord. Thus the posterior roots of the nerves 
 in Elasmobranchs 1 arise, in the exact manner which might have 
 been anticipated, were the spinal canal due to such a folding as 
 I have suggested. 
 
 The argument from the position of the outgrowth of nerves 
 becomes the more striking from its great peculiarity, and forms 
 a feature which would be most perplexing without some such 
 explanation as I have proposed. The central epithelium of the 
 neural canal, according to this view, represents the external 
 skin, and its ciliation in certain cases may, perhaps, be ex- 
 plained as a remnant of the ciliation of the external skin still 
 found amongst many of the lower Annelids. 
 
 I have employed the comparison of the Vertebrate and 
 Annelidan nervous cords, not so much to prove a genetic rela- 
 tion between the two, as to shew the a priori possibility of the 
 formation of a spinal cord, and the a posteriori evidence we 
 have of the vertebrate canal having been formed in the way 
 indicated. I have not made use of what is really my strongest 
 argument, viz. that the embryological mode of formation of the 
 spinal canal by a folding in of the external epiblast is the very 
 method by which I supposed the spinal canal to have been 
 formed in the ancestors of Vertebrates. My object has been to 
 suggest a meaning for the peculiar primitive position of the 
 posterior roots, rather than to attempt to explain in full the 
 origin of the spinal canal. 
 
 Although the homologies between the Vertebrate and the 
 Annelidan nervous systems are not necessarily involved in the 
 questions which arise with reference to the formation of the 
 spinal canal, they have nevertheless considerable bearings on it. 
 
 Two views have recently been put forward on this subject. 
 
 1 There are strong reasons for regarding the posterior roots as the primitive ones. 
 These are spoken of later, but I may state that they depend : 
 
 (1) On the fact that only posterior roots exist in the brain. 
 
 (2) That only posterior roots exist in Amphioxus. 
 
 (3) That the posterior roots develop at an earlier period than the anterior. 
 
DR DOHRN'S HYPOTHESIS. 389 
 
 Professor Gegenbaur 1 looks upon the central nervous system of 
 Vertebrates as equivalent to the superior cesophageal ganglia 
 of Annelids and Arthropods only, while Professors Leydig 2 and 
 Semper 3 and Dr Dohrn 4 compare it with the whole Annelidan 
 nervous system. 
 
 The first of these two views is only possible on the suppo- 
 sition that Vertebrates are descended from unsegmented an- 
 cestors, and even then presents considerable difficulties. If the 
 ancestors of Vertebrates were segmented animals, and several of 
 the recent researches tend to shew that they were, they must 
 almost certainly have possessed a nervous cord like that of ex- 
 isting Annelids. If such were the case, it is almost inconceivable 
 that the greater portion of the nervous system which forms the 
 ventral cord can have become lost, and the system reduced to 
 the superior cesophageal ganglia. Dr Dohrn 5 , who has specu- 
 lated very profoundly on this matter, has attempted to explain ' 
 and remove some of the difficulties which arise in comparing 
 the nervous systems of Vertebrates and Annelids. He supposes 
 that the segmented Annelids, from which Vertebrates are de- 
 scended, were swimming animals. He further supposes that 
 their alimentary canal was pierced by a number of gill-slits, 
 and that the anterior amongst these served for the introduction 
 of nutriment into the alimentary canal, in fact as supplementary 
 mouths as well as for respiration. Eventually the old mouth 
 and throat atrophied, and one pair of coalesced gill-slits came 
 to serve as the sole mouth. Thus it came about that on the 
 disappearance of that portion of the alimentary canal, which 
 penetrated the cesophageal nervous ring, the latter structure 
 ceased to be visible as such, and no part of the alimentary 
 
 1 Grundriss d. vergleichenden Anat. p. 264. 
 
 2 Bmi des thierischen Korpers. 
 
 * Stammesvenuandschaft d. Wirbelthiere u. Wirbellosen and Die Venvandschafts- 
 beziehungen d. gegliederten Thiere. This latter work, for a copy of which I return my 
 best thanks to the author, came into my hands after what follows was written, and I 
 much regret only to have been able to make one or two passing allusions to it. The 
 work is a most important contribution to the questions about to be discussed, and 
 contains a great deal that is very suggestive ; some of the conclusions with reference 
 to the Nervous System appear to me however to be directly opposed to the observa- 
 tions on Spinal Nerves above recorded. 
 
 4 Ursprung d. Wirbelthiere u. Princip des Functionswechsds. 
 
 5 Loc. at. 
 
39O DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 canal was any longer enclosed by a commissure of the central 
 nervous system. With the change of mouth Dr Dohrn also sup- 
 poses that there took place a change, which would for a swim- 
 ming animal be one of no great difficulty, of the ventral for the 
 dorsal surface. This general explanation of Dr Dohrn's, apart 
 from the considerable difficulty of the fresh mouth, appears to 
 me to be fairly satisfactory. Dr Dohrn has not however in my 
 opinion satisfactorily dealt with the questions of detail which 
 arise in connection with this comparison. One of the most 
 important points for his theory is to settle the position where 
 the nervous system was formerly pierced by the oesophagus. 
 This position he fixes in the fourth ventricle, and supports his 
 hypothesis by the thinness of the roof of the spinal canal in this 
 place, and the absence (?) of nervous structures in it. 
 
 It appears to me that this thinness cannot be used as an 
 argument. In the first place, if the hypothesis I have suggested 
 as to the formation of the spinal canal be accepted, the forma- 
 tion of the canal must be supposed to have occurred in point 
 of time either after or before the loss of the primitive mouth. 
 If, on the one hand, the spinal canal made its appearance be- 
 fore the atrophy of the primitive mouth, the folding to form it 
 must necessarily have ceased behind the mouth ; and, on the 
 supposition of the cesophageal ring having been situated in the 
 region of the fourth ventricle, a continuation of the spinal canal 
 could not be present in front of this part. If, on the other 
 hand, the cerebro-spinal canal appeared after the disappearance 
 of the primitive mouth, its roof must necessarily also be a 
 formation subsequent to the atrophy of the mouth, and varieties 
 of structure in it can have no bearing upon the previous position 
 of the mouth. 
 
 But apart from speculations upon the origin of the spinal 
 cord, there are strong arguments against Dr Dohrn's view about 
 the fourth ventricle. In the first place, were the fourth ventricle 
 to be the part of the nervous system which previously formed 
 the cesophageal commissures, we should expect to find the 
 opening in the nervous system at this point to be visible at an 
 early period of development, and at a later period to cease to be 
 so. The reverse is however the case. In early embryonic life 
 the roof of the fourth ventricle is indistinguishable from other 
 
HOMOLOGIES OF THE VERTEBRATE NERVOUS SYSTEM. 39! 
 
 parts of the nervous system, and only thins out at a later 
 period. Further than this, any explanation of the thin roof of 
 the fourth ventricle ought also to elucidate the nearly similar 
 structure in the sinus rhomboidalis, and cannot be considered 
 satisfactory unless it does so. 
 
 The peculiarities of the cerebro-spinal canal in the region 
 of the brain appear to me to present considerable difficulties in 
 the way of comparing the central nervous system of Vertebrates 
 and segmented Annelids. The manner in which the cerebro- 
 spinal canal is prolonged into the optic vesicles, the cerebral and 
 the optic lobes is certainly opposed both to an intelligible expla- 
 nation of the spinal canal itself, and also to a comparison of the 
 two nervous systems under consideration. 
 
 Its continuation into the cerebral hemispheres and into the 
 optic lobes (mid-brain) may perhaps be looked upon as due to 
 peculiar secondary growths of those two ganglia, but it is very 
 difficult to understand its continuation into the optic vesicles. 
 
 If it be granted that the spinal canal has arisen from a 
 folding in of the external skin, then the present inner surface of 
 the optic vesicle must also have been its original outer surface, 
 and it follows as a necessary consequence that the present 
 position of the rods and cones behind and not in front of the 
 nervous structures of the retina was not the primitive one. The 
 rods and cones arise, as is well known, from the inner surface of 
 the outer portion of the optic vesicle, and must, according to 
 the above view, be supposed originally to have been situated 
 on the external surface, and have only come to occupy their 
 present position during the folding in, which resulted in the 
 spinal canal. On a priori grounds we should certainly expect 
 the rods and cones to have resulted from the differentiation of 
 a layer of cells external to the conducting nervous structures. 
 The position of the rods and cones posterior to these suggests 
 therefore that some peculiar infolding has occurred, and may be 
 used as an argument to prove that the medullary groove is no 
 mere embryonic structure, but the embryonic repetition of an 
 ancestral change. The supposition of such a change of position 
 in the rods and cones necessarily implies that the folding in 
 to form the spinal canal must have been a very slow one. It 
 must have given time to the refracting media of the eye 
 
392 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 gradually to travel round, so as still to maintain their primitive 
 position, while in successive generations a rudimentary spinal 
 furrow carrying with it the retina became gradually converted 
 into a canal 1 . 
 
 If Dr Dohrn's comparison of the vertebrate nervous system 
 with that of segmented Annelids be accepted, the following two 
 points must in my opinion be admitted : 
 
 (1) That the formation of the cerebro-spinal canal was sub- 
 sequent to the loss of the old mouth. 
 
 (2) That the position of the old mouth is still unknown. 
 The well-known view of looking at the pituitary and pineal 
 
 growths as the remnants of the primitive oesophagus, has no 
 doubt some features to recommend it. Nearly conclusive against 
 it is the fact that the pituitary involution is not, as used to be 
 supposed, a growth towards the infundibulum of the hypoblast 
 of the oesophagus, but of the epiblast of the mouth. It is almost 
 inconceivable that an involution from the present mouth can 
 have assisted in forming part of the old oesophagus. 
 
 There is a view not involving the difficulty of the cesophageal 
 ring, fresh mouth 2 , and of the change of the ventral to the dorsal 
 
 1 Professor Huxley informs me that he has for many years entertained somewhat 
 similar views to those in the text about the position of the rods and cones, and has 
 been accustomed to teach them in his lectures. 
 
 2 Professor Semper ("Die Verwandtschaftsbeziehungen d. gegliederten Thiere," 
 Arbeiten aus d. Zool.-zoot. Institut, Wiirzburg, 1876) has some interesting speculations 
 on the difficult question of the vertebrate mouth, which have unfortunately come to 
 my knowledge too late to be either fully discussed or incorporated in the text. These 
 speculations are founded on a comparison of the condition of the mouth in Turbel- 
 larians and Nemertines. He comes to the conclusion that there was a primitive 
 mouth on the cardiac side of the supra-resophageal ganglion, which is the existing 
 mouth of Turbellarians and Vertebrates and the opening of the proboscis of Nemer- 
 tines, but which has been replaced by a fresh mouth on the neural side in Annelids 
 and Nemertines. In Nemertines however the two mouths co-exist the vertebrate 
 mouth as the opening of the proboscis, and the Annelid mouth as the opening for the 
 alimentary tract. This ingenious hypothesis is supported by certain anatomical facts, 
 which do not appear to me of great weight, but for which the reader must refer 
 to the original paper. It no doubt avoids the difficulty of the present position of the 
 vertebrate mouth, but unfortunately at the same time substitutes an equal difficulty in 
 the origin of the Annelidan mouth. This Professor Semper attempts to get over by 
 an hypothesis which to my mind is not very satisfactory (p. 378), which, however, 
 and this Professor Semper does not appear to have noticed, could equally well be 
 employed to explain the origin of a Vertebrate mouth as a secondary formation subse- 
 quent to the Annelidan mouth. Under these circumstances this fresh hypothesis does 
 
ORIGIN OF THE VERTEBRATE NERVOUS SYSTEM. 393 
 
 surface, which, though so far unsupported by any firm basis of 
 observed facts, nevertheless appears to me worth suggesting. It 
 assumes that Vertebrates are descended not through the present 
 line of segmented Vermes, but through some other line which 
 has now, so far as is known, completely vanished. Thw line 
 must be supposed to have originated from the same unsegmented 
 Vermes as the present segmented Annelids. They therefore 
 acquired fundamentally similar segmental and other Annelidan 
 organs. 
 
 The difference between the two branches of the Vermes lay 
 in the nervous system. The unsegmented ancestors of the 
 present Annelids seem to have had a pair of super-cesophageal 
 ganglia, from which two main nervous stems extended back- 
 wards, one on each side of the body. Such a nervous system in 
 fact as is possessed by existing Nemertines or Turbellarians 1 . 
 As the Vermes became segmented and formed the Annelids, 
 these side nerves seem to have developed ganglia, corresponding 
 in number with the segments, and finally, approximating on the 
 ventral surface, to have formed the ventral cord 2 . 
 
 The other branch of Vermes which I suppose to have been 
 the ancestors of Vertebrates started from the same stock as 
 existing Annelids, but I conceive the lateral nerve-cords, instead 
 of approximating ventrally, to have done so dorsally, and thus a 
 dorsal cord to have become formed analogous to the ventral cord 
 of living Annelids, only without an cesophageal nerve-ring 3 . 
 
 not bring us very much nearer to a solution of the vertebrate-annelid mouth question, 
 but merely substitutes one difficulty for another; and does not appear to me so satis- 
 factory as the hypothesis suggested in the text. 
 
 At the same time Professor Semper's hypothesis suggests an explanation of that 
 curious organ the Nemertine proboscis. If the order of changes suggested by him 
 were altered it might be possible to suppose that there never was more than one 
 mouth for all Vermes, but that the proboscis in Nemertines gradually split itself off 
 from the oesophagus to which it originally belonged, and became quite free and pro- 
 vided with a separate opening and perhaps carried with it the so-called vagus of 
 Professors Semper and Leydig. 
 
 1 It is not of course to be supposed that the primitive nervous system was pierced 
 by a proboscis like that of the Nemertines. 
 
 2 This is Gegenbaur's view of the development of the ventral cord, and I regard 
 it in the meantime as the most probable view which has been suggested. 
 
 3 A dorsal instead of a ventral approximation of the lateral nerve-cords would be 
 possible in the descendants of such living segmented Vermes as Saccocirrus and Poly- 
 gordius. 
 
 B. 26 
 
394 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 It appears to me, (if the difficulties of comparing the 
 Annelidan ventral cord with the spinal cord of Vertebrates are 
 found to be insurmountable), that this hypothesis would involve 
 far fewer improbabilities than one which supposes the whole 
 central nervous system of Vertebrates to be homologous with 
 the super-oesophageal ganglia. The mode of formation of a 
 nervous system presupposed in my hypothesis, well accords with 
 what we know of the formation of the ventral cord in existing 
 Annelids. 
 
 The supposition of the existence of another branch of seg- 
 mented Vermes is not a very great difficulty. Even at the 
 present day we have possibly more than one branch of Vermes 
 which have independently acquired segmentation, viz.: the 
 Chaetopodous Annelids and the Hirudinea. If the latter is an 
 isolated branch, it is especially interesting from having inde- 
 pendently developed a series of segmental organs like those of 
 Chaetopodous Annelids, which we must suppose the ancestors of 
 Vertebrates also to have done if they too form an independent 
 branch. 
 
 In addition to the difficulty of imagining a fresh line of 
 segmented Vermes, there is another difficulty to my view, viz. : 
 the fact that in almost all Vermes, the blood flows forwards 
 in the dorsal vessel, and backwards in the ventral vessel. This 
 condition of the circulation very well suits the view of a change 
 of the dorsal for the ventral surfaces, but is opposed to these 
 surfaces being the same for Vertebrates and Vermes. I cannot 
 however regard this point as a very serious difficulty to my view, 
 considering how undefined is the circulation in the unsegmented 
 groups of the Vermes. 
 
 Sympathetic nervous system. 
 
 Between stages K and L there may be seen short branches 
 from the spinal nerves, which take a course towards the median 
 line of the body, and terminate in small irregular cellular masses 
 immediately dorsal to the cardinal veins (PL 18, fig. i, sy. g.}. 
 These form the first traces that have come under my notice of 
 the sympathetic nervous system. In the youngest of my embryos 
 in which I have detected these it has not been possible for me 
 
SYMPATHETIC NERVOUS SYSTEM. 395 
 
 either definitely to determine the antero-posterior limits of the 
 system, or to make certain whether the terminal masses of cells 
 which form the ganglia are connected by a longitudinal com- 
 missure. In a stage slightly younger than L the ganglia are 
 much more definite, the anterior one is situated in the cardiac 
 region close to the end of the intestinal branch of the vagus, and 
 the last of them quite at the posterior end of the abdominal 
 cavity. The anterior ganglia are the largest ; the commissural 
 cord, if developed, is still very indistinct. In stage L the com- 
 missural cord becomes definite, though not very easy to see even 
 in longitudinal sections, and the ganglia become so considerable 
 as not to be easily overlooked. They are represented in PI. 13, 
 % l > s y- S- a d in PL 1 8, fig. 2, in the normal position immediately 
 above the cardinal veins. The branches connecting them with 
 the trunks of the spinal nerves may still be seen without difficulty. 
 In later stages these branches cannot so easily be made out in 
 sections, but the ganglia themselves continue as fairly conspicuous 
 objects. The segmental arrangement of the ganglia is shewn in 
 PL 1 8, fig. 3, a longitudinal and vertical section of an embryo 
 between stages L and M with the junctions of the sympathetic 
 ganglia and spinal nerves. The ganglia occupy the intervals 
 between the successive segments of the kidneys. 
 
 The sympathetic system only came under my notice at a 
 comparatively late period in my investigations, and the above 
 facts do not in all points clear up its development 1 . My obser- 
 vations seem to point to the sympathetic system arising as an 
 off-shoot from the cerebrospinal system. Intestinal branches 
 would seem to be developed on the main nerve stems of this in 
 the thoracic and abdominal regions, each of these then developes 
 a ganglion, and the ganglia become connected by a longitudinal 
 commissure. On this view a typical spinal nerve has the follow- 
 ing parts: two roots, a dorsal and ventral, the dorsal one 
 ganglionated, and three main branches, (i) a ramus dorsalis, 
 (2) a ramus ventralis, and (3) a ramus intestinalis. This scheme 
 may be advantageously compared with that of a typical cranial 
 nerve according to Gegenbaur. It may be noted that it brings 
 
 1 The formation out of the sympathetic ganglia of the so-called paired suprarenal 
 bodies is dealt with in connection with the vascular system. The original views of 
 Leydig on these bodies are fully borne out by the facts of their development. 
 
 26 2 
 
396 DEVELOPMENT OF ELASMOBRANCII FISHES. 
 
 the sympathetic nervous system into accord with the other 
 parts of the nervous system as a product of the epiblast, and 
 derived from outgrowths from the neural axis. It is clear, how- 
 ever, that my investigations, though they may naturally be 
 interpreted in this way, do not definitely exclude a completely 
 different method of development for the sympathetic system. 
 
 EXPLANATION OF PLATE 14. 
 
 This Plate illustrates the Formation of the Spinal Nerves. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 a r. Anterior root of a spinal nerve, ch. Notochord. com. Commissure connect- 
 ing the posterior roots of the spinal nerves, i. Mesoblastic investment of spinal cord. 
 m p. Muscle-plate. n. Spinal nerve, nc. Neural canal, pr. Posterior root of a 
 spinal nerve, spg. Ganglion on posterior root of spinal nerve, v r. Vertebral rudi- 
 ment, w. White matter of spinal cord. y. Point where the spinal cord became 
 segmented off from the superjacent epiblast. 
 
 Figs, i , 2, and 3. Three sections of a Pristiurus embryo belonging to stage 1. 
 Fig. i passes through the heart, fig. 2 through the anterior part of the dorsal region, 
 fig. 3 through a point slightly behind this. (Zeiss CC, ocul. 2.) In fig. 3 there is 
 visible a slight proliferation of cells from the dorsal summit of the neural canal. In 
 fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (pr) the 
 rudiments of the posterior nerve-roots, both attached to the dorsal summit of the 
 spinal cord. In fig. r the rudiments of the posterior roots are of considerable length. 
 
 Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older 
 than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the 
 formation of a pair of dorsal nerve-rudiments (pr) and a ventral nerve-rudiment (a r). 
 The latter is shewn in its youngest condition, and is not distinctly cellular. 
 
 Fig- 5- Section through the dorsal region of a Torpedo embryo slightly younger 
 than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The 
 rudiment of the ganglion (spg) on the posterior root has appeared, and the junction of 
 posterior root with the cord is difficult to detect. The anterior root forms an elonga- 
 ted cellular structure. 
 
 Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage K. 
 (Zeiss CC, ocul. 2.) The section especially illustrates the attachment of the posterior 
 root to the spinal cord. 
 
 Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. r.) The 
 section contains an anterior root, which takes its origin at a point opposite the interval 
 between two posterior roots. 
 
 Fig. 8. A series of posterior roots with their central ends united by a dorsal 
 commissure, from a longitudinal and vertical section of a Scyllium embryo belonging 
 to a stage intermediate between L and M. The embryo was hardened in a mixture 
 of osmic and chromic acids. 
 
 Fig. 9. The central end of a posterior nerve-root from the same embryo, with the 
 commissure springing out from it on either side. 
 
CHAPTER IX. 
 THE DEVELOPMENT OF THE ORGANS IN THE HEAD. 
 
 The Development of the Brain. 
 
 General History. In stage G the brain presents a very simple 
 constitution (PI. 8, fig. G),and is in tact little more than a dilated 
 termination to the cerebro-spinal axis. Its length is nearly one- 
 third that of the whole body, being proportionately very much 
 greater than in the adult. 
 
 It is divided by very slight constrictions into three lobes, 
 the posterior of which is considerably the largest. These are 
 known as the fore-brain, the mid-brain, and the hind-brain. 
 The anterior part of the brain is bent slightly downwards about 
 an axis passing through the mid-brain. The walls of the brain, 
 composed of several rows of elongated columnar cells, have a 
 fairly uniform thickness, and even the roof of the hind-brain 
 is as thick as any other part. Towards the end of stage G 
 the section of the hind-brain becomes somewhat triangular with 
 the apex of the triangle directed downwards. 
 
 In Pristiurus during stage H no very important changes take 
 place in the constitution of the brain. In Scyllium, however, 
 indications appear in the hind-brain of its future division into a 
 cerebellum and medulla oblongata. The cavity of the anterior 
 part dilates and becomes rounded, while that of the posterior 
 part assumes in section an hour-glass shape, owing to an increase 
 in the thickness of the lateral parts of the walls. At the same 
 time the place of the original thick roof is taken by a very thin 
 layer, which is formed not so much through a change in the 
 character and arrangements of the cells composing the roof, as 
 
398 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 by a divarication of the two sides of the hind-brain, and the 
 simultaneous introduction of a fresh structure in the form of a 
 thin sheet of cells connecting dorsally the diverging lateral halves 
 of this part of the brain. By stage I, the hind-brain in Pristiurus 
 also acquires an hour-glass shaped section, but the roof has 
 hardly begun to thin out (PL 15, figs. 4^ and 4^). 
 
 During stages I and K the cranial flexure becomes more and 
 more pronounced, and causes the mid-brain definitely to form 
 the termination of the long axis of the embryo (PI. 15, figs. I, 2, 
 etc.), and before the close of stage K a thin coating of white 
 matter has appeared on the exterior of the whole brain, but no 
 other histological changes of interest have occurred. 
 
 During stage L an apparent rectification of the cranial flexure 
 commences, and is completed by stage Q. The changes involved 
 in this process may be advantageously studied by comparing 
 the longitudinal sections of the brain during stages L, P, and Q, 
 represented in PI. 16, figs, la, 5 and /. 
 
 It will be seen, first of all, that so far from the flexure of the 
 brain itself being diminished, it is increased, and in P (fig. 5) 
 the angle in the floor of the mid-brain becomes very acute 
 indeed ; in other words, the anterior part of the brain has been 
 bent upon the posterior through nearly two right angles, and the 
 infundibulum, or primitive front end of the brain, now points 
 nearly directly backwards. At the same time the cerebral hemi- 
 spheres have grown directly forwards, and if figures la and 
 5 in PI. 1 6 be compared it will be seen that in the older brain of 
 the two the cerebral hemispheres have assumed a position which 
 might be looked on as the result of their having been pushed 
 dorsalwards and forwards against the mid-brain, and Having in 
 the process pressed in and nearly obliterated the original thala- 
 mencephalon. The thalamencephalon in fig. \a, belonging to 
 stage L, is relatively large, but in fig. 5, belonging to stage P, it 
 only occupies a very small space between the front wall of the 
 mid-brain and the hind wall of the cerebral hemispheres. It is 
 therefore in part by the change in position of the cerebral hemi- 
 spheres that the angle between the trabeculae and parachordals 
 becomes increased, i.e. their flexure diminished, while at the 
 same time the flexure of the brain itself is increased. More 
 important perhaps in the apparent rectification of the cranial 
 
THE FORE-BRAIN. 399 
 
 flexure than any of the previously mentioned points, is the 
 appearance of a bend in the hind-brain which tends to correct 
 the original cranial flexure. The gradual growth of this fresh 
 flexure can be studied in the longitudinal sections which have 
 been represented. It is at its maximum in stage Q. This_short 
 preliminary sketch of the development of the brain as a whole 
 will serve as an introduction to the history of the individual 
 divisions of the brain. 
 
 Fore-brain. In its earliest condition the fore-brain forms 
 a single vesicle without a trace of separate divisions, but buds 
 off very early the optic vesicles, whose history is described with 
 that of the eye (PI. 15, fig. 3 op. v). Between stages I and K 
 the posterior part of the fore-brain sends outwards a papilliform 
 process towards the exterior, which forms the rudiment of the 
 pineal gland (PI. 15, fig. !,/) Immediately in front of the 
 rudiment a constriction appears, causing a division of the fore- 
 brain into a large anterior and a small posterior portion. This 
 constriction is shallow at first, but towards the close of stage K 
 becomes much deeper (PI. 15, fig. 2 and fig. i6#), leaving however 
 the two cavities of the two divisions of the fore-brain united 
 ventrally by a somewhat wide canal. 
 
 The posterior of the two divisions of the fore-brain forms 
 the thalamencephalon. Its anterior wall adjoining the cerebral 
 rudiment becomes excessively thin (PI. 15, fig. 11) ; and its base 
 till the close of stage K is in close contact with the mouth 
 involution, and presents but a very inconspicuous prominence 
 which marks the eventual position of the infundibulum (PI. 15, 
 figs, ga, 12, 1 6, in). The anterior and larger division of the fore- 
 brain forms the rudiment of the cerebral hemispheres and 
 olfactory lobes. Up to stage K this rudiment remains perfectly 
 simple, and exhibits no signs, either externally or internally, of a 
 longitudinal constriction into two lobes. From the canal uniting 
 the two divisions of the fore-brain (which eventually forms part 
 of the thalamencephalon) there spring the hollow optic nerves. 
 A slight ventral constriction separating the cerebral rudiment 
 from that part of the brain where these are attached appears 
 even before the close of stage K (PI. 15, fig. 1 1, op. ). 
 
 During stage L the infundibulum becomes much produced, 
 and forms a wide sack in contact with the pituitary body, and 
 
4OO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 its cavity communicates with that of the third ventricle by an 
 elongated slit-like aperture. This may be seen by comparing 
 PI. 16, figs. \a and ic. In fig. ic taken along the middle line, 
 there is present a long opening into the infundibulum (/;/), which 
 is shewn to be very narrow by being no longer present in fig. \a 
 representing a section slightly to one side of the middle line. 
 During the same stage the pineal gland grows into a sack-like 
 body, springing from the roof of the thalamencephalon, fig. ib,pn. 
 This latter (the thalamencephalon) is now dorsally separated 
 from the cerebral rudiment by a deep constriction, and also 
 ventrally by a less well marked constriction. At its side also a 
 deep constriction is being formed in it, immediately behind the 
 pineal gland. The cerebral rudiment is still quite unpaired and 
 exhibits no sign of becoming constricted into two lobes. 
 
 During the next two stages the changes in the fore-brain are 
 of no great importance, and I pass at once to stage O. The 
 infundibulum is now nearly in the same condition as during 
 stage L, though (as is well shewn in the figure of a longitudinal 
 section of the next stage) it points more directly backwards 
 than before. The remaining parts of the thalamencephalon 
 have however undergone considerable changes. The more im- 
 portant of these are illustrated by a section of stage O, PI. 16, 
 fig. 3, transverse to the long axis of the embryo, and therefore, 
 owing to the cranial flexure, cutting the thalamencephalon 
 longitudinally and horizontally; and for stage P in a longi- 
 tudinal and vertical section through the brain (PI. 16, fig. 5). 
 In the first place the roof of the thalamencephalon has become 
 very much shortened by the approximation of the cerebral 
 rudiment to the mid-brain. The pineal sack has also become 
 greatly elongated, and its somewhat dilated extremity is 
 situated between the cerebral rudiment and the external skin. 
 It opens into the hind end of the third ventricle, and its 
 posterior wall is continuous with the front wall of the mid- 
 brain. The sides of the thalamencephalon have become much 
 thickened, and form distinct optic thalami (op.} united by a very 
 well marked posterior commissure (pc.}. The anterior wall of 
 the thalamencephalon as well as its roof are very thin. The 
 optic nerves have become by stage O quite solid except at their 
 roots, into which the ventricles of the fore-brain are for a short 
 
THE CEREBRAL HEMISPHERES. 401 
 
 distance prolonged. This solidification is arrived at, so far as I 
 have determined, without the intervention of a fold. The 
 nerves are fibrous, and a commencement of the chiasma is 
 certainly present. From the chiasma there appears to pass out 
 on each side a band of fibres, which runs near the outer surface 
 of the brain to the base of the optic lobes (mid-brain), and here 
 the fibres of the two sides again cross. 
 
 By stage O important changes are perceptible in the cerebral 
 rudiment. In the first place there has appeared a slight fold at 
 its anterior extremity (PI. 16, fig. 3, x), destined to form a 
 vertical septum dividing it into two hemispheres, and secondly, 
 lateral outgrowths (vide PI. 16, fig. 2, ol. I), to form the olfactory 
 lobes. Its thin posterior wall presents on each side a fold which 
 projects into the central cavity. From the peripheral end of 
 each olfactory lobe a nerve similar in its histological con- 
 stitution to any other cranial nerve makes its appearance (PI. 16, 
 fig. 2) ; this divides into a number of branches, one of which 
 passes into the connective tissue between the two layers of 
 epithelium in each Schneiderian fold. On the root of this 
 nerve there is a large development of ganglionic cells. I have 
 not definitely observed its origin, but have no reason to doubt 
 that it is a direct outgrowth from the olfactory lobe, exactly 
 similar in its mode of development to any other nerve of the body. 
 
 The cerebral rudiment undergoes great changes during stage 
 P. In addition to a great increase in the thickness of its walls, 
 the fold which appeared in the last stage has grown backwards, 
 and now divides it in front into two lobes, the rudiments of the 
 cerebral hemispheres. The greater and posterior section is still 
 however quite undivided, and the cavities of the lobes (lateral 
 ventricles) though separated in front are still quite continuous 
 behind. At the same time, the olfactory lobes, each containing 
 a prolongation of the ventricle, have become much more pro- 
 nounced (vide PI. 1 6, figs. 40 and 4^ ol.l). The root of the 
 olfactory nerve is now very thick, and the ganglion cells it con- 
 tains are directly prolonged into the ganglionic portion of the 
 olfactory bulb ; in consequence of which it becomes rather 
 difficult to fix on the exact line of demarcation between the bulb 
 and the nerve. 
 
 Stage Q is the latest period in which I have investigated the 
 
4O2 DEVELOPMENT OF ELASiMOBRANCH FISHES. 
 
 development of the brain. Its structure is represented for this 
 stage in general view in PI. 16, figs. 6a, 6b, 6c, in longitudinal 
 section in PI. 16, figs, ja, jb, and in transverse section PL 16, 
 figs. 8a d. The transverse sections are taken from a some- 
 what older embryo than the longitudinal. In the thalamence- 
 phalon there is no fresh point of great importance to be noticed. 
 The pineal gland remains as before, and has become, if any- 
 thing, longer than it was, and extends further forwards over the 
 summit of the cerebrum. It is situated, as might be expected, 
 in the connective tissue within the cranial cavity (fig. Sa, pn], 
 and does not extend outside the skull, as it appears to do, 
 according to Gotte's investigations, in Amphibians. Gotte 1 
 compares the pineal gland with the long persisting pore which 
 leads into the cavity of the brain in the embryo of Amphioxus, 
 and we might add the Ascidians, and calls it "ein Umbildungs- 
 produkt einer letzten Verbindung des Hirns mit der Oberhaut" 
 This suggestion appears to me a very good one, though no facts 
 have come under my notice which confirm it. The sacci vas- 
 culosi are perhaps indicated at this stage in the two lateral 
 divisions of the trilobed ventricle of the infundibulum (fig. 8c). 
 
 The lateral ventricles (fig. 8a) are now quite separated by a 
 median partition, and a slight external constriction marks the 
 lobes of the two hemispheres ; these, however, are still united 
 by nervous structures for the greater part of their extent. The 
 olfactory lobes are formed of a distinct bulb and stalk (fig. Sa, 
 oU), and contain, as before, prolongations of the lateral ventricles. 
 The so called optic chiasma is very distinct (fig. 8b, op.ii), but 
 the fibres from the optic nerves appear to me simply to cross 
 and not to intermingle. 
 
 The mid-brain. The mid-brain is at first fairly marked off 
 from both the fore and hind brains, but less conspicuously from 
 the latter than from the former. Its roof becomes progressively 
 thinner and its sides thicker up to stage P, its cavity remaining 
 quite simple. The thinness of the roof gives it, in isolated 
 brains of stage P, a bilobed appearance (vide PI. 16, fig. 4$, mb, 
 in which the distinctness of this character is by no means 
 exaggerated). During stage Q it becomes really bilobed through 
 
 1 Ent. d. Unite, p. 304. 
 
THE HIND-BRAIN. 4 3 
 
 the formation in its roof of a shallow median furrow (PI. 16, fig. 
 8). Its cavity exhibits at the same time the indication of a 
 division into a central and two lateral parts. 
 
 The hind-brain. The hind-brain has at first a fairly uniform 
 structure, but by the close of stage I, the anterior part becomes 
 distinguished from the remainder by the fact, that its roof does 
 not become thin as does that of the posterior part. This anterior, 
 and at first very insignificant portion, forms the rudiment of 
 the cerebellum. Its cavity is quite simple and is continued 
 uninterruptedly into that of the remainder of the hind-brain. 
 The cerebellum assumes in the course of development a greater 
 and greater prominence, and eventually at the close of stage Q 
 overlaps both the optic lobes in front and the medulla behind 
 (PL 16, fig. "ja]. It exhibits in surface-views of the hardened 
 brain of stages P and Q the appearance of a median con- 
 striction, and the portion of the ventricle contained in it is 
 prolonged into two lateral outgrowths (PI. 16, figs. Sc and 
 84 cb\ 
 
 The posterior section of the hind-brain which forms the me- 
 dulla undergoes changes of a somewhat complicated character. 
 In the first place its roof becomes in front very much extended 
 and thinned out. At the raphe, where the two lateral halves 
 of the brain originally united, a separation, as it were, takes 
 place, and the two sides of the brain become pushed apart, 
 remaining united by only a very thin layer of nervous matter 
 (PI. 15, fig. 6, iv. v.). As a result of this peculiar growth in 
 the brain, the roots of the nerves of the two sides which were 
 originally in contact at the dorsal summit of the brain become 
 carried away from one another, and appear to rise at the sides 
 of the brain (PI. 15, figs. 6 and 7). Other changes also take 
 place in the walls of the brain. Each lateral wall presents two 
 projections towards the interior (PI. 15, fig. 50). The ventral 
 of these vanish, and the dorsal approximate so as nearly to 
 divide the cavity of the hind-brain, or fourth ventricle, into a 
 large dorsal and a small ventral channel (PI. 15, fig. 6), and 
 this latter becomes completely obliterated in the later stages. 
 The dorsal pair, while approximating, also become more promi- 
 nent, and stretch into the dorsal moiety of the fourth ventricle 
 (PI. 15, fig. 6). They are still very prominent at stage Q (PI. 16, 
 
404 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 fig. %d, ft], and correspond in position with the fasciculi teretes 
 of human anatomy. Part of the root of the seventh nerve 
 originates from them. They project freely in front into the 
 cavity of the fourth ventricle (PI. 16, fig. 7ft). 
 
 By stage Q restiform tracts are indistinctly marked off from 
 the remainder of the brain, and are anteriorly continued into the 
 cerebellum, of which they form the peduncles. Near their junction 
 with the cerebellum they form prominent bodies (PI. 16, fig. 7 a, 
 rt}, which are regarded by Miklucho-Maclay 1 as representing the 
 true cerebellum. 
 
 By stage O the medulla presents posteriorly, projecting into 
 its cavity, a series of lobes which correspond with the main roots 
 (not the branches) of the vagus and glosso-pharyngeal nerves 
 (PI. 17, fig. 5). There appear to me to be present seven or eight 
 projections : their number cannot however be quite certainly 
 determined. The first of them belongs to the root of the glosso- 
 pharyngeal, the next one is interposed between the glosso- 
 pharyngeal and the first root of the vagus, and is without any 
 corresponding nerve-root. The next five correspond to the 
 five main roots of the vagus. For each projection to which a 
 nerve pertains there is a special nucleus of nervous matter, from 
 which the root springs. These nuclei do not stain like the 
 remainder of the walls of the medulla, and stand out accordingly 
 very conspicuously in stained sections. 
 
 The coating of white matter which appeared at the end of 
 stage K, on the exterior of each lateral half of the hind-brain, 
 extends from a point just dorsal to the attachment of the nerve- 
 roots to the ventral edge of the medulla, and is specially con- 
 nected with the tissue of the upper of the two already described 
 projections into the fourth ventricle. 
 
 A rudiment of the tela vasculosa makes its appearance during 
 stage Q, and is represented by the folds in the wall of the fourth 
 ventricle in my figure of that stage (PI. 16, fig. ja, tv). 
 
 The development of the brain in Elasmobranchs has already 
 been worked out by Professor Huxley, and a brief but in many 
 respects very complete account of it is given in his recent paper 
 
 1 Das Gehirn d. Selachier, Leipzig, 1870. 
 
THE VIEWS OF MIKLUCHO-MACLAY. 405 
 
 on Ceratodus 1 . He says, pp. 30 and 31, " The development of 
 the cerebral hemispheres in Plagiostome Fishes differs from the 
 process by which they arise in the higher Vertebrata. In a very 
 early stage, when the first and second visceral clefts of the 
 embryo Scyllium are provided with only a few short branchial 
 filaments, the anterior cerebral vesicle is already distinctly divided 
 into the thalamencephalon (from which the large infundibulum 
 proceeds below, and the small tubular peduncle of the pineal 
 gland above, while the optic nerve leaves its sides) and a large 
 single oval vesicle of the hemispheres. On the ventral face of 
 the integument covering these are two oval depressions, the 
 rudimentary olfactory sacs. 
 
 " As development proceeds the vesicle of the hemispheres 
 becomes divided by the ingrowth of a median longitudinal septum, 
 and the olfactory lobes grow out from the posterior lateral regions 
 of each ventricle thus formed, and eventually rise on to the 
 dorsal faces of the hemispheres, instead of, as in most Vertebrata, 
 remaining on their ventral sides. I may remark, that I cannot 
 accept the views of Miklucho-Maclay, whose proposal to alter 
 the nomenclature of the parts of the Elasmobranch's brain, appears 
 to me to be based upon a misinterpretation of the (acts of develop- 
 ment." 
 
 The last sentence of the paragraph brings me to the one 
 part on which it is necessary to say a few words, viz. the views of 
 Miklucho-Maclay. His views have not received any general 
 acceptance, but the facts narrated in the preceding pages shew, 
 beyond a doubt, that he has 'misinterpreted' the facts of develop- 
 ment, and that the ordinary view of the homology of the parts is 
 the correct one. A comparison of the figures I have given of 
 the embryo brain with similar figures of the brain of higher 
 Vertebrates shews this point conclusively. Miklucho-Maclay 
 has been misled by the large size of the cerebellum, but, as we 
 have seen, this body does not begin to be conspicuous till late in 
 embryonic life. Amongst the features of the embryonic brain of 
 Elasmobranchs, the long persisting unpaired condition of the 
 cerebral hemisphere, upon which so much stress has already been 
 laid by Professor Huxley, appears to me to be one of great 
 
 1 Proceedings of the Zoological Society, 1876, Pt. I. pp. 30 and 31. 
 
406 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 importance, and may not improbably be regarded as a real 
 ancestral feature. Some observations have recently been pub- 
 lished by Professor B. G. Wilder * upon this point, and upon the 
 homologies and development of the olfactory lobes. Fairly good 
 figures are given to illustrate the development of the cerebral 
 hemispheres, but the conclusions arrived at are in part opposed 
 to my own results. Professor Wilder says : " The true hemi- 
 spheres are the lateral masses, more or less completely fused in 
 the middle line, and sometimes developing at the plane of union 
 a bundle of longitudinal commissural fibres. The hemispheres 
 retain their typical condition as anterior protrusions of the 
 anterior vesicle ; but they lie mesiad of the olfactory lobes, and 
 in Mustelus at least seem to be formed after them'' The italics 
 are my own. From what has been said above, it is clear that 
 the statement italicised, for Scyllium at least, completely reverses 
 the order of development. Still more divergent from my con- 
 clusions are Professor Wilder's statements on the olfactory lobes. 
 He says : " The true olfactory lobe, or rhinencephalon, seems, 
 therefore, to embrace only the hollow base of the crus, more 
 or less thickened, and more or less distinguishable from the main 
 mass as a hollow process. The olfactory bulb, with the more or 
 less elongated crus of many Plagiostomes, seems to be developed 
 independently, or in connection with the olfactory sack, as are 
 the general nerves ;" and again, " But the young and adult brains 
 since examined shew that the ventricle (i.e. the ventricle of the 
 olfactory lobe) ends as a rounded cul-de-sac before reaching the 
 ' lobe.'" 
 
 The majority of the statements contained in the above 
 quotations are not borne out by my observations. Even the 
 few preparations of which I have given figures, appear to me to 
 prove that (i) the olfactory lobes (crura and bulbs) are direct 
 outgrowths from the cerebral rudiment, and develope quite in- 
 dependently of the olfactory sack ; (2) that the ventricle of the 
 cerebral rudiment does not stop short at the base of the crus ; 
 (3) that from the bulb a nerve grows out which has a centrifugal 
 growth like other nerves of the body, and places the central 
 olfactory lobe in communication with the peripheral olfactory 
 
 1 "Anterior brain-mass with Sharks and Skates," American Journal of Science 
 and Arts, Vol. xil. 1876. 
 
THE OLFACTORY ORGAN. 407 
 
 sack. In some other Vertebrates this nerve seems hardly to be 
 developed, but it is easily intelligible, that if in the ordinary 
 course of growth the olfactory sack became approximated to the 
 olfactory lobe, the nerve which grew out from the latter to the 
 sack might become so short as to escape detection. 
 
 Organs of Sense. 
 
 T/te olfactory organ. The olfactory pit is the latest formed 
 of the three organs of special sense. It appears during a stage 
 intermediate between / and K, as a pair of slight thickenings of 
 the external epiblast, in the normal vertebrate position on the 
 under side of the fore-brain immediately in front of the mouth 
 (PI. 15, figs, i and 2, ol). 
 
 The epiblast cells which form this thickening are very co- 
 lumnar, but present no special peculiarities. Each thickened 
 patch of skin soon becomes involuted as a shallow pit, which 
 remains in this condition till the close of the stage K. The 
 epithelium very early becomes raised into a series of folds 
 (Schneiderian folds). These are bilaterally symmetrical, and 
 diverge like the barbs of a feather from a median line (PI. 15, 
 fig. 14). The nasal pits at the close of stage K are still separated 
 by a considerable interval from the walls of the brain, and no 
 rudiment of an olfactory lobe arises till a later period ; but a 
 description of the development of this as an integral part of the 
 brain has already been given, p. 401. 
 
 Eye. The eye does not present in its early development any 
 very special features of interest. The optic vesicles arise as 
 hollow outgrowths from the base of the fore-brain (PI. 15, fig. 
 3, op. v), from which they soon become partially constricted, and 
 form vesicles united to the base of the brain by comparatively 
 narrow hollow stalks, the rudiments of the optic nerves. The con- 
 striction to which the stalk or optic nerve is due takes place 
 from above and backwards, so that the optic nerves open into 
 the base of the front part of the thalamencephalon (PI. 15, fig. 
 130, op.n}. After the establishment of the optic nerves, there 
 take place the formation of the lens and the pushing in of the 
 anterior wall of the optic vesicle towards the posterior. 
 
408 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The lens arises in the usual vertebrate fashion. The epiblast 
 in front of the optic vesicle becomes very much thickened, and 
 then involuted as a shallow pit, which eventually deepens and 
 narrows. The walls of the pit are soon constricted off as a nearly 
 'spherical mass of cells enclosing a very small central cavity, in 
 some cases indeed so small as to be barely recognizable (PI. 15, 
 fig- 7> 0- The pushing in of the anterior wall of the optic vesicle 
 towards the posterior takes place in quite the normal manner ; 
 but, as has been already noticed by Gotte 1 and others, is not a 
 simple mechanical result of the formation of the lens, as is shewn 
 by the fact that the vesicle assumes a flattened form even before 
 the appearance of the lens. The whole exterior of the optic 
 cup becomes invested by mesoblast, but no mesoblastic cells grow 
 in between the lens and the adjoining wall of the optic c^^p. 
 
 Round the exterior of the lens, and around the exterior and 
 interior of the optic cup, there appear membrane-like structures, 
 similar to those already described round the spinal cord and 
 other organs. These membrane-like structures appear with a 
 varying distinctness, but at the close of stage K stand out with 
 such remarkable clearness as to leave no doubt that they are 
 not artificial products (PI. 15, fig. I3) 2 . They form the rudi- 
 ments of the hyaloid membrane and lens capsule. Similar, 
 though less well marked membranes, may often be seen lining 
 the central cavity of the lens and the space between the two 
 walls of the optic cup. The optic cup is at first very shallow, 
 but owing to the rapid growth of the free edge of its walls soon 
 becomes fairly deep. The growth extends to the whole circum- 
 ference of the walls except the point of entrance of the optic 
 nerve (PL 15, fig. 13^), where no growth takes place; here accord- 
 ingly a gap is left in the walls which forms the well-known 
 choroid slit. While this double walled cup is increasing in size, 
 the wall lining the cavity of the cup becomes thick, and the 
 outer wall very thin (fig. 1 30). No further differentiations arise 
 before the close of stage K. 
 
 The lens is carried outwards with the growth of the optic 
 cup, leaving the cavity of the cup quite empty. It also grows in 
 size, and its central cavity becomes larger. Still later its anterior 
 
 1 Entwicklungsgeschichte d. Unke. 
 
 2 The engraver has not been very successful in rendering these membranes. 
 
THE PROCESSUS FALCIFORMIS. 409 
 
 wall becomes very thin, and its posterior wall thick, and doubly 
 convex (fig. 1 3#). Its changes, however, so exactly correspond 
 to those already known in other Vertebrates, that a detailed 
 description of them would be superfluous. 
 
 No mesoblast passes into tJie optic cup round its edge, ~bttt a 
 process of mesoblast, accompanied by a blood-vessel, passes into 
 the space between the lens and the wall of the optic cup through 
 the choroid slit (fig. 1 3#, cli). This process of tissue is very easily 
 seen, and swells out on entering the optic cup into a mushroom- 
 like expansion. It forms the processus falciformis, and from it 
 is derived the vitreous humour. 
 
 About the development of the parts of the eye, subsequently 
 to stage K, I shall not say much. The iris appears during 
 stage O, as an ingrowing fold of both layers of the optic cup 
 with a layer of mesoblast on its outer surface, which tends to 
 close over the front of the lens. Both the epiblast layers com- 
 prising the iris are somewhat atrophied, and the outer one is 
 strongly pigmented. At stage O the mesoblast first also grows 
 in between the external skin and the lens to form the rudiment 
 of the mesoblastic structures of the eye in front of the lens. The 
 layer, when first formed, is of a great tenuity. 
 
 The points in my observations, to which I attach the 
 greatest importance, are the formation of the lens capsule and 
 the hyaloid membrane ; with the development of these may be 
 treated also that of the vitreous humour and rudimentary pro- 
 cessus falciformis. The development of these parts in Elasmo- 
 branchs has recently been dealt with by Dr Bergmeister 1 , and 
 his observations with reference to the vitreous humour and 
 processus falciformis, the discovery of which in embryo Elas- 
 mobranchs is due to him, are very complete. I cannot, however, 
 accept his view that the hyaloid membrane is a mesoblastic pro- 
 duct. Through the choroid slit there grows, as has been said, 
 a process of mesoblast, the processus falciformis, which on 
 entering the optic cup dilates, and therefore appears mushroom- 
 shaped in section. At the earliest stage (K) a blood-vessel 
 appeared in connection with it, but no vascular structure came 
 under my notice in the later stages. The structure of this 
 process during stage P is shewn in PI. 17, fig. 6, /. fal. ; it 
 
 1 "Emhryologie d. Coloboms," Sitz. d. k. Akad. IVien, Bd. LXXi. 1875. 
 B. 27 
 
4IO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 is there seen to be composed of mesoblast-cells with fibrous 
 prolongations. The cells, as has been noticed by Bergmeister, 
 form a special border round its dilated extremity. This pro- 
 cess is formed much earlier than the vitreous humour, which is 
 first seen in stage O. In hardened specimens this latter appears 
 either as a gelatinous mass with a meshwork of fibres or (as 
 shewn in PI. 17, fig. 6) with elongated fibres proceeding from 
 the end of the processus falciformis. These fibres are probably 
 a product of the hardening reagent, but perhaps represent some 
 preformed structure in the vitreous humour. I have failed to 
 detect in it any cellular elements. It is more or less firmly 
 attached to the hyaloid membrane. 
 
 On each side of the processus falciformis in stage P a slight 
 fold of the optic cup is to be seen, but folds so large as those 
 represented by Bergmeister have, never come under my notice, 
 though this may be due to my not having cut sections of such 
 late embryos as he has. The hyaloid membrane appears long 
 before the vitreous humour as a delicate basement membrane 
 round the inner surface of the optic cup (PI. 15, fig. 13^), which 
 is perfectly continuous with a similar membrane round the outer 
 surface. In the course of development the hyaloid membrane 
 becomes thicker than the membrane outside the optic cup, with 
 which however it remains continuous. This is very clear in my 
 sections of stage M. By stage C> the membrane outside the cup 
 has ceased to be distinguishable, but the hyaloid membrane 
 may nevertheless be traced to the very edge of the cup round 
 the developing iris ; but does not unite with the lens capsule. 
 It can also be traced quite to the junction of the two layers of 
 the optic cup at the side of the choroid slit (PI. 17, fig. 6, ky. m). 
 When the vitreous humour becomes artificially separated from 
 the retina, the hyaloid membrane sometimes remains attached 
 to the former, but at other times retains in preference its attach- 
 ment to the retina. My observations do not throw any light 
 upon the junction of the hyaloid membrane and lens capsule 
 to form the suspensory ligament, nor have I ever seen (as de- 
 scribed by Bergmeister) the hyaloid membrane extending across 
 the free end of the processus falciformis and separating the 
 latter from the vitreous humour. This however probably ap- 
 pears at a period subsequent to the latest one investigated by 
 
THE VITREOUS HUMOUR. 411 
 
 me. The lens capsule arises at about the same period as the 
 hyaloid membrane, and is a product of the cells of the lens. It 
 can be very distinctly seen in all the stages subsequent to its 
 first formation. The proof of its being a product of the epi- 
 blastic lens, and not of the mesoblast, lies mainly in the fact of 
 there being no mesoblast at hand to give rise to it at the time of 
 its formation, vide PI. 15, fig. 130. If the above observations 
 are correct, it is clear that the hyaloid membrane and lens 
 capsule are respectively products of the retina and lens ; so that 
 it becomes necessary to go back to the older views of Kolliker 
 and others in preference to the more modern ones of Lieberkuhn 
 and Arnold. It would take me too far from my subject to 
 discuss the arguments used by the later investigators' to main- 
 tain their view that the hyaloid membrane and lens capsule are 
 mesoblastic products ; but it will suffice to say that the con- 
 tinuity of the hyaloid membrane over the pecten in birds is no 
 conclusive argument against its retinal origin, considering the 
 great amount of apparently independent growth which mem- 
 branes, when once formed, are capable of exhibiting. 
 
 Bergmeister's and my own observations on the vitreous 
 humour clearly prove that this is derived from an ingrowth 
 through the choroid-slit. On the other hand, the researches 
 of Lieberkuhn and Arnold on the Mammalian Eye appear to 
 demonstrate that a layer of mesoblast becomes in Mammalia 
 involuted with the lens, and from this the vitreous humour 
 (including the membrana capsjilo-pupillaris) is said to be in part 
 formed. Lieberkuhn states that in Birds the vitreous humour 
 is formed in a similar fashion. I cannot, however, accept his 
 results on this point. It appears, therefore, that, so far as is known, 
 all groups of Vertebrata, with the exception of Mammalia, con- 
 form to the Elasmobranch type. The differences between the 
 types of Mammalia and remaining Vertebrata are, however, not 
 so great as might at first sight appear. They are merely de- 
 pendent on slight differences in the manner in which the mesoblast 
 enters the optic cup. In the one case it grows in round one 
 specialized part of the edge of the cup, i.e. the choroid-slit ; in 
 the other, round the whole edge, including the choroid-slit. Per- 
 haps the mode of formation of the vitreous humour in Mammalia 
 may be correlated with the early closing of the choroid-slit. 
 
 27 2 
 
412 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Auditory Organ. With reference to the development of the 
 organ of hearing I have very little to say. Opposite the in- 
 terval between the seventh and the glosso-pharyngeal nerves 
 the external epiblast becomes thickened, and eventually in- 
 voluted as a vesicle which remains however in communication 
 with the exterior by a narrow duct. Towards the close of stage 
 K the auditory sack presents three protuberances one pointing 
 forwards, a second backwards, and a third outwards. These are 
 respectively the rudiments of the anterior and posterior vertical 
 and external horizontal semicircular canals. These rudiments 
 are easily visible from the exterior (PI. 15, fig. 2). 
 
 As has been already pointed out, the epiblast of Elasmo- 
 branchs during the early periods of development exhibits no 
 division into an epidermic and a nervous layer, and in accord- 
 ance with its primitive undifferentiated condition, those portions 
 of the organs of sense which are at this time directly derived 
 from the external integument are formed indiscriminately from 
 the whole, and not from an inner or so-called nervous part of it 
 only. In the Amphibians the auditory sack and lens are de- 
 rived from the nervous division of the epiblast only, while the 
 same division of the layer plays the major part in forming the 
 olfactory organ. It is also stated that in Birds and Mammals 
 the part of the epiblast corresponding to the nervous layer is 
 alone concerned in the formation of the lens, though this does 
 not appear to be the case with the olfactory or auditory organs 
 in these groups of Vertebrates. 
 
 Mouth involution and Pituitary body. 
 
 The development of the mouth involution and the pituitary 
 body is closely related to that of the brain, and may con- 
 veniently be dealt with here. The epiblast in the angle formed 
 by the cranial flexure becomes involuted as a hollow process 
 situated in close proximity to the base of the brain. This hollow 
 process is the mouth' involution, and it is bordered on its pos- 
 terior surface by the front wall of the alimentary tract, and on 
 its anterior by the base of the fore-brain. 
 
THE PITUITARY BODY. 413 
 
 The uppermost end of this does not till near the close of 
 stage K become markedly constricted off from the remainder, 
 but is nevertheless the rudiment of the pituitary body. PL 15, 
 figs. 9 a and 12 m shew in a most conclusive manner the cor- 
 rectness of the above account, and demonstrate that it is^from 
 the mouth involution, and not, as has usually been stated, from 
 the alimentary canal, that the pituitary body is derived. 
 
 This fact was mentioned in my preliminary account of Elas- 
 mobranch development 1 ; and has also been shewn to be the 
 case in Amphibians by Gotte 2 ; and in Birds by Mihalkowics 3 . 
 The fact is of considerable importance with reference to specula- 
 tions as to the meaning of this body. 
 
 Plate 15, fig. 7 represents a transverse section through the 
 head during a stage between I and K ; but, owing to the cranial 
 flexure, it cuts the fore part of the head longitudinally and hori- 
 zontally, and passes through both the fore-brain (fb) and the 
 hind-brain (iv. v.}. Close to the base of the fore-brain are seen 
 the mouth (m), and the pituitary involution from this (pt). In 
 contact with the pituitary involution is the blind anterior ter- 
 mination of the throat, which a little way back opens to the 
 exterior by the first visceral cleft (l. v.c.}. This figure alone 
 suffices to demonstrate the correctness of the above account of 
 the pituitary body ; but the truth of this is still further con- 
 firmed by other figures on the same plate (figs. 9 a and 12 m] ; 
 in which the mouth involution is in contact with, but still 
 separated from, the front end of the alimentary tract. By the 
 close of stage K, the septum between the mouth and throat 
 becomes pierced, and the two are placed in communication. 
 This condition is shewn in PI. 15, fig. i6a, and PI. 16, figs, i a, 
 i c, pt. In these figures the pituitary involution has become 
 very partially constricted off from the mouth involution, though 
 still in direct communication with it. In later stages the 
 pituitary involution becomes longer and dilated terminally, 
 while the passage connecting it with the mouth becomes nar- 
 
 1 Quarterly Journal of Microscopic Science, Oct. 1874. 
 
 ' 2 Ent-wicklungsgeschichte der Unke. Gotte was the first to draw attention to this 
 fact. His observations were then shewn to hold true for Elasmobranchs by myself, 
 and subsequently for Birds by Mihalkowics. 
 
 :! Arch. f. mift: Anat. Vol. XI. 
 
4H DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 rower and narrower, and is finally reduced to a solid cord, 
 which in its turn disappears. The remaining vesicle then be- 
 comes divided into lobes, and connects itself closely with the 
 infundibulum (PL 16, figs. 5 and 6 pt}. The later stages for 
 Elasmobranchs are fully described by W. Miiller in his im- 
 portant memoir on the Comparative Anatomy and development 
 of this organ 1 . 
 
 Development of the Cranial Nerves. 
 
 The present section deals with the whole development (so 
 far as I have succeeded in elucidating it) of the cranial nerves 
 (excluding the optic and olfactory nerves and the nerves of the 
 eye-muscles) from their first appearance to their attainment of 
 the adult condition. My description commences with the first 
 development of the nerves, to this succeeds a short description 
 of the nerves in the adult Scyllium, and the section is completed 
 by an account of the gradual steps by which the adult condition 
 is attained. 
 
 Early Development of the Cranial Nerves. Before the close 
 of stage H the more important of the cranial nerves make their 
 appearance. The fifth and the seventh are the first to be 
 formed. The fifth arises by stage G (PL 15, fig. 3 v), near the 
 anterior end of the hind-brain, as an outgrowth from the extreme 
 dorsal summit of tJie brain, in identically the same way as the 
 dorsal root of a spinal nerve. 
 
 The roots of the two sides sprout out from the summit of 
 the brain, in contact with each other, and grow ventralwards, 
 one on each side of the brain, in close contact with its walls. I 
 have failed to detect more than one root for the two embryonic 
 branches of the fifth (ophthalmic and mandibular), and no trace of 
 an anterior or ventral root has been tnet with in any of my sections. 
 
 The seventh nerve is formed nearly simultaneously with or 
 shortly after the fifth, and some little distance behind and inde- 
 pendently of it, opposite the anterior end of the thickening of 
 the epiblast to form the auditory involution. It arises precisely 
 
 1 W. Miiller, "Ueber Entwicklung und Bau d. Hypophysis u. d. Processus in- 
 fundibuli cerebri," Jenaische Zeitschrift, Bd. vi. 
 
FIRST FORMATION OF CRANIAL NERVES. 415 
 
 like the fifth, from the extreme dorsal summit of the neural axis 
 (PI. 15, fig. 4, vn). So far as I have been able to determine, 
 the auditory nerve and the seventh proper possess only a single 
 root common to the two. There is no anterior root for the 
 seventh any more than for the fifth. 
 
 Behind the auditory involution, at a stage subsequent to that 
 in which the fifth and seventh nerves appear, there arise a series 
 of roots from the dorsal summit of the hind-brain, which form 
 the rudiments of the glosso-pharyngeal and vagus nerves. These 
 roots are formed towards the close of stage H, but are still quite 
 short at the beginning of stage I. Their manner of development 
 resembles that of the previously described cranial nerves. The 
 central ends of the roots of the opposite sides are at first in 
 contact with each other, and there is nothing to distinguish the 
 roots of the glosso-pharyngeal and of the vagus nerves from the 
 dorsal roots of spinal nerves. Like the dorsal roots of the spinal 
 nerves, they appear as a series of ventral prolongations of a 
 continuous outgrowth from the brain, which outgrowth is more- 
 over continuous with that for the spinal nerves 1 . The outgrowth 
 of the vagus and glosso-pharyngeal nerves is not continuous 
 with that of the seventh nerve. This is shewn by PI. 15, figs. 4# 
 and 4& The outgrowth of the seventh nerve though present in 
 40 is completely absent in 4^ which represents a section just 
 behind 4^. 
 
 Thus, by the end of stage I, there have appeared the rudi- 
 ments of the 5th, 7th, 8th, Qth and loth cranial nerves, all of 
 which spring from the hind-brain. These nerves all develope 
 precisely as do the posterior roots of the spinal nerves, and it is 
 a remarkable fact tliat hitherto I have failed to find a trace in the 
 brain of a root of any cranial nerve arising from the ventral 
 corner of the brain as do the anterior roots of the spinal nerves*. 
 
 1 In the presence of this continuous outgrowth of the brain from which spring the 
 separate nerve stems of the vagus, may perhaps be found a reconciliation of the 
 apparently conflicting statements of Gbtte and myself with reference to the vagus 
 nerve. Gotte regards the vagus as a single nerve, from its originating as an undivided 
 rudiment ; but it is clear from my researches that, for Elasmobranchs at least, this 
 method of arguing will not hold good, since it would lead to the conclusion that all 
 the spinal nerves were branches of one single nerve, since they too spring as pro- 
 cesses from a continuous outgrowth from the brain ! 
 
 - The conclusion here arrived at with reference to the anterior roots, is opposed 
 to the observations of both Gegenbaur on Hexanchus, Jenaisckc Zeifsc/irift, Vol. VI., 
 
416 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 It is admittedly difficult to prove a negative, and it may still 
 turn out that there are anterior roots of the brain similar to 
 those of the spinal cord ; in the mean time, however, the balance 
 of evidence is in favour of there being none such. This at first 
 sight appears a somewhat startling conclusion, but a little con- 
 sideration shews that it is not seriously opposed to the facts 
 which we know. In the first place it has been shewn by myself 1 
 that in Amphioxus (whose vertebrate nature I cannot doubt) only 
 dorsal nerve-roots are" present. Yet the nerves of Amphioxus 
 are clearly mixed motor and sensory nerves, and it appears to 
 me far more probable that Amphioxus represents a phase of 
 development in which the nerves had not acquired two roots, 
 rather than one in which the anterior root has been lost. In 
 other words, the condition of the nerves in Amphioxus appears 
 to me to point to the conclusion that primitively the cranio-spinal 
 nerves of vertebrates were nerves of mixed function with one root 
 only, and that root a dorsal one ; and that the present anterior or 
 ventral root is a secondary acquisition. This conclusion is further 
 supported by the fact that the posterior roots develope in point 
 of time before the anterior roots. If it be admitted that the 
 vertebrate nerves primitively had only a single root, then the 
 retention of that condition in the brain implies that this became 
 differentiated from the remainder of the nervous system at a 
 very early period before the acquirement of anterior nerve-roots, 
 and that these eventually become developed only in the case of 
 spinal nerves, and not in the case of the already highly modified 
 cranial nerves. 
 
 Subsequent Changes of the Nerves. To simplify my descrip- 
 tion of the subsequent growth of the cranial nerves, I have 
 inserted a short description of their distribution in the adult. 
 
 and of Jackson and Clarke on Echinorhinus, Journal of Anatomy and Physiology, 
 Vol. x. These morphologists identify certain roots springing from the medulla below 
 and behind the main roots of the vagus as true anterior roots of this nerve. The 
 existence of these roots is not open to question, but without asserting that it is im- 
 possible for me to have failed to detect such roots had they been present in the embryo, 
 I think I may maintain if these anterior roots are not present in the embryo, their 
 identification as vagus roots must be abandoned ; and they must be regarded as be- 
 longing to spinal nerves. This point is more fully spoken of at p. 428. 
 1 Journal of Anatomy and Physiology, Vol. x. [This Edition, No. ix.] 
 
CRANIAL NERVES IN THE ADULT. 417 
 
 This is taken from a dissection of Scyllium stellare, which like 
 other species has some individualities of its own not found in 
 the other Elasmobranchs. For points not touched on in this 
 description I must refer the reader to the more detailed accounts 
 of my predecessors, amongst whom may specially be mentioned 
 Stannius 1 for Carcharias, Spinax, Raja, Chimaera, &c. ; Gegen- 
 oaur 2 for Hexanchus ; Jackson and Clarke 3 for Echinorhinus. 
 
 The ordinary nomenclature has been employed for the 
 branches of the fifth and seventh nerves, though embryological 
 data to be adduced in the sequel throw serious doubts upon it. 
 Since I am without observations on the origin of the nerves to 
 the muscles of the eyes, all account of these is omitted. 
 
 The fifth nerve arises from the brain by three roots 4 : (i) an anterior more 
 or less ventral root; (2) a root slightly behind, but close to the former 5 , 
 formed by the coalescence of two distinct strands, one arising from a dorsal 
 part of the medulla, and a second and larger from the ventral ; (3) a dorsal 
 and posterior root, in its origin quite distinct and well separated from the 
 other two, and situated slightly behind the dorsal strand of the second root. 
 This root a little way from its attachment becomes enclosed for a short dis- 
 tance in the same sheath as the dorsal part of the second root, and a slight 
 mixture of fibres seems to occur, but the majority of its fibres have no con- 
 nection with those of the second root. The first and second roots of the fifth 
 appear to me partially to unite, but before their junction the ramus ophthal- 
 micus profundus is given off from the first of them. 
 
 The fifth nerve, according to the usual nomenclature, has three main 
 divisions. The first of these is the ophthalmic. It is formed by the coales- 
 cence of two entirely independent branches of the fifth, which unite on 
 leaving the orbit. The dorsalmost of these, or ramus ophthalmicus super- 
 ficialis, originates from the third and posterior of the roots of the fifth, nearly 
 the whole of which appears to enter into its formation. This root is situated 
 on the dorsal part of the " lobi trigemini," at a point posterior to that of the 
 other roots of the fifth or even of the seventh nerve. The branch itself enters 
 the orbit by a separate foramen, and, keeping on the dorsal side of it, reenters 
 the cartilage at its anterior wall, and is there joined by the ramus ophthal- 
 micus prof undiis. This latter nerve arises from the anterior root of the fifth, 
 separately pierces the wall of the orbit, and takes a course slightly ventral to 
 the superior ophthalmic nerve, but does not (as is usual with Elasmobranchs) 
 
 1 Neruensystem d. Fische, Rostock, 1849. 
 - Jenaische Zeitschrifl, Vol. vi. 
 
 3 Journal of Anatomy and Physiology, Vol. X. 
 
 4 My results with reference to these roots accord exactly, so far as they go, with 
 the more carefully worked out conclusions of Stannius, loc. cit. pp. 29 and 30. 
 
 5 The root of the seventh nerve cannot properly be distinguished from this root. 
 
41 8 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 run below the superior rectus and superior oblique muscles of the eye. The 
 nerve formed by the coalescence of the superficial and deep ophthalmic 
 branches courses a short way below the surface, and supplies the mucous 
 canals of the front of the snout. It is a purely sensory nerve. Strong 
 grounds will be adduced in the sequel for regarding the ramus ophthalmicus 
 superficial, though not the ophthalmicus profundus, as in reality a branch 
 of the seventh, and not of the fifth nerve. 
 
 The second division of the fifth nerve is the superior maxillary, which 
 appears to me to arise from both the first and second roots of the fifth, though 
 mainly from the first. It divides once into two main branches. The first of 
 these the buccal nerve of Stannius after passing forwards along the base 
 of the orbit takes its course obliquely across the palatine arch and behind 
 and below the nasal sack, supplying by the way numerous mucous canals, 
 and dividing at last into two branches, one of these passing directly forwards 
 on the ventral surface of the snout, and the second keeping along the front 
 border of the mouth. The second division of the superior maxillary nerve 
 (superior maxillary of Stannius), after giving off a small branch, which passes 
 backwards in company with a branch from the inferior maxillary nerve to 
 the levator maxillae superioris, itself keeps close to the buccal nerve, and 
 eventually divides into numerous fine twigs to the mucous canals of the skin 
 at the posterior region of the upper jaw. It anastomoses with the buccal 
 nerve. The inferior maxillary nerve arises mainly from the second root of 
 the fifth. After sending a small branch to the levator maxillae superioris, it 
 passes outwards along the line separating the musculus adductor mandibulas 
 from the musculus levator labii superioris, and after giving branches to 
 these muscles takes a course forward along the border of the lower jaw. It 
 appears to be a mixed motor and sensory nerve. 
 
 The seventh or facial nerve arises by a root close to, but behind and below 
 the second root of the fifth, and is intimately fused with this. It divides 
 almost at once into .a small anterior branch and large posterior. 
 
 The anterior branch is the palatine nerve. It gives off at first one or two 
 very small twigs, which pursue a course towards the spiracle, and probably 
 represent the spiracular nerves of other Elasmobranchs. Immediately 
 after giving off these branches it divides into two stems, a posterior smaller 
 and an anterior -larger one. The former eventually takes a course which 
 tends towards the. angle of the jaw, and is distributed to the mucous mem- 
 brane of the roof of the mouth, while the larger one bends forwards and 
 supplies the mucous membrane at the edge of the upper jaw. The main 
 stem of the seventh, after giving off a branch to the dorsal section of the 
 musculus constrictor superficialis, passes outwards to the junction of the 
 upper and lower jaws, where it divides into tv/o branches, an anterior superficial 
 branch, which runs immediately below the skin on the surface of the lower 
 jaw, and a second branch, which takes a deep course along the posterior 
 border of the lower jaw, between it and the hyoid, and sends a series of 
 branches backwards to the ventral section of the musculus constrictor super- 
 ficialis. The main stem of the facial is mixed motor and sensory. I have 
 
DEVELOPMENT OF THE FIFTH NERVE. 419 
 
 not noticed a dorsal branch, similar to that described by Jackson and 
 Clarke. 
 
 The auditory nerve arises immediately behind the seventh, but requires 
 no special notice here. A short way behind the auditory is situated the root 
 of the glossopharyngeal nerve. This nerve takes an oblique course back- 
 wards through the skull, and gives off in its passage a very small dorsal 
 branch, which passes upwards and backwards through the cartilage towards 
 the roof of the skull. At the point where the main stem leaves the cartilage 
 it divides into two branches, an anterior smaller branch to the hinder border 
 of the hyoid arch, and a posterior and larger one to anterior border of the 
 first branchial arch. It forks, in fact, over the first visceral, cleft. 
 
 The vagus arises by a great number of distinct strands from the sides of 
 the medulla. In the example dissected there were twelve in all. The an- 
 terior three of these were the largest ; the middle one having the most ventral 
 origin. The next four were very small and in pairs, and were separated by 
 a considerable interval from the next four, also very small, and these again 
 by a marked interval from the hindermost strand. 
 
 The common stem formed by the junction of these gives off immediately 
 on leaving the skull a branch which forks on the second branchial cleft : a 
 second for the third cleft is next given off; the main stem then divides into a 
 dorsal branch the lateral nerve and a ventral one the branchio-intestinal 
 nerve which, after giving off the branches for the two last branchial clefts, 
 supplies the heart and intestinal tract. The lateral nerve passes back 
 towards the posterior end of the body, internal to the lateral line, and between 
 the dorso-lateral and ventro-lateral muscles. It gives off at its origin a fine 
 nerve, which has a course nearly parallel to its own. The main stem of the 
 vagus, at a short distance from its central end, receives a nerve which springs 
 from the ventral side of the medulla, on about a level with the most pos- 
 terior of the true roots of the vagus. This small nerve corresponds with the 
 ventral or anterior roots of the vagus described by Gegenbaur, Jackson, and 
 Clarke (though in the species investigated by the latter authors these roots 
 did not join the vagus, but the anterior spinal nerves). Similar roots are 
 also mentioned by Stannius, who found two of them in the Elasmobranchs 
 dissected by him; it is possible that a second may be present in Scyllium, 
 but have been overlooked by me, or perhaps may have been exceptionally 
 absent in the example dissected. 
 
 TJie FiftJi Nerve. The thinning of the roof of the brain, in 
 the manner already described, produces a great change in the 
 apparent position of the roots of all the nerves. The central 
 ends of the rudiments of the two sides are, as has been men- 
 tioned, at first in contact dorsally ; but, when by the growth of 
 the roof of the brain its two lateral halves become pushed apart, 
 the nerves also shift their position and become widely separated. 
 The roots of the fifth nerve are so influenced by these changes 
 
420 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 that they spring from the brain about half way up its sides, and 
 a little ventral to the border of its thin roof. While this change 
 has been taking place in the point of attachment of the fifth 
 nerve, it has not remained in other respects in a stationary con- 
 dition. 
 
 During stage H it already exhibits two distinct branches 
 known as the mandibular and ophthalmic. These branches first 
 lie outside a section of the body cavity which exists in the front 
 part of the head. The ophthalmic branch of the fifth being 
 situated near the anterior end of this, and the mandibular near 
 the posterior end. 
 
 In stage I the body cavity in this part becomes divided into 
 two parts one behind the other, the posterior being situated in 
 the mandibular arch. The bifurcation of the nerve then takes 
 place over the summit of the posterior of the two divisions of 
 the body cavity, PI. 15, figs. 9 b, V. and 10, V, &c., and at first 
 both branches keep close to the sides of this. 
 
 The anterior or ophthalmic branch of the fifth soon leaves the 
 walls of the cavity just spoken of and tends towards the eye, 
 and there comes in close contact with the most anterior section 
 of the body cavity which exists in the head. These relations it 
 retains unchanged till the close of stage K. Between stages I 
 and K it may easily be seen from the surface ; but, before the 
 close of stage K, the increased density of the tissues renders it 
 invisible in the living embryo. 
 
 The posterior branch of the fifth extends downwards into the 
 mandibular arch in close contact with the posterior and outer 
 wall of the body space already alluded to. At first no branches 
 from it can be seen, but I have detected by the close of stage K, 
 by an examination of the living embryo, a branch springing 
 from it a short way from its central extremity, and passing for- 
 wards, PI. 15, fig. 2, V. This branch I take to be the rudiment 
 of the superior maxillary division of the fifth nerve. It is shewn 
 in section, PI. 15, fig. 15 a, V. 
 
 In the stages after K the anatomy of the nerves becomes 
 increasingly difficult to follow, and accordingly I must plead 
 indulgence for the imperfections in my observations on all the 
 nerves subsequently to this date. In the fifth I find up to 
 stage O a single ophthalmic branch (PI. 17, fig. 4 b, V op. th.}, 
 
SEVENTH AND AUDITORY NERVES. 42! 
 
 which passes forwards slightly dorsal to the eye and parallel 
 and ventral to a branch of the seventh, which will be described 
 when I come to that nerve. I have been unable to observe that' 
 this branch divides into a ramus superficialis and ramus pro- 
 fundus, and subsequently to stage O I have no observations- on it. 
 
 By stage O the fifth may be observed to have two very 
 distinct roots, and a large ganglionic mass is developed close 
 to their junction (Gasserian ganglion), PI. 17, fig. 4 a. But in 
 addition to this ganglionic enlargement, all of the branches have 
 special ganglia of their own, PL 17, fig. 4 b 
 
 Summary. The fifth nerve has almost from the beginning 
 two branches, the ophthalmic (probably the inferior ophthalmic 
 of the adult) and the inferior maxillary. The superior maxillary 
 nerve arises later than the other two as a branch from the in- 
 ferior, originating comparatively far from its root. There is at 
 first but a single root for the whole nerve, which subsequently 
 becomes divided into two. Ganglionic swellings are developed 
 on the common stem and main branches of the nerve. 
 
 A general view of the nerve is shewn in the diagram in 
 PL 17, fig. i. 
 
 Seventh and A uditory Nerves. There appears in my earliest 
 sections a single large rudiment in the position of the seventh 
 and auditory nerves ; but in longitudinal sections of an embryo 
 somewhat older than stage I, in which the auditory organ forms 
 a fairly deep pit, still widely open to the exterior, there are to 
 be seen immediately in front of the ear the rudiments of two 
 nerves, which come into contact where they join the brain and 
 have their roots still closely connected at the end of stage K 
 (PL 15, figs. 10 and 15 a and 15 b}. The anterior of these pur- 
 sues a straight course to the hyoid arch (PL 15, fig. 10, VII.), the 
 second of the two (PL 15, fig. 10, ait* .), which is clearly the 
 rudiment of the auditory nerve, developes a ganglionic enlarge- 
 ment and, turning backward, closely hugs the ventral wall of the 
 auditory involution. 
 
 The observation just recorded appears to lead to the fol- 
 lowing conclusions with reference to the development of the 
 auditory nerve. A single rudiment arises from the brain for 
 the auditory and seventh nerves. This rudiment subsequently 
 
422 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 becomes split into two parts, an anterior to form the seventh 
 nerve, and a posterior to form the auditory nerve. The gan- 
 glionic part of the auditory nerve is derived from the primitive 
 outgrowths from the brain, and not from the auditory involu- 
 tion. I do not feel perfectly confident that an independent 
 origin of the auditory nerve might not have escaped my notice ; 
 but, admitting the correctness of the view which attributes to 
 the seventh and auditory a common origin, it follows that the 
 auditory nerve primitively arose in connection with the seventh, 
 of which it' may either, as Gegenbaur believes, be a distinct 
 part the ramus dorsalis or else may possibly have formed 
 part of a commissure, homologous with that uniting the dorsal 
 roots of the spinal nerves, connecting the seventh with the 
 glossopharyngeal nerve. In either case it must be supposed 
 secondarily to have become separate and independent in con- 
 sequence of the development of the organ of hearing. 
 
 My sections of embryos of stage K and the subsequent 
 stages do not bring to light many new facts with reference to 
 the auditory nerve : they demonstrate however that its gan- 
 glionic part increases greatly in size, and in stage O there is a 
 distinct root for the auditory nerve in contact with that for the 
 seventh. 
 
 The history of the seventh nerve in its later stages presents 
 points of great interest. Near the close of stage K there may 
 be observed, in the living embryos and in sections, two branches 
 of the seventh in addition to the original trunk to the hyoid 
 arch, both arising from its anterior side ; one passes straight 
 forwards close to the external skin, but is at first only traceable 
 a short way in front of the fifth, and a second passes downwards 
 into the mandibular arch in such a fashion, that the seventh 
 nerve forks over the hyomandibular cleft (vide PI. 15, fig. 2, VII. ; 
 15 a, VII.). My sections shew both these branches with great 
 clearness. A third branch has also come under my notice, 
 whose course leads me to suppose that it supplies the roof of 
 the palate. 
 
 In the later stages my attention has been specially directed 
 to the very remarkable anterior branch of the seventh. This 
 may, in stages L to O, be traced passing on a level with the 
 root of the fifth nerve above the eye, and apparently termi- 
 
RAMUS OPHTHALMICUS SUPERFICIALIS. 423 
 
 nating in branches to the skin in front of the eye (PI. 17, figs. 3, 
 VII. ; 4. a, VII. a). It courses close beneath the skin (though this does 
 not appear in the sections represented on account of their ob- 
 liqueness), and runs parallel and dorsal to the ophthalmic branch 
 of the fifth nerve, and may easily be seen in this position in 
 longitudinal sections belonging to stage O ; but its changes 
 after this stage have hitherto baffled me, and its final fate is 
 therefore, to a certain extent, a matter of speculation. 
 
 The two other branches of the seventh, viz., the hyoid or 
 main branch and mandibular branch, retain their primitive 
 arrangement till the close of stage O. 
 
 The fate of the remarkable anterior branch of the seventh 
 nerve is one of the most interesting points which has started 
 up in the course of my investigations on the development of 
 the cranial nerves, and it is a matter of very great regret to me 
 that I have not been able to clear up for certain its later 
 history. 
 
 Its primitive distribution leads to the supposition that it 
 becomes the nerve known in the adult as the ramus opthal- 
 micus superficialis of the fifth nerve, and this is the view which I 
 admit myself to be inclined to adopt. There are several points 
 in the anatomy of this nerve in the adult which tell in favour of 
 accepting this view with reference to it. In the first place, the 
 ramus ophthalmicus superficialis rises from the brain (vide 
 description above, p. 417), quite independently of the ramus 
 ophthalmicus profundus, and not in very close connection with 
 the other branches of the fifth, and also considerably behind 
 these, quite as far back indeed as the ventral root of the 
 seventh. There is therefore nothing in the position of its root 
 opposed to its being regarded as a branch of the seventh nerve. 
 Secondly, its distribution, which might at first sight be regarded 
 as peculiar, presents no very strange features if it is looked on 
 as a ramus dorsalis of the seventh, whose apparent anterior 
 instead of dorsal course is due to the cranial flexure. If, how- 
 ever, the distribution of the ramus ophthalmicus superficialis is 
 used as an argument against my view, a satisfactory reply is 
 to be found in the fact that a branch of the seventh nerve cer- 
 tainly has the distribution in question in tJie embryo, and that 
 there is no reason why it should not retain it in the adult. 
 
424 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Finally, the junction of the two rami ophthalmici, most re- 
 markable if they are branches of a single nerve, would present 
 nothing astonishing when they are regarded as branches of two 
 separate nerves. 
 
 If this view be adopted, certain modifications of the more 
 generally accepted views of the morphology of the cranial 
 nerves will be necessitated ; but this subject is treated of at the 
 end of this section. 
 
 Some doubt hangs over the fate of the other branches of 
 the seventh nerve, but their destination is not so obscure as that 
 of the anterior branch. The branch to the roof of the mouth 
 can be at once identified as the ' palatine nerve ', and it only 
 remains to speak of the mandibular branch. 
 
 It may be noticed first of all with reference to this branch, 
 that the seventh behaves precisely like the less modified succeed- 
 ing cranial nerves. It forks in fact over a visceral cleft (the 
 hyomandibular) the two sides of which it supplies ; the branch 
 at the anterior side of the cleft is the later developed and smaller 
 of the two. There cannot be much doubt that the mandibular 
 branch must be identified with the spiracular nerve (prae-spira- 
 cular branch Jackson and Clarke) of the adult, and if the chorda 
 tympani of Mammals is correctly regarded as the mandibular 
 branch of the seventh nerve, then the spiracular nerve must 
 represent it. Jackson and Clarke 1 take a different view of the 
 homology of the chorda tympani, and regard it as equivalent to 
 the ramus mandibularis internus (one of the two branches into 
 which the seventh eventually divides), because this nerve takes 
 its course over the ligament connecting the mandible with the 
 hyoid. This view I cannot accept so long as it is admitted that 
 the chorda tympani is the branch of a cranial nerve supplying 
 the anterior side of a cleft. The ramus mandibularis internus, 
 instead of forming with the main branch of the seventh a fork 
 over the spiracle, passes to its destination completely behind 
 and below the spiracle, and therefore fails to fulfil the conditions 
 requisite for regarding it as a branch to the anterior wall of 
 a visceral cleft. It is indeed clear that the ramus mandibularis 
 internus cannot be identified with the embryonic mandibular 
 branch of the seventh (which passes above the spiracle or 
 
 1 Loc. tit. 
 
THE GLOSSOPHARYNGEAL AND VAGUS NERVES. 425 
 
 hyomandibular cleft) when there is present in the adult another 
 nerve (the spiracular nerve), which exactly corresponds in 
 distribution with the embryonic nerve in question. My view 
 accords precisely with that already expressed by Gegenbaur 
 in his masterly paper on the nerves of Hexanchus, in Tvhich 
 he distinctly states that he looks upon the spiracular nerve as 
 the homologue of an anterior branchial branch of a division 
 of the vagus. In the adult the spiracular nerve is sometimes 
 represented by one or two branches of the palatine, e.g. Scyllium, 
 but at other times arises independently from the main stem 
 of the seventh 1 . The only difficulty in my identification of the 
 embryonic mandibular branch with the adult spiracular nerve, 
 is the extremely small size of the latter in the adult, compared 
 with the size of mandibular in the embryo ; but it is hardly 
 surprising to find an atrophy of the spiracular nerve accompany- 
 ing an atrophy of the spiracle itself. The palatine appears to 
 me to have been rightly regarded by Jackson and Clarke as the 
 great superficial petrosal of Mammals. 
 
 On the common root of the branches of the seventh nerve, 
 as well as on its hyoid branch, ganglionic enlargements are 
 present at an early period of development. 
 
 The Glossopharyngeal and Vagus Nerves. Behind the ear 
 there are formed a series of five nerves which pass down to 
 respectively the first, second, third, fourth and fifth visceral, 
 arches. 
 
 For each arch there is thus one nerve, whose course lies 
 close to the posterior margin of the preceding cleft, a second 
 anterior branch being developed later. These nerves are con- 
 nected with the brain (as I have determined by transverse 
 sections) by roots at first attached to the dorsal summit, but 
 eventually situated about half-way down the sides (PI. 15, 
 fig. 6,) nearly opposite the level of the process which divides 
 the ventricle of the hind-brain into a dorsal and a ventral moiety. 
 The foremost of these nerves is the glossopharyngeal. The 
 next four are, as has been shewn by Gegenbaur 2 , equivalent 
 to four independent nerves, but form, together with the glosso- 
 pharyngeal, a compound nerve, which we may briefly call the 
 vagus. 
 
 1 Hexanchus, Gegenbaur, Jenaische Zeitschrift, Vol. VI. - Loc. cit. 
 
 B. 28 
 
426 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 This compound nerve by stage K attains a very complicated 
 structure, and presents several remarkable and unexpected 
 features. Since it has not been possible for me completely 
 to elucidate the origin of all its various parts, it will conduce 
 to clearness if I give an account of its structure during stage K 
 or L, and then return to what facts I can mention with reference 
 to its development. Its structure during these stages is repre- 
 sented on the diagram, PI. 17, fig. I. There are present five 
 branches, viz. the glossopharyngeal and four branches of the 
 vagus, arising probably by a considerably greater number of 
 strands from the brain 1 . All the strands from the brain are 
 united together by a thin commissure, Vg. com., continuous with 
 the commissure of the posterior roots of the spinal nerves, and 
 from this commissure the five branches are continued obliquely 
 ventralwards and backwards, and each of them dilates into a 
 ganglionic swelling. They all become again united together 
 by a second thick commissure, which is continued backwards as 
 the intestinal branch of the vagus nerve Vg. in. The nerves, 
 however, are continued ventralwards each to its respective arch. 
 From the hinder part of the intestinal nerve springs the lateral 
 nerve n.l., at a point whose relations to the branches of the vagus 
 I have not certainly determined. 
 
 The whole nerve-complex formed by the glossopharyngeal 
 and the vagus nerves cannot of course be shewn in any single 
 section. The various roots are shewn in PL 17, fig. 5. The 
 dorsal commissure is represented in longitudinal section in PL 1 5, 
 fig. 15 b, com., and in transverse section in PL 17, fig. 2 Vg, com. 
 The lower commissure continued as the intestinal nerve is shewn 
 in PL 15, fig. 15 a, Vg., and as seen in the living embryo in 
 PL 15, figs, i and 2. The ganglia are seen in PL 15, fig. 6, Vg. 
 The junction of the vagus and glossopharyngeal 'nerves is shewn 
 in PL 15, fig. 10. My observations have not taught me much 
 with reference to the origin of the two commissures, viz. the 
 dorsal one and the one which forms the intestinal branch of the 
 vagus. Very possibly they originate as a single commissure 
 which becomes longitudinally segmented. It deserves to be 
 noticed that the dorsal commissure has a long stretch, from 
 
 1 In the diagram there are only five strands represented. This is due to the fact 
 that I have not certainly made out their true number. 
 
THE ROOTS OF THE VAGUS NERVE. 427 
 
 the last branch of the vagus to the first spinal nerve, during 
 which it is not connected with the root of any nerve ; vide 
 fig. 15 b, coin. This space probably contained originally the 
 now lost branches of the vagus. In many transverse sections 
 where the dorsal commissure might certainly be expected- to 
 be present it cannot be seen, but this is perhaps due to its 
 easily falling out of the sections. I have not been able to prove 
 that the commissure is continued forwards into the auditory nerve. 
 
 The relation of the branches of the vagus and glossopharyn- 
 geal to the branchial clefts requires no special remark. It is 
 fundamentally the same in the embryo as in the adult. The 
 branches at the posterior side of the clefts are the first to appear, 
 those at the anterior side of the clefts being formed subsequently 
 to stage K. 
 
 One of the most interesting points with reference to the 
 vagus is the number of separate strands from the brain which 
 unite to form it. The questions connected with these have been 
 worked out in a masterly manner, both from an anatomical and 
 a theoretical standpoint, by Professor Gegenbaur 1 . It has not 
 been possible for me to determine the exact number of these in 
 my embryos, nor have I been able to shew whether they are as 
 numerous at the earliest appearance of the vagus as at a later 
 embryonic period. The strands are connected (PL 17, fig. 5) 
 with separate ganglionic centres in the brain, though in several 
 instances more than one strand is connected with a single 
 centre. In an embryo between stage O and P more than a 
 dozen strands are present. In an adult Scyllium I counted 
 twelve separate strands, but their number has been shewn by 
 Gegenbaur to be very variable. It is possible that they are 
 remnants of the roots of the numerous primary branches of the 
 vagus which have now vanished ; and this perhaps is the ex- 
 planation of their variability, since in the case of all organs 
 which are on the way to disappear variability is a precursor of 
 disappearance. 
 
 A second interesting point is the presence of the two connect- 
 ing commissures spoken of above. It was not till comparatively 
 late in my investigations that I detected the dorsal one. This 
 has clearly the same characters as the dorsal commissure already 
 
 1 Loc. cit. 
 
 282 
 
428 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 described as connecting the roots of all the spinal nerves, and is 
 indeed a direct prolongation of this. It becomes gradually 
 thinner and thinner, and finally ceases to be observable by 
 about the close of stage L. It is of importance as shewing 
 the similarity of the branches of the vagus to the dorsal roots 
 of the spinal nerves. The ventral of the two commissures 
 persists in the adult as the common stem from which all the 
 branches of the vagus successively originate, and is itself continued 
 backwards as the intestinal branch of the vagus. The glosso- 
 pharyngeal nerve alone becomes eventually separated from the 
 succeeding branches. Stannius and Gegenbaur have, as was 
 mentioned above, detected in adult Elasmobranchs roots which 
 join the vagus, and which resemble the anterior or ventral roots 
 of spinal nerves ; and I have myself described one such root 
 in the adult Scyllium. I have searched for these in my embryos, 
 but without obtaining conclusive results. In the earliest stages 
 I can find no trace of them, but I have detected in stage L 
 one anterior root on debatable border-land, which may conceivably 
 be the root in question, but which I should naturally have put 
 down for the root of a spinal nerve. Are the roots in question 
 to be regarded as proper roots of the vagus, or as ventral roots 
 of spinal nerves whose dorsal roots have been lost ? The latter 
 view appears to me the most probable one, partly from the 
 embryologrcal evidence furnished by my researches, which is 
 clearly opposed to the existence of anterior roots in the brain, 
 and partly from the condition of these roots in Echinorhinus, in 
 which they join the succeeding spinal nerves and not the vagus 1 . 
 The similar relations of the apparently homologous branch or 
 branches in many Osseous Fish may also be used as an argument 
 for my view. 
 
 If, as seems probable, the roots in question become the 
 hypoglossal nerve, this nerve must be regarded as formed from 
 the anterior roots of one or more spinal nerves. Without embryo- 
 logical evidence it does not however seem possible to decide 
 whether the hypoglossal nerve contains elements only of anterior 
 roots or of both anterior and posterior roots. 
 
 1 Vide Jackson and Clarke, loc. cit. The authors take a different view to that 
 here advocated, and regard the ventral roots described by them as having originally 
 belonged to the vagus. 
 
MYOTOMES OF THE HEAD. 429 
 
 Mesoblast of the Head. 
 
 Body Cavity and Myotomes of the Head. During stage F the 
 appearance of a cavity on each side in the mesoblast of the head 
 was described. (Vide PI. 10, figs. 3 b and 6//.) These cavities 
 end in front opposite the blind anterior extremity of the alimen- 
 tary canal ; behind they are continuous with the general body- 
 cavity. I propose calling them the head-cavities. The cavities 
 of the two sides have no communication with each other. 
 
 Coincidently with the formation of an outgrowth from the 
 throat to form the first visceral cleft, the head-cavity on each 
 side becomes divided into a section in front of the cleft and a 
 section behind the cleft (vide PI. 15, figs. 4 a and ^b pp.); and 
 during stage H it becomes, owing to the formation of a second 
 cleft, divided into three sections: (i) a section in front of the 
 first or hyomandibular cleft ; (2) a section in the hyoid arch 
 between the hyomandibular cleft and the hyobranchial or first 
 branchial cleft ; (3) a section behind the first branchial cleft. 
 
 The section in front of the hyomandibular cleft stands in a 
 peculiar relation to the two branches of the fifth nerve. The 
 ophthalmic branch of the fifth lies close to the outer side of its 
 anterior part, the mandibular branch close to the outer side of its 
 posterior part. During stage I this front section of the head- 
 cavity grows forward, and becomes divided, without the inter- 
 vention of a visceral cleft, into an anterior and posterior division. 
 The anterior lies close to the eye, and in front of the commencing 
 mouth involution, and is connected with the ophthalmic branch 
 of the fifth nerve. The posterior part lies completely within the 
 mandibular arch, and is closely connected with the mandibular 
 division of the fifth nerve. 
 
 As the rudiments of the successive visceral clefts are formed, 
 the posterior part of the head-cavity becomes divided into suc- 
 cessive sections, there being one section for each arch. Thus 
 the whole head-cavity becomes on each side divided into (i) a 
 premandibular section ; (2) a mandibular section ; (3) a hyoid 
 section ; (4) sections in the branchial arches. 
 
 The first of these divisions forms a space of a considerable 
 size, with epithelial walls of somewhat short columnar cells. It 
 
430 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 is situated close to the eye, and presents a rounded or sometimes 
 triangular figure in sections (PL 15, figs. 7, 9 b and \6b, I pp.}. 
 The ophthalmic branch of the fifth nerve passes close to its 
 superior and outer wall. 
 
 Between stages I and K the anterior cavities of the two sides 
 are prolonged ventralwards and meet below the base of the 
 fore-brain (PI. 15, fig. 8. i //.). The connection between the two 
 cavities appears to last for a considerable time, and still persists 
 at the close of stage L. The anterior or premandibular pair of 
 cavities are the only parts of the body-cavity within the head 
 which unite ventrally. In the trunk, however, the primitively 
 independent lateral halves of the body-cavity always unite in 
 this way. The section of the head-cavity just described is so 
 similar to the remaining posterior sections that it must be con- 
 sidered as equivalent to them. 
 
 The next division of the head-cavity, which from its position 
 may be called the mandibular cavity, presents during the stages 
 I and K a spatulate shape. It forms a flattened cavity, dilated 
 dorsally, and produced ventrally into a long thin process parallel 
 to the hyomandibular gill-cleft, PI. 15, fig. I //. and fig. 7, 9 b 
 and 15 a, 2 pp. Like the previous space it is lined by a short 
 columnar epithelium. 
 
 The fifth nerve, as has already been mentioned, bifurcates 
 over its dorsal summit, and the mandibular branch of that nerve 
 passes down on its posterior and outer side. The mandibular 
 aortic arch is situated close to its inner side, PI. 15, fig. 7. To- 
 wards the close of this period the upper part of the cavity 
 atrophies. Its lower part also becomes much narrowed, but its 
 walls of columnar cells persist and lie close to one another. 
 The outer or somatic wall becomes very thin indeed, the splanch- 
 nic wall, on the other hand, thickens and forms a layer of several 
 rows of elongated cells. This thicker wall is on its inner side 
 separated from the surrounding tissue by a small space lined 
 by a membrane-like structure. In each of the remaining arches 
 there is a segment of the original body-cavity fundamentally 
 similar to that in the mandibular arch. A dorsal dilated portion 
 appears, however, to be present in the third or hyoid section 
 alone, and even there disappears by the close of stage K. The 
 cavities in the posterior parts of the head become much reduced 
 
MYOTOMES OF THE HEAD. 431 
 
 like those in its anterior part, though at rather a later period. 
 Their walls however persist, and become more columnar. In 
 PI. 15, fig. 13 b,pp., is represented the cavity in the last arch but 
 one, at a period when the cavity in the mandibular arch has 
 become greatly reduced. It occupies the same position -en- the 
 outer side of the aortic trunk of its arch as does the cavity in 
 the mandibular arch (PI. 15, fig. 7, 2pp). In Torpedo embryos 
 the head-cavity is much smaller, and atrophies earlier than in 
 the embryos of Pristiurus and Scyllium. 
 
 It has been shewn that, with the exception of the most 
 anterior, the divisions of the body-cavity in the head become 
 atrophied, not so however tJieir walls. The cells forming these 
 become elongated, and by stage N become distinctly developed 
 into muscles. Their exact history I have not followed in its 
 details, but they almost unquestionably become the musculus 
 constrictor superficialis and musculus interbranchialis 1 ; and pro- 
 bably also musculus levator mandibuli and other muscles of the 
 front part of the head. 
 
 The most anterior cavity close to the eye remains unaltered 
 much longer than the remaining cavities, and its two halves are 
 still in communication at the close of stage L. I have not yet 
 succeeded in tracing the subsequent fate of its walls, but think 
 it probable that they develope into the muscles of the eye. The 
 morphological importance of the sections of the body-cavity in 
 the head cannot be over-estimated, and the fact that the walls 
 become developed into the muscular system of the head renders 
 it almost certain that we must regard them as equivalent to the 
 muscle-plates of the body, which originally contain, equally with 
 those of the head, sections of the body-cavity. If this determination 
 is correct, there can be no doubt that they ought to serve as 
 valuable guides to the number of segments which have coalesced 
 to form the head. This point is, however, discussed in a sub- 
 sequent section. 
 
 General mesoblast of the head. In stage G no mesoblast is 
 present in the head, except that which forms the walls of the 
 head-cavity. 
 
 During stage H a few cells of undifferentiated connective 
 
 1 Vide Vetter, " Die Kiemen und Kiefermusculatw d. Fische." Jenaische Zeit- 
 schrift, Vol. VI I. 
 
432 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 tissue appear around the stalk of the optic vesicle, and in the 
 space between the front end of the alimentary tract and the 
 base of the brain in the angle of the cranial flexure. They are 
 probably budded off from the walls of the head-cavities. Their 
 number rapidly increases, and they soon form an investment 
 surrounding all the organs of the head, and arrange themselves 
 as a layer, between the walls of the roof of the fore and mid- 
 brain and the external skin. At the close of stage K they are 
 still undifferentiated and embryonic, each consisting of a large 
 nucleus surrounded by a very delicate layer of protoplasm pro- 
 duced into numerous thread-like processes. They form a regular 
 meshwork, the spaces of which are filled up by an intercellular 
 fluid. 
 
 I have not worked out the development of the cranial and 
 visceral skeleton ; but this has been made the subject of an 
 investigation by Mr Parker, who is more competent to deal with 
 it than any other living anatomist. His results were in part made 
 known in his lectures before the Royal College of Surgeons 1 , and 
 will be published in full in the Transactions of the Zoological 
 Society. 
 
 All my efforts have hitherto failed to demonstrate any seg- 
 mentation in the mesoblast of the head, other than that in- 
 dicated by the sections of the body-cavity before-mentioned ; 
 but since these, as above stated, must be regarded as equivalent 
 to muscle-plates, any further segmentation of mesoblast could 
 not be anticipated. To this statement the posterior part of the 
 head forms an apparent exception. Not far behind the auditory 
 involution there are visible at the end of period K a few longi- 
 tudinal muscles, forming about three or four muscle-plates, the 
 ventral part of which is wanting. I have not the means of de- 
 ciding whether they properly belong to the head, or may not 
 really be a part of the trunk system of muscles which has, to a 
 certain extent, overlapped the back part of the head, but am 
 inclined to accept the latter view. These cranial muscle-plates 
 are shewn in PI. 15, fig. 15 b, and in PI. 17, fig. 2. 
 
 1 A report of the lectures appeared in Nature, 
 
THE GILL-SLITS. 433 
 
 Notochord in the Head. 
 
 The notochord during stage G is situated for its whole length 
 close under the brain, and terminates opposite the base of the 
 mid-brain. As the cranial flexure becomes greater and meso- 
 blast is collected in the angle formed by this, the termination of 
 the notochord recedes from the base of the brain, but remains 
 in close contact with the front end of the alimentary canal. At 
 the same time its terminal part becomes very much thinner than 
 the remainder, ends in a point, and exhibits signs of a retro- 
 gressive metamorphosis. It also becomes bent upon itself in a 
 ventral direction through an angle of 180; vide PL 15, figs. 90 
 and 1 6 a. In some cases this curvature is even more marked 
 than is represented in these figures. 
 
 The bending of the end of the notochord is not directly 
 caused by the cranial flexure, as is proved by the fact that the 
 end of the notochord becomes bent through a far greater angle 
 than does the brain. During the stages subsequent to K the 
 ventral flexure of the notochord disappears, and its terminal 
 part acquires by stage O a distinct dorsal curvature. 
 
 Hypoblast of the Head. 
 
 The only feature of the alimentary tract in the head which 
 presents any special interest is the formation of the gill-slits and 
 of the thyroid body. In. the present section the development of 
 the former alone is dealt with : the latter body will be treated 
 in the section devoted to the general development of the ali- 
 mentary tract. 
 
 The gill-slits arise as outgrowths of the lining of the throat 
 towards the external skin. In the gill-slits of Torpedo I .have 
 observed a very slight ingrowth of the external skin towards 
 the hypoblastic outgrowth in one single case. In all other cases 
 observed by me, the outgrowth from the throat meets the 
 passive external skin, coalesces with it, and then, by the dis- 
 solution of the wall separating the lumen of the throat from the 
 exterior, a free communication from the throat outwards is 
 effected ; vide PL 15, figs. 5 a and b, and 13 b. Thus it happens 
 
434 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 that the walls lining the clefts are entirely formed of hypoblast. 
 The clefts are formed successively 1 , the anterior appearing first, 
 and it is not till after the rudiments of three have appeared, that 
 any of them become open to the exterior. 
 
 In stage K, four if not five are open to the exterior, and the 
 rudiments of six, the full number, have appeared 2 . Towards the 
 close of stage K there arise, from the walls of the 2nd, 3rd and 
 4th clefts, very small knob-like processes, the rudiments of the 
 external gills. These outgrowths are formed both by the lining 
 of the gill-cleft and by the adjoining mesoblast 3 . 
 
 From the mode of development of the gill-clefts, it appears 
 that their walls are lined externally by hypoblast, and therefore 
 that the external gills are processes of the walls of the alimen- 
 tary tract, i.e. are covered by an hypoblastic, and not an epiblastic 
 layer. It should be remembered, however, that after the gill- 
 slits become open, the point where the hypoblast joins the 
 epiblast ceases to be determinable, so that some doubt hangs 
 over the above statement. 
 
 The identification of the layer to which the gills belong is not 
 without interest. If the external gills have an epiblastic origin, 
 they may be reasonably regarded 4 as homologous with the ex- 
 ternal gills of Annelids ; but, if derived from the hypoblast, this 
 view becomes, to say the least, very much less probable. 
 
 Segmentation of the Head. 
 
 The nature of the vertebrate head and its relation to the 
 trunk forms some of the oldest questions of Philosophical 
 Morphology. 
 
 The answers of the older anatomists to these questions are 
 of a contradictory character, but within the last few years it has 
 been more or less generally accepted that the head is, in part at 
 least, merely a modified portion of the trunk, and composed, like 
 
 1 Vide Plate 8. 
 
 2 The description of stage K and L, pp. 292 and 293, is a little inaccurate with 
 reference to the number of the visceral clefts, though the number visible in the 
 hardened embryos is correctly described. 
 
 3 Vide on the development of the gills, Schenk, Sitz. d. k. Akad. IVien, Vol. 
 LXXI. 1875. 
 
 4 Vide Dohrn, Ur sprung d. Wirbelthiere. 
 
SEGMENTATION OF THE HEAD. 435 
 
 that, of a series of homodynamous segments 1 . While the 
 researches of Huxley, Parker, Gegenbaur, Gotte, and other 
 anatomists, have demonstrated in an approximately conclusive 
 manner that the head is composed of a series of segments, great 
 divergence of opinion still exists both as to the number of these 
 segments, and as to the modifications which they have under- 
 gone, especially in the anterior part of the head. The questions 
 involved are amongst the most difficult in the whole range 
 of morphology, and the investigations recorded in the preceding 
 pages do not, I am very well aware, go far towards definitely 
 solving them. At the same time my observations on the nerves 
 and on the head-cavities appear to me to throw a somewhat 
 new light upon these questions, and it has therefore appeared 
 to me worth while shortly to state the results to which a con- 
 sideration of these organs points. There are three sets of organs, 
 whose development has been worked out, each of which presents 
 more or less markedly a segmental arrangement: (i) The 
 cranial nerves ; (2) the visceral clefts ; (3) the divisions of the 
 head-cavity. 
 
 The first and second of these have often been employed in 
 the solution of the present problem, while the third, so far as is 
 known, exists only in the embryos of Elasmobranchs. 
 
 The development of the cranial nerves has recently been 
 studied with great care by Dr Gotte, and his investigations have 
 led him to adopt very definite views on the segments of head. 
 The arrangement of the cranial nerves in the adult has frequently 
 been used in morphological investigations about the skull, but 
 there are to my mind strong grounds against regarding it as 
 affording a safe basis for speculation. The most important of 
 these depends on the fact that nerves are liable to the greatest 
 modification on any changes taking place in the organs they 
 supply. On this account it is a matter of great difficulty, amount- 
 ing in many cases to actual impossibility, to determine the 
 morphological significance of the different nerve-branches, or the 
 nature of the fusions and separations which have taken place at 
 the roots of the nerves. It is, in fact, only in those parts of the 
 
 1 Semper, in his most recent work, maintains, if I understand him rightly, that 
 the head is in no sense a modified part of the trunk, but admits that it is segmented 
 in a similar fashion to the trunk. 
 
436 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 head which have, relatively speaking, undergone but slight 
 modifications, and which require no special elucidation from the 
 nerves, that these sufficiently retain in the adult their primitive 
 form to serve as trustworthy morphological guides. 
 
 I propose to examine separately the light thrown on the 
 segmentation of the head by the development of (i) the nerves, 
 (2) the visceral clefts, (3) the head-cavities ; and then to compare 
 the three sets of results so 'obtained. 
 
 The post-auditory nerves present no difficulties ; they are all 
 organized in the same fashion, and, as was first pointed out by 
 Gegenbaur, form five separate nerves, each indicating a seg- 
 ment. A comparison of the post-auditory nerves of Scyllium 
 and other typical Elasmobranchs with those of Hexanchus and 
 Heptanchus proves, however, that other segments were originally 
 present behind those now found in the more typical forms. And 
 the presence in Scyllium of numerous (twelve) strands from 
 the brain to form the vagus, as well as the fact that a large 
 section of the commissure connecting the vagus roots with the 
 posterior roots of the spinal nerves is not connected with the 
 brain, appear to me to shew that all traces of the lost nerves 
 have not yet vanished. 
 
 Passing forwards from the post-auditory nerves, we come to 
 the seventh and auditory nerves. The embryological evidence 
 brought forward in this paper is against regarding these nerves 
 as representing two segments. Although it must be granted 
 that my evidence is not conclusive against an independent 
 formation of these two nerves, yet it certainly tells in favour of 
 their originating from a common rudiment, and Marshall's results 
 on the origin of the two nerves in Birds (published in the 
 Journal of Anatomy and Physiology, Vol. XI. Part 3) support, 
 I have reason to believe, the same conclusion. Even were 
 it eventually to be proved that the auditory nerve originated 
 independently of the seventh, the general relations of this 
 nerve, embryological and otherwise, are such that, provisionally 
 at least, it could not be regarded as belonging to the same 
 category as the facial or glossopharyngeal nerves, and it has 
 therefore no place in a discussion on the segmentation of the 
 head. 
 
 The seventh nerve of the embryo (PI. 17, fig. I, VII.) is 
 
SEGMENTATION OF THE HEAD. 437 
 
 formed by the junction of three conspicuous branches, (i) an 
 anterior dorsal branch which takes a more or less horizontal 
 course above the eye (VII. a) ; (2) a main branch. to the hyoid 
 arch (VII. hy) ; (3) a smaller branch to the posterior edge of the 
 mandibular arch (vn. m?t): The first of these branches can 
 clearly be nothing else but the typical "ramus dorsalis," of which 
 however the auditory may perhaps be a specialized part. The 
 fact that this branch pursues an anterior and not a directly 
 dorsal course is probably to be explained as a consequence of 
 the cranial flexure. The two other branches of the seventh 
 nerve are the same as those present in all the posterior nerves, 
 viz. the branches to the two sides of a branchial cleft, in the 
 present instance the spiracle ; the seventh nerve being clearly 
 the nerve of the hyoid arch. 
 
 The fifth nerve presents in the arrangement of its branches 
 a similarity to the seventh nerve so striking that it cannot be 
 overlooked. This similarity is at once obvious from an inspec- 
 tion of the diagram of the nerves on PI. 17, fig. I, V., or from an 
 examination of the sections representing these nerves (PI. 17, 
 figs. 3 and 4). It divides like the seventh nerve into three main 
 branches : (i) an anterior and dorsal branch (r. ophthalmicus 
 profundus), whose course lies parallel to but ventral to that of 
 the dorsal branch of the seventh nerve ; (2) a main branch to 
 the mandibular arch (r. maxillae inferioris) ; and (3) an anterior 
 branch to the palatine arcade (r. maxillae superioris). I was at 
 first inclined to regard the anterior branch of the fifth (ophthal- 
 mic) as representing a separate nerve, and was supported in this 
 view by its relation to the most anterior of the head-cavities ; 
 but the unexpected discovery of an exactly similar branch in the 
 seventh nerve has induced me to modify this view, and I am now 
 constrained to view the fifth as a single nerve, whose branches 
 exactly correspond with those of the seventh. The anterior 
 branch of the fifth is, like the corresponding branch of the 1 
 seventh, the ramus dorsalis, and the two other branches are the 
 equivalent of the branches of the seventh, which fork over the 
 spiracle, though in the case of the fifth nerve no distinct cleft is 
 present unless we regard the mouth as such. Embryology thus 
 appears to teach us that the fifth nerve is a single nerve supply- 
 ing the mandibular arch, and not, as has been usually thought, a 
 
DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 complex nerve resulting from the coalescence of two or three 
 distinct nerves. My observations do not embrace the origin or 
 history of the third, fourth, and sixth nerves, but it is hardly 
 possible to help suspecting that in these we have the nerve of 
 one or more segments in front of that supplied by the fifth 
 nerve ; a view which well accords with the most recent morpho- 
 logical speculations of Professor Huxley 1 . 
 
 From this enumeration of the nerves the optic nerve is ex- 
 cluded for obvious reasons, and although it has been shewn 
 above that the olfactory nerve developes like the other nerves 
 as an outgrowth from the brain, yet its very late appearance 
 and peculiar relations are, at least for the present, to my mind 
 sufficient grounds for excluding it from the category of seg- 
 mental cranial nerves. 
 
 The nerves then give us indications of seven cranial seg- 
 ments, or, if the nerves to the eye-muscles be included, of at the 
 least eight segments, but to these must be added a number of 
 segments now lost, but which once existed behind the last of 
 those at present remaining. 
 
 The branchial clefts have been regarded as guides to seg- 
 mentation by Gegenbaur, Huxley, Semper, etc., and this view 
 cannot I think be controverted. In Scyllium there are six 
 clefts which give indications of seven segments, viz., the seg- 
 ments of the mandibular arch, hyoid arch, and of the five 
 branchial arches. If, following the views of Dr Dohrn 2 , we 
 regard the mouth as representing a cleft, we shall have seven 
 clefts and eight segments ; and it is possible, as pointed out in 
 Dr Dohrn's very suggestive pamphlet, that remnants of a still 
 greater number of praeoral clefts may still be in existence. 
 Whatever may be the value of these speculations, such forms 
 as Hexanchus and Heptanchus and Amphioxus make it all but 
 certain that the ancestors of Vertebrates had a number of clefts 
 behind those now developed. 
 
 The last group of organs to be dealt with for our present 
 question is that of the Head-Cavities. 
 
 The walls of the spaces formed by the cephalic prolongations 
 
 1 Preliminary note upon the brain and skull of Amphioxus, Proc. of the Royal 
 Society, Vol. xxir. 
 
 8 Ursprung d. Wirbelthiere. 
 
SEGMENTATION OF THE HEAD. 
 
 439 
 
 of the body-cavity develope into muscles and resemble the 
 muscle-plates of the trunk, and with these they must be identi- 
 fied, as has been already stated. As equivalent to the muscle- 
 plates, they clearly are capable of serving as very valuable guides 
 for determining the segmentation of the head. There are then 
 a pair of these in front of the mandibular arch, a pair in the 
 mandibular arch, and a pair in each succeeding arch. In all 
 there are eight pairs of these cavities representing eight seg- 
 ments, the first of them praeoral. As was mentioned above, 
 each of the sections of the head-cavity (except perhaps the first) 
 stands in a definite relation to the nerve and artery of the arch 
 in which it is situated. 
 
 The comparative results of these three independent methods 
 of determining the segmentation of the head are in the sub- 
 joined table represented in a form in which they can be com- 
 pared : 
 
 Table of the Cephalic Segments as determined by the Nerves, Visceral 
 Arches, and Head- Cavities. 
 
 Segments 
 
 Nerves 
 
 Visceral Arches 
 
 Head-Cavities or 
 Cranial Muscle-Plates 
 
 PicEoral i 
 Postoral -2 
 
 3rd and 4th and ? 6th 
 nerves (perhaps repre- 
 senting more than one 
 segment) 
 
 5th nerve 
 
 7th nerve 
 Glossopharyngeal nerve 
 ist branch of vagus 
 2nd branch of vagus 
 3rd branch of vagus 
 4th branch of vagus 
 
 (?) 
 
 Mandibular 
 
 Hyoid 
 ist branchial arch 
 2nd branchial arch 
 3rd branchial arch 
 4th branchial arch 
 5th branchial arch 
 
 ist head-cavity 
 (in my figures i //.) 
 
 2nd head-cavity 
 (in my figures 2 pp.) 
 
 3rd head-cavity 
 4th head-cavity 
 5th head-cavity 
 6th head-cavity 
 7th head-cavity 
 8th head-cavity 
 
 3 
 
 ' 4 
 
 5 
 - f\ 
 
 7 
 
 
 In the above table the first column denotes the segments of 
 the head as indicated by a comparison of the three sets of 
 organs employed. The second column denotes the segments as 
 
44O DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 obtained by an examination of the nerves ; the third column is 
 for the visceral arches (which lead to the same results as, but are 
 more convenient for our table than, the visceral clefts), and the 
 fourth column is for the head-cavities. It may be noticed that 
 from the second segment backwards the three sets of organs 
 lead to the same results. The head-cavities indicate one seg- 
 ment in front of the mouth, and now that the ophthalmic branch 
 of the fifth has been dethroned from its position as a separate 
 nerve, the eye-nerves, or one of them, may probably be regarded 
 as belonging to this segment. If the suggestion made above 
 (p. 431), that the walls of the first cavity become the eye- 
 muscles, be correct, the eye-nerves would perhaps after all be 
 the most suitable nerves to regard as belonging to the segment 
 of the first head-cavity. 
 
 EXPLANATION OF PLATES 15, 16, 17. 
 
 PLATE 15. (THE HEAD DURING STAGES G K.) 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 i aa, iaa, etc. * ist, 2d, etc. aortic arch. acv. Anterior cardinal vein. al. Ali- 
 mentary canal, ao. Aorta. an. Thickening of epiblast to form the auditory pit. 
 aun. Auditory nerve, aup. Auditory pit. auv. Auditory vesicle, b. Wall of 
 brain, bb. Base of brain. cb. Cerebellum, cer. Cerebrum. Ch. Choroid slit. 
 ch. Notochord. com. Commissure connecting roots of vagus nerve, i, 2, 3 etc. 
 eg. External gills, ep. External epiblast. fb. Fore-brain, gl. Glossopharyngeal 
 nerve, h b. Hind-brain, ht. Heart, hy. Hyaloid membrane. In. Infundibulum. 
 /. Lens. M. Mouth involution, m. Mesoblast at the base of the brain, m b. Mid- 
 brain, mn. v. Mandibular branch of fifth, ol. Olfactory pit. op. Eye. opn. Optic 
 nerve, opv. Optic vesicle, opth v. Ophthalmic branch of fifth, p. Posterior root 
 of spinal nerve, pn. Pineal gland. 1,2 etc. pp. First, second, etc. section of body- 
 cavity in the head. pt. Pituitary body. so. Somatopleure. sp. Splanchnopleure. 
 spc. Spinal cord. Th. Thyroid body. v. Blood-vessel, iv. v. Fourth ventricle, 
 v. Fifth nerve. V ' c. Visceral cleft. Vg. Vagus, vii. Seventh or facial nerve. 
 
 Fig. i. Head of a Pristiurus embryo of stage K viewed as a transparent object. 
 
 The points which deserve special attention are: (i) The sections of the body- 
 cavity in the head (pp) : the first or premandibular section being situated close to the 
 eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. 
 The third at the summit of the hyoid arch. 
 
 The cranial nerves and the general appearance of the brain are well shewn in the 
 figure. 
 
EXPLANATION OF PLATE 15. 44! 
 
 The notochord cannot be traced in the living embryo so far forward as it is repre- 
 sented. It has been inserted according to the position which it is seen to occupy in 
 sections. 
 
 Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K, 
 viewed as a transparent object. 
 
 The figure shews the condition of the brain ; the branches of the fifth and sevchtll 
 nerves (v. vii.) ; the rudiments of the semicircular canals ; and the commencing 
 appearance of the external gills as buds on both walls of 2nd, 3rd, and 4th clefts. 
 The external gills have not appeared on the first cleft or spiracle. 
 
 Fig. 3. Section through the head of a Pristiurus embryo during stage G. It 
 shews (i) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the 
 brain. (2) The optic vesicles not yet constricted off from the fore-brain. 
 
 Figs. 4 a and ^b. Two sections through the head of a Pristiurus embryo of 
 stage I. They shew (i) the appearance of the seventh nerve. (2) The portion of the 
 body cavity belonging to the first and second visceral arches. (3) The commencing 
 thickening of epiblast to form the auditory involution. 
 
 In 4 b, the posterior of the two sections, no trace of an auditory nerve is to be seen. 
 
 Figs. 5 a and 5 b. Two sections through the head of a Torpedo embryo with 3 
 visceral clefts. Zeiss A, ocul. i. 
 
 5 a shews the formation of the thin roof of the fourth ventricle by a divarication of 
 the two lateral halves of the brain. 
 
 Both sections shew the commencing formation of the thyroid body (ffi) at the base 
 of the mandibular arch. 
 
 They also illustrate the formation of the visceral clefts by an outgrowth from the 
 alimentary tract without any corresponding ingrowth of the external epiblast. 
 
 Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. 
 Zeiss A, ocul. i. 
 
 The section shews (i) the attachment of a branch of the vagus to the walls of the 
 hind-brain. (2) The peculiar form of the hind-brain. 
 
 Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to 
 a stage intermediate between I and K, passing through both the fore-brain and the 
 hind -brain. Zeiss A, ocul. i. 
 
 The section illustrates (i) the formation of the pituitary body (//) from the mouth 
 involution (;), and proves that, although the wall of the throat (al) is in contact with 
 the mouth involution, there is by this stage no communication between the two. 
 (2) The eye. (3) The sections of the body-cavity in the head (i//, 2//). (4) The 
 fifth nerve (v.) and the seventh nerve (vii.). 
 
 Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. 
 It shews the ventral junction of the anterior sections of the body-cavity in the head 
 
 ('//) 
 
 Figs. 9 a and gb. Two longitudinal sections through the brain of a Pristiurus 
 embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. i. 
 
 9 a is taken through the median line, but is reconstructed from two sections. It 
 shews (i) The divisions of the brain The cerebrum and thalamencephalon in the 
 fore-brain ; the mid-brain ; the commencing cerebellum in the hind-brain. (2) The 
 relation of the mouth involution to the infundibulum. (3) The termination of the 
 notochord. 
 
 B. 29 
 
442 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 gb is a section to one side of the same brain. It shews (i) The divisions of the 
 brain. (2) The point of outgrowth of the optic nerves (ppri). (3) The sections of 
 the body-cavity in the head and the bifurcation of the optic nerve over the second of 
 these. 
 
 Fig. 10. Longitudinal section through the head of a Pristiurus embryo somewhat 
 younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of the nerves and the 
 junction of the fifth, seventh, and auditory nerves with the brain. 
 
 Fig. n. Longitudinal section through the fore-brain of a Pristiurus embryo of 
 stage K, slightly to one side of the middle line. It shews the deep constriction 
 separating the thalamencephalon from the cerebral hemispheres. 
 
 Fig. 12. Longitudinal section through the base of the brain of an embryo of a 
 stage intermediate between I and K. 
 
 It shews (i) the condition of the end of the notochord; (2) the relation of the 
 mouth involution to the infundibulum. 
 
 Fig. 130. Longitudinal and horizontal section through part of the head of a 
 Pristiurus embryo rather older than K. Zeiss A, ocul. i. 
 
 The figure contains the eye cut through in the plane of the choroid slit. Thus the 
 optic nerve (op n) and choroid slit (ch) are both exhibited. Through the latter is 
 seen passing mesoblast accompanied by a blood-vessel (v). Op represents part of the 
 optic vesicle to one side of the choroid slit. 
 
 No mesoblast can be seen passing round the outside of the optic cup ; and the only 
 mesoblast which enters the optic cup passes through the choroid slit. 
 
 Fig. 13^. Transverse section through the last arch but one of the same embryo 
 as 130. Zeiss A, ocul. i. 
 
 The figure shews ( i ) The mode of formation of a visceral cleft without any involu- 
 tion of the external skin. (2) The head-cavity in the arch and its situation in relation 
 to the aortic arch. 
 
 Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13, con- 
 siderably magnified. The specimen was prepared by removing the nasal pit, flattening 
 it out and mounting in glycerine after treatment with chromic acid. It shews the 
 primitive arrangement of the Schneiderian folds. One side has been injured. 
 
 Figs. i5 and 15 3. Two longitudinal and vertical sections through the head of a 
 Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3. 
 
 15 is the most superficial section of the two. It shews the constitution of the 
 seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior 
 branch of the seventh nerve deserves a special notice. 
 
 15 mainly illustrates the dorsal commissure of the vagus nerve (com) continuous 
 with the dorsal commissures of the posterior root of the spinal nerves. 
 
 Fig. 1 6. Two longitudinal and vertical sections of the head of a Pristiurus 
 embryo belonging to the end of stage K. Zeiss a, ocul. i. 
 
 i6a passes through the median line of the brain and shews the infundibulum, 
 notochord and pituitary body, etc. 
 
 The pituitary body still opens into the mouth, though the septum between the 
 mouth and the throat is broken through. 
 
 \6b is a more superficial section shewing the head-cavities// i, 2, 3, and the 
 lower vagus commissure. 
 
EXPLANATION OF PLATE 1 6. 443 
 
 PLATE 16. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 au v. Auditory vesicle, cb. Cerebellum, cer. Cerebral hemispheres, ch, Notcv 
 chord, cin. Internal carotid, ft. Fasciculi teretes. in, Infundibulum. Iv. 
 Lateral ventricle, m b. Mid-brain, or optic lobes, md. Medulla oblongata. mn, 
 Mandible. ol. Olfactory pit. oil. Olfactory lobe, op. Eye. opn. Optic nerve. 
 opth. Optic thalamus. pc. Posterior commissure, pel. Posterior clinoid. pn, 
 Pineal gland, pt. Pituitary body, r t, Restiform tracts, t v. Tela vasculosa of the 
 roof of the fourth ventricle, iv. v. Fourth ventricle, vii. Seventh nerve, x. Rudi- 
 ment of septum which will grow backwards and divide the unpaired cerebral rudiment 
 into the two hemispheres. 
 
 Figs, i a, i b, ic. Longitudinal sections of the brain of a Scyllium embryo 
 belonging to stage L. Zeiss a, ocul I. 
 
 i a is taken slightly to one side of the middle line, and shews the general features 
 of the brain, and more especially the infundibulum (in) and pituitary body (ft). 
 
 i b is through the median line of the pineal gland. 
 
 i c is through the median line of the base of the brain, and shews the notochord 
 (ch) and pituitary body (pt) ; the latter still communicating with the mouth. It also 
 shews the wide opening of the infundibulum in the middle line into the base of the 
 brain. 
 
 Fig. 2. Section through the unpaired cerebral rudiment during stage O, to shew 
 the origin of the olfactory lobe and the olfactory nerve. The latter is seen to divide 
 into numerous branches, one of which passes into each Schneiderian fold. At its 
 origin are numerous ganglion cells represented by dots. Zeiss a, ocul. 2. 
 
 Fig. 3. Horizontal section through the three lobes of the brain during stage O. 
 Zeiss a, ocul. 2. 
 
 The figure shews (i) the very slight indications which have appeared by this 
 stage of an ingrowth to divide the cerebral rudiment into two lobes (x) : (2) the optic 
 thalami united by a posterior commissure, and on one side joining the base of the 
 mid-brain, and behind them the pineal gland : (3) the thin posterior wall of the 
 cerebral rudiment with folds projecting into the cerebral cavity. 
 
 Figs. 4 a, 4^, \c. Views from the side, from above, and from below, of a brain 
 of Scyllium canicula during stage P. In the view from the side the eye (op) has not 
 been removed. 
 
 The bilobed appearance both of the mid-brain and cerebellum should be noticed. 
 
 Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage P. 
 Zeiss a, ocul. 2. 
 
 There should be noticed ( i ) the increase in the flexure of the brain accompanying 
 a rectification of the cranial axis ; (2) the elongated pineal gland, and (3) the structure 
 of the optic thalamus. 
 
 Figs. 6 a, 6 1>, 6c, Views from the side, from above, and from below, of a brain 
 of Scyllium stellare during a slightly later stage than Q. 
 
 29 2 
 
444 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Figs. 7 a and 7 b. Two longitudinal sections through the brain of a Scyllium 
 embryo during stage Q. Zeiss a, ocul. i. 
 
 ja cuts the hind part of the brain nearly through the middle line ; while ib cuts 
 the cerebral hemispheres and pineal gland through the middle. 
 
 In 7 the infundibulum (i), cerebellum (2), the passage of the restiform tracts (rt) 
 into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7 b 
 the septum between the two lobes of the cerebral hemispheres (i), the pineal gland (2), 
 and the relations of the optic thalami (3) are shewn. 
 
 Figs. 8 a, 8 b, 8 c, 8 d. Four transverse sections of the brain of an embryo slightly 
 older than Q. Zeiss a, ocul. i . 
 
 8 a passes through the cerebral hemispheres at their junction with the olfactory 
 lobes. On the right side is seen the olfactory nerve coming off from the olfactory 
 lobe. At the dorsal side of the hemispheres is seen the pineal gland (fn). 
 
 8 b passes through the mid-brain now slightly bilobed, and the opening into the 
 infundibulum (in). At the base of the section are seen, the optic nerves and their 
 chiasma. 
 
 8 c passes through the opening from the ventricle of the mid-brain into that of the 
 cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of 
 the sacci vasculosi. 
 
 8 d passes through the front end of the medulla, and shews the roots of the seventh 
 pair of nerves, and the overlapping of the medulla by the cerebellum. 
 
 PLATE 17. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 vii. a. Anterior branch of seventh nerve, a r. Anterior root of spinal nerve. 
 auv. Auditory vesicle, cer. Cerebrum, ch. Notochord. eh. Epithelial layer of 
 choroid membrane, gl. Glossopharyngeal nerve, vii. hy. Hyoid branch of seventh 
 nerve, hym. Hyaloid membrane. //. Lateral line. v. mn. Ramus mandibularis 
 of fifth nerve, vii. mn. Mandibular (spiracular) branch of seventh nerve, v. mx. 
 Ramus maxillae superioris of fifth nerve, n I. Nervus lateralis. ol. Olfactory pit. 
 op. Eye. v. op th. Ramus ophthalmicus of fifth nerve. / ch. Parachordal cartilage. 
 pfal. Processus falciformis. pp. Head cavity, pr. Posterior root of spinal nerve. 
 rt. Retina, sp. Spiracle, v. Fifth nerve, vii. Seventh nerve, v c. Visceral cleft. 
 vg. Vagus nerve, vgbr. Branchial branch of vagus, vgcom. Commissure uniting 
 the roots of the vagus, and continuous with commissure uniting the posterior roots of 
 the spinal nerves, vgr. Roots of vagus nerves in the brain, vgin. Intestinal branch 
 of vagus, v h. Vitreous humour. 
 
 Fig. i. Diagram of cranial nerves at stage L. 
 
 A description of the part of this referring to the vagus and glossopharyngeal 
 nerves is given at p. 426. It should be noticed that there are only five strands 
 indicated as springing from the spinal cord to form the vagus and glossopharyngeal 
 nerves. It is however probable that there are even from the first a greater number 
 of strands than this. 
 
EXPLANATION OF PLATE I/. 
 
 445 
 
 Fig. 2. Section through the hinder part of the medulla oblongata, stage between 
 K and L. Zeiss A, ocul. 2. 
 
 It shews (i) the vagus commissure with branches on one side from the medulla : 
 (i) the intestinal branch of the vagus giving off a nerve to the lateral line. 
 
 Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo 
 of stage L. Zeiss a, ocul. 2. 
 
 It shews the course of the anterior branch of the seventh nerve (vii.) ; especially 
 with relation to the ophthalmic branch of the fifth nerve (v. o th). 
 
 Figs. 4 a and 4^. Two horizontal and longitudinal sections through the head of a 
 Scyllium embryo belonging to stage O. Zeiss a, ocul. i. 
 
 4 a is the most dorsal of the two sections, and shews the course of the anterior 
 branch of the seventh nerve above the eye. 
 
 4 b is a slightly more ventral section, and shews the course of the fifth nerve. 
 
 Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, 
 shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B, 
 ocul. 2. 
 
 There appears to be one root in the brain for the glossopharyngeal, and at least 
 six for the vagus. The fibres from the roots divide in many cases into two bundles 
 before leaving the brain. Swellings of the brain towards the interior of the fourth 
 ventricle are in connection with the first five roots of the vagus, and the glosso- 
 pharyngeal root ; and a swelling is also intercalated between the first vagus root and 
 the glossopharyngeal root. 
 
 Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B, 
 ocul. 2. 
 
 The figure shews (i) the rudimentary processus falciformis (pfal) giving origin to 
 the vitreous humour; and (2) the hyaloid membrane (Ay m) which is seen to adhere 
 to the retina, and not to the vitreous humour or processus falciformis. 
 
CHAPTER X. 
 THE ALIMENTARY CANAL. 
 
 THE present Chapter completes the history of the primitive 
 alimentary canal, whose formation has already been described. 
 In order to economise space, no attempt has been made to give 
 a full account of the alimentary canal and its appendages, but 
 only those points have been dealt with which present any 
 features of special interest. 
 
 The development of the following organs is described in 
 order. 
 
 (1) The solid oesophagus. 
 
 (2) The postanal section of the alimentary tract. 
 
 (3) The cloaca and anus. 
 
 (4) The thyroid body. 
 
 (5) The pancreas. 
 
 (6) The liver. 
 
 (7) The subnotochordal rod. 
 
 The solid oesophagus. 
 
 A curious point which has turned up in the course of my 
 investigations is the fact that for a considerable period of em- 
 bryonic life a part of the oesophagus remains quite solid and 
 without a lumen. The part of the oesophagus to undergo this 
 peculiar change is that which overlies the heart, and extends 
 from the front end of the stomach to the branchial region. At 
 first, this part of the oesophagus has the form of a tube with 
 a well- developed lumen like the remainder of the alimentary 
 
POSTANAL SECTION OF ALIMENTARY CANAL. 447 
 
 tract, but at a stage slightly younger than K its lumen becomes 
 smaller, and finally vanishes, and the original tube is replaced 
 by a solid rod of uniform and somewhat polygonal cells. A 
 section of it in this condition is represented in PI. n, fig. 8 a. 
 
 At a slightly later stage its outermost cells become more 
 columnar than the remainder, and between stages K and L it 
 loses its cylindrical form and becomes much more flattened. 
 By stage L the external layer of columnar cells is more definitely 
 established, and the central rounded cells are no longer so 
 numerous (PI. 18, fig. 4, s ces.}. 
 
 In the succeeding stages the solid part of the oesophagus 
 immediately adjoining the stomach is carried farther back 
 relatively to the heart and overlies the front end of the liver. 
 A lumen is not however formed in it by the close of stage Q, 
 and beyond that period I have not carried my investigations, 
 and cannot therefore state the exact period at which the lumen 
 reappears. The limits of the solid part of the oesophagus are 
 very satisfactorily shewn in longitudinal and vertical sections. 
 
 The solidification of the oesophagus belongs to a class of 
 embryological phenomena which are curious rather than in- 
 teresting, and are mainly worth recording from the possibility 
 of their turning out to have some unsuspected morphological 
 bearings. 
 
 Up to stage Q there are no signs of a rudimentary air- 
 bladder. 
 
 The postanal section of the alimentary tract. 
 
 An account has already been given (p. 307) of the posterior 
 continuity of the neural and alimentary canals, and it was there 
 stated that Kowalevsky was the discoverer of this peculiar 
 arrangement. Since that account was published, Kowalevsky 
 has given further details of his investigations on this point, and 
 more especially describes the later history of the hindermost 
 section of the alimentary tract. He says 1 : 
 
 The two germinal layers, epiblast and hypoblast, are continuous with 
 each other at the border of the germinal disc. The primitive groove or 
 
 1 Archiv f. Mic. Anat. Vol. XIII. pp. 194, 195. 
 
448 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 furrow appears at the border of the germinal disc and is continued from the 
 upper to the lower side. By the closing of the groove there is formed the 
 medullary canal above, while the part of the groove on the under surface 
 directed below is chiefly converted into the hind end of the alimentary 
 tract. The connection of the two tubes in Acanthias persists till the for- 
 mation of the anus, and the part of the nervous tube which lies under the 
 chorda passes gradually upwards to the dorsal side of the chorda, and per- 
 sists there for a long time in the form of a large thin-walled vesicle. 
 
 The last part of the description beginning at " The con- 
 nection of" does not hold good for any of the genera which I 
 have had an opportunity of investigating, as will appear from 
 the sequel. 
 
 In a previous section 1 the history of the alimentary tract was 
 completed up to stage G. 
 
 In stage H the point where the anus will (at a very much 
 later period) appear, becomes marked out by the alimentary 
 tract sending down a papilliform process towards the skin. 
 This is shewn in PI. 8, figs. H and /, an. 
 
 That part of the alimentary tract which is situated behind 
 this point may, for convenience, be called the postanal section. 
 During stage H the postanal section begins to develope a 
 terminal dilatation or vesicle, connected with the remainder of 
 the canal by a narrower stalk. The relation in diameter be- 
 tween the vesicle and the stalk may be gathered by a com- 
 parison of figs. 30 and 3^, PI. n. The diameter of the vesicle 
 represented in section in PI. n, fig. 3, is O'328 Mm. 
 
 The walls both of the vesicle and stalk are formed of a fairly 
 columnar epithelium. The vesicle communicates in front by a 
 narrow passage (PI. n, fig. $a) with the neural canal, and 
 behind is continued into two horns (PI. 11, fig. 2, al.) cor- 
 responding with the two caudal swellings spoken of above 
 (p. 288). Where the canal is continued into these two horns, 
 its walls lose their distinctness of outline, and become con- 
 tinuous with the adjacent mesoblast. 
 
 In the succeeding stages up to K the tail grows longer and 
 longer, and with it grows the postanal section of the alimen- 
 tary tract, without however altering in any of its essential 
 characters. 
 
 1 P- 33 et sec l- 
 
POSTANAL SECTION OF ALIMENTARY CANAL. 449 
 
 Its features at stage K are illustrated by an optical section 
 of the tail of an embryo (PI. 18, fig. 5) and by a series of trans- 
 verse sections through the tail of another embryo in PL 18, 
 figs. 6a, 6b, 6c, 6d. In the optical section there is seen a terminal 
 vesicle (alv.) opening into the neural canal, and connected with 
 the remainder of the alimentary tract. The terminal vesicle 
 causes the end of the tail to be dilated, as is shewn in PL 8, 
 fig. K. The length of the postanal section extending from the 
 abdominal paired fins to the end of the tail (equal to rather less 
 than one-third of the whole length of the embryo), may be 
 gathered from the same figure. 
 
 The most accurate method of studying this part of the 
 alimentary canal is by means of transverse sections. Four 
 sections have been selected for illustration (PL 18, figs. 6a, 6b, 
 6c, and 6d} out of a fairly-complete series of about one hundred 
 and twenty. 
 
 Posteriorly (fig. 6a) there is present a terminal vesicle 
 25 Mm. in diameter, and therefore rather smaller than in the 
 earlier stage, whose walls are formed of columnar epithelium, 
 and which communicates dorsally by a narrow opening with the 
 neural canal ; to this is attached a stalk in the form of a tube, 
 also lined by columnar epithelium, and extending through 
 about thirty sections (PL 18, fig. 6b}. Its average diameter is 
 about '084 Mm. Overlying its front end is the subnotochordal 
 rod (fig. 6b, x.}, but this does not extend as far back as the 
 terminal vesicle. 
 
 The thick-walled stalk of the vesicle is connected with the 
 cloacal section of the alimentary tract by a very narrow thin- 
 walled tube (PL 1 8, 6c, al.}. This for the most part has a fairly 
 uniform calibre, and a diameter of not more than '035 Mm. 
 Its walls are formed of a flattened epithelium. At a point not 
 far from the cloaca it becomes smaller, and its diameter falls 
 to '03 Mm. In front of this point it rapidly dilates again, and, 
 after becoming fairly wide, opens on the dorsal side of the 
 cloacal section of the alimentary canal just behind the anus 
 (fig. &/). 
 
 Near the close of stage K at a point shortly behind the 
 anus, where the postanal section of the canal was thinnest in 
 the early part of the stage, the alimentary canal becomes solid 
 
4SO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 (PI. 1 1, fig. <)d}, and a rupture here occurs in it at a slightly later 
 period. 
 
 In stage L the posterior part of the postanal section of the 
 canal is represented by a small rudiment near the end of the 
 tail. The rudiment no longer has a terminal vesicle, nor does 
 it communicate with the neural canal. It was visible in one 
 series for about 40 sections, and was continued forwards by a 
 few granular cells, lying between the aorta and the caudal vein. 
 The portion of the postanal section of the alimentary tract just 
 behind the cloaca, was in the same embryo represented by a 
 still smaller rudiment of the dilated part which at an earlier 
 period opened into the cloaca. 
 
 Later than stage L no trace of the postanal section of the 
 alimentary canal has come under my notice, and I conclude that 
 it vanishes without becoming converted into any organ in the 
 adult. Since my preliminary account of the development of 
 Elasmobranch Fishes was written, no fresh light appears to 
 have been thrown on the question of the postanal section of the 
 alimentary canal being represented in higher Vertebrata by the 
 allantois. 
 
 The cloaca and anus. 
 
 Elasmobranchs agree closely with other Vertebrates in the 
 formation of the cloaca and anus, and in the relations of the 
 cloaca to the urinogenital ducts. 
 
 The point where the anus, or more precisely the external 
 opening of the cloaca, will be formed, becomes very early 
 marked out by the approximation of the wall of the alimentary 
 tract and external skin. This is shewn for stages H and I in 
 PI. 8 an. 
 
 Between stages I and K the alimentary canal on either side 
 of this point, which we may for brevity speak of as the anus, is 
 far removed from the external skin, but at the anus itself the 
 lining of the alimentary canal and the skin are in absolute 
 contact. There is, however, no involution from the exterior, 
 but, on the contrary, the position of the anus is marked by a 
 distinct prominence. Opposite the anus the alimentary canal 
 dilates and forms the cloaca. 
 
CLOACA AND ANUS. 451 
 
 During stage K, just in front of the prominence of the anus, 
 a groove is formed between two downgrowths of the body-wall. 
 This is shewn in PI. n, fig. ga. During the same stage the 
 segmental ducts grow downwards to the cloaca, and open into it 
 in the succeeding stage (PI. n, fig. gb). Up to stage" K the 
 cloaca is connected with the prseanal section of the alimentary 
 canal in front, and the postanal section behind ; the latter, how- 
 ever, by stage L, as has been stated above, atrophies, with the 
 exception of a very small rudiment. In stage L the posterior 
 part of the cloaca is on a level with the hind end of the kidneys, 
 and is situated behind the posterior horns of the body-cavity, 
 which are continued backwards to about the point where the 
 segmental ducts open into the cloaca, and though very small at 
 their termination rapidly increase in size anteriorly. 
 
 Nothing very worthy of note takes place in connection with 
 the cloaca till stage O. By this stage we have three important 
 structures developed, (i) An involution from the exterior to 
 form the mouth of the cloaca or anus. (2) A perforation leading 
 into the cloaca at the hind end of this. (3) The rudiments of 
 the abdominal pockets. All of these structures are shewn in 
 PI. 19, figs, i a, ib, ic. 
 
 The mouth of the cloaca is formed by an involution of the 
 skin, which is deepest in front and becomes very shallow behind 
 (PI. 19, figs. I a, ib). At first only the mucous layer of the skin 
 takes part in it, but when the involution forms a true groove, 
 both layers of the skin serve to line it. At its posterior part, 
 where it is shallowest, there is present, at stage O, a slit-like 
 longitudinal perforation, leading into the posterior part of the 
 cloaca (PI. 19, fig. ic) and forming its external opening. Else- 
 where the wall of the cloaca and cloacal groove are merely in 
 contact but do not communicate. On each side of the external 
 opening of the cloaca there is present an involution (PI. 19, fig. 
 ic, ab.p.} of the skin, which resembles the median cloacal involu- 
 tion, and forms the rudiment of an abdominal pocket. These 
 two rudiments must not be confused with two similar ones, which 
 are present in all the three sections represented, and mark out 
 the line which separates the limbs from the trunk. These latter 
 are not present in the succeeding stages. The abdominal 
 pockets are only found in sections through the opening into 
 
452 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the cloaca ; and are only visible in the hindermost of my three 
 .sections. 
 
 All the structures of the adult cloaca appear to be already 
 constituted by stage O, and the subsequent changes, so far as I 
 have investigated them, may be dealt with in very few words. 
 The perforation of the cloacal involution is carried slowly for- 
 wards, so that the opening into the cloaca, though retaining 
 its slit-like character, becomes continuously longer ; by stage Q 
 its size is very considerable. The cloacal involution, relatively 
 to the cloaca, recedes backwards. In stage O its anterior end is 
 situated some distance in front of the opening of the segmental 
 duct into the cloaca ; by stage P the front end of the cloacal 
 involution is nearly opposite this opening, and by stage Q is 
 situated behind it. 
 
 As I have shewn elsewhere 1 , the so-called abdominal pores 
 of Scyllium are simple pockets open to the exterior, but without 
 any communication with the body-cavity. By stage Q they are 
 considerably deeper than in stage O, and retain their original 
 position near the hind end of the opening into the cloaca. The 
 opening of the urinogenital ducts into the cloaca will be described 
 in the section devoted to the urinogenital system. 
 
 In Elasmobranchs, as in other Vertebrata, that part of the 
 cloaca which receives the urinogenital ducts, is in reality the 
 hindermost section of the gut and not the involution of epiblast 
 which eventually meets this. Thus the urinogenital ducts at 
 first open into the alimentary canal and not to the exterior. 
 This fact is certainly surprising, and its meaning is not quite 
 clear to me. 
 
 The very late appearance of the anus may be noticed as a 
 point in which Elasmobranchs agree with other Vertebrata, 
 notably the Fowl 2 . The abdominal pockets, as might be anti- 
 cipated from their structure in the adult, are simple involutions 
 of the epiblast. 
 
 The thyroid body. 
 
 The earliest trace of the thyroid body has come under 
 my notice in a Torpedo embryo slightly older than I. In this 
 
 1 This Edition, No. vn. p. 152. 
 
 3 Vide Gasser, Entwicklungsgeschichte der Allantois, etc. 
 
THE THYROID BODY. 453 
 
 embryo it appeared as a diverticulum from the ventral surface 
 of the throat in the region of the mandibular arch, and extended 
 from the border of the mouth to the point where the ventral 
 aorta divided into the two aortic branches of the mandibular 
 arch. In front it bounded a groove (PI. 15, fig. $a, T/i.}, directly 
 continuous with the narrow posterior pointed end of the mouth 
 and open to the throat, while behind it became a solid rod 
 attached to the ventral wall of the oesophagus (PI. 15, fig. $b, 
 Th.). In a Scyllium embryo belonging to the early part of 
 stage K, the thyroid gland presented the same arrangement as 
 in the Torpedo embryo just described, with the exception that 
 no solid posterior section of it was present. 
 
 Towards the close of stage K the thyroid body begins to 
 elongate and become solid, though it still retains its attachment 
 to the wall of the oesophagus. The solidification is effected by 
 the columnar cells which line the groove elongating and meeting 
 in the centre. As soon as the lumen is by these means obliterated* 
 small cells make their appearance in the interior of the body, 
 probably budded off from the original columnar cells. 
 
 The gland continues to grow in length, and by stage L 
 assumes a long sack-like form with a layer of columnar cells 
 bounding it externally, and a core of rounded cells filling up its 
 interior. Anteriorly it is still attached to the throat, and its 
 posterior extremity lies immediately below the end of the ven- 
 tral aorta. The cells of the gland contain numerous yellowish 
 concretionary pigment bodies, which are also present in the later 
 stages. 
 
 Up to stage P the thyroid gland retains its original position. 
 Its form and situation are shewn in PI. 19, fig. 3, th., in longitu- 
 dinal and vertical section for a stage between O and P. The 
 external layer of columnar cells has now vanished, and the gland 
 is divided up by the ingrowth of connective-tissue septa into a 
 number of areas or lobules the rudiments of the future follicles. 
 These lobules are perfectly solid without any trace of a lumen. 
 A capillary network following the septa is present. 
 
 By stage Q the rudimentary follicles are more distinctly 
 marked, but still without a lumen, and a connective-tissue sheath 
 indistinctly separated from the surrounding tissue has been 
 formed. My sections do not shew a junction between the gland 
 
454 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 and the epithelium of the throat ; but the two are so close 
 together, that I am inclined to think that such a junction still 
 exists. It is certainly present up to stage P. 
 
 Dr MUller 1 , in his exhaustive memoir on the thyroid body, 
 gives an account of its condition in two Acanthias embryos. In 
 his earliest embryo (which, judging from the size, is perhaps 
 about the same age as my latest) the thyroid body is discon- 
 nected from the throat, yet contains a lumen, and is not divided 
 up into lobules. It is clear from this account, that there must 
 be considerable differences of detail in the development of the 
 thyroid body in Acanthias and Scyllium. 
 
 In the Bird Dr Muller's figures shew that the thyroid body 
 developes in the region of the hyoid arch, whereas, in Elasmo- 
 branchs, it developes in the region of the mandibular arch. 
 Dr Gotte's 2 account of this body in Bombinator accords very 
 completely with my own, both with reference to the region in 
 which it developes, and its mode of development. 
 
 The pancreas. 
 
 The pancreas arises towards the close of stage K as a some- 
 what rounded hollow outgrowth from the dorsal side of that 
 part of the gut which from its homologies may be called the 
 duodenum. In the region where the pancreas is being formed 
 the appearances presented in a series of transverse sections are 
 somewhat complicated (PL 18, fig. i), owing to the several parts 
 of the gut and its appendages which may appear in a single 
 section, but I have detected no trace of other than a single out- 
 growth to form the pancreas. 
 
 By stage L the original outgrowth from the gut has become 
 elongated longitudinally, but transversely compressed : at the 
 same time its opening into the duodenum has become some- 
 what narrowed. 
 
 Owing to these changes the pancreas presents in longitudinal 
 and vertical section a funnel-shaped appearance (PL 19, fig. 4). 
 From the expanded dorsal part of the funnel, especially from 
 its anterior end, numerous small tubular diverticula grow out 
 
 1 Jenaische Zeitsckrift, Vol. vi. 
 
 2 Entwicklungsgeschichte d. Unke. 
 
THE LIVER. 455 
 
 into the mesoblast. The apex of the funnel leads into the 
 duodenum. From this arrangement it results that at this period 
 the original outgrowth from the duodenum serves as a recep- 
 tacle into which each ductule of the embryonic gland opens 
 separately. I have not followed in detail the further growth of 
 the gland. It is, however, easy to note that while the ductules 
 grow longer and become branched, vascular processes grow in 
 between them, and the whole forms a compact glandular body 
 in the mesentery on the dorsal side of the alimentary tract, and 
 nearly on a level with the front end of the spiral valve. The 
 funnel-shaped receptacle loses its original form, and elongating, 
 assumes the character of a duct. 
 
 From the above account it follows that the glandular part 
 of the pancreas, and not merely its duct, is derived from the 
 original hypoblastic outgrowth from the gut. This point is 
 extremely clear in my preparations, and does not, in spite of 
 Schenk's observations to the contrary 1 , appear to me seriously 
 open to doubt. 
 
 The liver. 
 
 The liver arises during stage I as a ventral outgrowth from 
 the duodenum immediately in front of the opening of the 
 umbilical canal (duct of the yolk-sack) into the intestine. 
 Almost as soon as it is formed this outgrowth developes two 
 lateral diverticula opening into a median canal. 
 
 The two diverticula are the rudimentary lobes of the liver, 
 and the median duct is the rudiment of the common bile-duct 
 (ductus choledochus) and gall-bladder (PL n, fig. 9). 
 
 By stage K the hepatic diverticula have begun to bud out a 
 number of small hollow knobs. These rapidly increase in length 
 and number, and form the so-called hepatic cylinders. They 
 anastomose and unite together, so that by stage L there is con- 
 structed a regular network. As the cylinders increase in length 
 their lumen becomes very small, but appears never to vanish 
 (PL 19, ng. 5). 
 
 The mode of formation of the liver parenchyma by hollow 
 and not solid outgrowths agrees with the suggestion made in 
 
 1 Lehrbuch d. vergleichenden Embryologie. 
 
DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the Elements of Embryology, p. 133, and also with the results 
 of Gotte on the Amphibian liver. Schenk has thrown doubts 
 upon the hypoblastic nature of the secreting tissue of the liver, 
 but it does not appear to me, from my own investigations, that 
 this point is open to question. 
 
 Coincidently with the formation of the hepatic network, the 
 umbilical vein (PI. II, fig. 9, u. v.) which unites with the sub- 
 intestinal or splanchnic vein (PL n, fig. 8 V.) breaks up into a 
 series of channels, which form a second network in the spaces 
 of the hepatic network. These vascular channels of the liver 
 appear to me to have from the first distinct walls of delicate 
 spindle-shaped cells, and I have failed to find a stage similar to 
 that described by Gotte for Amphibians in which the blood- 
 channels are simply lacunar spaces in the hepatic parenchyma. 
 
 The changes of the median duct of the liver are of rather a 
 passive nature. By stage O its anterior end has dilated into 
 a distinct gall-bladder, whose duct receives in succession the 
 hepatic ducts, and so forms the ductus choledochus. The duc- 
 tus choledochus opens on the ventral side of the intestine im- 
 mediately in front of the commencement of the spiral valve. 
 
 It may be noted that the liver and pancreas are correspond- 
 ing ventral and dorsal appendages of the part of the alimentary 
 tract immediately in front of its junction with the yolk-sack. 
 
 The subnotochordal rod. 
 
 The existence of this remarkable body in Vertebrata was 
 first made known by Dr Gotte 1 , who not only demonstrated its 
 existence, but also gave a correct account of its development. 
 Its presence in Elasmobranchs and mode of development were 
 mentioned by myself in my preliminary account of the devel- 
 opment of these fishes 2 , and it has been independently ob- 
 served and described by Professor Semper 3 . No plausible 
 suggestion as to its function has hitherto been made, and it is 
 therefore a matter of some difficulty to settle with what group 
 
 1 Archiv fur Micros. Anatomic, Bd. V., and Entwicklungsgeschichte d. Unke. 
 
 2 Quarterly Journal of Microscopic Science, Oct, 1874. [This Edition, No. V.] 
 
 3 " Stammverwandschaft d. Wirbelthiere u. Wirbellosen " and " Das Urogenital- 
 system d. Plagiostomen," Arb. Zool. Zoot. Institut. z. Wiirzburg, Bd. 11. 
 
THE SUBNOTOCHORDAL ROD. 457 
 
 of organs it ought to be treated. In the presence of this 
 difficulty it seemed best to deal with it in this chapter, since it 
 is unquestionably developed from the wall of the alimentary 
 canal. 
 
 At its full growth this body forms a rod underlying the 
 notochord, and has nearly the same longitudinal extension as 
 this. It is indicated in most of my sections by the letter x. 
 We may distinguish two sections of it, the one situated in the 
 head, the other in the trunk. The junction between the two 
 occurs at the hind border of the visceral clefts. 
 
 The section in the trunk is the first to develope. It arises 
 during stage H in the manner illustrated in PI. 1 1, figs. I and la. 
 The wall of the alimentary canal becomes thickened (PI. 11, 
 fig. i) along the median dorsal line, or else produced into a 
 ridge into which there penetrates a narrow prolongation of the 
 lumen of the alimentary canal. In either case the cells at the 
 extreme summit of the thickening become gradually constricted 
 off as a rod, which lies immediately dorsal to the alimentary 
 tract, and ventral to the notochord. The shape of the rod 
 varies in the different regions of the body, but it is always 
 more or less elliptical in section. Owing to its small size and 
 soft structure it is easily distorted in the process of preparing 
 sections. 
 
 In the hindermost part of the body its mode of formation 
 differs somewhat from that above described. In this part the 
 alimentary wall is very thick and undergoes no special growth 
 prior to the formation of the subnotochordal rod ; on the con- 
 trary, a small linear portion of the wall becomes scooped out 
 along the median dorsal line, and eventually separates from the 
 remainder as the rod in question. In the trunk the splitting off 
 of the rod takes place from before backwards, so that the an- 
 terior part of it is formed before the posterior. 
 
 The section of the subnotochordal rod in the head would 
 appear from my observations on Pristiurus to develope in the 
 same way as in the trunk, and the splitting off from the throat 
 proceeds from before backwards (PL 15, fig. 40 x). 
 
 In Torpedo, this rod developes very much later in the Head 
 than in the trunk ; and indeed my conclusion that it developes 
 in the head at all is only based on grounds of analogy, since in 
 
 B. 30 
 
458 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 my oldest Torpedo embryo (just younger than K) there is no 
 trace of it present. In a Torpedo embryo of stage I the sub- 
 notochordal rod of the trunk terminated anteriorly by uniting 
 with the wall of the throat. The junction was effected by a 
 narrow pedicle, so that the rod appeared mushroom-shaped in 
 section, the stalk representing the pedicle of attachment. 
 
 On the formation of the dorsal aorta, the subnotochordal rod 
 becomes separated from the wall of the gut and the aorta in- 
 terposed between the two. 
 
 The subnotochordal rod attains its fullest development 
 during stage K. Anteriorly it terminates at a point well in 
 front of the ear, though a little behind the end of the noto- 
 chord ; posteriorly it extends very nearly to the extremity of 
 the tail and is almost co-extensive with the postanal section of 
 the alimentary tract, though it does not quite reach so far back 
 as the caudal vesicle (PI. 18, fig. 6bx). In stage L it is still 
 fairly large in the tail, though it has begun to atrophy an- 
 teriorly. We may therefore conclude that its atrophy, like its 
 development, takes place from before backwards. In the suc- 
 ceeding stages I have failed to find any trace of it, and con- 
 clude, as does Professor Semper, that it disappears completely. 
 
 Gotte 1 is of opinion that the subnotochordal rod is con- 
 verted into the dorsal lymphatic trunk, and regards it as the 
 anterior continuation of the postanal gut, which he believes to 
 be also converted into a lymphatic trunk. My observations 
 afford no support to these views, and the fact already men- 
 tioned, that the subnotochordal rod is nearly co-extensive with 
 the postanal section of the gut, renders it improbable that both 
 these structures are connected with the lymphatic system. 
 
 1 Entwicklungsgeschichte d. Unke, p. 775. 
 
EXPLANATION OF PLATE 1 8. 459 
 
 EXPLANATION OF PLATE 18. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 Nervous System. 
 
 a r. Anterior root of spinal nerve, n c. Neural canal. / r. Posterior root of 
 spinal nerve, sp n. Spinal nerve, sy g. Sympathetic ganglion. 
 
 Alimentary Canal. 
 
 al. Alimentary canal, al v. Caudal vesicle of the postanal gut. d al. Cloacal 
 section of alimentary canal, du. Duodenum, hpd. Ductus choledochus. pan. 
 pancreas, sees. Solid oesophagus, spv. Intestine with rudiment of spiral valve. 
 urn c. Umbilical canal. 
 
 General. 
 
 ao. Dorsal aorta, aur. Auricle of heart, ca v. Cardinal vein. ch. Notochord. 
 eppp. Epithelial lining of the body-cavity, ir. Interreual body. me. Mesentery. 
 mp. Muscle-plate, m p I. Muscle-plate sending a prolongation into the limb, p o. 
 Primitive ovum. pp. Body-cavity, s d. Segmental duct. st. Segmental tube. 
 ts. Tail swelling, v cau. Caudal vein. x. Subnotochordal rod. 
 
 Fig. i. Transverse section through the anterior abdominal region of an embryo 
 of a stage between K and L. Zeiss B, ocul. 2. Reduced one- third. 
 
 The section illustrates the junction of a sympathetic ganglion with a spinal nerve 
 and the sprouting of the muscle-plates into the limbs (mpl). 
 
 Fig. 2. Transverse section through the abdominal region of an embryo belonging 
 to stage L. Zeiss B, ocul. 2. Reduced one-third. 
 
 The section illustrates the junction of a sympathetic ganglion with a spinal nerve, 
 and also the commencing formation of a branch from the aorta (still solid) which will 
 pass through the sympathetic ganglion, and forms the first sign of the conversion 
 of part of a sympathetic ganglion into one of the suprarenal bodies. 
 
 Fig. 3. Longitudinal and vertical section of an embryo of a stage between L and 
 M, shewing the successive junctions of the spinal nerves and sympathetic ganglia. 
 
 Fig. 4. Section through the solid oesophagus during stage L. Zeiss A, ocul. i. 
 The section is taken through the region of the heart, so that the cavity of the auricle 
 (aur) lies immediately below the oesophagus. 
 
 Fig. 5. Optical section of the tail of an embryo between stages I and K, shewing 
 the junction between the neural and alimentary canals. 
 
 Fig. 6. Four sections through the caudal region of an embryo belonging to stage 
 K, shewing the condition of the postanal section of the alimentary tract. Zeiss A, 
 ocul. 2. An explanation of these figures is given on p. 449. 
 
 Fig. 7. Section through the interrenal body of a Scyllium embryo belonging to 
 stage Q. Zeiss C, ocul. 2. 
 
 Fig. 8. Portion of a section of the interrenal body of an adult Scyllium. Zeiss 
 C, ocul. i. 
 
 302 
 
CHAPTER XI. 
 THE VASCULAR SYSTEM AND VASCULAR GLANDS. 
 
 THE present chapter deals with the early development of the 
 heart, the development of the general circulatory system, es- 
 pecially the venous part of it, and the circulation of the yolk- 
 sack. It also contains an account of two bodies which I shall 
 call the suprarenal and interrenal bodies, which are generally 
 described as vascular glands. 
 
 The heart. 
 
 The first trace of the heart becomes apparent during stage 
 G, as a cavity between the splanchnic mesoblast and the wall 
 of the gut immediately behind the region of the visceral clefts 
 (PL 11, fig. 4,^.). 
 
 The body-cavity in the region of the heart is at first double, 
 owing to the two divisions of it not having coalesced ; but even 
 in the earliest condition of the heart the layers of splanchnic 
 mesoblast of the two sides have united so as to form a com- 
 plete wall below. The cavity of the heart is circumscribed by a 
 more or less complete epithelioid (endothelial) layer of flattened 
 cells, connected with the splanchnic wall of the heart by pro- 
 toplasmic processes. The origin of this lining layer I could not 
 certainly determine, but its connection with the splanchnic 
 mesoblast suggests that it is probably a derivative of this 1 . In 
 
 1 From observations on the development of the heart in the Fowl, I have been 
 able to satisfy myself that the epithelioid lining of the heart is derived from the 
 splanchnic mesoblast. When the cavity of the heart is being formed by the separation 
 of the splanchnic mesoblast from the hypoblast, a layer of the former remains close to 
 the hypoblast, but connected with the main mass of the splanchnic mesoblast by 
 
THE HEART. 461 
 
 front the cavity of the heart is bounded by the approximation 
 of the splanchnic mesoblast to the wall of the throat, and be- 
 hind by the stalk connecting the alimentary canal with the 
 yolk-sack. 
 
 As development proceeds the ventral wall of the heart~be- 
 comes bent inwards on each side on a level with the wall of the 
 gut (Plate ii, fig. 4), and eventually becomes so folded in as 
 to form for the heart a complete muscular wall of splanchnic 
 mesoblast. The growth inwards of the mesoblast to form the 
 dorsal wall of the heart does not, as might be expected, begin in 
 front and proceed backwards, but commences behind and is 
 gradually carried forwards. 
 
 From the above account it is clear that I have failed to 
 find in Elasmobranchs any traces of two distinct cavities co- 
 alescing to form the heart, such as have been recently de- 
 scribed in Mammals and Birds ; and this, as well as the other 
 features of the formation of the heart in Elasmobranchs, are in 
 very close accordance with the careful description given by 
 Gb'tte 1 of the formation of the heart in Bombinator. The di- 
 vergence which appears to be indicated in the formation of so 
 important an organ as the heart between Pisces and Amphi- 
 bians on the one hand, and Aves and Mammalia on the other, 
 is certainly startling, and demands a careful scrutiny. The 
 most complete observations 6n the double formation of the 
 heart in Mammalia have been made by Hensen, Gotte and 
 Kolliker. These observations lead to the conclusion (i) that 
 the heart arises as two independent splits between the splanchnic 
 mesoblast and the hypoblast, each with an epithelioid (endo- 
 thelial) lining. (2) That the heart is first formed at a period 
 when the folding in of the splanchnopleure to form tJw throat has 
 
 protoplasmic processes. A second layer next becomes split from the splanchnic 
 mesoblast, connected with the first layer by the above-mentioned protoplasmic pro- 
 cesses. These two layers form the epithelioid lining of the heart ; between them is 
 the cavity of the heart, which soon loses the protoplasmic trabeculae which at first 
 traverse it. 
 
 1 Bischoff has recently stated, Historisch-kritische Bemerkungen il.d. Entwickelung 
 d. Satigethiereier, that Gb'tte has found a double formation of the heart in Bombinator. 
 It may seem bold to question the accuracy of Bischoff's interpretation of writings in 
 his own language, but I have certainly failed to gather this either from Dr Gotte's text 
 or figures. 
 
462 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 not commenced, and when therefore it would be impossible for it 
 to be formed as a single tube. 
 
 In Birds almost every investigator since von Baer has de- 
 tected more or less clearly the coalescence of two halves to 
 form the unpaired heart 1 . Most investigators have however 
 believed that there was from the first an unpaired anterior sec- 
 tion of the heart, and that only the posterior part was formed 
 by the coalescence of two lateral halves. Professor Darlste His, 
 and more recently Kolliker, have stated that there is no such 
 unpaired anterior section of the heart. My own recent ob- 
 servations confirm their conclusions as to the double formation 
 of the heart, though I find that the heart has from the first a 
 A-shaped form. At the apex of the A the two limbs are only 
 separated by a median partition and are not continuous with 
 the aortic arches, which do not arise till a later period' 2 . In 
 the Bird the heart arises just behind the completed throat, and a 
 double formation of the heart appears, in fact, in all instances to 
 be most distinctly correlated with the non-closure of the throat, a 
 non-closure which it must be noted would render it impossible 
 for the heart to arise otherwise than as a double cavity. 
 
 In the instances in which the heart arises as a double cavity 
 it is formed before the complete closttre of the throat, and in those 
 in which it arises as a single cavity it is formed subsequently to 
 the complete formation of the throat. There is thus a double 
 coincidence which renders the conclusion almost certain, that 
 the formation of the heart as two cavities is a secondary change 
 which has been brought about by variations in the period of the 
 closing in of the wall of the throat. 
 
 If the closing in of the throat were deferred and yet the 
 primitive time of formation of the heart retained, it is clear that 
 such a condition as may be observed in Birds and Mammals 
 must occur, and that the two halves of the heart must be formed 
 widely apart, and only eventually united on the folding in of 
 
 1 Vide Elements of Embryology, Foster and Balfour, pp. 64-66. 
 
 2 Professor Bischoff (loc. cit.) throws doubts upon the double formation of the 
 heart, and supports his views by Dr Foster's and my failure to find any trace of a 
 double formation of the heart in the chick. Professor Bischoff must, I think, have 
 misunderstood our description, which contains a clear account of the double formation 
 of the heart. 
 
THE HEART. 463 
 
 the wall of the throat. We may then safely conclude that the 
 double formation of the heart has no morphological significance, 
 and does not, as might at first sight be supposed, imply that the 
 ancestral Vertebrate had two tubes in the place of the present 
 unpaired heart. I have spoken of this point at considerable 
 length, on account of the morphological importance which has 
 been attached to the double formation of the heart. But the 
 views above enunciated are not expressed for the first time. In 
 the Elements of Embryology we say, p. 64, " The exact mode of 
 development (of the heart) appears according to our present 
 knowledge to be very different in different cases ; and it seems 
 probable that the differences are in fact the result of variations 
 in the mode of formation and time of closure of the alimentary 
 canal." Gotte again in his great work 1 appears to maintain 
 similar views, though I do not perfectly understand all his state- 
 ments. In my review of Kolliker's Embryology 2 this point is 
 still more distinctly enunciated in the following passage : " The 
 primitive wide separation and complete independence of the two 
 halves of the heart is certainly surprising ; but we are inclined, 
 provisionally at least, to regard it as a secondary condition due 
 to the late period at which the closing of the throat takes place 
 in Mammals." 
 
 
 
 The general circulation. 
 
 The chief points of interest in connection with the general 
 circulation centre round the venous system. The arterial arches 
 present no peculiarities : the dorsal aorta, as in all other Ver- 
 tebrates, is at first double (PI. II, fig. 6 ad), and, generally 
 speaking, the arrangement of the arteries accords with what is 
 already known in other forms. The evolution of the venous 
 system deserves more attention. 
 
 The cardinal veins are comparatively late developments. 
 There is at first one single primitive vein continuous in front 
 with the heart and underlying the alimentary canal through its 
 pfaeanal and postanal sections. This vein is shewn in section in 
 PI. 11, fig. 8, V. It may be called either the subintestinal or 
 
 1 Entwicklungsgeschichte d. Unite, pp. 779, 780, 781. 
 - Journal of Anatomy and Physiology, Vol. X. p. 794. 
 
464 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 splanchnic vein. At the cloaca, where the gut enlarges and 
 comes in contact with the skin, this vein is compelled to bi- 
 furcate (PI. 1 8, fig. 6 d, v. cau^}, and usually the two branches 
 into which it divides are unequal in size. The two branches 
 meet again behind the cloaca and take their course ventral to 
 the postanal section of the gut, and terminate close to the end of 
 the tail, PI. 18, fig. 6 c, v. can. In the tail they form what is 
 usually known as the caudal vein. The venous system of Scyl- 
 lium or Pristiurus, during the early parts of stage K, presents 
 the simple constitution just described. 
 
 Before proceeding to describe the subsequent changes which 
 take place in it, it appears to me worth pointing out the re- 
 markable resemblance which the vascular system of an Elas- 
 mobranch presents at this stage to that of an ordinary Annelid 
 and Amphioxus. It consists, as does the circulatory system, in 
 Annelids, of a neural vessel (the aorta) and an intestinal vessel, 
 the blood flowing backwards in the latter and forwards in the 
 former. The two in Elasmobranchs communicate posteriorly 
 by a capillary system, and in front by the arterial arches, con- 
 nected like the similar vessels in Annelids with the branchiae. 
 Striking as is this resemblance, there is a still closer resemblance 
 between the circulation of the Scyllium embryo at stage K and 
 that of Amphioxus. The two systems are in fact identical ex- 
 cept in very small details. The subintestinal vessel, absent or 
 only represented by the caudal vein and in part by the ductus 
 venosus in higher Vertebrates and adult Fish, forms the main 
 and only posterior venous trunk of Amphioxus and the embryo 
 Scyllium. The only noteworthy point of difference between 
 Amphioxus and the embryo Scyllium is the presence of a portal 
 circulation in the former, absent at this stage in the latter ; but 
 even this is acquired in Scyllium before the close of stage K, 
 and does not therefore represent a real difference between the 
 two types. 
 
 The cardinal veins make their appearance before the close 
 of stage K, and very soon unite behind with the unpaired 
 section of the caudal vein (PI. 11, fig. 9 b, p. cav. and v.}. On 
 this junction being effected retrogressive changes take place in 
 the original subintestinal vessel. It breaks up in front into a 
 number of smaller vessels ; the lesser of the two branches con- 
 
THE VENOUS SYSTEM. 465 
 
 necting it round the cloaca with the caudal vein first vanishes 
 (PI. n, fig. 9 a, v), and then the larger; and the two cardinals 
 are left as the sole forward continuations of the caudal vein. 
 This latter then becomes prolonged forwards, and the two pos- 
 terior cardinals open into it some little distance in fronForthe 
 hind end of the kidneys. By these changes and by the dis- 
 appearance of the postanal section of the gut the caudal vein is 
 made to appear as a superintestinal and not a subintestinal 
 vessel, and as the direct posterior continuation of the cardinal 
 veins. Embryology proves however that the caudal vein is a 
 true subintestinal vessel 1 , and that its connection with the car- 
 dinals is entirely secondary. 
 
 The invariably late appearance of the cardinal veins in the 
 embryo and their absence in Amphioxus leads me to regard 
 them as additions to the circulatory system which appeared 
 in the Vertebrata themselves, and were not inherited from their 
 ancestors. It would no doubt be easy to point to vessels in 
 existing Annelids which might be regarded as their equivalent, 
 but to do so would be in my opinion to follow an entirely false 
 morphological scent. 
 
 The circti/ation of the yolk-sack. 
 
 The observations recorded on this subject are so far as I 
 am acquainted with them very imperfect, and in most cases the 
 arteries and veins appear to have been transposed. 
 
 Professor Wyman 2 , however, gives a short description of the 
 circulation in Raja Batis, in which he rightly identifies the 
 arteries, though he regards the arterial ring which surrounds the 
 vascular area as equivalent to the venous sinus terminalis of the 
 Bird. 
 
 The general features of the circulation are clearly portrayed 
 in the somewhat diagrammatic figures on PL 9, in which the 
 arteries are represented red, and the veins blue 3 . 
 
 1 The morphological importance of this point is considerable. It proves, for 
 instance, that the haemal arches of the vertebrae in the tail (vide pp. 373 and 374) 
 potentially, at any rate, encircle the gut and enclose the body-cavity as completely as 
 the ribs which meet in the median ventral line may be said to do anteriorly. 
 
 2 Memoirs of the American Academy of Arts and Sciences, Vol. ix. 
 
 3 I may state that my determinations of the arrangement of the circulation were 
 made by actual observation of the flow of the blood under the microscope. 
 
466 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 I shall follow the figures on this plate in my descriptions. 
 
 Fig. i represents my earliest stage of the circulation of the 
 yolk-sack. At this stage there is visible a single aortic trunk 
 passing forwards from the embryo and dividing into two branches. 
 No venous trunk could be detected with the simple microscope, 
 but probably venous channels were present in the thickened 
 edge of the blastoderm. 
 
 In fig. 2 the circulation was greatly advanced 1 . The blasto- 
 derm has now nearly completely enveloped the yolk, and there 
 remains only a small circular space (yk) not enclosed by it The 
 arterial trunk is present as before, and divides in front of the 
 embryo into two branches which turn backwards and nearly 
 form a complete ring round the embryo. In general appearance 
 it resembles the sinus terminalis of the area vasculosa of the 
 Bird, but in reality bears quite a different relation to the circula- 
 tion. It gives off branches only on its inner side. 
 
 A venous system of returning vessels is now fully developed, 
 and its relations are very remarkable. There is a main venous 
 ring round the thickened edge of the blastoderm, which is 
 connected with the embryo by a single stem which runs along 
 the seam where the edges of the blastoderm have coalesced. 
 Since the venous trunks are only developed behind the embryo, 
 it is only the posterior part of the arterial ring which gives off 
 branches. 
 
 The succeeding stage, fig. 3, is also one of considerable 
 interest. The arterial ring has greatly extended, and now 
 embraces nearly half the yolk, and sends off trunks on its inner 
 side along its whole circumference. 
 
 More important changes have taken place in the venous 
 system. The blastoderm has now completely enveloped the 
 yolk, and as a result of this, the venous ring no longer exists, 
 but at the point where it vanished there may be observed a 
 number of smaller veins diverging in a brush-like fashion from 
 the termination of the unpaired trunk which originally connected 
 the venous ring with the heart. This point is indicated in the 
 figure by the letter y. The brush-like divergence of the veins is 
 
 1 My figure may be compared with that of Leydig, Rochen und Haie, Plate in. 
 fig. 6. Leydig calls the arterial ring the sinus terminalis, and appears to regard it as 
 venous, but his description is so short that this point is not quite clear. 
 
THE CIRCULATION OF THE YOLK-SACK. 467 
 
 a still more marked feature in a blastoderm of a succeeding 
 stage (fig. 4). 
 
 The circulation in the succeeding stage (fig. 4) (projected in 
 my figure) only differs in details from that of the previous stage. 
 The arterial ring has become much larger, and the portion of 
 the yolk not embraced (x) by it is quite small. Instead of all 
 the branches from the ring being of nearly equal size, two of 
 them are especially developed. The venous system has under- 
 gone no important changes. 
 
 In fig. 5 the circulation is represented at a still later stage. 
 The arterial ring has come to embrace the whole yolk, and as 
 a result of this, has in its turn vanished as did the venous ring 
 before it. At this stage of the circulation there is present a 
 single arterial and a single venous trunk. The arterial trunk is 
 a branch of the dorsal aorta, and the venous trunk originally 
 falls into the heart together with the subintestinal or splanchnic 
 vein, but on the formation of the liver enters this and breaks up 
 into capillaries in it. The venous trunk leaves the body on the 
 right side, and the arterial on the left. 
 
 The most interesting point to be noticed in connection with 
 the yolk-sack circulation of Scyllium is the fact of its being formed 
 on a completely different type to that of the Amniotic Verte- 
 brates. 
 
 THE VASCULAR GLANDS. 
 
 There are in Scyllium two structures which have gone under 
 the name of the suprarenal body. The one of these is an 
 unpaired rod-like body lying between the dorsal aorta and the 
 caudal vein in the region of the posterior end of the kidneys. 
 This body I propose to call the interrenal body. The other is 
 formed by a series of paired bodies situated dorsal to the cardinal 
 veins on branches of the aorta, and arranged segmentally. These 
 bodies I shall call the suprarenal bodies. I propose treating the 
 literature of these bodies together, since they have usually been 
 dealt with in this way, and indeed regarded as parts of the same 
 system. As I hope to shew in the sequel, the origin of these 
 bodies is very different. The interrenal body appears to be 
 
468 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 developed from the mesoblast ; while my researches on the 
 suprarenal bodies confirm the brilliant investigations of Leydig, 
 shewing that they are formed out of the sympathetic ganglia. 
 
 The most important investigations on these bodies have been 
 made by Leydig 1 . In his first researches, RocJien u. Haie, pp. 
 71, 72, he gives an account of the position and histology of what 
 is probably my interrenal body 2 . 
 
 The position and relations of the interrenal body vary some- 
 what according to Leydig in different cases. He makes the fol- 
 lowing statement about its histology. " Fat molecules form the 
 chief mass of the body, which causes its white, or ochre-yellow 
 colour, and one finds freely embedded in them clear vesicular 
 nuclei." He then proceeds to state that this structure is totally 
 dissimilar to that of the Mammalian suprarenal body, and gives 
 it as his opinion that it is not the same body as this. In his 
 later researches 3 he abandons this opinion, and adopts the view 
 that the interrenal body is part of the same system as the supra- 
 renal bodies to be subsequently spoken of. Leydig describes 
 the suprarenal bodies as paired bodies segmentally arranged 
 along the ventral side of the spinal column situated on the 
 successive arteriae axillares, and in close connection with one or 
 more sympathetic ganglia. He finds them formed of lobes, 
 consisting of closed vesicles full of nuclei and cells. Numerous 
 nerve-fibres are also described as present. With reference to the 
 real meaning of these bodies he expresses a distinct view. He 
 says 4 , " As the pituitary body is an integral part of the brain, so 
 are the suprarenal bodies part of the sympathetic system." He 
 re-affirms with still greater emphasis the same view in his Fische 
 u. Reptilien. Though these views have not obtained much 
 
 1 Rochen und Haie and Untersuchung. u. Fische u. Reptilien. 
 
 2 I do not feel sure that Leydig's unpaired suprarenal body is really my interrenal 
 body, or at any rate it alone. The point could no doubt easily be settled with fresh 
 specimens, but these I unfortunately cannot at present obtain. My doubts rest partly 
 on the fact that, in addition to my interrenal body, other peculiar masses of tissue 
 (which may be called lymphoid in lieu of a better name) are certainly present around 
 some of the larger vessels of the kidneys which are not identical in structure and 
 development with my interrenal body, and partly that Stannius' statements (to be 
 alluded to directly) rather indicate the existence of a second unpaired body in con- 
 nection with the kidneys, though I do not fully understand his descriptions. 
 
 3 Fische u. Reptilien, p. 14. 
 
 4 Rochen u. ffaie, p. 18. 
 
THE VASCULAR GLAND. 469 
 
 acceptance, and the accuracy of the histological data on which 
 they are grounded has been questioned, yet I hope to shew in 
 the sequel not only that Leydig's statements are in the main 
 true, but that development proves his conclusions to have been 
 well founded. 
 
 Stannius alludes 1 to both these bodies, and though he does 
 not contribute much to Leydig's previous statements, yet he 
 accepts Leydig's position with reference to the relation of the 
 sympathetic and suprarenal bodies 2 . 
 
 The general text-books of Histology, Kolliker's work, and 
 Eberth's article in Strieker's Histology, do not give much in- 
 formation on this subject; but Eberth, without apparently having 
 examined the point, questions the accuracy of Leydig's state- 
 ments with reference to the anatomical relations of the sympa- 
 thetic ganglia and suprarenal bodies. 
 
 The last author who has dealt with this subject is Professor 
 Semper 8 . He records observations both on the anatomy and 
 development of these organs. His anatomical observations are 
 in the main confirmatory of those of Leydig, but he shews still 
 more clearly than did Leydig the segmental arrangement of the 
 suprarenal bodies. He definitely regards the interrenal and 
 suprarenal bodies as parts of the same system, and states that 
 in many forms they are continuous (p. 228) : 
 
 " Hier freilich gehen sie bei manchen Formen...in einen 
 Korper iiber, welcher zwischen den Enden d. beiden Nieren 
 liegend dicht an der einfachen Caudalvene sitzt." 
 
 With reference to their development he says : " They arise 
 then also completely independently of the kidneys, as isolated 
 segmentally arranged groups of mesoderm cells between the con- 
 volutions of the segmental organs ; only anteriorly do they stretch 
 beyond them, and extend quite up to the pericardium." 
 
 To Semper's statements I shall return, but now pass on to 
 my own observations. The paired suprarenal bodies are dealt 
 with first. 
 
 1 Vergleichende Anatomie, II. Auflage. 
 
 2 Stannius' description is not quite intelligible, but appears to point to the ex- 
 istence of a third kind of body connected with the kidney. From my own observations 
 (vide above), I am inclined to regard it as probable that such a third body exists. 
 
 :i " Urogenitalsystem d. Plagiostomen." Arb. zool. zoot. fnst. z. lViirzburg,Vo\.\\. 
 
470 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The siiprarenal bodies. 
 
 My observations on these bodies in the adult Scyllium have 
 only been made with specimens hardened in chromic acid, and 
 there are many points which deserve a fuller investigation than 
 I have been able to give them. 
 
 The general position and relations of the suprarenal bodies 
 have been fully given by Leydig and Semper, and I have nothing 
 to add to their statements. They are situated on branches of 
 the aorta, segmentally arranged, and extend on each side of the 
 vertebral column from close behind the heart to the posterior 
 part of the body-cavity. The anterior pair are the largest, and 
 are formed apparently from the fusion of two bodies 1 . When 
 these bodies are examined microscopically, their connection with 
 the sympathetic ganglia becomes at once obvious. Bound up 
 in the same sheath as the anterior one is an especially large 
 ganglion already alluded to by Leydig, and sympathetic ganglia 
 are more or less distinctly developed in connection with all the 
 others. There is however considerable irregularity in the develop- 
 ment and general arrangement of the sympathetic ganglia, which 
 are broken up into a number of small ganglionic swellings, on 
 some of which an occasional extra suprarenal body is at times 
 developed. As a rule it may be stated that there is a much 
 smaller ganglionic development in connection with the posterior 
 suprarenal bodies than with the anterior. 
 
 The different suprarenal bodies exhibit variations in structure 
 mainly dependent on the ganglion cells and nerves in them, 
 and their typical structure is best exhibited in a posterior one, 
 in which there is a comparatively small development of nervous 
 elements. 
 
 A portion of a section through one of these is represented on 
 PL 19, fig. 6, and presents the following features. Externally 
 there is present a fibrous capsule, which sends in the septa, im- 
 perfectly dividing up the body into a series of alveoli or lobes. 
 Penetrating and following the septa there is a rich capillary 
 network. The parenchyma of the body itself exhibits a well- 
 
 1 There is a very good figure of them in Semper's paper, PI. xxi. fig. 3. 
 
THE SUPRARENAL BODIES. 4/1 
 
 marked distinction in the majority of instances into a cortical 
 and medullary substance. The cortical substance is formed of 
 rather irregular columnar cells, for the most part one row deep, 
 arranged round the periphery of the body. Its cells measure 
 on about an average '03 Mm. in their longest diameter."" The 
 medullary substance is more or less distinctly divided into 
 alveoli, and is formed of irregularly polygonal cells ; and though 
 it is difficult to give an estimate of their size on account of 
 their irregularity, 'O2i Mm. may be taken as probably about 
 the diameter of an average cell. The character of the cortical 
 and medullary cells is nearly the same, and the cells of the two 
 strata appear rather to differ in shape than in any other essential 
 point. The protoplasm of both has a markedly yellow tinge, 
 giving to the suprarenal bodies a yellowish brown colour. The 
 nuclei are small compared to the size of the cells, being about 
 009 Mm. in both cortical and medullary cells. In the anterior 
 suprarenal body there is a less marked distinction between the 
 cortical and the medullary layers, and a less pronounced yellow 
 coloration of the whole, than in the posterior bodies. The 
 suprarenal bodies are often partially or completely surrounded 
 by a lymphoid tissue, which is alluded to in the account of their 
 development. 
 
 The most interesting features of my sections of the anterior 
 bodies are the relations they bring to light between the sympa- 
 thetic ganglia and the suprarenal bodies. In the case of one of 
 the posterior suprarenal bodies, a small ganglion is generally 
 found attached to both ends of the body, and invested in the 
 same sheath ; in addition to this a certain number of ganglion 
 cells (very conspicuous by their size and other characters) are to 
 be found scattered through the body. In the anterior suprarenal 
 bodies the development of ganglion cells is very much greater. 
 If a section is taken through the region where the large sympa- 
 thetic ganglion (already mentioned) is attached to the body, one 
 half of the section is composed mainly of sympathetic ganglion 
 cells and nerve fibres, and the other of suprarenal tissue, but 
 the former spread in considerable numbers into the latter. A 
 transverse section through the suprarenal body in front of, or 
 behind this point, is still more instructive. One of these is 
 represented in PI. 19, fig. 7. The suprarenal tissue is not 
 
4/2 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 inserted, but fills up the whole space within the outline of the 
 body. At one point a nerve (n) is seen to enter. In connection 
 with this are a number of ganglion cells, the exact distribution 
 of which has been reproduced. They are scattered irregularly 
 throughout the suprarenal body, but are more concentrated at 
 the smaller than at the large end. It is this small end which, 
 in succeeding sections, is entirely replaced by a sympathetic 
 ganglion. Wavy fibres (which I take to be nervous) are dis- 
 tributed through the suprarenal body in a manner which, roughly 
 speaking, is proportional to the number of ganglion cells. At 
 the large end of the body, where there are few nerve cells, the 
 typical suprarenal structure is more or less retained. Where 
 the nerve fibres are more numerous at the small end of the 
 section, they give to the tissue a somewhat peculiar appearance, 
 though the individual suprarenal cells retain their normal struc- 
 ture. In a section of this kind the ganglion and nerves are 
 clearly so intimately united with the suprarenal body as not to 
 be separable from it. 
 
 The question naturally arises as to whether there are cells of 
 an intermediate character between the ganglion cells and the 
 cells of the suprarenal body. I have not clearly detected any 
 such, but my observations are of too limited a character to settle 
 the point in an adverse sense. 
 
 The embryological part of my researches on these bodies is 
 in reality an investigation of later development of the sym- 
 pathetic ganglia. The earliest stages in the development of 
 these have already been given 1 , and I take them up here as they 
 appear during stage L, and shall confine my description to the 
 changes they undergo in the anterior part of the trunk. They 
 form during stage L irregular masses of cells with very con- 
 spicuous branches connecting them with the spinal nerves (PI. 
 1 8, fig. 3). There may be noticed at intervals solid rods of cells 
 passing from the bodies to the aorta, PI. 18, fig. 2. These rods 
 are the rudiments of the aortic branches to which the suprarenal 
 bodies are eventually attached. 
 
 In a stage between M and N the trunks connecting these 
 bodies with the spinal nerves are much smaller and less easy to 
 see than during stage L. In some cases moreover the nerves 
 
 1 Antea, pp. 394 396. 
 
THE SUPRARENAL BODIES. 473 
 
 appear to attach themselves more definitely to a central and 
 inner part of the ganglia than to the whole of them. This is 
 shewn in PI. 19, fig. 8, and I regard it as the first trace of a 
 division of the primitive ganglia into a suprarenal part and a 
 ganglionic part. The branches from the aorta have now" a 
 definite lumen, and take a course through the centre of these 
 bodies, as do the aortic branches in the adult. 
 
 By stage O these bodies have acquired a distinct mesoblastic 
 investment, which penetrates into their interior, and divides it, 
 especially in the case of the- anterior bodies, into a number of 
 distinct alveoli. These alveoli are far more distinct in some 
 parts of the bodies than in others. The nerve-trunks uniting 
 the bodies with the spinal nerves are (at least in specimens 
 hardened in picric and chromic acids) very difficult to see, and 
 I have failed to detect that they are connected with special parts 
 of the bodies, or that the separate alveoli differ much as to the 
 nature of their constituent cells. The aortic branches to the 
 bodies are larger than in the previous stage, and the bodies them- 
 selves fairly vascular. 
 
 By stage Q (PI. 19, fig. 9) two distinct varieties of cells are 
 present in these bodies. One of these is large, angular, and 
 strikingly resembles the ganglion cells of the spinal nerves at 
 the same period. This variety is found in separate lobules or 
 alveoli on the inner border of the bodies. I take them to be 
 true ganglion cells, though I have not seen them in my sections 
 especially connected with the nerves. The cells of the second 
 variety are also aggregated in special lobules, and are very 
 markedly smaller than the ganglionic cells. They form, I 
 imagine, the cells of the true suprarenal tissue. At this and 
 the earlier stage lymphoid tissue, like that surrounding the supra- 
 renal bodies in the adult, is found adjacent to these bodies. 
 
 Stage Q forms my last embryonic stage, and it may perhaps 
 be asked on what grounds I regard these bodies as suprarenal 
 bodies at all and not as simple sympathetic ganglia. 
 
 My determination mainly rests on three grounds: (i) That 
 a branch from the aorta penetrates these bodies and maintains 
 exactly the same relations to them that the same branches of 
 the aorta do in the adult to the true suprarenal bodies. (2) That 
 the bodies are highly vascular. (3) That in my last stage they 
 B. 3I 
 
474 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 become divided into a ganglionic and a non-ganglionic part, 
 with the same relations as the ganglia and suprarenal tissue in 
 the adult. These grounds appear to me to afford ample justifica- 
 tion for my determinations, and the evidence adduced above 
 appears to me to render it almost certain that the suprarenal 
 tissue is a product of the primitive ganglion and not introduced 
 from the mesoblast without, though it is not to be denied that 
 a more complete investigation of this point than it has been 
 possible for me to make would be very desirable. 
 
 Professor Semper states that he only made a very slight 
 embryological investigation of these bodies, and probably has 
 only carefully studied their later stages. He has accordingly 
 overlooked the branches connecting them with the spinal nerves, 
 and has not therefore detected the fact that they develope as 
 parts of the sympathetic nervous system. I feel sure that if he 
 re-examines his sections of younger embryos he will not fail to 
 discover the nerve-branches described by me. His descriptions 
 apart from this point accord fairly well with my own. The 
 credit of the discovery that these bodies are really derivatives 
 of the sympathetic nervous system is entirely Leydig's : my 
 observations do no more than confirm his remarkable observa- 
 tions and well-founded conclusions. 
 
 Interrenal body. 
 
 My investigations on the interrenal body in the adult are 
 even less complete than those on the suprarenal bodies. Lfind 
 the body forming a small rod elliptical in section in the poste- 
 rior region of the kidney between the dorsal aorta and unpaired 
 caudal vein. Some little distance behind its front end (and 
 probably not at its thickest point) it measured in one example, 
 of which I have sections, a little less than a millimetre in its 
 longest diameter. Anteriorly it overlaps the suprarenal bodies, 
 and I failed to find any connection between them and it. On 
 this point my observations do not accord with those of Professor 
 Semper. I have however only been able to examine hardened 
 specimens. 
 
 It is, vide PI. 18, fig. 8, invested by a fairly thick tunica 
 propria, which sends in septa, dividing it into rather well-marked 
 
THE INTERRENAL BODY. 475 
 
 lobules or alveoli. These are filled with polygonal cells, which 
 form the true parenchyma of the body. These cells are in my 
 hardened specimens not conspicuous by the number of oil- 
 globules they contain, as might have been expected from Leydig's 
 description 1 . They are rather granular in appearance, and are 
 mainly peculiar from the somewhat large size of the nucleus. 
 The diameter of an average cell is about '015 Mm., and that of 
 the nucleus about 'Oi to -012. The nuclei are remarkably 
 granular. The septa of the body are provided with a fairly rich 
 capillary network. 
 
 At the first glance there is some resemblance in structure 
 between the tissues of the suprarenal and interrenal bodies, but 
 on a closer inspection this resemblance resolves itself into both 
 bodies being divided up into lobules by connective-tissue septa. 
 There is in the interrenal body no distinction between cortical 
 and medullary layers as in the suprarenal. The cells of the 
 two bodies have very different characters, as is demonstrated by 
 a comparison of the relative -diameters of the nuclei and the 
 cells. The cells of the suprarenal bodies are considerably larger 
 than those of the interrenal ('021 to '03 as compared to -015), yet 
 the nuclei of the larger cells of the former body do not equal in 
 size those of the smaller cells of the latter (-009 as compared to 
 01). 
 
 My observations both on the coarser anatomy and on the 
 histology of the interrenal body in the adult point to its being 
 in no way connected with the suprarenal bodies, and are thus 
 in accordance with the earlier and not the later views of Leydig. 
 
 The embryology of this body (under the title of suprarenal 
 body) was first described in my preliminary account of the 
 development of the Elasmobranch Fishes 2 . A short account of 
 its embryonic structure was given, and I stated that although I 
 had not fully proved the point, yet I believed it to be derived 
 from the wall of the alimentary canal. As will be shewn in the 
 sequel this belief was ill-founded, and the organ in question is 
 derived from the mesoblast. Allusion has also been made to it 
 
 1 Perhaps the body I am describing is not identical with Leydig's posterior supra- 
 renal body. I do not, as mentioned above, feel satisfied that it is so from Leydig's 
 description. 
 
 2 Quarterly Journal of Microscopic Science, October, 1874. [This edition No. V.] 
 
 312 
 
476 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 by Professor Semper, who figures it at an early stage of develop- 
 ment, and implies that it arises in the mesoblast and in connection 
 with the suprarenal body. It appears at stage K as a rod-like 
 aggregate of mesoblast cells, rather more closely packed than 
 their neighbours, between the two kidneys near their hinder 
 ends (Plate n, fig. ga, sit). The posterior and best marked part 
 of it does not extend further forwards than the front end of the 
 large intestine, and reaches backwards nearly as far as the 
 hinder end of the kidneys. This part of the body lies between 
 the caudal vein and dorsal aorta. 
 
 At about the point where the unpaired caudal vein divides 
 into the two cardinals, the interrenal body becomes less well 
 marked off from the surrounding tissue, though it may be traced 
 forward for a considerable distance in the region of the small 
 intestine. It retains up to stage Q its original extension, but 
 the anterior part becomes quite definite though still of a smaller 
 calibre than the posterior. In one of my examples of stage O 
 the two divisions were separated by a small interval, and not as 
 in other cases continuous. I have not determined whether this 
 was an accidental peculiarity or a general feature. I have never 
 seen any signs of the interrenal body becoming continuous with 
 the suprarenal bodies, though, as in the adult, the two bodies 
 overlap for a considerable distance. 
 
 The histology of the interrenal body in the embryonic periods 
 is very simple. At first it is formed of cells differing from those 
 around in being more circular and more closely packed. By 
 stage L its cells have acquired a character of their own. They 
 are still spherical or oval, but have more protoplasm than before, 
 and their nucleus becomes very granular. At the same time the 
 whole body becomes invested by a tunic of spindle-shaped 
 mesoblast cells. By stage O it begins to be divided into a 
 number of separate areas or lobes by septa formed of nucleated 
 fibres. These become more distinct in the succeeding stages up 
 to Q (PI. 1 8, fig. 7), and in them a fair number of capillaries are 
 formed. 
 
 From the above description it is clear that embryology lends 
 no more countenance than does anatomy to the view that the 
 interrenal bodies belong to the same system as the suprarenal, 
 and it becomes a question with which (if of either) of these two 
 
EXPLANATION OF PLATE 19. 477 
 
 bodies the suprarenal bodies of the higher Vertebrata are homo- 
 logous. This question I shall not attempt to answer in a definite 
 way. My own decided belief is that the suprarenal bodies of 
 Scyllium are homologous with the suprarenal bodies of Mammalia, 
 and a good many points both in their structure and position 
 might be urged in favour of this view. In the mean time, how- 
 ever, it appears to me better to wait before expressing a definite 
 opinion till the embryonic development of the suprarenal bodies 
 has been worked out in the higher Vertebrata. 
 
 EXPLANATION OF PLATE 19. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 Nervous System, 
 n. Nerve, spn. Spinal nerve, sy g. Sympathetic ganglion. 
 
 Alimentary Canal. 
 
 d. Cloaca. in cl. Cloacal involution, ce ep. OZsophageal epithelium, pan. 
 Pancreas, th. Thyroid body. 
 
 General. 
 
 abp. Abdominal pocket (pore), anr. Auricle. ca v. Cardinal vein. cauv. 
 Caudal vein. ly. Lymphoid tissue, mm. Muscles, od. Oviduct, pc. Pericardium. 
 pp. Body cavity, s r. Suprarenal body. it. Ureter, v ao. Ventral aorta (anterior 
 continuation of bulbus arteriosus). ven. Ventricle, wd. Wolffian duct. 
 
 Figs, i a, 1 1>, ic. Three sections through the cloacal region of an embryo belong- 
 ing to stage O. i a is the anterior of the three sections. Zeiss A, ocul. 2. Reduced 
 one-third. 
 
 i a shews the cloacal involution at its deepest part abutting on the cloacal section 
 of the alimentary tract. 
 
 i d is a section through a point somewhat behind this close to the opening of the 
 Wolffian ducts into the cloaca. 
 
 i c shews the opening to the exterior in the posterior part of the cloaca, and also 
 the rudiments of the two abdominal pockets (abp). 
 
 Fig. 2. Section through the cloacal region of an embryo belonging to stage P. 
 Zeiss A, ocul. 2. 
 
 The figure shews the solid anterior extremity of the cloacal involution. 
 
 Fig. 3. Longitudinal vertical section through the thyroid body in a stage between 
 O and P. Zeiss aa, ocul. i. 
 
 The figure shews the solid thyroid body (th) connected in front with throat, and 
 terminating below the bulbus arteriosus. 
 
478 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Fig. 4. Pancreas (pan) and adjoining part of the alimentary tract in longitudinal 
 section, from an embryo between stages L and M. Zeiss A, ocul 2. 
 
 Fig. 5. Portion of liver network^ of stage L. Zeiss C, ocul. i. The section is 
 intended to illustrate the fact that the tubules or cylinders of which the liver is 
 composed are hollow and not solid. Between the liver tubules are seen blood spaces 
 with distinct walls, and blood corpuscles in their interior. 
 
 Fig. 6. Section through part of one of the suprarenal bodies of an adult Scyllium 
 hardened in chromic acid. Zeiss C, ocul. i. The section shews the columnar cells 
 forming the cortex and the more polygonal cells of the medulla. 
 
 Fig. 7. Transverse section through the anterior suprarenal body of an adult 
 Scyllium. Zeiss B, ocul. 2. Reduced one- third. The tissue of the suprarenal body 
 has not been filled in, but only the sympathetic ganglion cells which are seen to be 
 irregularly scattered through the substance of the body. The entrance of the nerve 
 (n) is shewn, and indications are given of the distribution of the nerve-fibres. 
 
 Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo between 
 stages M and N, shewing the connecting trunk between the suprarenal body and the 
 spinal nerve (sf n), and the appearance of an indication in the ganglion of a portion 
 more directly connected with the nerve. Zeiss D, ocul. 2. 
 
 Fig. o,. Section through one of the anterior sympathetic ganglia of an embryo of 
 stage Q, shewing its division into a true ganglionic portion (sy g), and a suprarenal 
 body (sr). Zeiss C, ocul 2. 
 
CHAPTER XII. 
 THE ORGANS OF EXCRETION. 
 
 THE earliest stages in the development of the excretory 
 system have already been described in a previous chapter 1 of this 
 memoir, and up to the present time no investigator, with the 
 exception of Dr Alex. Schultz 2 , has gone over the same ground. 
 Dr Schultz' descriptions are somewhat brief, but differ from my 
 own mainly in stating that the segmental duct arises from an 
 involution instead of as a solid knob. This discrepancy is, 
 I believe, due to Dr Schultz drawing his conclusions as to the 
 development of the segmental duct from its appearance at a 
 comparatively late stage. He appears to have been unac- 
 quainted with my earlier descriptions. 
 
 The adult anatomy and later stages in the development of 
 the excretory organs form the subject of the present chapter, 
 and stand in marked contrast to the earlier stages in that they 
 have been dealt with in a magnificent monograph 3 by Professor 
 Semper, whose investigations have converted this previously 
 almost unknown field of vertebrate embryology into one of the 
 most fully explored parts of the whole subject. Reference is 
 frequently made to this monograph in the succeeding pages, but 
 my references, numerous as they are, give no adequate idea of 
 the completeness and thoroughness of Professor Semper's in- 
 vestigations. In Professor Semper's monograph are embodied 
 the results of a considerable number of preliminary papers pub- 
 lished by him in his Arbeiten and in the Centralblatt. The 
 excretory organs of Elasmobranchs have also formed the sub- 
 
 1 Chapter vi. p. 345, et set/. 
 
 2 Archiv f. Micr. Anat. Bd. XI. 
 
 * " Urogenital System d. Plagiostomen," Semper, Arbeiten, Vol. n. 
 
480 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 ject of some investigations by Dr Meyer 1 and by myself 2 . Their 
 older literature is fully given by Professor Semper. In addition 
 to the above-cited works, there is one other paper by Dr Spengel 3 
 on the Urinogenital System of Amphibians, to which reference 
 will frequently be made in the sequel, and which, though only 
 indirectly connected with the subject of this chapter, deserves 
 special mention both on account of the accuracy of the investi- 
 gations of which it forms the record, and of the novel light 
 which it throws on many of the problems of the constitution of 
 the urinogenital system of Vertebrates. 
 
 Excretory organs and genital ducts in the adult. 
 
 The kidneys of Scyllium canicula are paired bodies in con- 
 tact along the *nedian line. They are situated on the dorsal 
 wall of the abdominal cavity, and extend from close to the 
 diaphragm to a point a short way behind the anus. Externally, 
 each appears as a single gland, but by the arrangement of its 
 ducts may be divided into two distinct parts, an anterior and a 
 posterior. The former will be spoken of as the Wolffian body, 
 and the latter as the kidney, from their respective homology 
 with the glands so named in higher Vertebrates. The grounds 
 for these determinations have already been fully dealt with both 
 by Semper 4 and by myself. . 
 
 Externally both the Wolffian body and the kidney are more 
 or less clearly divided into segments, and though the breadth of 
 both glands as viewed from the ventral surface is fairly uniform, 
 yet the hinder part of the kidney is very much thicker and 
 bulkier than the anterior part and than the whole of the Wolffian 
 body. In both sexes the Wolffian body is rather longer than 
 the kidney proper. Thus in a male example, 33 centimetres 
 
 1 Sitzungsberichte d. Natiirfor. Gas. Leipzig, 1875. No. i. 
 
 2 " Preliminary account of the development of Elasmobranch Fishes," Qiiarterly 
 Journal of Microscopical Science, 1874. " Origin and History of the Urinogenital 
 Organs of Vertebrates," Journal of Anat. and Physiol. Vol. x. 
 
 8 Arbeiten, Semper, Vol. in. 
 
 * Though Professor Semper has come to the same conclusion as myself with 
 respect to these homologies, yet he calls the Wolffian body Leydig's gland after its 
 distinguished discoverer, and its duct Leydig's duct. 
 
EXCRETORY ORGANS IN THE ADULT. 481 
 
 long, the two glands together measured 8 centimetres and the 
 kidney proper only 3^. In the male the Wolffian bodies ex- 
 tend somewhat further forwards than in the female. Leaving 
 the finer details of the glands for subsequent treatment, I pass 
 at once to their ducts. These differ slightly in the twg~scxes, 
 so that it will be more convenient to take the male and female 
 separately. 
 
 A partly diagrammatic representation of the kidney and 
 Wolffian body of the male is given on PI. 20, fig. I. The se- 
 cretion of the Wolffian body is carried off by a duct, the Wolffian 
 duct (w. d.}, which lies on the ventral surface of the gland, and 
 receives a separate ductule from each segment (PI. 20, fig. 5). 
 The main function of the Wolffian duct in the male is, how- 
 ever, that of a vas deferens. The testicular products are brought 
 to it through the coils of the anterior segments of the Wolffian 
 body by a number of vasa efferentia, the arrangement of which 
 is treated of on pp. 487, 488. The section of the Wolffian duct 
 which overlies the Wolffian body is much contorted, and in 
 adult individuals at the generative period enormously so. The 
 duct often presents one or two contortions beyond the hind end 
 of the Wolffian body, but in the normal condition takes a 
 straight course from this point to the unpaired urinogenital 
 cloaca, into which it falls independently of its fellow of the 
 opposite side. It receives no feeders from the kidney proper. 
 
 The excretion of the kidney proper is carried off not by a 
 single duct, but by a series of more or less independent ducts, 
 which, in accordance with Prof. Semper's nomenclature, will be 
 spoken of as ureters. These are very minute, and their in- 
 vestigation requires some care. I have reason, from my ex- 
 aminations of this and other species of Elasmobranchs, to be- 
 lieve that they are, moreover, subject to considerable variations, 
 and the following description applies to a definite individual. 
 Nine or possibly ten distinct ureters, whose arrangement is 
 diagrammatically represented in fig. I, PI. 20, were present on 
 each side. It will be noticed that, whereas the five hindermost 
 are distinct till close to their openings into the urinogenital 
 cloaca, the four anterior ones appear to unite at once into a 
 single duct, but are probably only bound up in a common 
 sheath. The ureters fall into the common urinogenital cloaca, 
 
482 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 immediately behind the opening of the Wolffian duct (so far as 
 could be determined), by four apertures on each side. In a 
 section made through the part of the wall of the cloaca con- 
 taining the openings of the ureters of both sides, there were 
 present on the left side (where the section passed nearer to the 
 surface than on the right) four small openings posteriorly, viz. 
 the openings of the ureters and one larger one anteriorly, viz. 
 the opening of the Wolffian duct. On the other side of the 
 section where the level was rather deeper, there were five dis- 
 tinct ducts cut through, one of which was almost on the point of 
 dividing into two. This second section proves that, in this in- 
 stance at least, the two ureters did not unite till just before 
 opening into the urinogenital cloaca. The same section also 
 appeared to shew that one of the ureters fell not into the cloaca 
 but into the Wolffian duct. 
 
 As stated above both the Wolffian duct and the ureters fall 
 into an unpaired urinogenital cloaca. This cloaca communicates 
 at one end with the general cloaca by a single aperture situated 
 at the point of a somewhat conspicuous papilla, just behind the 
 anus (PI. 20, fig. i, o), and on the other it opens freely into a 
 pair of bladders, situated in close contact with each other, on 
 the ventral side of the kidney (PI. 20, fig. I, sb). To these 
 bladders Professor Semper has given the name uterus mascu- 
 linus, from having supposed them to correspond with the lower 
 part of the oviducts of the female. This homology he now 
 admits to be erroneous, and it will accordingly be better to drop 
 the name uterus masculinus, for which may be substituted 
 seminal bladder a name which suits their function, since they 
 are usually filled with semen at the generation season. The 
 seminal bladders communicate with the urinogenital cloaca by 
 wide openings, and it is on the borders of these openings that 
 the mouths of the Wolffian duct and ureters must be looked for. 
 My embryological investigations, though they have not been 
 specially directed to this point, seem to shew that the seminal 
 bladders do not arise during embryonic life, and are still absent 
 in very young individuals. It seems probable that both the 
 bladders and the urinogenital cloaca are products of the lower 
 extremities of the Wolffian duct. The only other duct requiring 
 any notice in the male is the rudimentary oviduct. As was first 
 
URINARY DUCTS OF THE FEMALE. 483 
 
 shewn by Semper, rudiments of the upper extremities of the 
 oviducts, with their abdominal openings, are to be found in the 
 male in the same position as in the female, on the front surface 
 of the liver. 
 
 In the female the same ducts are present as in the male, 
 viz. the Wolffian duct and the ureters. The part of the Wolffian 
 duct which receives the secretion of the Wolffian body is not 
 contorted, but is otherwise similar to the homologous part of 
 the Wolffian duct in the male. The Wolffian ducts of the two 
 sides fall independently into an unpaired urinal cloaca, but 
 their lower ends, instead of remaining simple as in .the male, 
 become dilated into urinary bladders. Vide PL 20, fig. 2. There 
 were nine ureters in the example dissected, whose arrangement 
 did not differ greatly from that in the male the hinder ones 
 remaining distinct from each other, but a certain amount of 
 fusion, the extent of which could not be quite certainly ascer- 
 tained, taking place between the anterior ones. The arrange- 
 ment of the openings of these ducts is not quite the same as in 
 the male. A somewhat magnified representation of it is given 
 in PL 20, fig. 3, o. u. The two Wolffian ducts meet at so acute 
 an angle that their hindermost extremities are only separated 
 by a septum. In the region of this septum on the inner walls 
 of the two Wolffian ducts were situated the openings of the 
 ureters, of which there were five on each side arranged linearly. 
 In a second example, also adult, I found four distinct openings 
 on each side similarly arranged to those in the specimen de- 
 scribed. Professor Semper states that all the ureters in the 
 female unite into a single duct before opening into the Wolffian 
 duct. It will certainly surprise me to find such great variations 
 in different individuals of this species as is implied by the dis- 
 crepancy between Professor Semper's description and my own. 
 
 The main difference between the ureters in the male and 
 female consists in their falling into the urinogenital cloaca in 
 the former and into the Wolffian duct in the latter. Since, 
 however, the urinogenital cloaca is a derivative of the Wolffian 
 duct, this difference between the two sexes is not a very im- 
 portant one. The urinary cloaca opens, in the female, into the 
 general cloaca by a median papilla of somewhat smaller di- 
 mensions than the corresponding papilla in the male. Seminal 
 
484 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 bladders are absent in the female, though possibly represented 
 by the bladder-like dilatations of the Wolffian duct. The ovi- 
 ducts, whose anatomy is too well known to need description, 
 open independently into the general cloaca. 
 
 Since the publication of Professor Semper's researches on 
 the urinogenital system of Elasmobranch fishes, it has been well 
 known that, in most adult Elasmobranchs, there are present a 
 series of funnel-shaped openings, leading from the perivisceral 
 cavity, by the intermediation of a short canal, into the glandular 
 tubuli of the kidney. These openings are called by Professor 
 Semper, Segmentaltrichter, and by Dr Spengel, in his valuable 
 work on the urogenital system of Amphibia, Nephrostomen. In 
 the present work the openings will be spoken of as segmental 
 openings, and the tubes connected with them as segmental 
 tubes. Of these openings there are a considerable number in 
 the adults of both sexes of Scy. canicula, situated along the 
 inner border of each kidney. The majority of them belong to 
 the Wolffian body, though absent in the extreme anterior part 
 of this. In very young examples a few certainly belong to 
 the region of the kidney proper. Where present, there is one 
 for each segment 1 . It is not easy to make certain of their 
 exact number. In one male I counted thirteen. In the female 
 it is more difficult than in the male to make this out with cer- 
 tainty, but in one young example, which had left the egg but a 
 short time, there appeared to be at least fourteen present. Ac- 
 cording to Semper there are thirteen funnels in both sexes a 
 number which fairly well agrees with my own results. In the 
 male, rudiments of segmental tubes are present in all the an- 
 terior segments of the Wolffian body behind the vasa efferentia, 
 but it is not till about the tenth segment that the first complete 
 one is present. In the female a somewhat smaller number of 
 the anterior segments, six or seven, are without segmental tubes, 
 or only possess them in a rudimentary condition. 
 
 A typical segment of the Wolffian body or kidney, in the 
 sense in which this term has been used above, consists of a 
 number of factors, each of which will be considered in detail 
 with reference to its variations. On PL 20, fig. 5, is represented 
 
 1 The term segment will be more accurately defined below. 
 
SEGMENTAL TUBES. 485 
 
 a portion of the Wolffian body with three complete segments 
 and part of a fourth. If one of these be selected, it will be seen 
 to commence with (i) a segmental opening, somewhat oval in . 
 form (st. o) and leading directly into (2) a narrow tube, the seg- 
 mental tube, which takes a more or less oblique course back- 
 wards, and, passing superficially to the Wolffian duct (w.d], 
 opens into (3) a Malpighian body (/. mg) at the anterior ex- 
 tremity of an isolated coil of glandular tubuli. This coil forms 
 the fourth section of each segment, and starts from the Mal- 
 pighian body. It consists of a considerable number of rather 
 definite convolutions, and after uniting with tubuli from one or 
 two (according to size of the segment) accessory Malpighian 
 bodies (a. mg), smaller than the one into which the segmental 
 tube falls, eventually opens by a (5) narrowish tube into the 
 Wolffian duct at the posterior end of the segment. Each seg- 
 ment is completely isolated (except for certain rudimentary 
 structures to be alluded to shortly) from the adjoining ones, and 
 never has more than one segmental tube and one communication 
 with the Wolffian duct. 
 
 The number and general arrangement of the segmental 
 tubes have already been spoken of. Their openings into the 
 body-cavity are. in Scyllium, very small, much more so than in 
 the majority of Elasmobranchs. The general appearance of a 
 segmental tube and its opening is somewhat that of a spoon, in 
 which the handle represents the segmental tube, and the bowl 
 the segmental opening. Usually amongst Elasmobranchs the 
 openings and tubes are ciliated, but I have not determined 
 whether this is the case in Scy. canicula, and Semper does not 
 speak definitely on this point. From the segmental openings 
 proceed the segmental tubes, which in the front segments have 
 nearly a transverse direction, but in the posterior ones are 
 directed more and more obliquely backwards. This statement 
 applies to both sexes, but the obliquity is greater in the female 
 than in the male. 
 
 As has been said, each segmental tube normally opens into a 
 Malpighian body, from which again there proceeds the tubulus, 
 the convolutions of which form the main mass of each segment. 
 This feature can be easily seen in the case of the Malpighian 
 bodies of the anterior part of the Wolffian gland in young 
 
486 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 examples, and sometimes fairly well in old ones, of either sex 1 . 
 There is generally in each segment a second Malpighian body, 
 which forms the commencement of a tubulus joining that from 
 the primary Malpighian body, and, where the segments are 
 larger, there are three, and possibly in the hinder segments of 
 the Wolffian gland and segments of the kidney proper, more 
 than three Malpighian bodies. 
 
 The accessory Malpighian bodies, or at any rate one of them, 
 appear to have curious relations to the segmental tubes. The 
 necks of some of the anterior segmental tubes (PL 20, fig. 5) 
 close to their openings into the primary Malpighian bodies are 
 provided with a small knob of cells which points towards the 
 preceding segment and is usually connected with it by a fibrous 
 band. This knob is most conspicuous in the male, and in very 
 young animals or almost ripe embryos. In several instances in 
 a ripe male embryo it appeared to me to have a lumen, and to 
 be continued directly forwards into the accessory Malpighian 
 body of the preceding segment. One such case is figured in 
 the middle segment on PI. 20, fig. 5. In this embryo segmental 
 tubes were present in the segments immediately succeeding 
 those connected with the vasa efferentia, and at the same time 
 these segments contained ordinary and accessory Malpighian 
 bodies. The segmental tubes of these segments were not. how- 
 ever, connected with the Malpighian body of their proper seg- 
 ment, but instead, turned forwards and entered the segment 
 in front of that to which they properly belonged. I failed to 
 trace them quite definitely to the accessory Malpighian body 
 of the preceding segment, but, in one instance at least, there 
 appeared to me to be present a fibrous connection, which is 
 shewn in the figure already referred to, PI. 20, fig. 5, r. st. In 
 any case it can hardly be doubted that this peculiarity of the 
 foremost segmental tubes is related to what would seem to be 
 the normal arrangement in the next few succeeding segments, 
 where each segmental tube is connected with a Malpighian body 
 in its own segment, and more or less distinctly with an accessory 
 Malpighian body in the preceding segment. 
 
 1 My observations on this subject completely disprove, if it is necessary to do so 
 after Professor Semper's investigations, the statement of Dr Meyer, that segmental 
 tubes in Scyllium open into lymph organs. 
 
THE VASA EFFERENTIA. 487 
 
 In the male the anterior segmental tubes, which even in the 
 embryo exhibit signs of atrophy, become in the adult completely 
 aborted (as has been already shewn by Semper), and remain as 
 irregular tubes closed at both ends, which for the most part do 
 not extend beyond the Wolffian duct (PI. 20, fig. 4, r. str}. - In 
 the adult, the first two or three segments with these aborted 
 tubes contain only accessory Malpighian bodies ; the remaining 
 segments, with aborted segmental tubes, both secondary and 
 primary Malpighian bodies. In neither case are the Malpighian 
 bodies connected with the aborted tubes. 
 
 The Malpighian bodies in Scyllium present no special 
 peculiarities. The outer layer of their capsule is for the most 
 part formed of flattened cells ; but, between the opening of the 
 segmental tube and the efferent tubulus of the kidney, their cells 
 become columnar. Vide PI. 20, fig. 5. The convoluted tubuli 
 continuous with them are, I believe, ciliated in their proximal 
 section, but I have not made careful investigations with refer- 
 ence to their finer structure. Each segment is connected with 
 the Wolffian duct by a single tube at the hinder end of the 
 segment. In the kidney proper, these tubes become greatly 
 prolonged, and form the ureters. 
 
 It has already been stated that the semen is carried by vasa 
 efferentia from the testes to the anterior segments of the Wolf- 
 fian body, and thence through the coils of the Wolffian body to 
 the Wolffian duct. The nature of the vasa will be discussed in 
 the embryological section of this chapter : I shall here confine 
 myself to a simple description of their anatomical relations. The 
 consideration of their connections naturally falls under three 
 heads: (i) the vasa efferentia passing from the testes to the 
 Wolffian body, (2) the mode in which these are connected with 
 the Wolffian body, and (3) with the testis. 
 
 In PI. 20, fig. 4, drawn for me from nature by my friend 
 Mr Haddon, are shewn the vasa efferentia and their junctions 
 both with the testes and the kidney. This figure illustrates 
 better than any description the anatomy of the various parts. 
 Behind there are two simple vasa efferentia (v. e.) and in front 
 a complicated network of vasa, which might be regarded as 
 formed of either two or four main vessels. It will be shewn 
 in the sequel that it is really formed of four distinct vessels. 
 
488 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Professor Semper states that there is but a single vas efferens in 
 Scyllium canicula, a statement which appears to me unquestion- 
 ably erroneous. All the vasa efferentia fall into a longitudinal 
 duct (I. c), which is connected in succession with the several 
 segments of the Wolffian body (one for each vas efferens) which 
 appertain to the testis. The hind end of the longitudinal duct 
 is simple, and ends blindly close to its junction with the last vas 
 efferens ; but in front, where the vasa efferentia are complicated, 
 the longitudinal duct also has a complicated constitution, and 
 forms a network rather than a simple tube. It typically sends 
 off a duct to join the coils of the Wolffian body between each 
 pair of vasa efferentia, and is usually swollen where this duct 
 parts from it. A duct similar to this has been described by 
 Semper as Nierenrandcanal in several Elasmobranchs, but its 
 existence is expressly denied in the case of Scyllium ! It is 
 usually found in Amphibia, as we know from Bidder and Spengel's 
 researches. Spengel calls it Langscanal des Hoden ; the vessels 
 from it into the kidney he calls vasa efferentia, and the vessels to 
 it, which I speak of as vasa efferentia, he calls Quercanale. 
 
 The exact mode of junction of the separate vasa efferentia 
 with the testis is difficult to make out on account of the opacity 
 of the basal portion of the testis. My figure shews that there 
 is a network of tubes (formed of four main tubes connected 
 by transverse branches) which is a continuation of the anterior 
 vasa efferentia, and joined by the two posterior ones. These 
 tubes receive the tubuli coming from the testicular ampullae. 
 The whole network may be called, with Semper, the testicular 
 network. While its general relations are represented in my 
 figure, the opacity of the testes was too great to allow of all 
 the details being with certainty filled in. 
 
 The kidneys of Scyllium stellare, as might be expected, 
 closely resemble those of Scy. canicula. The ducts of the kidney 
 proper, have, in the former species, a larger number of distinct 
 openings into the urinogenital cloaca. In two male examples 
 I counted seven distinct ureters, though it is not impossible 
 that there may have been one or two more present. In one 
 of my examples the ureters had seven distinct openings into the 
 cloaca, in the other five openings. In a female I counted eleven 
 ureters opening into the Wolffian duct by seven distinct openings 
 
THE VASA EFFERENTIA. 
 
 In the remaining parts of the excretory organs the two species 
 of Scyllium resemble each other very closely. 
 
 As may be gathered from Prof. Samper's monograph, the 
 excretory organs of Scyllium canicula are fairly typical for Elas- 
 mobranchs generally. The division into kidney and Wolrrian 
 body is universal. The segmental openings may be more 
 numerous and larger, e.g. Acanthias and Squatina, or absent in 
 the adult, e.g. Mustelus and Raja. Bladder-like swellings of the 
 Wolrrian duct in the female appear to be exceptional, and 
 seminal bladders are not always present. The variations in the 
 ureters and their openings are considerable, and in some cases 
 all the ureters are stated to fall into a single duct, which may be 
 spoken of as the ureter par excellence^, with the same relations 
 to the kidneys as the Wolffian duct bears to the Wolffian body. 
 In some cases Malpighian corpuscles are completely absent in 
 the Wolffian body, e.g. Raja. 
 
 The vasa efferentia of the testes in Scyllium are very typical, 
 but there are some forms in which they are more numerous 
 as well as others in which they are less so. Perhaps the vasa 
 efferentia are seen in their most typical form in Centrina as 
 described and figured (PI. XXI) by Professor Semper, or in Squatina 
 vulgaris, as I find it, and have represented it on PI. 20, fig. 8. 
 From my figure, representing the anterior part of the Wolffian 
 body of a nearly ripe embryo, it will be seen that there are five 
 vasa efferentia (y. e) connected on the one hand with a longitudinal 
 canal at the base of the testes (n. t) and orj the other with a 
 longitudinal canal in the Wolffian body. Connected with the 
 second longitudinal canal are four Malpighian bodies, three 
 of them stalked and one sessile ; from which again proceed 
 tubes forming the commencements of the coils of the anterior 
 segments of the Wolffian body. These Malpighian bodies are 
 clearly my primary Malpighian bodies, but there are in Squatina, 
 even in the generative segments, secondary Malpighian bodies. 
 What Semper has described for Centrina and one or two other 
 genera, closely correspond with what is present in Squatina. 
 
 1 I feel considerable hesitation in accepting Semper's descriptions of the ureters 
 and their openings. It has been shewn above that for Scyllium his statements are 
 probably inaccurate, and in other instances, e.g. Raja, I cannot bring my dissections to 
 harmonise with his descriptions. 
 
 B. 32 
 
490 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Development of tfie Segmental Tubes. 
 
 On p. 345, et seq. an account was given of the first formation 
 of the segmental tubes and the segmental duct, and the history 
 of these bodies was carried on till nearly the period at which it 
 is taken up in the exhaustive Memoir of Professor Semper. 
 Though the succeeding narration traverses to a great extent the 
 same ground as Semper's Memoir, yet many points are treated 
 somewhat differently, and others are dealt with which do not 
 find a place in the latter. In the majority of instances, attention 
 is called to points on which my results either agree with, or are 
 opposed to, those of Professor Semper. 
 
 From previous statements it has been rendered clear that at 
 first the excretory organs of Elasmobranchs exhibit no division 
 into Wolffian body or kidney proper. Since this distinction 
 is merely a question of the ducts, and does not concern the 
 glandular tubuli, no allusion is made to its appearance in the 
 present section, which deals only with the glandular part of the 
 kidneys and not with their ducts. 
 
 Up to the close of stage K the urinogenital organs consist 
 of a segmental duct opening in front into the body-cavity, and 
 terminating blindly behind in close contact with the cloaca, and 
 of a series of segmental tubes, each opening into the body-cavity 
 on the inner side of the segmental duct, but ending blindly at 
 their opposite extremities. It is with these latter that we have 
 at present to deal. They are from the first directed obliquely 
 backwards, and coil close round the inner and dorsal sides of the 
 segmental duct. Where they are in contact (close to their open- 
 ings into the body-cavity) with the segmental duct, the lumen of 
 the latter diminishes and so comes to exhibit regular alternations 
 of size. This is shewn in PI. 12, fig. 18^. d. At the points where 
 the segmental duct has a larger lumen, it eventually unites with 
 the segmental tubes. 
 
 The segmental tubes rapidly undergo a series of changes, the 
 character of which may be investigated, either by piecing together 
 transverse sections, or more easily from longitudinal and vertical 
 sections. They acquire a A -shaped form with an anterior limb 
 opening into the body-cavity and posterior limb, resting on a 
 
THE SEGMENTAL TUBES. 49 1 
 
 dilated portion of the segmental duct. The next important 
 change which they undergo consists in a junction being effected 
 between their posterior limbs and the segmental duct. In the 
 anterior part of the body these junctions appear before the 
 commencement of stage L. A segmental tube at this stage 4s 
 shewn in longitudinal section on PI. 21, fig. ja, and in transverse 
 section on PI. 18, fig. 2. In the former the actual openings 
 into the body-cavity are not visible. In the transverse section 
 only one limb of the A is met with on either side of the section ; 
 the limb opening into the body-cavity is seen on the left side, 
 and that opening into the segmental duct on the right side. 
 This becomes quite intelligible from a comparison with the 
 longitudinal section, which demonstrates that it is clearly not 
 possible to see more than a single limb of the A in any transverse 
 section. 
 
 After the formation of their junctions with the segmental 
 duct, other changes soon take place in the segmental tubes. By 
 the close of stage L four distinct divisions may be noticed in 
 each tube. Firstly, there is the opening into the body-cavity, 
 with a somewhat narrow stalk, to which the name segmental 
 tube will be strictly confined in the future, while the whole pro- 
 ducts of the original segmental tube will be spoken of as a seg- 
 ment of the kidney. This narrow stalk opens into a vesicle 
 (PL 1 8, fig. 2, and 21, fig. 6), which forms the second division. 
 From the vesicle proceeds a narrower section forming the third 
 division, which during stage L remains very short, though in 
 later stages it grows with great rapidity. It leads into the 
 fourth division, which constitutes the posterior limb of the A, 
 and has the form of a dilated tube with a narrow opening into 
 the segmental duct. 
 
 The subsequent changes of each segment do not for the 
 most part call for much attention. They consist mainly in the 
 elongation of the third division, and its conversion into a coiled 
 tubulus, which then constitutes the main mass of each segment of 
 the kidney. There are, however, two points of some interest, 
 viz. (i) the formation of the Malpighian bodies, and (2) the 
 establishment of the connection between each segmental tube 
 and the tubulus of the preceding segment which was alluded 
 to in the description on p. 486. The development of the 
 
 32 2 
 
492 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Malpighian body is intimately linked with that of the secondary 
 connection between two segments. They are both products of 
 the metamorphosis of the vesicle which forms the termination of 
 the segmental tube proper. 
 
 At about stage O this vesicle grows out in two directions 
 (PL 21, fig. 10), viz. towards the segment in front (p.x) and 
 posteriorly into the segment of which it properly forms a part 
 (mg). That portion which grows backward remains continuous 
 with the third division of its proper segment, and becomes con- 
 verted into a Malpighian body. It assumes (PL 21, figs. 6 and 
 10) a hemispherical form, while near one edge of it is the opening 
 from a segmental tube, and near the other the opening leading 
 into a tubulus of the kidney. The two-walled hemisphere soon 
 grows into a nearly closed sphere, with a central cavity into 
 which projects a vascular tuft. For this tuft the thickened inner 
 wall of cells forms a lining, and at the same time the outer wall 
 becomes thinner, and formed of flattened cells, except in the in- 
 terval between the openings of the segmental tube and kidney 
 tubulus, where its cells remain columnar. 
 
 The above account of the formation of the Malpighian 
 bodies agrees very well with the description which Pye 1 has 
 given of the formation of these bodies in the embryonic Mam- 
 malian kidney. My statements also agree with those of Semper, 
 in attributing the formation of the Malpighian body to a 
 metamorphosis of part of the vesicle at the end of the seg- 
 mental tube. Semper does not however enter into full details 
 on this subject. 
 
 The elucidation of the history of the second outgrowth from 
 the original vesicle towards the preceding segment is fraught 
 with considerable difficulties, which might no doubt be over- 
 come by a patient investigation of ample material, but which I 
 have not succeeded in fully accomplishing. 
 
 The points which I believe myself to have determined are 
 illustrated by fig. 10, PL 21, a longitudinal vertical section 
 through a portion of the kidney between stages O and P. In 
 this figure parts of three segments of the kidney are repre- 
 sented. In the hindermost of the three the one to the right 
 
 1 Journal of Anatomy and Physiology, Vol. IX. 
 
THE MALPIGHIAN BODIES. 493 
 
 there is a complete segmental tube (s. t] which opens at its 
 upper extremity into an irregular vesicle, prolonged behind into 
 a body which is obviously a developing Malpighian body, m.g, 
 and in front into a wide tube cut obliquely in the section and 
 ending apparently blindly (p.x). In the preceding segment 
 there is also a segmental tube (s. t} whose opening into the body- 
 cavity passes out of the plane of the section, but which is again 
 connected with a vesicle dilating behind into a Malpighian 
 body (m.g) and in front into the irregular tube (p.x), as in the 
 succeeding segment, but this tube is now connected (and this 
 could be still more completely seen in the segment in front of 
 this) with a vesicle which opens into tJie thick-walled collecting 
 tnbe (fourth division} of the preceding segment close to the 
 opening of the latter into the Wolffian duct. The fact that the 
 anterior prolongation of the vesicle ends blindly in the hinder- 
 most segment is due of course to its terminal part passing out 
 of the plane of the section. Thus we have established betwem 
 stages O and P a connection between each segmental tube and 
 the collecting tube of tJie segment in front of that to which it 
 properly belongs ; and it further appears that in consequence of 
 this each segment of the kidney contains two distinct coils of 
 tubuli whicli only unite close to their common opening into the 
 Wolffian duct! 
 
 This remarkable connection is not without morphological 
 interest, but I am unfortunately only able to give in a frag- 
 mentary manner its further history. During the greater part of 
 embryonic life a large amount of interstitial tissue is present in 
 the embryonic kidneys, and renders them too opaque -to be 
 advantageously studied as a whole ; and I have also, so far, 
 failed to prepare longitudinal sections suitable for the study of 
 this connection. It thus results that the next stage I have 
 satisfactorily investigated is that of a nearly ripe embryo 
 already spoken of in connection with the adult, and represented 
 on PI. 20, fig. 5. This figure shews that each segmental tube, 
 while distinctly connected with the Malpighian body of its own 
 segment, also sends out a branch towards the secondary Mal- 
 pighian body of the preceding segment. This branch in most 
 cases appeared to be rudimentary, and in the adult is certainly 
 not represented by more than a fibrous band, but I fancy that I 
 
494 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 have been able to trace it (though not with the distinctness I 
 could desire) in surface views of the embryonic kidney of 
 stage Q. The condition of the Wolffian body represented on 
 PL 20, fig. 5 renders it probable that the accessory Malpighian 
 body in each segment is developed in connection with the anterior 
 groivth from tJie original vesicle at the end of the segmental tube of 
 the sitcceeding segment. How the third or fourth accessory Mal- 
 pighian bodies, when present, take their origin I have not made 
 out. It is, however, fairly certain that they form the com- 
 mencement of two additional coils which unite, like the coil 
 connected with the first accessory Malpighian body, with the 
 collecting tube of the primitive coil close to its opening into the 
 Wolffian duct or ureter. 
 
 The connection above described between two successive 
 kidney segments appears to have escaped Professor Semper's 
 notice, though I fancy that the peculiar vesicle he describes, 
 loc. cit. p. 303, as connected with the end of each segmental 
 tube, is in some way related to it. It seems possible that the 
 secondary connection between the segmental tube and the pre- 
 ceding segment may explain a peculiar observation of Dr 
 Spengel 1 on the kidney of the tailless Amphibians. He finds 
 that, in this group, the segmental tubes do not open into Mal- 
 pighian bodies, but into the fourth division of the kidney tube. 
 Is it not just possible that in this case the primitive attachment 
 of the segmental tubes may have become lost, and a secondary 
 attachment, equivalent to that above described, though without 
 the development of a secondary Malpighian body, have been 
 developed ? In my embryos the secondary coil of the seg- 
 mental tubes opens, as in the Anura, into the fourth section of a 
 kidney tubulus. 
 
 Development of the Mullerian and Wolffian ducts. 
 
 The formation of the Mullerian and Wolfnan ducts out of 
 the original segmental duct has been dealt with in a masterly 
 manner by Professor Semper, but though I give my entire 
 assent to his general conclusions, yet there are a few points on 
 
 1 Loc. cit. pp. 85-89. 
 
MULLERIAN AND WOLFFIAN DUCTS. 495 
 
 which I differ from him. These are for the most part of a 
 secondary importance ; but they have a certain bearing on the 
 homology between the Miillerian duct of higher Vertebrates 
 and that of Elasmobranchs. The following account refers to 
 Scy. canicula, but so far as my observations go, the changes in 
 Scy. stellare are nearly identical in character. 
 
 I propose treating the development of these ducts in the two 
 sexes separately, and begin with the female. 
 
 Shortly before stage N a horizontal split arises in the seg- 
 mental duct 1 , commencing some little distance from its anterior 
 extremity, and extending backwards. This split divides the 
 duct into a dorsal section and a ventral one. The dorsal section 
 forms the Wolffian duct, and receives the openings of the seg- 
 mental tubes, and the ventral one forms the Mullerian duct or 
 oviduct, and is continuous with the unsplit anterior part of the 
 primitive segmental duct, which opens into the body-cavity. 
 The nature of the splitting may be gathered from the woodcut, 
 fig. 6, p. 511, where x represents the line along which the sg- 
 mental duct is divided. The splitting of the primitive duct 
 extends slowly backwards, and thus there is for a considerable 
 period a single duct behind, which bifurcates in front. A series 
 of transverse sections through the point of bifurcation always 
 exhibits the following features. Anteriorly two separate ducts 
 are present, next two ducts in close juxtaposition, and immedi- 
 ately behind this a single duct. A series of sections through 
 the junction of two ducts is represented on Plate 21, figs. I A, 
 i B, i C, i D. 
 
 In my youngest example, in which the splitting had com- 
 menced, there were two separate ducts for only 14 sections, and 
 in a slightly older one for- about 18. In the second of these 
 embryos the part of the segmental duct anterior to the front 
 end of the Wolffian duct, which is converted directly into the 
 oviduct, extended through 48 sections. In the space included 
 in these 48 sections at least five, and I believe six, segmental 
 tubes with openings into the body-cavity were present. These 
 segmental tubes did not however unite with the oviduct, or at best, 
 but one or two rudimentary junctions were visible, and the evi- 
 dence of my earlier embryos appears to shew that the segmental 
 
 1 For the development of the segmental duct, vide p. 345, et seq. 
 
496 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 tubes in front of the Wolffian duct never become in the female 
 united with the segmental duct. The anterior end of the 
 Wolffian duct is very much smaller than the oviduct adjoining 
 it, and as the reverse holds good in the male, an easy method is 
 afforded of distinguishing the two sexes even at the earliest 
 period of the formation of the Wolffian duct. 
 
 Hitherto merely the general features of the development of 
 the oviduct and Wolffian duct have been alluded to, but a 
 careful inspection of any good series of sections, shewing the 
 junction of these two ducts, brings to light some features worth 
 noticing in the formation of the oviduct. It might have been 
 anticipated that, where the two ducts unite behind as the seg- 
 mental duct, their lumens would have nearly the same diameter, 
 but normally this appears to be far from the case. 
 
 To illustrate the formation of the oviduct I have represented 
 a series of sections through a junction in an embryo in which 
 the splitting into two ducts had only just commenced (PI. 21, 
 fig. i), but I have found that the features of this series of 
 sections are exactly reproduced in other series in which the 
 splitting has extended as far back as the end of the small intes- 
 tine. In the series represented (PI. 21) I A is the foremost 
 section, and I D the hindermost. In I A the oviduct (o d) is as 
 large or slightly larger than the Wolffian duct (w. d), and in the 
 section in front of this (which I have not represented) was con- 
 siderably the larger of the two ducts. In i B the oviduct has 
 become markedly smaller, but there is no indication of its lumen 
 becoming united with that of the Wolffian duct the two ducts, 
 though in contact, are distinctly separate. In i C the walls of 
 the two ducts have fused, and the oviduct appears merely as a 
 ridge on the under surface of the Wolffian duct, and its lumen, 
 though extremely minute, shews no sign of becoming one with 
 that of the Wolffian duct. Finally, in i D the oviduct can 
 merely be recognised as a thickening on the under side of the 
 segmental duct, as we must now call the single duct, but a slight 
 bulging downwards of the lumen of the segmental duct appears 
 to indicate that the lumens of the two ducts may perhaps have 
 actually united. But of this I could not be by any means 
 certain, and it seems quite possible that the lumen of the oviduct 
 never does open into that of the segmental duct. 
 
MULLERIAN AND WOLFFIAN DUCTS. 497 
 
 The above series of sections goes far to prove that the 
 posterior part of the oviduct is developed as a nearly solid ridge 
 split off from the under side of the segmental duct, into which 
 at the utmost a very small portion of the lumen of the latter 
 is continued. One instance has however occurred amongst 
 my sections which probably indicates that the lumen of the 
 segmental duct may sometimes, in the course of the formation 
 of the oviduct and Wolffian duct, become divided into two parts, 
 of which that for the oviduct, though considerably smaller than 
 that for the Wolffian duct, is not so markedly so as in normal 
 cases (PI. 21, fig. 2). 
 
 Professor Semper states that the lumen of the part of the 
 oviduct split off from the hindermost end of the segmental duct 
 becomes continuously smaller, till at last close to the cloaca it is 
 split off as a solid rod of cells without a lumen, and thus it comes 
 about that the oviduct, when formed, ends blindly, and does not 
 open into the cloaca till the period of sexual maturity. My own 
 sections do not include a series shewing the formation of a 
 terminal part of the oviduct, but Semper's statements accord 
 precisely with what might probably take place if my account of 
 the earlier stages in the development of the oviduct is correct. 
 The presence of a hymen in young female Elasmobranchs was 
 first made known by Putmann and Garman 1 , and subsequently 
 discovered independently by Semper 2 . 
 
 The Wolffian duct appears to receive its first segmental tube 
 at its anterior extremity. 
 
 In the male the changes of the original segmental duct have 
 a somewhat different character to those in the female, although 
 there is a fundamental agreement between the two sexes. As in 
 the female, a horizontal split makes its appearance a short way 
 behind the front end of the segmental duct, and divides this into 
 a dorsal Wolffian duct and a ventral Miillerian duct, the latter 
 continuous with the anterior section of the segmental duct, 
 which carries the abdominal opening. The differences in deve- 
 lopment between the two sexes are, in spite of a general similarity, 
 
 1 "On the Male and Female Organs of Sharks and Skates, with special reference 
 to the use of the claspers," Proceed. American Association for Advancement of Science, 
 
 1X74- 
 
 2 Loc. cit. 
 
498 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 very obvious. In the first place, the ventral portion split off 
 from the segmental duct, instead of being as in the female 
 larger in front than the Wolffian duct, is very much smaller ; 
 while behind it does not form a continuous duct, but in some 
 parts a lumen is present, and in others again absent- (PI. 2 1 , fig. 6). 
 It does not even form an unbroken cord, but is divided in dis- 
 connected portions. Those parts with a lumen do not appear to 
 open into the Wolffian duct. 
 
 The process of splitting extends gradually backwards, so that 
 .there is a much longer rudimentary Mullerian duct by stage O 
 than by stage N. By stage P the posterior portions of the 
 Mullerian ducts have vanished. The anterior parts remain, 
 as has been already stated, till adult life. A second difference 
 between the male and female depends on the fact that, in the 
 male, the splitting of the segmental duct into Mullerian duct 
 and Wolffian duct never extends beyond the hinder extremity 
 of the small intestine. A third and rather important point 
 of difference consists in the splitting commencing far nearer 
 the front end of the segmental duct in the male than in the 
 female. In the female it was shewn that about 48 sections 
 intervened between the front end of the segmental duct and 
 the point where this became split, and that this region included 
 five or six segmental tubes. In the male the homologous space 
 only occupies about 7 to 12 sections, and does not contain the 
 rudiment of more tJian a single segmental tube. Although my 
 sections have not an absolutely uniform thickness, yet the above 
 figures suffice to shew in a conclusive manner that the splitting, 
 of the segmental duct commences far further forwards in the 
 male than in the female. This difference accounts for two facts 
 which were mentioned in connection with the excretory organs 
 of the adult, viz. (i) the greater length of the Wolffian body 
 in the male than in the female, and (2) the fact that although a 
 nearly similar number of segmental tubes persist in the adults 
 of both sexes, yet that in the male there are five or six more 
 segments in front of the first fully developed segmental opening 
 than in the female. 
 
 The above description of the formation of the Mullerian duct 
 in the male agrees very closely with that of Professor Semper 
 for Acanthias. For Scyllium however he denies, as it appears to 
 
MULLERIAN DUCT IN BIRDS. 499 
 
 me erroneously, the existence of the posterior rudimentary parts 
 of the Mullerian duct. He further asserts that the portions of 
 the Mullerian duct with a lumen open into the Wolffian duct. 
 The most important difference, however, between Professor 
 Semper's and my own description consists in his having failed to 
 note that the splitting of the segmental duct commences much 
 further forwards in the male than in the female. 
 
 I have attempted to shew that the oviduct in the female, 
 with the exception of the front extremity, is formed as a nearly 
 solid cord split off from the ventral surface of the segmental 
 duct, and not by a simple splitting of the segmental duct into 
 two equal parts. If I am right on this point, it appears to me 
 far easier to understand the relationship between the oviduct 
 or Mullerian duct of Elasmobranchs and the Mullerian duct of 
 Birds, than if Professor Semper's account of the development of 
 the oviduct is the correct one. Both Professor Semper and my- 
 self have stated our belief in the homology of the ducts in the 
 two cases, but we have treated their relationship in a very 
 different way. Professor Semper 1 finds himself compelled to 
 reject, on theoretical grounds, the testimony of recent observers 
 on the development of the Mullerian duct in Birds, and to assert 
 that it is formed out of the Wolffian duct, or, according to my 
 nomenclature, 'the segmental duct.' In my account 2 , the ordinary 
 statements with reference to the development of the Mullerian 
 duct in Birds are accepted ; but it is suggested that the indepen- 
 dent development of the Mullerian duct may be explained 
 by the function of this duct in the adult having, as it were, more 
 and more impressed itself upon the embryonic development, 
 till finally all connection, even during embryonic life, between 
 the oviduct and the segmental duct (Wolffian duct) became lost. 
 
 Since finding what a small portion of the segmental duct 
 became converted into the Mullerian duct in Elasmobranchs, I 
 have reexamined the development of the Mullerian duct in the 
 Fowl, in the hope of finding that its posterior part might develope 
 nearly in the same manner as in Elasmobranchs, at the expense 
 of a thickening of cells on the outer surface of the Wolffian duct. 
 
 1 Loc. cit. pp. 412, 413. 
 
 4 " The Urinogenital Organs of Vertebrates," Journal of Anatomy and Physiology* 
 Vol. X. p. 47. [This edition, p. 164.] 
 
5<DO DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 I have satisfied myself, in conjunction with Mr Sedgwick, that 
 this is not the case, and that the general account is in the main 
 true ; but at the same time we have obtained evidence which 
 tends to shew that the cells which form the Mullerian duct are 
 in part derived from the walls of the Wolffian duct. We propose 
 giving a full account of our observations on this point, so that I 
 refrain from mentioning further details here. It may however 
 be well to point out that, apart from observations on the actual 
 development of the Miillerian duct in the Bird, the fact of 
 its abdominal opening being situated some way behind the 
 front end of the Wolffian duct, is of itself a sufficient proof that 
 it cannot be the metamorphosed front extremity of the Wolffian 
 (= segmental) duct, in the same way that the abdominal opening 
 of the Mullerian duct is the front extremity of the segmental 
 duct in Elasmobranchs. 
 
 Although the evidence I can produce in the case of the 
 Fowl of a direct participation of the Wolffian duct in the for- 
 mation of the Mullerian is not of an absolutely conclusive kind, 
 yet I am inclined to think that the complete independence of 
 the two ducts, if eventually established as a fact, would not of 
 itself be sufficient (as Semper is inclined to think) to disprove 
 the identity of the Mullerian duct in Birds and Elasmobranchs. 
 
 We have, no doubt, almost no knowledge of the magnitude of 
 the changes which can take place in the mode of development of 
 the same organ in different types, yet this would have to be placed 
 at a very low figure indeed in order to exclude the possibility 
 of a change from the mode of development of the Mullerian 
 duct in Elasmobranchs to that in Birds. We have, it appears 
 to me, in the smallness of the portion of the segmental duct 
 which goes to form the Mullerian duct in Elasmobranchs, evidence 
 that a change has already appeared in this group in the direction 
 of a development of the Mullerian duct independent of the 
 segmental duct, and therefore of the Wolffian duct ; and it has 
 been in view of this consideration, that I have devoted so much 
 attention to the apparently unimportant point of how much 
 of the segmental duct was concerned in the formation of the 
 Mullerian duct. An analogous change, in a somewhat different 
 direction, would seem to be taking place in the development 
 of the rudimentary Mullerian duct in the male Elasmobranchs. 
 
URINAL CLOACA. 50 1 
 
 It is, perhaps, just worth pointing out, that the blindness of 
 the oviduct of female Elasmobranchs, and its mode of develop- 
 ment from an imperfect splitting of the segmental duct, may 
 probably be brought into connection with the blindness of the 
 extremity of the Mullerian duct or oviduct which so often occurs 
 in both sexes of Sturgeons (Accipenser). 
 
 I may, perhaps, at this point, be permitted to say a few 
 words about my original account of the development of the 
 Wolffian duct. This account was incorrect, and based upon a 
 false interpretation of an imperfect series of sections, and I took 
 the opportunity, in a general account of the urinogenital system 
 of Vertebrates, to point out my mistake 1 . Professor Semper 
 has, however, subsequently done me the honour to discuss, at 
 considerable length, my original errors, and to attempt to ex- 
 plain them. Since it appears to me improbable that the con- 
 tinuation of such a discussion can be of much general interest, 
 it will suffice to say now, that both Professor Semper's and my 
 own original statements on the development of the Wolffian 
 duct were erroneous ; but that both of us have now recognised 
 our mistakes ; and that the first morphologically correct account 
 of the development was given by him. 
 
 With reference to the formation of the urinal cloaca there is 
 not much to say. The originally widely separated openings of 
 the two Wolffian ducts gradually approximate in both sexes. 
 By stage O (PI. 19, fig. I b} they are in close contact, and the 
 lower ends of the two ducts actually coalesce at a somewhat 
 later period, and open by a single aperture into the common 
 cloaca. The papilla on which this is situated begins to make its 
 appearance considerably before the actual fusion of the lower 
 extremities of the two ducts. 
 
 Formation of Wolffian Body and Kidney proper. 
 
 Between stages L and M the hindermost ten or eleven seg- 
 ments of the primitive undivided excretory organ commence to 
 undergo changes which result in their separation from the 
 
 1 Journal of Anatomy and Physiology, Vol. X. 1875. [This edition, No. VII.] 
 
502 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 anterior segments as a distinct gland, which was spoken of in 
 the description of the adult as the kidney proper, while the 
 unaltered preceding segments of the kidney were spoken of as 
 the Wolffian body. 
 
 It will be remembered that each segment of the embryonic 
 kidney consists of four divisions, the last or fourth of which 
 opens into the Wolffian duct. The changes which take place 
 in the hindermost ten or eleven segments, and cause them to 
 become distinguished as the kidney proper, concern alone the 
 fourth division of each segment, which becomes prolonged back- 
 wards, and its opening into the Wolffian duct proportionately 
 shifted. These changes affect the foremost segments of the 
 kidney much more than the hindermost, so that the fourth 
 division in the foremost segments becomes very much longer 
 than in the hindermost, and at last all the prolongations of the 
 kidney segments come to open nearly on the same level, close 
 to the cloacal termination of the Wolffian duct (PI. 21, fig. 8). 
 The prolongations of the fourth division of the kidney-segments 
 have already (p. 481) been spoken of in the description of the 
 adult as ureters, and this name will be employed for them in the 
 present section. 
 
 The exact manner in which the changes, that have been 
 briefly related, take place is rather curious, and very difficult 
 to unravel without the aid of longitudinal sections. First of all, 
 the junction between each segment of the kidney and the 
 Wolffian duct becomes so elongated as to occupy the whole 
 interval between the junctions of the two neighbouring seg- 
 ments. The original opening of each tube into the Wolffian 
 duct is situated at the anterior end of this elongated attach- 
 ment, the remaining part of the attachment being formed solely 
 of a ridge of cells on the dorsal side of the Wolffian duct. The 
 general character of this growth will be understood by com- 
 paring figs. 7 a and 7 b, PI. 21 two longitudinal vertical sec- 
 tions through part of the kidneys. Fig. 7 a shews the normal 
 junction of a segmental tube with the Wolffian duct in the 
 Wolffian body, while in figure 7 b (r. u) is shewn the modified 
 junction in the region of the kidney proper in the same embryo. 
 The latter of these figures (fig. 7 b) appears to me to prove that 
 the elongation of the attachments between the segmental tubes 
 
THE URETERS. 503 
 
 and Wolffian duct takes place entirely at the expense of the 
 former. Owing to the length of this attachment, every trans- 
 verse section through the .kidney proper at this stage either 
 presents a solid ridge of cells closely adhering to the dorsal side 
 of the Wolffian duct, or else passes through one of the openings 
 into the Wolffian duct. 
 
 During stage M the original openings of the segmental tubes 
 into the Wolffian duct appear to me to become obliterated, and 
 at the same time the lumen of each ureter is prolonged into the 
 ridge of cells on .the dorsal wall of the duct. 
 
 Both of these changes are illustrated in my figures. The 
 fact of the obliteration of the original opening into the Wolffian 
 duct is shewn in longitudinal section in PI. 21, fig. 9, u, but 
 more conclusively in the series of transverse sections represented 
 on PI. 21, figs. 3 A, 3 B, 3 C. In the hindermost of these (3 C) 
 is seen the solid terminal point of a ureter, while the same 
 ureter possesses a lumen in the two previous sections, but ex- 
 hibits no signs of opening into the Wolffian duct. Sections 
 may however be met with which appear to shew that in some 
 instances the ureters still continue to open into the Wolffian 
 duct, but these I find to be rare and inconclusive, and am in- 
 clined to regard them as abnormalities. The prolongation of 
 the lumen of the ureters takes place in a somewhat peculiar 
 fashion. The lumen is not, as might be expected, completely 
 circumscribed by the wall of the ureter, but only dorsally and 
 to the sides. Ventrally it is closed in by the dorsal wall of the 
 Wolffian duct. In other words, each ureter is at first an in- 
 complete tube. This peculiarity is clearly shewn in the middle 
 figure of the series on PI. 21, fig. 3 B. 
 
 During stages M and N the ureters elongate considerably, 
 and, since the foremost ones grow the most rapidly, they soon 
 come to overlap those behind. As each ureter grows in length 
 it remains an incomplete tube, and its lumen, though pro- 
 portionately prolonged, continues to present the same general 
 relations as at first. It is circumscribed by its proper walls 
 only dorsally and laterally ; its floor being formed in the case 
 of the front ureter by the Wolffian duct, and in the case of each 
 succeeding ureter by the dorsal wall of the ureter in front. 
 This is most easily seen in longitudinal sections, and is repre- 
 
504 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 sented on PL 21, fig. 9, or on a larger scale in fig. 9 A. In the 
 latter figure it is especially clear that while the wall on the 
 dorsal side of the lumen of each ureter is continuous with the 
 dorsal wall of the tubulus of its own segment, the wall on the 
 ventral side is continuous with the dorsal wall of the ureter of 
 the preceding segment. This feature in the ureters explains the 
 appearance of transverse sections in which the ureters are not 
 separate from each other, but form together a kind of ridge on 
 the dorsal side of the Wolffian duct, in which there are a series 
 of perforations representing the separate lumens of the ureters 
 (PI. 21, fig. 4). The peculiarities in the appearance of the 
 dorsal wall of the Wolffian duct in fig. 9 A, and the difference 
 between the cells composing it and those of the ventral wall, 
 become intelligible on comparing this figure with the repre- 
 sentation of transverse section in figs. 3 B and 3 C, and especially 
 in fig. 4. Most of the ureters continue to end blindly at the 
 close of stage N, and appear to have solid posterior terminations 
 like that of the Miillerian duct in Birds. 
 
 By stage O all the ureters have become prolonged up to the 
 cloacal end of the Wolffian duct, so that the anterior one has a 
 length equal to that of the whole kidney proper. For the most 
 part they acquire independent openings into the end section of 
 the Wolffian duct, though some of them unite together before 
 reaching this. The general appearance of the hindermost of 
 them between stages N and O is shewn in longitudinal and 
 vertical section in PI. 21, fig. 8, u. 
 
 They next commence to develope into complete and in- 
 dependent tubes by their side walls growing inwards and meet- 
 ing below so as to completely enclose their lumen. This is seen 
 already to have occurred in most of the posterior ureters in 
 PI. 21, fig. 8. 
 
 Before stage P the ureters cease to be united into a con- 
 tinuous ridge, and each becomes separated from its neighbours 
 by a layer of indifferent tissue : by this stage, in fact, the ureters 
 have practically attained very nearly their adult condition. The 
 general features of a typical section through them are shewn on 
 PL 21, fig. 5. The figure represents the section of a female 
 embryo, not far from the cloaca. Below is the oviduct (od} t 
 Above this again is the Wolffian duct (w. d), and still dorsal to 
 
THE VASA EFFERENTIA. 505 
 
 this are four ureters (#). In female embryos more than four 
 ureters are not usually to be seen in a single section. This is 
 probably owing to the persistence, in some instances, of the 
 intimate connection between the ureters found at an earlier 
 stage of development, and results in a single ureter coming 
 to serve as the collecting duct for several segments. A section 
 through a male embryo of stage P would mainly differ from 
 that through a female in the absence of the oviduct, and in the 
 presence of probably six 1 , instead of four, ureters. 
 
 The exact amount of fusion which takes place between the 
 ureters, and the exact number of the ureters, cannot easily be 
 determined from sections, but the study of sections is chiefly 
 of value in shewing the general nature of the changes which take 
 place in the process of attaining the adult condition. 
 
 It may be noticed, as a consequence of the above account, 
 that the formation of the ureters takes place by a growth of the 
 original segmental tubes, and not by a splitting off of parts of the 
 wall of the Wolffian duct. 
 
 The formation of ureters in Scyllium, which has been only 
 very cursorily alluded to by Professor Semper, appears to differ 
 very considerably from that in Acanthias as narrated by him. 
 
 The Vasa Efferentia. 
 
 A comparison of the results of Professor Semper on Elasmo- 
 branchs, and Dr Spengel on Amphibians, suggests several 
 interesting questions with reference to the development of the 
 vasa efferentia, and the longitudinal canal of the Wolffian body. 
 
 Professor Semper was the first to describe the adult anatomy 
 and development of vasa efferentia in Elasmobranchs, and 
 the following extracts will fully illustrate his views with reference 
 to them. 
 
 " In 2 dem friihesten Stadium finden sich wie friiher angegeben 
 ungefahr 34 Trichter in der Leibeshohle, von diesen gehen die 
 27 hintersten in die persistirenden Segmentaltrichter u'ber, von 
 denen 4 beim erwachsenen Thiere auf dem Mesorchium stehen. 
 
 1 This at least holds good for one of my embryos at this stage, which is labelled 
 Scy. canicula, but which may possibly be Scy. stellare. 
 '-' I.oc. cit. p. 364. 
 
 B. 33 
 
5O6 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 Die tibrigen 7 schliessen sich vollstandig ab zu den erwahnten 
 langlichen und spater mannigfach auswachsenden varicosen 
 Trichterblasen ; von diesen sind es wiederum 3 4 welche unter- 
 einander in der Langsrichtung verwachsen und dadurch den in 
 der Basis der Hodenfalte verlaufenden Centralcanal des Hodens 
 bilden. Ehe aber diese Verwachsung zu einem mehr oder 
 minder geschlangelten Centralcanal vollstandig wird, hat sich 
 einmal das Lumen der Trichterblasen fast vollstandig geschlossen 
 und ausserdem von ihnen aus durch Verwachsung und Knospung 
 die erste Anlage des rete vasculosum Halleri gebildet (Taf. XX. 
 Figs, i, 2 c). Es erstreckt sich namlich mehr oder minder weit 
 in die Genitalfalte hinein ein unregelmassiges von kleinen Zellen 
 begranztes Canalnetz welches zweifellos mit dem noch nicht 
 ganz vollstandigen Centralcanale des Hodens (Taf. XX. Fig. 2 c] 
 in Verbindung steht. Von diesem letzteren aus gehen in regel- 
 massigen Abstanden die Segmentalgange (Taf. XX. Fig. 2 sg.} 
 gegen die Niere hin ; da sie meist stark geneigt oder selbst 
 geschlangelt (bei 6 ctm langen Embryonen) gegen die Niere zu 
 verlaufen, wo sie sich an die primaren Mafyighi'schen Korper- 
 chen und deren Bildungsblasen ansetzen, so kann ein verticaler 
 Querschnitt auch nie einen solchen nun zum vas efferens gewor- 
 denen Segmentalgang seiner ganzen Lange nach treffen. Gegen 
 die Trichterfurche zu aber steht namentlich am hinteren Theile 
 der Genitalfalte der Centralcanal haufig noch durch einen kurzen 
 Zellstrang mit dem Keimepithel der Trichterfurche in Ver- 
 bindung; mitunter findet sich hier sogar noch eine kleine 
 Hohlung, Rest des ursprunglich hier vorhandenen weiten 
 Trichters" (Taf. XX. Fig. 3^). 
 
 And again : " Dieser 1 Gegensatz in der Umbildung der Seg- 
 mentalgange an der Hodenbasis scheint nun mit einem anderen 
 Hand in Hand zu gehen. Es bildet sich namlich am Innenrande 
 der Niere durch Sprossung und Verwachsung der Segmentalgange 
 vor ihrer Insertion an das primare Malpighi'sche Korperchen 
 ein Canal beim Mannchen aus. den ich als Nierenrandcanal oben 
 bezeichnet habe. Ich habe denselben bei Acanthias Centrina 
 (Taf. XXI. Fig. 13) und Mustelus (Taf. XV. Fig. 8) gefunden. 
 Bei Centrina ist er ziemlich lang und vereinigt mindestens 7 
 Segmentalgange, aber von diesen letzteren stehen nur 5 mit dem 
 
 1 Loc. cit. p. 395. 
 
THE VASA EFFERENTIA. $7 
 
 Hodennetz in Verbindung. Dort nun wo diese letzteren sich an 
 den Niercnrandcanal ansetzen (Taf. XXI. Fig. 13 sg. t sg. 6 ) findet 
 sich jedesmal ein typisch ausgebildetes Malpightsc\\es Korper- 
 chen, mit dem aber nun nicht mehr wie ursprtinglich nur 2 Canale 
 verbunden sind (Taf. XXI. Fig. 14) sondern 3. Einer dieser 
 letzteren ist derjenige Ast des Nierenrandcanals welcher die Ver- 
 bindung mit dem nachst folgenden Segmentalgang zu besorgen 
 hat. An den Stellen aber wo sich an den Nierenrandcanal die 
 hinteren blind gegen den Hoden hin endenden Segmentalgange 
 ansetzen fehlen diese Malpighischen Korperchen (Taf. XXI. Fig. 
 13 sg 7 ] vollstandig. Auch bei Mustelus (Taf. XV. Figs. 8, 10) findet 
 genau dasselbe Verhaltniss statt; da aber hier nur 2 (oder 3) 
 Segmentalgange zu vasa efferentia umgewandelt werden, so 
 stehen hier am kurzen Randcanal der Niere auch nur 2 oder 3 
 Malpighi'schz Korperchen. Diese aber sind typisch ausgebildet" 
 (Taf. xv. Fig. 10). 
 
 From these two extracts it is clear that Semper regards both 
 the vasa efferentia, and central canal of the testis network, as 
 well as the longitudinal canal of the Wolffian body, as products 
 of the anterior segmental tubes. 
 
 The appearance of these various parts in the fully grown 
 embryos or adults of such genera as Acanthias and Squatina 
 strongly favours this view, but Semper appears to have worked 
 out the development of these structures somewhat partially and 
 by means of sections, a method not, in Scyllium at least, very 
 suitable for this particular investigation. I myself at first 
 unhesitatingly accepted Semper's views, and it was not till after 
 the study of the paper of Dr Spengel on the Amphibian kidney 
 that I came to have my doubts as to their accuracy. The 
 arrangement of the parts in most Amphibians is strikingly similar 
 to that in Elasmobranchs. From the testis come transverse 
 canals corresponding with my vasa efferentia ; these fall into a 
 longitudinal canal of the kidneys, from which again, as in Squatina 
 (PL 20, fig. 8), Mustelus and Centrina, canals (the vasa efferentia 
 of Spengel) pass off to Malpighian bodies. So far there is no 
 difficulty, but Dr Spengel has made the extremely important 
 discovery, that in young Amphibians each Malpighian body 
 in the region of the generative ducts, in addition to receiving 
 the vasa efferentia, is connected with a fully developed segmental 
 
 332 
 
508 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 tube opening into the body-cavity. In Amphibians, therefore, 
 it is improbable that the vasa efferentia are products of the open 
 extremities of the segmental tubes, considering that these latter 
 are found in their unaltered condition at the same time as the 
 vasa efferentia. When it is borne in mind how strikingly similar 
 in most respects is the arrangement of the testicular ducts in 
 Amphibia and Elasmobranchs, it will not easily be credited that 
 they develope in entirely different methods. Since then we find 
 in Amphibians fully developed segmental tubes in the same 
 segments as the vasa efferentia, it is difficult to believe that 
 in Elasmobranchs the same vasa efferentia have been developed 
 out of the segmental tubes by the obliteration of their openings. 
 
 I set myself to the solution of the origin of the vasa effe- 
 rentia by means of surface views, after the parts had been made 
 transparent in creosote, but I have met with great difficulties, and 
 so far my researches have only been partially successful. From 
 what I have been able to see of Squatina and Acanthias, I am 
 inclined to think that the embryos of either of these genera 
 would form far more suitable objects for this research than 
 Scyllium. I have had a few embryos of Squatina which were 
 unfortunately too old for my purpose. 
 
 Very early the vasa efferentia are fully formed, and their 
 arrangement in an embryo eight centimetres long is shewn 
 in PI. 20, fig. 6, v.e. It is there seen that there are six if not 
 seven vasa efferentia connected with a longitudinal canal along 
 the base of the testes (Semper's central canal of the testis), and 
 passing down like the segmental tubes to spaces between the 
 successive segments of the Wolffian body. They were probably 
 connected by a longitudinal canal in the Wolffian body, but this 
 could not be clearly seen. In the segment immediately behind 
 the last vas efferens was a fully developed segmental tube. This 
 embryo clearly throws no light on the question at issue except 
 that on the whole it supports Semper's views. I further failed to 
 make out anything from an examination of still younger embryos. 
 
 In a somewhat older embryo there was connected with the 
 anterior vas efferens a peculiar structure represented on PI. 20, 
 fig. 7, r. st? which strangely resembled the opening of an 
 ordinary segmental tube, but as I could not find it in the 
 younger embryo, this suggestion as to its nature, is, at the best, 
 
THE VASA EFFERENTIA. 509 
 
 extremely hazardous. If, however, this body really is the 
 remnant of a segmental opening, it would be reasonable to con- 
 clude that the vasa efferentia are buds from the segmental tubes 
 as opposed to their openings ; a mode of origin which is not 
 incompatible with the discoveries of Dr Spengel. I have noticed 
 a remnant, somewhat similar to that in the Scyllium embryo, 
 close to the hindermost vas efferens in an embryo Squatina 
 (PI. 20, fig. 8, r. st ?). 
 
 With reference to the development of the longitudinal canal 
 of the Wolffian body, I am without observations, but it appears 
 to me to be probably a further development of the outgrowths 
 of the vesicles of each segmental tube, which were described in 
 connection with the development of the segmental tubes, p. 492. 
 Were an anterior outgrowth of one vesicle to meet and coalesce 
 with the posterior outgrowth of the preceding vesicle, a longi- 
 tudinal canal such as actually exists would be the result. The 
 central canal of the base of the testes and the network connected 
 with it in the adult (PI. 20, fig. 4), appear to be derivatives of 
 the vasa efferentia. 
 
 I am thus compelled to leave open the question of the real 
 nature of the vasa efferentia, but am inclined to regard them as 
 outgrowths from the anterior segmental tubes, though not from 
 their open terminations. 
 
 My views upon the homologies of the various parts of the 
 urinogenital system, the development of which has been described 
 in the present chapter, have already been expressed in a paper 
 on Urinogenital organs of Vertebrates 1 . Although Kolliker's 2 
 discovery of the segmental tubes in Aves, and the researches of 
 Spengel 3 , Gasser 4 , Ewart 6 and others, have rendered necessary 
 a few corrections in my facts, I still adhere in their entirety to 
 the views expressed in that paper, and feel it unnecessary to 
 
 1 Journal of Anatomy and Physiology, Vol. X. [This edition, No. vn.] 
 
 2 Entwicklungsgeschichte des Menschen u. der hoheren Thiere. 
 
 3 Loc. cit. 
 
 4 Beitrage zur Entwicklungsg. d. Allantois d. Muller 'schen Gange u. d. Afters. 
 
 6 "Abdominal Pores and Urogenital Sinus of Lam prey, "Journal of Anatomy and 
 Physiology, Vol. X. p. 488. 
 
5io 
 
 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 repeat them in this place. I conclude the chapter with a resume 
 of the development of the urinogenital organs in Elasmobranchs 
 from their first appearance to their permanent condition. 
 
 Resume. The first trace of the urinary system makes its 
 appearance as a knob springing from the intermediate cell-mass 
 opposite the fifth protovertebra (woodcut, fig. $A.,p.d}. This 
 knob is the rudiment of the abdominal opening of the segmental 
 duct, and from it there grows backwards to the level of the anus 
 a solid column of cells, which constitutes the rudiment of the 
 segmental duct itself (woodcut, fig. 5 B, p. d]. The knob projects 
 
 FIG. 5. 
 Two SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. 
 
 spn 
 
 spn 
 
 The sections illustrate the development of the segmental duct (pd} or primitive 
 duct of the kidneys. In A (the anterior of the two sections) this appears as a solid 
 knob (pd) projecting towards the epiblast. In B is seen a section of the column 
 which has grown backwards from the knob in A. 
 
 spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord ; X. string 
 of cells below the notochord; mp. muscle-plate; mp' '. specially developed portion of 
 muscle-plate; ao. dorsal aorta; pd. segmental duct; so. somatopleure ; sp. splanchno- 
 pleure; pp. pleuroperitoneal or body-cavity; ep. epiblast; al. alimentary canal. 
 
 towards the epiblast, and the column connected with it lies 
 between the mesoblast and epiblast. The knob and column do 
 not long remain solid, but the former acquires an opening into 
 the body-cavity continuous with a lumen, which makes its 
 appearance in the latter. 
 
 While the lumen is gradually pushing its way backwards 
 along the solid rudiment of the segmental duct, the first traces 
 
RESUME OF URINOGENITAL SYSTEM. 511 
 
 of the segmental tubes, or proper excretory organs, make their 
 appearance in the form of solid outgrowths of the intermediate 
 cell-mass, which soon become hollow and open into the body- 
 cavity. Their blind ends curl obliquely backwards round the 
 inner and dorsal side of the segmental duct. One segmental 
 tube makes its appearance for each protovertebra, commencing 
 with that immediately behind the abdominal opening of the 
 segmental duct, the last tube being situated a short way behind 
 the anus. Soon after their formation the blind ends of the 
 segmental tubes open into the segmental duct, and each of them 
 becomes divided into four parts. These are (woodcut 7) (i) 
 a section carrying the abdominal opening or segmental tube 
 proper, (2) a dilated vesicle into which this opens, (3) a coiled 
 tubulus proceeding from (2) and terminating in (4), a wider portion 
 opening into the segmental duct. At the same time, or shortly 
 before this, each segmental duct unites with and opens into 
 one of the horns of the cloaca, and also retires from its primitive 
 position between the epiblast and mesoblast, and assumes a 
 position close to the epithelium lining the body-cavity. The 
 general features of the excretory organs at this period are dia- 
 grammatically represented on the woodcut, fig. 6. In this fig. 
 
 FIG. 6. 
 
 DIAGRAM OK THE PRIMITIVE CONDITION OF THE KIDNEY IN AN ELASMOBRANCH 
 
 EMBRYO. 
 
 pd. segmental duct. It opens at o into the body-cavity and at its other extremity 
 into the cloaca; x. line along which the division appears which separates the 
 segmental duct into the Wolffian duct above and the Mullerian duct below; st. 
 segmental tubes. They open at one end into the body-cavity, and at the other into 
 the segmental duct. 
 
 p.d is the segmental duct and o its abdominal opening, s.t points 
 to the segmental tubes, the finer details of whose structure are 
 not represented in the diagram. The kidneys thus form at this 
 period an unbroken gland composed of a series of isolated coiled 
 
512 
 
 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 tubes, one extremity of each of which opens into the body- 
 cavity, and the other into the segmental duct, which forms the 
 only duct of the kidney, and communicates at one end with the 
 body-cavity, and at the other with the cloaca. 
 
 The next important change concerns the segmental duct, 
 which becomes longitudinally split into two complete ducts in 
 the female, and one complete duct and parts of a second in the 
 male. The manner in which this takes place is diagrammatically 
 represented in woodcut 6 by the clear line x, and in transverse 
 section in woodcut 7. The resulting ducts are the (i) Wolffian 
 duct dorsally, which remains continuous with the excretory 
 
 FIG. 7. 
 
 DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A SCYLLIUM 
 EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MULLERIAN 
 DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT. 
 
 me. medullary canal; mp. muscle-plate; ch. notochord ; ao. aorta; ca v. 
 cardinal vein ; st. segmental tube. On the one side the section passes through the 
 opening of a segmental tube into the body-cavity. On the other this opening is 
 represented by dotted lines, and the opening of the segmental tube into the Wolffian 
 duct has been cut through; w. d. Wolffian duct; m. d. Miillerian duct. The 
 section is taken through the point where the segmental duct and Wolffian duct have 
 just become separate; gr. The germinal ridge with the thickened germinal 
 epithelium ; /. liver ; i. intestine with spiral valve. 
 
RESUME OF URINOGENITAL SYSTEM. 513 
 
 tubules of the kidney, and ventrally (2) the oviduct or Mullerian 
 duct in the female, and the rudiments of this duct in the male. 
 In the female the formation of these ducts takes place by a nearly 
 solid rod of cells, being gradually split off from the ventral side 
 of all but the foremost part of the original segmental duct, with 
 the short undivided anterior part of which duct it is continuous 
 in front. Into it a very small portion of the lumen of the original 
 segmental duct is perhaps continued (PI. 21, fig. I A, etc.). The 
 remainder of the segmental duct (after the loss of its anterior 
 section and the part split off from its ventral side) forms the 
 Wolffian duct. The process of formation of the ducts in the 
 male chiefly differs from that in the female in the fact of the 
 anterior undivided part of the segmental duct, which forms 
 the front end of the Mullerian duct, being shorter, and in the 
 column of cells with which it is continuous being from the first 
 incomplete. 
 
 The tubuli of the primitive excretory organ undergo further 
 important changes. The vesicle at the termination of each 
 segmental tube grows forwards towards the preceding tubulus, 
 and joins the fourth section of it close to the opening into the 
 Wolffian duct (PI. 21, fig. 10). The remainder of the vesicle 
 becomes converted into a Malpighian body. By the first of 
 these changes a connection is established between the successive 
 segments of the kidney, and though this connection is certainly 
 lost (or only represented by fibrous bands) in the anterior 
 part of the excretory organs in the adult, and very probably 
 in the hinder part, yet it seems most probable that traces of 
 it are to be found in the presence of the secondary Malpighian 
 bodies of the majority of segments, which are most likely 
 developed from it. 
 
 Up to this time there has been no distinction between the 
 anterior and posterior tubuli of the primitive excretory organ 
 which alike open into the Wolffian duct. The terminal division 
 of the tubuli of a considerable number of the hindermostof these 
 (ten or eleven in Scyllium canicula), either in some species 
 elongate, overlap, and eventually open by apertures (not usually 
 so numerous as the separate tubes), on nearly the same level, 
 into the hindermost section of the Wolffian duct in the female, 
 or into the urinogenital cloaca, formed by the coalesced terminal 
 
514 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 parts of the Wolffian ducts, in the male ; or in other species 
 become modified in such a manner as to pour their secretion into 
 a single duct on each side, which opens in a position correspond- 
 ing with the numerous ducts of the other type (woodcut, fig. 8). 
 It seems that both in Amphibians and Elasmobranchs the type 
 with a single duct, or approximations to it, are more often found 
 in the females than in the males. The subject requires however 
 to be more worked out in Elasmobranchs 1 . In both groups the 
 modified posterior kidney-segments are probably equivalent to 
 the permanent kidney of the amniotic Vertebrates, and for this 
 reason the numerous ducts of the first group or single duct of 
 the second were spoken of as ureters. The anterior tubuli of 
 the primitive excretory organ retain their early relation to the 
 Wolffian duct, and form the Wolffian body. 
 
 The originally separate terminal extremities of the Wolffian 
 ducts always coalesce, and form a urinal cloaca, opening by 
 a single aperture situated at the extremity of a median papilla 
 behind the anus. Some of the abdominal openings of the 
 segmental tubes in Scyllium, or in other cases all the openings, 
 become obliterated. 
 
 In the male the anterior segmental tubes undergo remark- 
 able modifications. There appear to grow from the first three 
 or four or more of them (though the point is still somewhat 
 obscure) branches, which pass to the base of the testis and there 
 unite into a longitudinal canal, form a network, and receive 
 the secretion of the testicular ampullae (woodcut 9, nf). These 
 ducts, the vasa efferentia, carry the semen to the Wolffian body, 
 but before opening into the tubuli of this they unite into the 
 longitudinal canal of the Wolffian body (l.c], from which pass off 
 ducts equal in number to the vasa efferentia, each of which 
 normally ends in a Malpighian body. From the Malpighian 
 body so connected start the convoluted tubuli of what may be 
 called the generative segments of the Wolffian body along 
 which the semen is conveyed to the Wolffian duct (v. d). The 
 Wolffian duct itself becomes much contorted and acts as vas 
 deferens. 
 
 1 The reverse of the above rule is the case with Raja, in the male of which a closer 
 approximation to the single-duct type is found than in the female. 
 
RESUME OF URINOGENITAL SYSTEM. 
 
 515 
 
 FIG. 8. 
 
 DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT 
 FEMALE ELASMOBRANCH. 
 
 in. d. Mullerian duct; iv. d. Wolffian duct; s. t. glandular tubuli ; five of 
 them are represented with openings into the body-cavity; d. duct of the posterior 
 segmental tubes ; ov. ovary. 
 
 In the woodcuts, figs. 8 and 9, are diagrammatically repre- 
 sented the chief constituents of the adult urinogenital organs in 
 the two sexes. In the adult female, fig. 8, there are present the 
 following parts : 
 
 (1) The oviduct or Mullerian duct (m.d] split off from the 
 segmental duct of the kidneys. Each oviduct opens at its an- 
 terior extremity into the body-cavity, and behind the two ovi- 
 ducts have independent communications with the general cloaca. 
 
 (2) The Wolffian ducts (w. d}, the other product of the seg- 
 mental ducts of the kidneys. They end in front by becoming 
 continuous with the tubulus of the anterior segment of the 
 Wolman body on each side, and unite behind to open by a com- 
 mon papilla into the cloaca. The Wolffian duct receives the 
 secretion of the anterior part of the primitive kidney which 
 forms the Wolffian body. 
 
 (3) The ureter (d) which carries off the secretion of the 
 kidney proper. It is represented in my diagram in its most 
 rare and differentiated condition as a single duct. 
 
 (4) The glandular tubuli (s. /), some of which retain their 
 original openings into the body-cavity, and others are without 
 them. They are divided into two groups, an anterior forming 
 
5 i6 
 
 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 the Wolffian body, which pour their secretion into the Wolffian 
 duct, and a posterior group forming the kidney proper, which 
 are connected with the ureter. 
 
 FIG. 9. 
 
 DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT 
 MALE ELASMOBRANCH. 
 
 tc 
 
 m. d. rudiment of Miillerian duct; w. d. Wolffian duct, marked vd in front 
 and serving as vas deferens ; 5/. glandular tubuli ; two of them are represented 
 with openings into the body-cavity; d. ureter; t. testis; nt. central canal at 
 the base of the testis; VE. vasa efferentia; Ic. longitudinal canal of the 
 Wolffian body. 
 
 In the male the following parts are present (woodcut 9) : 
 
 (1) The Miillerian duct (md), consisting of a small rudi- 
 ment attached to the liver representing the foremost end of the 
 oviduct of the female. 
 
 (2) The Wolffian duct (w, d} which precisely corresponds to 
 the Wolffian duct of the female, but, in addition to functioning 
 as the duct of the Wolffian body, also acts as a vas deferens (vd). 
 In the adult male its foremost part has a very tortuous course. 
 
 (3) The ureter (d), which has the same fundamental consti- 
 tution as in the female. 
 
 (4) The segmental tubes (st). The posterior of these have 
 the same arrangement in both sexes, but in the male modifica- 
 tions take place in connection with the anterior ones to fit them 
 to act as transporters of the testicular products. 
 
 Connected with the anterior ones there are present (i) the 
 vasa efferentia (VE), united on the one hand with (2) the central 
 canal in the base of the testis (nt), and on the other with the 
 
POSTSCRIPT. 517 
 
 longitudinal canal of the Wolffian body (l.c}. From the latter 
 are seen passing off the successive tubuli of the anterior seg- 
 ments of the Wolffian body in connection with which Malpighian 
 bodies are typically present, though not represented in my 
 diagram. 
 
 Postscript. 
 
 It was my original intention to have given an account of 
 the development of the generative organs. In the course, how- 
 ever, of my work a number of novel and unexpected points 
 turned up, which have considerably protracted my investiga- 
 tions, and it has appeared to me better no longer to delay the 
 appearance of this monograph, but to publish elsewhere my 
 results on the generative organs. In chapter VI. p. 349 et seq. 
 the early stages of the generative organs are described, but in 
 contemplation of the completion of the account no allusion was 
 made to their literature, and more especially to Professor 
 Semper's important contributions. I may perhaps say that I 
 have been able to confirm the most important result to which he 
 and other anatomists have nearly simultaneously arrived with 
 respect to Vertebrates, viz. that the primitive ova give rise to both 
 the male and female generative products. 
 
5l8 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 EXPLANATION OF PLATES 20 AND 21. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 a mg. Accessory Malpighian body. cav. Cardinal vein. ge. Germinal epithelium. 
 k. True kidney. /. c. Longitudinal canal of the Wolffian body connected with vasa 
 efferentia. mg. Malpighian body. nt. Network and central canal at the base of 
 the testis. o. External aperture of urinal cloaca, od. Oviduct or Miillerian duct 
 of the female, od' . Miillerian duct of the male. ou. Openings of ureters in Wolffian 
 duct in the female (fig. 3). pmg. Primary Malpighian body. px. Growth from 
 vesicle at the end of a segmental tube to join the collecting tube of the preceding 
 segment, r st. Rudimentary segmental tube. tu. Ureter commencing to be formed. 
 s b. Seminal bladder, s d. Segmental duct, s t. Segmental tube, st o. Opening of 
 segmental tube into body-cavity, sur. Suprarenal body. t. Testis. it. Ureters. 
 ve. Vas efferens. w b. Wolffian body, w d. Wolffian duct. 
 
 PLATE 20. 
 
 Fig. r. Diagrammatic representation of excretory organs on one side of a male 
 Scyllium canicula, natural size. 
 
 Fig. 2. Diagrammatic representation of the kidney proper on one side of a female 
 Scyllium canicula, natural size, shewing the ducts of the kidney and the dilated por- 
 tion of the Wolffian duct. 
 
 Fig. 3. Opening of the ureters into the Wolffian duct of a female Scyllium 
 canicula. The figure represents the Wolffian ducts (w d) with ventral portion removed 
 so as to expose their inner surface, and shews the junction of the two W. ducts to 
 form the common urinal cloaca, the single external opening of this (o), and openings 
 of ureters into one Wolffian duct (ou). 
 
 Fig. 4. Anterior extremity of Wolffian body of a young male Scyllium canicula 
 shewing the vasa efferentia and their connection with the kidneys and the testis. The 
 vasa efferentia and longitudinal canal are coloured to render them distinct. They are 
 intended to be continuous with the uncoloured coils of the Wolffian body, though this 
 connection has not been very successfully rendered by the artist. 
 
 Fig. 5. Part of the Wolffian body of a nearly ripe male embryo of Scyllium 
 canicula as a transparent object. Zeiss a a, ocul. 3. The figure shews two segmental 
 tubes opening into the body-cavity and connected with a primary Malpighian body, 
 and also, by a fibrous connection, with a secondary Malpighian body of the preceding 
 segment. It also shews one segmental tube (r st) imperfectly connected with the 
 accessory Malpighian body of the preceding segment of the kidney. The coils of the 
 kidney are represented somewhat diagrammatically. 
 
 Fig. 6. Vasa efferentia of a male embryo of Scyllium canicula eight centimetres 
 in length. Zeiss a a, ocul. 2. 
 
 There are seen to be at the least six and possibly seven distinct vasa going to as 
 many segments of the Wolffian body and connected with a longitudinal canal in the 
 base of the testis. They were probably also connected with a longitudinal canal iri 
 the Wolffian body, but this could not be clearly made out. 
 
EXPLANATION OF PLATES 2O AND 21. 519 
 
 Fig. 7. The anterior four vasa efferentia of a nearly ripe embryo. Connected 
 with the foremost one is seen a body which looks like the remnant of a segmental 
 tube and its opening (r st ?). 
 
 Fig. 8. Testis and anterior part of Wolffian body of an embryo of Squatina 
 vulgaris. 
 
 The figure is intended to illustrate the arrangement of the vasa efferentia. Tliere 
 are five of these connected with a longitudinal canal in the base of the testis, and 
 with another longitudinal canal in the Wolffian body. From the second longitudinal 
 canal there pass off four ducts to as many Malpighian bodies. Through the Mal- 
 pighian bodies these ducts are continuous with the several coils of the Wolfnan body, 
 and so eventually with the Wolffian duct. Close to the hindermost vas efferens is 
 seen a body which resembles a rudimentary segmental tube (rst?). 
 
 PLATE 21. 
 
 Figs, i A, i B, i C, i D. Four sections from a female Scyllium canicula of a stage 
 between M and N through the part where the segmental duct becomes split into 
 Wolffian duct and oviduct. Zeiss B, ocul. 2. r A is the foremost section. 
 
 The sections shew that the oviduct arises as a thickening on the under surface of 
 the segmental duct into which at the utmost a very narrow prolongation of the lumen 
 of the segmental duct is carried. The small size of the lumen of the Wolffian duct in 
 the foremost section is due to the section passing through nearly its anterior blind 
 extremity. 
 
 Fig. 2. Section close to the junction of the Wolffian duct and oviduct in a female 
 embryo of Scyllium canicula belonging to stage N. Zeiss B, ocul. 2. 
 
 The section represented shews that in some instances the formation of the oviduct 
 and Wolffian duct is accompanied by a division of the lumen of the segmental duct 
 into -two not very unequal parts. 
 
 Figs. 3 A, 3 B, 3 C. Three sections illustrating the formation of a ureter in a 
 female embryo belonging to stage N. Zeiss B, ocul. 2. 
 
 3 A is the foremost section. 
 
 The figures shew that the lumen of the developing ureter is enclosed in front by 
 an independent wall (fig. 3 A), but that further back the lumen is partly shut in by 
 the subjacent Wolffian duct, while behind no lumen is present, but the ureter ends as 
 a solid knob of cells without an opening into the Wolffian duct. 
 
 Fig. 4. Section through the ureters of the same embryo as fig. 3, but nearer the 
 cloaca. Zeiss B, ocul. 2. 
 
 The figure shews the appearance of a transverse section through the wall of cells 
 above the Wblffian duct formed by the overlapping ureters, the lumens of which 
 appear as perforations in it. It should be compared with fig. 9 A, which represents a 
 longitudinal section through a similar wall of cells. 
 
 Fig. 5. Section through the ureters, the Wolffian duct and the oviduct of a female 
 embryo of Scy. canicula belonging to stage P. Zeiss B, ocul. 2. 
 
 Fig. 6. Section of part of the Wolffian body of a male embryo of Scyllium 
 canicula belonging to stage O. Zeiss B, ocul. 2. 
 
520 DEVELOPMENT OF ELASMOBRANCH FISHES. 
 
 The section illustrates (i) the formation of a Malpighian body (nig) from the 
 dilatation at the end of a segmental tube, (2) the appearance of the rudiment of the 
 Miillerian duct in the male (od 1 ). 
 
 Figs. 7 a, 7 b. Two longitudinal and vertical sections through part of the kidney 
 of an embryo between stages L and M. Zeiss B, ocul. i. 
 
 7 a illustrates the parts of a single segment of the Wolffian body at this stage, vide 
 p. 491. The segmental tube and opening are not in the plane of the section, but the 
 dilated vesicle is shewn into which the segmental tube opens. 
 
 7 b is taken from the region of the kidney proper. To the right is seen the opening 
 of a segmental tube into the body-cavity, and in the segment to the left the commenc- 
 ing formation of a ureter, vide p. 502. 
 
 Fig. 8. Longitudinal and vertical section through the posterior part of the kidney 
 proper of an embryo of Scy Ilium canicula at a stage between N and O. Zeiss A, 
 ocul. 2. 
 
 The section shews the nearly completed ureters, developing Malpighian bodies, &c. 
 
 Fig. 9. Longitudinal and vertical section through the anterior part of the kidney 
 proper of the same embryo as fig. 8. Zeiss A, ocul. 2. 
 
 The figure illustrates the mode of growth of the developing ureters. 
 9 A. More highly magnified portion of the same section as fig. 9. 
 Compare with transverse section fig. 4. 
 
 Fig. 10. Longitudinal and vertical section through part of the Wolffian body of 
 an embryo of Scyllium canicula at a stage between O and P. 
 
 The section contains two examples of the budding out of the vesicle of a segmental 
 tube to form a Malpighian body in its own segment and to unite with the tubulus of 
 the preceding segment close to its opening into the Wolffian duct, 
 
XI. ON THE PHENOMENA ACCOMPANYING THE MATURATION 
 AND IMPREGNATION OF THE OvuM 1 . 
 
 THE brilliant discoveries of Strasburger and Auerbach have 
 caused the attention of a large number of biologists to be turned 
 to the phenomena accompanying the division of nuclei and the 
 maturation and impregnation of the ovum. The results of the 
 recent investigations on the first of these points formed the sub- 
 ject of an article by Mr Priestley in the sixteenth volume of this 
 Journal, and the object of the present article is to give some 
 account of what has so far been made out with reference to the 
 second of them. The matters to be treated of naturally fall 
 under two heads: (i) the changes attending the ripening of the 
 ovum, wJiich are independent of impregnation ; (2) the changes 
 which are directly due to impregnation. 
 
 Every ovum as it approaches maturity is found to be composed 
 (Fig. i) of (i) a protoplasmic body or vitellus usually containing 
 yolk-spherules in suspension; (2) of a germinal vesicle or nucleus, 
 
 FIG. i. Unripe ovum of Toxopneustes lividus (copied from Hertwig). 
 1 From the Quarterly Journal of Microscopical Science, April, 1878. 
 
 . 34 
 
522 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 containing (3) one or more germinal spots or nucleoli. It is 
 with the germinal vesicle and its contents that we are especially 
 concerned. This body at its full development has a more 
 or less spherical shape, and is enveloped by a distinct membrane. 
 Its contents are for the most part fluid, but may be more or 
 less granular. Their most characteristic component is, however, a 
 protoplasmic network which stretches from the germinal spot to 
 the investing membrane, but is especially concentrated round 
 the former (Fig. i). The germinal spot forms a nearly homo- 
 geneous body, with frequently one or more vacuoles. It occupies 
 an often excentric position within the germinal vesicle, and is 
 usually rendered very conspicuous by its high refrangibility. In 
 many instances it has been shewn to be capable of amoeboid 
 movements (Auerbach, and Os. Hertwig), and is moreover more 
 solid and more strongly tinged by colouring reagents than the 
 remaining constituents of the germinal vesicle. These peculiari- 
 ties have caused the matter of which it is composed to be 
 distinguished by Auerbach and Hertwig as nuclear substance. 
 
 In many instances there is only one germinal spot, or one 
 main spot, and two or three accessory smaller spots. In other 
 cases, e.g. Osseous Fish, there are a large number of nearly equal 
 germinal spots. The eggs which have been most investigated 
 with reference to the changes of germinal vesicle are those with 
 a single germinal spot, and it is with these that I shall have more 
 especially to deal in the sequel. 
 
 The germinal vesicle occupies in the first instance a central 
 position in the ovum, but at maturity is almost always found in 
 close proximity to the surface. Its change of position in a large 
 number of instances is accomplished during the growth of the 
 ovum in the ovary, but in other cases does not take place till the 
 ovum has been laid. 
 
 The questions which many investigators have recently set 
 themselves to answer are the two following: (i) What becomes 
 of the germinal vesicle when the ovum is ready to be impregnated ? 
 (2) Is any part of it present in the ovum at the commencement 
 of segmentation ? According to their answers to these questions 
 the older embryologists roughly fall into two groups : (i) By 
 one set the germinal vesicle is stated to completely disappear 
 and not to be genetically connected with the subsequent nuclei 
 
MATURATION AND IMPREGNATION OF THE OVUM. 523 
 
 of the embryo. (2) According to the other set it remains in 
 the ovum and by successive divisions forms the parent nucleus 
 of all the nuclei in the body of the embryo. Though the second 
 of these views has been supported by several very distinguished 
 names the first view was without doubt the one most generally 
 entertained, and Haeckel (though from his own observations 
 he was originally a supporter of the second view) has even 
 enunciated the theory that there exists an anuclear stage, 
 after the disappearance of the germinal vesicle, which he regards 
 as an embryonic repetition of the monad condition of the 
 Protozoa. 
 
 While the supporters of the first view agree as to the dis- 
 appearance of the germinal vesicle they differ considerably as to 
 the manner of this occurrence. Some are of opinion that the 
 vesicle simply vanishes, its contents being absorbed in the ovum ; 
 others that it is ejected from the ovum and appears as the polar 
 cell or body, or RicJitungskorper of the Germans a small body 
 which is often found situated in the space between the ovum and 
 its membrane, and derives its name from retaining a constant 
 position in relation to the ovum, and thus serving as a guide in 
 determining the similar parts of the embryo through the different 
 stages. The researches of Oellacher (I5) 1 in this direction 
 deserve special mention, as having in a sense formed the founda- 
 tion of the modern views upon this subject. By a series of 
 careful observations upon the egg of the trout and subsequently 
 of the bird, he demonstrated that the germinal vesicle of the 
 ovum, while still in the ovary, underwent partial degeneration 
 and eventually became ejected. His observations were made to 
 a great extent by means of sections, and the general accuracy of 
 his results is fairly certain, but the nature of the eggs he worked 
 on, as well as other causes, prevented his obtaining so deep 
 an insight into the phenomena accompanying the ejection of 
 the germinal vesicle as has since been possible. Loven, Flemming 
 (6), and others have been led by their investigations to adopt 
 views similar in the main to Oellacher's. As a rule, however, 
 it is held by believers in the disappearance of the germinal 
 vesicle that it becomes simply absorbed, and many very accurate 
 
 1 The numbers appended to authors' names refer to the list of publications at the 
 end of the paper. 
 
 342 
 
524 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 accounts, so far as they go, have been given of the gradual 
 atrophy of the germinal vesicle. The description of Kleinenberg 
 (14) for Hydra, and Gotte for Bombinator, may perhaps be 
 selected as especially complete in this respect ; in both instances 
 the germinal vesicle' commences to atrophy at a relatively early 
 period. 
 
 Coming to the more modern period the researches of five 
 workers, viz. Biitschli, E. van Beneden, Fol, Hertwig, and 
 Strasburger have especially thrown light upon this difficult sub- 
 ject. It is now hardly open to doubt that while part of the 
 germinal vesicle is concerned in the formation of the polar cell 
 or cells, when such are present, and is therefore ejected from the 
 ovum, part also remains in the ovum and forms a nuclear body 
 which will be spoken of as \ho. female pronudeus , the fate of which 
 is recorded in the second part of this paper. The researches of 
 Biitschli and van Beneden have been especially instrumental in 
 demonstrating the relation between the polar bodies and the ger- 
 minal vesicle, and those of Hertwig and Fol, in shewing that part 
 of the germinal vesicle remained in the ovum. It must not, 
 however, be supposed that the results of these authors are fully 
 substantiated, or that all the questions connected with these 
 phenomena are settled. The statements we have are in many 
 points opposed and contradictory, and there is much that is still 
 very obscure. 
 
 In the sequel an account is first given of the researches of the 
 above-named authors, followed by a statement of those results 
 which appear to me the most probable. 
 
 The researches of van Beneden (3 and 4) were made on the 
 ovum of the rabbit and of Asterias, and from his observations 
 on both these widely separated forms he has been led to con- 
 clude that the germinal vesicle is either ejected or absorbed, 
 but that it has in no case a genetic connection with the first 
 segmentation sphere. He gives the following description of the 
 changes in the rabbit's ovum. The germinal vesicle is enclosed 
 by a membrane, and contains one main germinal spot, and a few 
 accessory ones, together with a granular material which he calls 
 nucleoplasma, which affects, as is usual in nuclei, a reticular 
 arrangement. The remaining space in the vesicle is filled by a 
 clear fluid. As the ovum approaches maturity the germinal 
 
MATURATION AND IMPREGNATION OF THE OVUM. 525 
 
 vesicle assumes an excentric position, and fuses with the peri- 
 pheral layer of the egg to constitute the cicatricular lens. The 
 germinal spot next travels to the surface of the cicatricular lens 
 and forms the nuclear disc: at the same time the membrane 
 of the germinal vesicle vanishes though it probably unites~with 
 the nuclear disc. The nucleoplasma then collects into a definite 
 mass and forms the nucleoplasmic body. Finally the nuclear 
 disc assumes an ellipsoidal form and becomes the nuclear 
 body. Nothing is now left of the original germinal vesicle but 
 the nuclear body and the nucleoplasmic body both still situated 
 within the ovum. In the next stage no trace of the germinal 
 vesicle can be detected in the ovum, but outside it, close to the 
 point where the modified remnants of the vesicle were previously 
 situated, there is present a polar body which is composed of two 
 parts, one of which stains deeply and resembles the nuclear 
 body, and the other does not stain but is similar to the nucleo- 
 plasmic body. Van Beneden concludes that the polar bodies 
 are the two ejected products of the germinal vesicle. In the 
 case of Asterias, van Beneden has not observed the mode 
 of formation of the polar bodies, and mainly gives an account 
 of the atrophy of the germinal vesicle, but adds very little 
 to what was already known to us from Kleinenberg's (14) 
 earlier observations. He describes with precision the breaking 
 up of the germinal spot into fragments and its eventual dis- 
 appearance. 
 
 Though there are reasons for doubting the accuracy of all the 
 above details on the ovum of the rabbit, nevertheless, the obser- 
 vations of van Beneden taken as a whole afford strong grounds 
 for concluding that the formation of the polar cells is connected 
 with the disappearance, partial or otherwise, of the germinal 
 vesicle. A very similar account of the apparent disappearance 
 of the germinal vesicle is given by Greeff (19) who states that 
 the apparent disappearance of the germinal spot precedes that 
 of the vesicle. 
 
 The observations of Butschli are of still greater importance in 
 this direction. He has studied with a view to elucidating the 
 fate of the germinal vesicle, the eggs of Ncphelis, Lymnaeus, 
 Cucullanus,and other Nematodes; and Rotifers. In all of these, 
 with the exception of Rotifers, he finds polar bodies, and in this 
 
526 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 respect his observations are of value as tending to shew the 
 wide-spread existence of these structures. Negative results with 
 reference to the presence of the polar bodies have, it may be re- 
 marked, only a very secondary value. Biitschli has made the 
 very important discovery that in perfectly ripe eggs of Nephelis, 
 Lymnaeus and Cucullanus and allied genera a spindle, similar to 
 that of ordinary nuclei in the act of division, appears close to 
 the surface of the egg. This spindle he regards as the meta- 
 morphosed germinal vesicle, and has demonstrated that it takes 
 part in the formation of the polar cells. He states that the 
 whole spindle is ejected from the egg, and that after swelling up 
 and forming a somewhat spherical mass it divides into three parts. 
 
 In the Nematodes generally. Biitschli has been unable to find 
 the spindle modification of the germinal vesicle, but he states 
 that the germinal vesicle undergoes degeneration, its outline be- 
 coming indistinct and the germinal spot vanishing. The position 
 of the germinal vesicle continues to be marked by a clear space 
 which gradually approaches the surface of the egg. When it is 
 in contact with the surface a small spherical body, the remnant 
 of the germinal vesicle, comes into view, and eventually becomes 
 ejected. The clear space subsequently disappears. This de- 
 scription of Biitschli resembles in some respects that given by 
 van Beneden of the changes in the rabbit's ovum, and not im- 
 possibly refers to a nearly identical series of phenomena. The 
 discovery by Biitschli of the spindle and its relation to the polar 
 body has been of very great value. 
 
 The publications of van Beneden, and more especially those 
 of Biitschli, taken by themselves lead to the conclusion that the 
 whole germinal vesicle is either ejected or absorbed. Nearly 
 simultaneously with their publications there appeared, however, 
 a paper by Oscar Hertwig (n) on the eggs of one of the com- 
 mon sea urchins (Toxopneustes lividus), in which he attempted to 
 shew that part of the germinal vesicle, at any rate, was con- 
 cerned in the formation of the first segmentation nucleus. He 
 believed (though he has himself now recognised that he was in 
 error on the point) that no polar cell was formed in Toxop- 
 neustes, and that the whole germinal vesicle was absorbed, with 
 the exception of the germinal spot which remained in the egg as 
 the female pronucleus. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 527 
 
 The following is the summary which he gives of his results, 
 
 PP- 3578- 
 
 " At the time when the egg is mature the germinal vesicle 
 undergoes a retrogressive metamorphosis and becomes carried 
 towards the surface of the egg by the contraction of the proto- 
 plasm. Its membrane becomes dissolved and its contents dis- 
 integrated and finally absorbed by the yolk. The germinal spot 
 appears, however, to remain unaltered and to continue in the 
 yolk and to become the permanent nucleus of the ripe ovum 
 capable of impregnation." 
 
 After the publication of Butschli's monograph, O. Hertwig (12) 
 continued his researches on the ova of Leeches (Hcemopis and 
 Ncphelis), and not only added very largely to our knowledge of 
 the history of the germinal vesicle, but was able to make a very 
 important rectification in Butschli's conclusions. The following 
 is a summary of his results : The germinal vesicle, as in other 
 cases, undergoes a form of degeneration, though retaining its 
 central position ; and the germinal spot breaks up into frag- 
 ments. The stages in which this occurs are followed by one 
 when, on a superficial examination, the ovum appears to be 
 absolutely without a nucleus ; but there can be demonstrated by 
 means of reagents in the position previously occupied by the 
 germinal vesicle a spindle nucleus with the usual suns at its 
 poles, which Hertwig believes to be a product of the fragments of 
 the germinal spot. This spindle travels towards the periphery of 
 the ovum and then forms the spindle observed by Butschli. At 
 the point where one of tlie apices of the spindle lies close to the 
 surface a small protuberance arises which is destined to form the 
 first polar cell. As the protuberance becomes more prominent 
 one half of the spindle passes into it. The spindle then divides 
 in the normal manner for nuclei, one half remaining in the pro- 
 tuberance, the other in the ovum, and finally the protuberance 
 becomes a rounded body united to the egg by a narrow stalk. 
 It is clear that if, as there is every reason to think, the above 
 description is correct, the polar cell is formed by a simple pro- 
 cess of cell-division and not, as Butschli believed, by the forcible 
 ejection of the spindle. 
 
 The portion of the spindle in the polar cell becomes a mass 
 of granules, and that in the ovum becomes converted without 
 
528 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 the occurrence of the usual nuclear stage into a fresh spindle. A 
 second polar cell is formed in the same manner as the first one, 
 and the first one subsequently divides into two. The portion of 
 the spindle which remains in the egg after the formation of the 
 second polar cell reconstitutes itself into a nucleus the female 
 pronucleus and travelling towards the centre of the egg un- 
 dergoes a fate which will be spoken of in the second part of this 
 paper. 
 
 The most obscure part of Hertwig's work is that which con- 
 cerns the formation of the spindle on the atrophy of the germinal 
 vesicle, and his latest paper, though it gives further details on 
 this head, does not appear to me to clear up the mystery. 
 Though Hertwig demonstrates clearly enough that this spindle 
 is a product of the metamorphoses of the germinal vesicle, he 
 does not appear to prove the thesis which he maintains, that it 
 is the metamorphosed germinal spot. 
 
 Fol, to whom we are indebted in his paper on the develop- 
 ment of Geryonia (7) for the best of the earlier descriptions of 
 the phenomena which attend the maturation of the egg, and 
 later for valuable contributions somewhat similar to those of 
 Biitschli with reference to the development of the Pteropod egg 
 (8), has recently given us a very interesting account of what 
 takes place in the ripe egg of Asterias glacialis (9). In reference 
 to the formation of the polar cells, his results accord closely 
 with those of Hertwig, but he differs considerably from this 
 author with reference to the preceding changes in the germinal 
 vesicle. He believes that the germinal spot atrophies more or 
 less completely, but that in any case its constituents remain 
 behind in the egg, though he will not definitely assert that it 
 takes no share in the formation of the spindle at the expense of 
 which both the polar cells and the female pronucleus are formed. 
 The spindle with its terminal suns arises, according to him, from 
 the contents of the germinal vesicle, loses its spindle character, 
 travels to the surface, and reacquiring a spindle character is con- 
 cerned in the formation of the polar cells in the way described 
 by Hertwig. 
 
 Giard (10) gives a somewhat different account of the be- 
 haviour of the germinal vesicle in Psammechinus miliaris. At 
 maturity the contents of the germinal vesicle and spot mix 
 
MATURATION AND IMPREGNATION OF THE OVUM. 529 
 
 together and form an amoeboid mass, which, assuming a spindle 
 form, divides into two parts, one of which travels towards the 
 centre of the egg and forms the female pronucleus, the other 
 remains at the surface and gives origin to two polar cells, both 
 of which are formed after the egg is laid. What Giard regards 
 as the female pronucleus is perhaps the lower of the two bodies 
 which take the place of the original germinal vesicle as de- 
 scribed by Fol. Vide the account of Fol's observations on p. 531. 
 
 Strasburger, from observations on Phallnsia, accepts in the 
 main Hertwig's conclusion with reference to the formation of 
 the polar bodies, but does not share Hertwig's view that either 
 the polar bodies or female pronucleus are formed at the expense 
 of the germinal spot alone. He has further shewn that the so- 
 called canal-cell of conifers is formed in the same manner as the 
 polar cells, and states his belief that an equivalent of the polar 
 cells is widely distributed in the vegetable subkingdom. 
 
 This sketch of the results of recent researches will, it is 
 hoped, suffice to bring into prominence the more important 
 steps by which the problems of this department of embryology 
 have been solved. The present aspects of the question may 
 perhaps be most conveniently displayed by following the 
 history of a single ovum. For this purpose the eggs of Asterias 
 glacialis, which have recently formed the subject of a series of 
 beautiful researches by Fol (9), may conveniently be selected. 
 
 The ripe ovum (fig. 2), when detached from the ovary, is 
 formed of a granular vitellus without a vitelline membrane, but 
 enveloped in a mucilaginous coat. It contains an excentrically 
 situated germinal vesicle and germinal spot. In the former is 
 present the usual protoplasmic reticulum. As soon as the ovum 
 reaches the sea water the germinal vesicle commences to un- 
 dergo a peculiar metamorphosis. It exhibits frequent changes 
 of form, its membrane becomes gradually absorbed and its out- 
 line indented and indistinct, and finally its contents become to a 
 certain extent confounded with the vitellus (Fig. 3). 
 
 The germinal spot at the same time loses its clearness of 
 outline and gradually disappears from view. 
 
 At a slightly later stage in the place of the original germinal 
 vesicle there may be observed in the fresh ovum two clear 
 spaces (fig. 4), one ovoid and nearer the surface, and the second 
 
53O MATURATION AND IMPREGNATION OF THE OVUM. 
 
 FIG. 2. Ripe ovum of Asterias glacialis enveloped in a mucilaginous envelope, and 
 containing an excentric germinal vesicle and germinal spot (copied from Fol). 
 
 FIG. 3. Two successive stages in the gradual metamorphosis of the germinal vesicle 
 and spot of the ovum of Asterias glacialis immediately after it is laid (copied 
 from Fol). 
 
 FIG. 4. Ovum of Asterias glacialis, shewing the clear spaces in the place of the 
 germinal vesicle. Fresh preparation (copied from Fol). 
 
 more irregular in form and situated rather deeper in the vitellus. 
 By treatment with reagents the first clear space is found to be 
 formed of a spindle with two terminal suns on the lower side of 
 which is a somewhat irregular body (Fig. 5). The second clear 
 space by the same treatment is shewn to contain a round body. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 531 
 
 FIG. 5. Ovum of Asterias glacialis, at the same stage as Fig. 4, treated with picric 
 acid (copied from Fol). 
 
 Fol concludes that the spindle is formed out of part of the 
 germinal vesicle and not of the germinal spot, while he sees in 
 the round body present in the lower of the two clear spaces the 
 metamorphosed germinal spot. He will not, however, assert 
 that no fragment of the germinal spot enters into the formation 
 of the spindle. It may be observed that Fol is here obliged to 
 fill up (so far at least as his present preliminary account enables 
 me to determine) a lacuna in his observations in a hypothetical 
 manner, and O. Hertwig's (13) most recent observations on the 
 ovum of the same or an allied species of Asterias tend to throw 
 some doubt upon Fol's interpretations. 
 
 The following is Hertwig's account of the changes in the 
 germinal vesicle. A quarter of an hour after the egg is laid the 
 protoplasm on the side of the germinal vesicle towards the 
 surface of the egg develops a prominence which presses inwards 
 the wall of the vesicle. At the same time the germinal spot 
 develops a large vacuole, in the interior of which is a body con- 
 sisting of nuclear substance, and formed of a firmer and more 
 refractive material than the remainder of the germinal spot. In 
 the above-mentioned prominence towards the germinal vesicle, 
 first one sun is formed by radial striae of protoplasm, and then a 
 second makes its appearance, while in the living ovum the 
 germinal spot appears to have vanished, the outline of the 
 germinal vesicle to have become indistinct, and its contents to 
 have mingled with the surrounding protoplasm. Treatment 
 with reagents demonstrates that in the process of disappearance 
 of the germinal spot the nuclear mass in the vacuole forms a 
 
532 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 rod-like body, the free end of which is situated between the two 
 suns which occupy the prominence of the germinal vesicle. At 
 a slightly later period granules may be seen at the end of the 
 rod and finally the rod itself vanishes. After these changes 
 there may be demonstrated by the aid of reagents a spindle 
 between the two suns, which Hertwig believes to grow in size as 
 the last remnants of the germinal spot gradually vanish, and he 
 maintains, as before mentioned, that the spindle is formed at the 
 expense of the germinal spot. Without following Hertwig so 
 far as this 1 it may be permitted to suggest that his observations 
 tend to shew that the body noticed by Fol in the median line, 
 on the inner side of his spindle, is in reality a remnant of the 
 germinal spot and not, as Fol supposes, part of the germinal 
 vesicle. Considering how conflicting is the evidence before us 
 it seems necessary to leave open for the present the question as 
 to what parts of the germinal vesicle are concerned in forming 
 the first spindle. 
 
 The spindle, however it be formed, has up to this time been 
 situated with its axis parallel to the surface of the egg, but not 
 long after the stage last described a spindle is found with one 
 end projecting into a protoplasmic prominence which makes its 
 appearance on the surface of the egg (Fig. 6). Hertwig believes 
 
 FIG. 6. Portion of the ovum of Asterias glacialis, shewing the spindle formed from 
 the metamorphosed germinal vesicle projecting into a protoplasmic prominence 
 of the surface of the egg. Picric acid preparation (copied from Fol). 
 
 that the spindle simply travels towards the surface, and while 
 doing so changes the direction of its axis. Fol finds, however, 
 that this is not the case, but that between the two conditions 
 
 1 Hertwig's full account of his observations, with figures, in the 4th vol. of the 
 Morphologische Jahrbuch, has appeared since the above was written. The figures 
 given strongly support Hertwig's views. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 533 
 
 of the spindle an intermediate one is found in which a spindle 
 can no longer be seen in the egg, but its place is taken by a com- 
 pact rounded body. He has not been able to arrive at a conclu- 
 sion as to what meaning is to be attached to this occurrence. In 
 any case the spindle which projects into the prominence on the 
 surface of the egg divides it into two parts, one in the prominence 
 and one in the egg (Fig. 7). The prominence itself with the 
 
 FK;. 7. Portion of the ovum of Asterias glacialis at the moment of the detachment 
 of the first polar body and the withdrawal of the remaining part of the spindle 
 within the ovum. Picric acid preparation (copied from Fol). 
 
 enclosed portion of the spindle becomes partially constricted off 
 from the egg as the first polar body (Fig. 8). The part of the 
 
 FIG. 8. Portion of the ovum of Asterias glacialis, with the first polar body as it 
 appears when living (copied from Fol). 
 
 spindle which remains in the egg becomes directly converted into 
 a second spindle by the elongation of its fibres without passing 
 through a typical nuclear condition. A second polar cell next 
 becomes formed in the same manner as the first (Fig. 9), and 
 
 FIG. 9. Portion of the ovum of Asterias glacialis immediately after the formation of 
 the second polar body. Picric acid preparation (copied from Fol). 
 
534 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 the portion of the spindle remaining in the egg becomes con- 
 verted into two or three clear vesicles (Fig. 10) which soon 
 unite to form a single nucleus, the female pronucleus (Fig. n). 
 
 FIG. 10. Portion of the ovum of Asterias glacialis after the formation of the second 
 polar cell, shewing the part of the spindle remaining in the ovum becoming 
 converted into two clear vesicles. Picric acid preparation (copied from Fol). 
 
 FIG. u. Ovum of Asterias glacialis with the two polar bodies and the female 
 pronucleus surrounded by radial striae, as seen in the living egg (copied from Fol). 
 
 The two polar cells appear to be situated between two membranes, 
 the outer of which is very delicate and only distinct where it 
 covers the polar cells, while the inner one is thicker and becomes, 
 after impregnation, more distinct and then forms what Fol speaks 
 of as the vitelline membrane. It is clear, as Hertwig has pointed 
 out, that the polar bodies originate by a regular cell division and 
 have the value of cells. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 535 
 
 General conclusions. 
 
 Considering how few ova have been adequately investigated 
 with reference to the behaviour of the germinal vesicle any 
 general conclusions which may at present be formed are to be 
 regarded as provisional, and I trust that this will be borne in 
 mind by the reader in perusing the following paragraphs. 
 
 There is abundant evidence that at the time of maturation of 
 the egg the germinal vesicle undergoes peculiar changes, which 
 are, in part at least, of a retrogressive character. These changes 
 may begin considerably before the egg has reached the period 
 of maturity, or may not take place till after it has been laid. 
 They consist in appearance of irregularity and obscurity in the 
 outline of the germinal vesicle, the absorption of its membrane, 
 the partial absorption of its contents in the yolk, and the break- 
 ing up and disappearance of the germinal spot. The exact fate 
 of the single germinal spot, or the numerous spots where they 
 are present, is still obscure; and the observations of Oellacheron 
 the trout, and to a certain extent my own on the skate, tend to 
 shew that the membrane of the germinal vesicle may in some 
 cases be ejected from the egg, but this conclusion cannot be 
 accepted without further confirmation. 
 
 The retrogressive metamorphosis of the germinal vesicle is 
 followed in a large number of instances by the conversion of 
 what remains into a striated spindle similar .in character to a 
 nucleus previous to division. This spindle travels to the surface 
 and undergoes division to form the polar cell or cells in the 
 manner above described. The part which remains in the egg 
 forms eventually the female pronucleus. 
 
 The germinal vesicle has up to the present time only been 
 observed to undergo the above series of changes in a certain 
 number of instances, which, however, include examples from 
 several divisions of the Coelenterata, the Echinodermata, and the 
 Mollusca, and also some of the Vermes (Nematodes, Hirudinea, 
 Sagitta). It is very possible, not to say probable, that it is uni- 
 versal in the animal kingdom, but the present state of our know- 
 ledge does not justify us in saying so. It may be that in the case 
 of the rabbit, and many Nematodes as described by van Beneden 
 
536 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 and by Biitschli, we have instances of a different mode of for- 
 mation of the polar cells. 
 
 The case of Amphibians, as described by Bambeke (2) and 
 Hertwig (12) cannot so far be brought into conformity with our 
 type, though observations are so difficult to make with such 
 opaque eggs that not much reliance can be placed upon the exist- 
 ing statements. In both of these types of possible exceptions it 
 is fairly clear that, whatever may be the case with reference to 
 the formation of the polar cells, part of the germinal vesicle 
 remains behind as the female pronucleus. 
 
 There are a large number of types, including the whole of the 
 Rotifera J and Arthropoda, with a few doubtful exceptions, in 
 which the polar cells cannot as yet be said to have been satis- 
 factorily observed. 
 
 Whatever may be the eventual result of more extended inves- 
 tigation, it is clear that the formation of polar cells according to 
 our type is a very constant occurrence. Its importance is also 
 very greatly increased by the discovery by Strasburger of the 
 existence of an analogous process amongst plants. Two questions 
 about it obviously present themselves for solution : (i) What are 
 the conditions of its occurrence with reference to impregnation ? 
 (2) What meaning has it in the development of the ovum or the 
 embryo ? 
 
 The answer to the first of these questions is not difficult 
 to find. The formation of the polar bodies is independent of 
 impregnation, and is the final act of the normal growth of the 
 ovum. In a few types the polar cells are formed while the ovum 
 is still in the ovary, as, for instance, in some species of Echini, 
 Hydra, &c., but, according to our present knowledge, far more 
 usually after the ovum has been laid. In some of the instances 
 the budding off of the polar cells precedes, and in others follows 
 impregnation ; but there is no evidence to shew that in the later 
 cases the process is influenced by the contact with the male 
 element. In Asterias, as has been shewn by O. Hertwig, the 
 
 1 Flemming (6) finds that, in the summer and probably parthenogenetic eggs of 
 Lacinularia socialis, the germinal vesicle approaches the surface and becomes invisible, 
 and that subsequently a slight indentation in the outline of the egg marks the point of 
 its disappearance. In the hollow of the indentation Flemming believes a polar cell to 
 be situated, though he has not definitely seen one. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 537 
 
 formation of the polar cells may indifferently either precede or 
 follow impregnation a fact which affords a clear demonstration 
 of the independence of the two occurrences. 
 
 To the second of the two questions it does not unfortunately 
 seem possible at present to give an answer which can be regarded 
 as satisfactory. 
 
 The retrogressive changes in the membrane of the germinal 
 vesicle which usher in the formation of the polar bodies may very 
 probably be viewed as a prelude to a renewed activity of the 
 contents of the vesicle; and are perhaps rendered the more neces- 
 sary from the thickness of the membrane which results from a 
 protracted period of passive growth. This suggestion does not, 
 however, help us to explain the formation of polar cells by a pro- 
 cess identical with cell division. The ejection of part of the 
 germinal vesicle in the formation of the polar cells may probably 
 be paralleled by the ejection of part or the whole of the original 
 nucleus which, if we may trust the beautiful researches of Biit- 
 schli, takes place during conjugation in Infusoria as a preliminary 
 to the formation of a fresh nucleus. This comparison is due to 
 Butschli, and according to it the formation of the polar bodies 
 would have to be regarded as assisting, in some as yet unknown 
 way, the process of regeneration of the germinal vesicle. Views 
 analogous to this are held by Strasburger and Hertwig, who 
 regard the formation of the polar bodies in the light of a process 
 of excretion or removal of useless material. Such hypotheses 
 do not unfortunately carry us very far. 
 
 I would suggest that in the formation of the polar cells 
 part of the constituents of the germinal vesicle which are requisite 
 for its functions as a complete and independent nucleus are 
 removed to make room for the supply of the necessary parts to 
 it again by the spermatic nucleus (vide p. 541). More light on 
 this, as on other points, may probably be thrown by further 
 investigations on parthenogenesis and the presence or absence 
 of a polar cell in eggs which develope parthenogenetically. 
 Curiously enough the two groups in which parthenogenesis most 
 frequently occurs in the ordinary course of development (Arthro- 
 poda and Rotifera) are also those in which polar cells, with the 
 possible exception mentioned above, of the parthenogenetic eggs 
 of Lacenularia, are stated to be absent. This curious coincidence, 
 
 B- 35 
 
538 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 should it be confirmed, may perhaps be explained on the 
 hypothesis, I have just suggested, viz. that a more or less essential 
 part of the nucleus is removed in the formation of tJie polar cells ; 
 so that in cases, e.g. Arthropoda and Rotifera, wJiere polar cells are 
 not formed, and an essential part of the nucleus not therefore 
 removed, parthenogenesis can much more easily occur titan when 
 polar cells are formed. 
 
 That the part removed in the formation of the polar cells is 
 not absolutely essential, seems at first sight to follow from the 
 fact of parthenogenesis being possible in instances where impreg- 
 nation is the normal occurrence. The genuineness of all the 
 observations on this head is too long a subject to enter into 
 here 1 , but after admitting, as we probably must, that there 
 are genuine cases of parthenogenesis, it cannot be taken for 
 granted without more extended observation that the occurrence 
 of development in these rare instances may not be due to the 
 polar cells not having been formed as usual, and that when the 
 polar cells are formed the development without impregnation is 
 less possible. 
 
 The remarkable observations of Professor Greeff (19) on the 
 parthenogenetic development of the eggs of Asterias rubens tell, 
 however, very strongly against this explanation. Greefif has 
 found that under normal circumstances the eggs of this species 
 of starfish will develope without impregnation in simple sea 
 water. The development is quite regular and normal though 
 much slower than in the case of impregnated eggs. It is not 
 definitely stated that polar cells are formed, but there can be no 
 doubt that this is implied. Professor Greeffs account is so 
 precise and circumstantial that it is not easy to believe that 
 any error can have crept in ; but neither Hertwig nor Fol 
 have been able to repeat his experiments, and we may be per- 
 mitted to wait for further confirmation before absolutely accepting 
 them. 
 
 1 The instances quoted by Siebold from Hensen and Oellacher are not quite 
 satisfactory. In Hensen's case impregnation would have been possible if we can 
 suppose the spermatozoa to be capable of passing into the body-cavity through the 
 open end of the uninjured oviduct ; and though Oellacher's instances are more 
 valuable, yet sufficient care seems hardly to have been taken, especially when it is not 
 certain for what length of time spermatozoa may be able to live in the oviduct. For 
 Oellacher's precautions, vide Zeit. fiir iviss. Zool, Bd. xxn. p. 202. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 539 
 
 It is possible that the removal of part of the protoplasm of 
 the egg in the formation of the polar cells may be a secondary 
 process due to an attractive influence of the nucleus on the cell 
 protoplasm, such as is ordinarily observed in cell division. 
 
 Impregnation of the Ovum. 
 
 A far greater amount of certainty appears to me to have been 
 attained as to the effects of impregnation than as to the changes 
 of the germinal vesicle which precede this, and there appears, 
 moreover, to be a greater uniformity in the series of resulting 
 phenomena. For convenience I propose to reverse the order 
 hitherto adopted and to reserve the history of the literature and 
 my discussion of disputed points till after my general account. 
 Fol's paper on Astcrias glacialis, is again my source of informa- 
 tion. The part of the germinal vesicle which remains in the egg, 
 after the formation of the second polar cell, becomes converted 
 into a number of small vesicles (Fig. 10), which aggregate them- 
 selves into a single clear nucleus which gradually travels toward 
 the centre of the egg and around which as a centre the protoplasm 
 becomes radiately striated (Fig. 11). This nucleus is known as 
 \.\\Q female pronudeus*. In Asterias glacialis the most favourable 
 period for fecundation is about an hour after the formation of 
 the female pronucleus. If at this time the spermatozoa are 
 allowed to come in contact with the egg, their heads soon 
 become enveloped in the investing mucilaginous coat. A pro- 
 minence, pointing towards the nearest spermatozoon, now arises 
 from the superficial layer of protoplasm of the egg and grows 
 till it comes in contact with the spermatozoon (Figs. 12 and 13). 
 Under normal circumstances the spermatozoon, which meets the 
 prominence, is the only one concerned in the fertilisation, and it 
 makes its way into the egg by passing through the prominence. 
 The tail of the spermatozoa, no longer motile, remains visible for 
 some time after the head has bored its way in, but its place is 
 soon taken by a pale conical body which is, however, probably 
 in part a product of the metamorphosis of the tail itself (Fig. 14). 
 This body vanishes in its turn. 
 
 1 According to Hert wig's most recent statement a nucleolus is present in this 
 nucleus 
 
 352 
 
540 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 FIG. 12. 
 
 FIGS. 12 and 13. Small portion of the ovum of Asterias glacialis. The spermatozoa 
 are shewn enveloped in the mucilaginous coat. In Fig. 12 a prominence is 
 rising from the surface of the egg towards the nearest spermatozoon ; and in Fig. 
 13 the spermatozoon and prominence have met. From living ovum (copied from 
 Fol). 
 
 At the moment of contact between the spermatozoon and the 
 egg the outermost layer of the protoplasm of the latter raises 
 itself as distinct membrane, which separates from the egg and 
 prevents the entrance of any more spermatozoa. At the point 
 where the spermatozoon entered a crater-like opening is left in 
 the membrane (Fig. 14). 
 
 FIG. 14. Portion of the ovum of Asterias glacialis after the entrance of a spermato- 
 zoon into the ovum. It shows the prominence of the ovum through which the 
 spermatozoon has entered. A vitelline membrane with a crater-like opening has 
 become distinctly formed. From living ovum (copied from Fol). 
 
 The head of the spermatozoon when in the egg forms a 
 nucleus for which the name male pronucleus may be conveniently 
 adopted. It grows in size by absorbing, it is said, material from 
 the ovum, though this may be doubted, and around it is formed 
 a clear space free from yolk-spherules. Shortly after its forma- 
 
MATURATION AND IMPREGNATION OF THE OVUM. 54! 
 
 tion the protoplasm in its neighbourhood assumes a radiate 
 arrangement (Fig. 15). At whatever point of the egg the 
 
 FIG. 15. Ovum of Asterias glacialis, with male and female pronucleus and a radial 
 striation of the protoplasm around the former. From living ovum (copied from 
 Fol). 
 
 spermatozoon may have entered, it gradually travels towards the 
 female pronucleus. This latter, around which the protoplasm 
 no longer has a radial arrangement, remains motionless till it 
 comes in contact with the rays of the male pronucleus, after 
 which its condition of repose is exchanged for one of activity, 
 and it rapidly approaches the male pronucleus, and eventually 
 fuses with it (Fig. 16). 
 
 FIG. 16. Three successive stages in the coalescence of the male and female pronu- 
 cleus in Asterias glacialis. From the living ovum (copied from Fol). 
 
 The product of this fusion forms the first segmentation nucleus 
 (Fig. 17), which soon, however, divides into the two nuclei of the 
 two first segmentation spheres. While the two pronuclei are 
 approaching one another the protoplasm of the egg exhibits 
 amoeboid movements. 
 
 Of the earlier observations on this subject there need perhaps 
 only be cited one of E. van Beneden, on the rabbit's ovum, 
 
542 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 FIG. 17. Ovum of Asterias glacialis, after the coalescence of the male and female 
 pronucleus (copied from Fol). 
 
 shewing the presence of two nuclei before the commencement 
 of segmentation. Biitschli was the earliest to state from ob- 
 servations on Rliabditis dolicJiura that the first segmentation 
 nucleus arose from the fusion of two nuclei, and this was sub- 
 sequently shewn with greater detail for A scans nigrovenosa, by 
 Auerbach (i). Neither of these authors gave at first the correct 
 interpretation of their results. At a later period Biitschli (5) 
 arrived at the conclusion that in a large number of instances 
 (Lymtuzus, Nephelis, Cucullanus, &c.), the nucleus in question 
 was formed by the fusion of two or more nuclei, and Strasburger 
 at first made a similar statement for P/iallusia, though he has 
 since withdrawn it. Though Biitschli's statements depend, as 
 it seems, upon a false interpretation of appearances, he never- 
 theless arrived at a correct view with reference to what occurs 
 in impregnation. Van Beneden (3) described in the rabbit 
 the formation of the original segmentation nucleus from two 
 nuclei, one peripheral and the other central, and he gave it 
 as his hypothetical view that the peripheral nucleus was derived 
 from the spermatic element. It was reserved for Oscar Hertwig 
 (11) to describe in Echinus lividus the entrance of a sperma- 
 tozoon into the egg and the formation from it of the male 
 pronucleus. 
 
 Though there is a general agreement between the most recent 
 observers, Hertwig, Fol, Selenka, Strasburger, &c., as to the 
 main facts connected with the entrance of one spermatozoon into 
 
MATURATION AND IMPREGNATION OF THE OVUM. 543 
 
 the egg, the formation of the male pronucleus, and its fusion 
 with the female pronucleus, there still exist differences of detail 
 in the different descriptions which partly, no doubt, depend upon 
 the difficulties of observation, but partly also upon the observa- 
 tions not having all been made upon the same species. HeTtwig 
 does not enter into details with reference to the actual entrance 
 of the spermatozoon into the egg, but in his latest paper points 
 out that considerable differences may be observed in occurrences 
 which succeed impregnation, according to the relative period at 
 which this takes place. When, in Asterias, the impregnation is 
 effected about an hour after the egg is laid and previously to the 
 formation of the polar cells, the male pronucleus appears at first 
 to exert but little influence on the protoplasm, but after the 
 formation of the second polar cell, the radial striae around it 
 become very marked, and the pronucleus rapidly grows in size. 
 When it finally unites with the female pronucleus it is equal in 
 size to the latter. In the case when the impregnation is deferred 
 for four hours the male pronucleus never becomes so large as the 
 female pronucleus. With reference to the effect of the time at 
 which impregnation takes place, Asterias would seem to serve as 
 a type. Thus in Hirudinea, Mollusca, and Ncmatodes impregna- 
 tion normally takes place before the formation of the polar 
 bodies is completed, and the male pronucleus is accordingly as 
 large as the female. In Echinus, on the other hand, where the 
 polar bodies are formed in the ovary, the male pronucleus is 
 always small. 
 
 Selenka, who has investigated the formation of the male 
 pronucleus in Toxopmustes variegatus, differs in certain points 
 from Fol. He finds that usually, though not always, a single 
 spermatozoon enters the egg, and that though the entrance may 
 be effected at any part of the surface, it generally occurs at the 
 point marked by a small prominence where the polar cell was 
 formed. The spermatozoon first makes its way through the 
 mucous envelope of the egg, within which it swims about, and 
 then bores with its head into the polar prominence. The head 
 of the spermatozoon on entering the egg becomes enveloped by 
 the superficial protoplasm, and travels inward with its envelope, 
 while the tail remains outside. As Fol has described, a delicate 
 membrane becomes formed shortly after the entrance of the 
 
544 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 spermatozoon. The head continues to make its way by means 
 of rapid oscillations, till it has traversed about one eighth of the 
 diameter of the egg, and then suddenly becomes still. The tail in 
 the meantime vanishes, while the neck swells up and forms the 
 male pronucleus. The junction of the male and female pronu- 
 cleus is described by Fol and Selenka in nearly the same manner. 
 
 Giard gives an account of impregnation which is not easily 
 brought into harmony with that of the other investigators. His 
 observations were made on Psammechinus miliaris. At one 
 point is situated a polar body and usually at the pole opposite to 
 it a corresponding prominence. The spermatozoa on gaining 
 access to the egg attach themselves to it and give it a rotatory 
 movement, but according to Giard none of them penetrate the 
 vitelline membrane which, though formed at an earlier period, 
 now retires from the surface of the egg. 
 
 Giard believes that the prominence opposite the polar cells 
 serves for the entrance of the spermatic material, which probably 
 passes in by a process of diffusion. Thus, though he regards 
 the male pronucleus as a product of impregnation, he does not 
 believe it to be the head of a spermatozoon. 
 
 Both Hertwig and Fol have made observations on the result 
 of the entrance into the egg of several spermatozoa. Fol finds 
 that when the impregnation has been too long delayed the 
 vitelline membrane is formed with comparative slowness and 
 several spermatozoa are thus enabled to penetrate. Each sper- 
 matozoon forms a separate pronucleus with a surrounding sun ; 
 and several male pronuclei usually fuse with the female pro- 
 nucleus. Each male pronucleus appears to exercise a repulsive 
 influence on other male pronuclei, but to be attracted by the 
 female pronucleus. When there are several male pronuclei the 
 segmentation is irregular and the resulting larva a monstrosity. 
 These statements of Fol and Hertwig are at first sight in con- 
 tradiction with the more recent results of Selenka. In Toxo- 
 pneustes variegatus Selenka finds that though impregnation is 
 usually effected by a single spermatozoon yet that several may 
 be concerned in the act. The development continues, however, 
 to be normal if three or even four spermatozoa enter the egg 
 almost simultaneously. Under such circumstances each sperma- 
 tozoon forms a separate pronucleus and sun. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 545 
 
 It may be noticed that, while the observations of Fol and 
 Hertwig were admittedly made upon eggs in which the impreg- 
 nation was delayed till they no longer displayed their pristine 
 activity, Selenka's were made upon quite fresh eggs ; and it 
 seems not impossible that the pathological symptoms in the 
 embryos reared by the two former authors may have been due 
 to the imperfection of the egg and not to the entrance of more 
 than one spermatozoon. This, of course, is merely a suggestion 
 which requires to be tested by fresh observations. We have not 
 as yet a sufficient body of observations to enable us to decide 
 whether impregnation is usually effected by a single spermato- 
 zoon, though in spite of certain conflicting evidence the balance 
 would seem to incline towards the side of a single spermato- 
 zoon 1 . 
 
 The discovery of Hertwig as to the formation of the male 
 pronucleus throws a flood of light upon impregnation. 
 
 The act of impregnation is seen essentially to consist in the 
 fusion of a male and female nucleus ; not only does this appear 
 in the actual fusion of the two pronuclei, but it is brought into 
 still greater prominence by the fact that the female pronucleus 
 is a product of the nucleus of a primitive ovum, and the male 
 pronucleus is the metamorphosed head of the spermatozoon 
 which is itself developed from the nucleus of a spermatic cell 2 . 
 The spermatic cells originate from cells (in the case of Verte- 
 brates at least) identical with the primitive ova, so that the 
 fusion which takes place is the fusion of morphologically similar 
 parts in the two sexes. 
 
 It must not, however, be forgotten, as Strasburger has pointed 
 out, that part of the protoplasm of the generative cells of the 
 two sexes also fuse, viz. the tail of the spermatozoon with the 
 protoplasm of the egg. But there is no evidence that the former 
 is of importance for the act of impregnation. The fact that 
 impregnation mainly consists in the union of two nuclei gives 
 an importance to the nucleus which would probably not have 
 been accorded to it on other grounds. 
 
 1 The recent researches of Calberla on the impregnation of the ovum of Petromyzon 
 Planeri support this conclusion. 
 
 8 This seems the most probable view with reference to the nature of the head of 
 the spermatozoon, though the point is not perhaps yet definitely decided. 
 
546 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 Hertwig's discovery is in no way opposed to Mr Darwin's 
 theory of pangenesis and other similar theories, but does not 
 afford any definite proof of their accuracy, nor does it in the 
 meantime supply any explanation of the origin of two sexes or 
 of the reasons for an embryo becoming male or female. 
 
 Summary. 
 
 In what may probably be regarded as a normal case the 
 following series of events accompanies the maturation and im- 
 pregnation of an egg : 
 
 (1) Transportation of the germinal vesicle to the surface of 
 the egg. 
 
 (2) Absorption of the membrane of the germinal vesicle 
 and metamorphosis of the germinal spot. 
 
 (3) Assumption of a spindle character by the remains of 
 germinal vesicle, these remains being probably largely formed 
 from the germinal spot. 
 
 (4) Entrance of one end of the spindle into a protoplasmic 
 prominence at the surface of the egg. 
 
 (5) Division of the spindle into two halves, one remaining 
 in the egg, the other in the prominence. The prominence 
 becomes at the same time nearly constricted off from the egg as 
 a polar cell. 
 
 (6) Formation of a second polar cell in same manner as 
 first, part of the spindle still remaining in the egg. 
 
 (7) Conversion of the part of the spindle remaining in the 
 egg after the formation of the second polar cell into a nucleus 
 the female pronucleus. 
 
 (8) Transportation of the female pronucleus towards the 
 centre of the egg. 
 
 (9) Entrance of one spermatozoon into the egg. 
 
 (10) Conversion of the head of the spermatozoon into a 
 nucleus the male pronucleus. 
 
 (u) Appearance of radial striae round the male pronucleus 
 which gradually travels towards the female pronucleus. 
 
 (12) Fusion of male and female pronuclei to form the first 
 segmentation nucleus. 
 
MATURATION AND IMPREGNATION OF THE OVUM. 547 
 
 List of important recent Publications on tJie Maturation and 
 Impregnation of t/ie Ovum. 
 
 1. Auerbach. Organologische Studien, Heft 2. 
 
 2. Bambeke. " Recherches s. Embryologie des Batraciens." Bull, 
 de I'Acad. royale de Belgiquc, 2me se*r., t. LXI, 1876. 
 
 3. E. Van Beneden. "La Maturation de 1'CEuf des Mammiferes." 
 Bull, de I'Acad. royale de Belgique, 2me se"r., t. XL, no. 12, 1875. 
 
 4. E. Van Beneden. " Contributions a PHistoire de la Vdsicule Ger- 
 minative, &c." Bull, de I'Acad. royale de Belgique, 2me seV., t. XLI, no. i, 
 1876. 
 
 5. Biitschli. Eiselle, Zelltheilung, und Conjugation der Infusorien. 
 
 6. Flemming. "Studien in d. Entwickelungsgeschichte der Najaden." 
 Sitz. d. k. Akad. Wien, B. LXXI, 1875. 
 
 7. Fol. "Die erste Entwickelung des Geryonideneies." Jenaische 
 Zcitschrift, Vol. VII. 
 
 8. Fol. "Sur le DeVeloppement des Pteropodes." Archives de 
 Zoologie Experimental et Gendrale, Vols. IV and v. 
 
 9. Fol. "Sur le Commencement de I'Hdnoge'nie." Archives des . 
 Sciences Physiques et Naturelle*. Geneve, 1877. 
 
 10. Giard. Note sur les premiers phtnomenes du developpement de 
 rOursin. 1877. 
 
 11. Hertwig, Oscar. "Beit. z. Kenntniss d. Bildung, &c., d. thier. 
 Eies." MorphologiscJies Jahrbuch, Bd. I. 
 
 12. Hertwig, Oscar. Ibid. Morphologisches Jahrbuch, Bd. in, Heft. i. 
 
 13. Hertwig, Oscar. "Weitere Beitrage, &c." Morphologisches 
 Jahrbuch, Bd. ill, Heft 3. 
 
 14. Klein enberg. Hydra. Leipzig, 1872. 
 
 15. Oellacher, J. " Beitrjige zur Geschichte des Keimblaschens im 
 Wirbelthiereie." Archiv f. micr. Anat., Bd. vin. 
 
 1 6. Selenka. Befruchtung u. Theihing des Eies von Toxopneustes 
 variegatus (Vorliiufige Mittheilung). Erlangen, 1877. 
 
 17. Strasburger. Ueber Zellbildung u. Zelltheilung. Jena, 1876. 
 
 1 8. Strasburger. Uebcr Befruchtung u. Zelltheilung. Jena, 1878. 
 
 19. R. Greeff. " Ucb. d. Bau u. d. Entwickelung d. Echinodermen." 
 Sitzitn. der Gesellschaft 2. Befbrderung d. gesammten Naturiviss, z. 
 Marburg, No. 5. 1876. 
 
548 MATURATION AND IMPREGNATION OF THE OVUM. 
 
 Postscript. Two important memoirs have appeared since this paper 
 was in type. One of these by Hertwig, Morphologisches Jahrbuch, Bd. IV, 
 contains a full account with illustrations of what was briefly narrated in his 
 previous paper (13); the other by Calberla, "Der Befruchtungsvorgang 
 beim Ei von Petromyzon Planeri" Zeit. fur wiss. Zool., Bd. XXX, shews 
 that the superficial layer of the egg is formed by a coating of protoplasm 
 free from yolk- spheres, which at one part is continued inwards as a column, 
 and contains the germinal vesicle. The surface of this column is in contact 
 with a micropyle in the egg-membrane. Impregnation is effected by the 
 entrance of the head of a single spermatozoon (the tail remaining outside) 
 through the micropyle, and then along the column of clear protoplasm to 
 the female pronucleus. 
 
XII. ON THE STRUCTURE AND DEVELOPMENT OF THE 
 VERTEBRATE OVARY*. 
 
 (With Plates 24, 25, 26.) 
 
 THE present paper records observations on the ovaries of but 
 two types, viz., Mammalia and Elasmobranchii. The main points 
 dealt with are three : i. The relation of the germinal epithelium 
 to the stroma. 2. The connection between primitive ova in 
 Waldeyer's sense and the permanent ova. 3. The homologies 
 of the egg membranes. 
 
 The second of these points seems to call for special attention 
 after Semper's discovery that the primitive ova ought really to 
 be regarded as primitive sexual cells, in' that they give rise to the 
 generative elements of both 'sexes. 
 
 THE DEVELOPMENT OF THE ELASMOBRANCH OVARY. 
 
 The development of the Elasmobranch ovary has recently 
 formed the subject of three investigations. The earliest of them, 
 by H. Ludvvig, is contained in his important work, on the 
 ' Formation of the Ovum in the Animal Kingdom V Ludwig 
 arrives at the conclusion that the ovum and the follicular epithe- 
 lium are both derived from the germinal epithelium, and enters 
 into some detail as to their formation. Schultz 3 , without appa- 
 rently being acquainted with Ludwig's observations, has come to 
 very similar results for Torpedo. 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. 18, 1878. 
 - Arbeit en a. d. zool.-zoot. Instil ut Wiirzburg, Bd. I. 
 3 Archivf. micr, Anat. Vol. xi. 
 
550 THE STRUCTURE AND DEVELOPMENT 
 
 Semper 1 , in his elaborate memoir on the urogenital system of 
 Elasmobranchs, has added very greatly to our knowledge on this 
 subject. In a general way he confirms Ludwig's statements, 
 though he shews that the formation of the ova is somewhat more 
 complicated than Ludwig had imagined. He more especially 
 lays stress on the existence of nests of ova (Ureierernester), 
 derived from the division of a single primitive ovum, and of 
 certain peculiarly modified nuclei, which he compares to spindle 
 nuclei in the act of division. 
 
 My own results agree with those of previous investigators, 
 in attributing to the germinal epithelium the origin both of the 
 follicular epithelium and ova, but include a number of points 
 which I believe to be new, and, perhaps, of some little interest ; 
 they differ, moreover, in many important particulars, both as to 
 the structure and development of the ovary, from the accounts 
 of my predecessors. 
 
 The history of the female generative organs may conveniently 
 be treated under two heads, viz. (i) the history of the ovarian 
 ridge itself, and (2) the history of the ova situated in it. I pro- 
 pose dealing in the first place with the ovarian ridge. 
 
 TJie Ovarian ridge in Scyllium. At the stage spoken of in my 
 monograph on Elasmobranch Fishes as stage L, the ovarian ridge 
 has a very small development, and its maximum height is about 
 O'l mm. It exhibits in section a somewhat rounded form, and is 
 slightly constricted along the line of attachment. It presents two 
 surfaces, which are respectively outer and inner, and is formed 
 of a layer of somewhat thickened germinal epithelium separated 
 by a basement membrane from a central core of stroma. The 
 epithelium is far thicker on the outer surface than on the inner, 
 and the primitive ova are entirely confined to the former. The 
 cells of the germinal epithelium are irregularly scattered around 
 the primitive ova, and have not the definite arrangement usually 
 characteristic of epithelial cells. Each of them has a large 
 nucleus, with a deeply staining small nucleolus, and a very scanty 
 protoplasm. In stage N the ovarian ridge has a pointed edge and 
 narrower attachment than in stage L. Its greatest height is 
 about 0*17 mm. There is more stroma, and the basement mem- 
 brane is more distinct than before ; in other respects no changes 
 
 1 Arbeiten a. d. zool.-zoot. Institut Wiirzburg, Bel. II. 
 
OF THE VERTEBRATE OVARY. 551 
 
 worth recording have taken place. By stage P a distinction is 
 observable between the right and left ovarian ridges ; the right 
 one has, in fact, grown more rapidly than the left, and the differ- 
 ence in size between the two ridges becomes more and more 
 conspicuous during the succeeding stages, till the left one ceases 
 to grow any larger, though it remains for a great part of life 
 as a small rudiment. 
 
 The right ovarian ridge, which will henceforth alone engage 
 our attention, has grown very considerably. Its height is now 
 about 0-4 mm. It has in section (vide PI. 24, fig. i) a triangular 
 form with constricted base, and is covered by a flat epithelium, 
 except for an area on the outer surface, in length co-extensive 
 with the ovarian ridge, and with a maximum breadth of about 
 0*25 mm. This area will be spoken of as the ovarian area or 
 region, since the primitive ova are confined to it. The epithelium 
 covering it has a maximum thickness of about O'O5 mm., and thins 
 off rather rapidly on both borders, to become continuous with the 
 general epithelium of the ovarian ridge. Its cells have the same 
 character as before, and are several layers deep. Scattered 
 irregularly amongst them are the primitive ova. The germinal 
 epithelium in the ovarian region is separated by a basement 
 membrane from the adjacent stroma. 
 
 In succeeding stages, till the embryo reaches a length of 7 
 centimetres, no very important changes take place. The ovariaTi 
 region grows somewhat in breadth, though in this respect different 
 embryos vary considerably. In two embryos of nearly the same 
 age, the breadth of the ovarian epithelium was 0*3 mm. in the 
 one and 0*35 mm. in the other. In the former of these em- 
 bryos, the thickness of the epithelium was slightly greater than 
 in the latter, viz. 0*09 mm. as compared with o - o8. In both 
 the epithelium was sharply separated from the subjacent stroma. 
 There were relatively more epithelial cells in proportion to 
 primitive ova than at the earlier date, and the individual cells 
 exhibited great variations in shape, some being oval, some 
 angular, others very elongated, and many of them applied to 
 part of an ovum and accommodating themselves to its shape. 
 In some of the more elongated cells very deeply stained nuclei 
 were present, which (in a favourable light and with high powers) 
 exhibited the spindle modification of Strasburger with great 
 
552 THE STRUCTURE AND DEVELOPMENT 
 
 clearness, and must therefore be regarded as undergoing division. 
 The ovarian region is at this stage bounded on each side by a 
 groove. 
 
 In an embryo of seven centimetres (PI. 24, fig. 2) the breadth 
 of the ovarian epithelium was O'5, but its height only o - o6 mm. 
 It was still sharply separated from the subjacent stroma, though 
 a membrane could only be demonstrated in certain parts. The 
 amount of stroma in the ovarian ridge varies greatly in different 
 individuals, and no reliance can be placed on its amount as 
 a test of the age of the embryo. In the base of the ovarian 
 ridge the cells were closely packed, elsewhere they were still 
 embryonic. 
 
 My next stage (PI. 24, fig. 3, and fig. 4), shortly before the 
 time of the hatching of the embryo, exhibits in many respects 
 an advance on the previous one. It is the stage during which a 
 follicular covering derived from the germinal epithelium is first 
 distinctly formed round the ova, in a manner which will be more 
 particularly spoken of in the section devoted to the development 
 of the ovum itself. The breadth of the ovarian region is 0^56 mm., 
 and its greatest height close to the central border, O'I2 mm. a 
 great advance on the previous stage, mainly, however, due to the 
 larger size of the ova. 
 
 The ovarian epithelium is still in part separated from the 
 subjacent stroma by a membrane close to its dorsal and ventral 
 borders, but elsewhere the separation is not so distinct, it being 
 occasionally difficult within a cell or so to be sure of the boundary 
 of the epithelium. The want of a clear line between the stroma 
 and the epithelium is rendered more obvious by the fact that 
 the surface of the latter is somewhat irregular, owing to pro- 
 jections formed by specially large ova, into the bays between which 
 are processes of the stroma. In an ovary about this stage, 
 hardened in osmic acid, the epithelium stains very differently 
 from the subjacent stroma, and the line of separation between 
 the two is quite sharp. A figure of the whole ovarian ridge, 
 shewing the relation between the two parts, is represented on 
 PL 24, fig- 5- 
 
 The layer of stroma in immediate contact with the epithelium 
 is very different from the remainder, and appears to be destined 
 to accompany the vascular growths into the epithelium, which 
 
OF THE VERTEBRATE OVARY. 553 
 
 will appear in the next stage. The protoplasm of the cells com- 
 posing it forms a loose reticulum with a fair number of oval or 
 rounded nuclei, with their long axis for the most part parallel to 
 the lower surface of the epithelium. It contains, even at this 
 stage, fully developed vascular channels. 
 
 The remainder of the stroma of the ovarian ridge has now 
 acquired a definite structure, which remains constant through 
 life, and is eminently characteristic of the genital ridge of both 
 sexes. The bulk of it (PI. 24, fig. 3, sfr) consists of closely 
 packed polygonal cells, of about 0*014 mm - with large nuclei of 
 about 0-009. These cells appear to be supported by a delicate 
 reticulum. The whole tissue is highly vascular, with the 
 numerous capillaries ; the nuclei in the walls of which stand out 
 in some preparations with great clearness. 
 
 In the next oldest ovary, of which I have sections, the 
 breadth of the ovarian epithelium is 07 mm. and its thickness 
 0-096. The ovary of this age was preserved in osmic acid, which 
 is the most favourable reagent, so far as I have seen, for observing 
 the relation of the stroma and epithelium. On PI. 24, fig. 6, is 
 represented a transverse section through the whole breadth of 
 the ovary, slightly magnified to shew the general relations of 
 the parts, and on PI. 24, fig. 7, a small portion of a section more 
 highly magnified. The inner surface of the ovarian epithelium 
 is more irregular than in the previous stage, and it may be 
 observed that the subjacent stroma is growing in amongst the 
 ova. From the relation of the two tissues it is fairly clear that 
 the growth which is taking place is a definite growth of the 
 stroma into the epithelium, and not a mutual intergrowth of the 
 two tissues. The ingrowths of the stroma are, moreover, 
 directed towards individual ova, around which, outside the 
 follicular epithelium, they form a special vascular investment in 
 the succeeding stages. They are formed of a reticular tissue 
 with comparatively few nuclei. 
 
 By the next stage, in my series of ovaries of Scy. canicnla, 
 important changes have taken place in the constitution of 
 ovarian epithelium. Fig. 8, PI. 24, represents a portion of the 
 ovarian epithelium, on the same scale as figs. I, 2, 3, &c, and 
 fig. 9 a section through the whole ovarian ridge slightly magni- 
 fied. Its breadth is now 1*3 mm., and its thickness 03 mm. 
 B. 36 
 
554 THE STRUCTURE AND DEVELOPMENT 
 
 The ova have grown very greatly, and it appears to me to be 
 mainly owing to their growth that the greater thickness of the 
 epithelium is due, as well as the irregularity of its inner surface 
 (vide fig. 9). 
 
 The general relation of the epithelium to the surrounding 
 parts is much the same as in the earlier stage, but two new 
 features have appeared (i) The outermost cells of the ovarian 
 region have more or less clearly arranged themselves as a 
 kind of epithelial covering for the organ ; and (2) the stroma 
 ingrowths of the previous stage have become definitely vascular, 
 and have penetrated through all parts of the epithelium. 
 
 The external layer of epithelium is by no means a very 
 marked structure, the character of its cells varies greatly in 
 different regions, and it is very imperfectly separated from the 
 subjacent layer. I shall speak of it for convenience as pseudo- 
 epitlielium. 
 
 The greater part of the germinal epithelium forms anasto- 
 mosing columns, separated by very thin tracts of stroma. The 
 columns are, in the majority of instances, continuous with the 
 pseudo-epithelium at the surface, and contain ova in all stages 
 of development. Many of the cells composing them naturally 
 form the follicular epithelium for the separate ova; but the 
 majority have no such relation. They have in many instances 
 assumed an appearance somewhat different from that which 
 they presented in the last stage, mainly owing to the individual 
 nuclei being more widely separated. A careful examination 
 with a high power shews that this is owing to an increase in the 
 amount of protoplasm of the individual cells, and it may be 
 noted that a similar increase in the size of the bodies of the cells 
 has taken place in the pseudo-epithelium and in the follicular 
 epithelium of the individual ova. 
 
 The stroma ingrowths form the most important feature of 
 the stage. In most instances they are very thin and delicate, 
 and might easily be overlooked, especially as many of the cells 
 in them are hardly to be distinguished, taken separately, from 
 those of the germinal epithelium. These features render the 
 investigation of the exact relation of the stroma and epithelium 
 a matter of some difficulty. I have, however, been greatly 
 assisted by the investigation of the ovary of a young example 
 
OF THE VERTEBRATE OVARY. 555 
 
 of Scylliuni stellare, i6 centimetres in length, a section of 
 which is represented in PI. 25, fig. 26. In this ovary, although 
 no other abnormalities were observable, the stroma ingrowths 
 were exceptionally wide ; indeed, quite without a parallel in my 
 series of ovaries in this respect. The stroma most clearly 
 divides up the epithelium of the ovary into separate masses, or 
 more probably anastomosing columns, the equivalents of the 
 egg-tubes of Pfluger. These columns are formed of normal cells 
 of the germinal epithelium, which enclose ovarian nests and ova 
 in all stages of development. A comparison of the section I 
 have represented, with those from previous stages, appears to 
 me to demonstrate that the relation of the epithelium and 
 stroma has been caused by an ingrowth or penetration of the 
 stroma into the epithelium, and not by a mutual intergrowth of 
 the two tissues. Although the ovary, of which fig. 26 represents 
 a section was from Scy. stellarc, and the previous ovaries have 
 been from Scy. canicula, yet the thickness of the epithelium may 
 still be appealed to in confirmation of this view. In the previous 
 stage the thickness was about O'(X)6 mm., in the present one it 
 is about o - i6 mm., a difference of thickness which can be easily 
 accounted for by the growth of the individual ova and the 
 additional tracts of stroma. A pseudo-epithelium is more or 
 less clearly formed, but it is continuous with the columns of 
 epithelium. In the stroma many isolated cells are present, 
 which appear to me, from a careful comparison of a series of 
 sections, to belong to the germinal epithelium. 
 
 The thickness of the follicular epithelium on the inner side 
 of the larger ova deserves to be noted. Its meaning is discussed 
 on p. 567. 
 
 Quite a different interpretation to that which I have given 
 has been put by Ludwig and Semper upon the parts of the 
 ovary at this stage. My pseudo-epWielium is regarded by them 
 as forming, together with the follicular epithelium of the ova, the 
 sole remnant of the original germinal epithelium; and the masses 
 of cells below the pseudo-epithelium, which I have attempted to 
 shew are derived from the original germinal epithelium, aie 
 regarded as parts of the ingrowths of the adjacent stroma. 
 
 Ludwig has assumed this interpretation without having had 
 an opportunity of working out the development of the parts, but 
 
 362 
 
556 THE STRUCTURE AND DEVELOPMENT 
 
 Semper attempts to bring forward embryological proofs in 
 support of this position. 
 
 If the series of ovaries which I have represented be ex- 
 amined, it will not, I think, be denied that the general appear- 
 ances are very much in favour of my view. The thickened 
 patch of ovarian epithelium can apparently be traced through 
 the whole series of sections, and no indications of its sudden 
 reduction to the thin pseudo-epithelium are apparent. The 
 most careful examination that I have been able to make brings 
 to light nothing tending to shew that the general appearances 
 are delusive. The important difference between us refers to 
 our views of the nature of the tissue subjacent to the pseudo- 
 epithelium. If my results be accepted, it' is clear that the whole 
 ovarian region is an epithelium interpenetrated by connective 
 tissue ingrowths, so that the region below the pseudo-epithelium 
 is a kind of honeycomb or trabecular net-work of germinal 
 epithelium, developing ova of all stages and sizes, and composed 
 of cells capable of forming follicular epithelium for developing 
 ova. Ludwig figures what he regards as the formation of the 
 follicular epithelium round primitive ova during their passage 
 into the stroma. It is quite clear to me, that his figures of the 
 later stages, 33 and 34, represent fully formed permanent ova 
 surrounded by a follicular epithelium, and that their situation in 
 contact with the pseudo-epithelium is, so to speak, an accident, 
 and it is quite possible that his figures 31 and 32 also represent 
 fully formed ova ; but I have little hesitation in asserting that 
 he has not understood the mode of formation of the follicular 
 epithelium, and that, though his statement that it is derived 
 from the germinal epithelium is quite correct, his account of the 
 process is completely misleading. The same criticism does not 
 exactly apply to Semper's statements. Semper has really 
 observed the formation of the follicular epithelium round young 
 ova ; but, nevertheless, he appears to me to give an entirely 
 wrong account of the relation of the stroma to the germinal 
 epithelium. The extent of the difference between Semper's and 
 my view may perhaps best be shewn by a quotation from 
 Semper, loc. cit., 465: " In females the nests of primitive 
 ova sink in groups into the stroma. In these groups one cell 
 enlarges till it becomes the ovum, the neighbouring cells 
 
OF THE VERTEBRATE OVARY. 557 
 
 increase and arrange themselves around the ova as follicle 
 cells." 
 
 Although the histological changes which take place in the 
 succeeding stages are not inconsiderable, they do not involve 
 any fundamental change in the constitution of the ovarian 
 region, and may be described with greater brevity than has been 
 so far possible. 
 
 In a half-grown female, with an ovarian region of 3 mm. in 
 breadth, and O'8 mm. in thickness, the stroma of the ovarian 
 region has assumed a far more formed aspect than before. It 
 consists (PI. 24, fig. 10) of a basis in most parts fibrous, but in 
 some nearly homogeneous, with a fair number of scattered cells. 
 Immediately below the pseudo-epithelium, there is an im- 
 perfectly developed fibrous layer, forming a kind of tunic, in 
 which are imbedded the relatively reduced epithelial trabeculae 
 of the previous stages. They appear in sections as columns, 
 either continuous with or independent of the pseudo-epithelium, 
 formed of normal cells of the germinal epithelium, nests of ova, 
 and permanent ova in various stages of development. Below 
 this there comes a layer of larger ova which are very closely 
 packed. A not inconsiderable number of the larger ova have, 
 however, a superficial situation, and lie in immediate contact 
 with the pseudo-epithelium. Some of the younger ova, enclosed 
 amongst epithelial cells continuous with the pseudo-epithelium, 
 are very similar to those figured by Ludwig. It is scarcely 
 necessary to insist that this fact does not afford any argument 
 in favour of his interpretations. Tire ovarian region is honey- 
 combed by large vascular channels with distinct walls, and 
 other channels which are perhaps lymphatic. 
 
 The surface of the ovarian region is somewhat irregular and 
 especially marked by deep oblique transverse furrows. It is 
 covered by a distinct, though still irregular pseudo-epithelium, 
 which is fairly columnar in the furrows but flattened along the 
 ridges. The cells of the pseudo-epithelium have one peculiarity 
 very unlike that of ordinary epithelial cells. Their inner ex- 
 tremities (vide fig. 10) are prolonged into fibrous processes 
 which enter the subjacent tissue, and bending nearly parallel 
 to the surface of the ovary, assist in forming the tunic spoken 
 of above. This peculiarity of the pseudo-epithelial cells seems 
 
558 THE STRUCTURE AND DEVELOPMENT 
 
 to indicate that they do not essentially differ from cells which 
 have the character of undoubted connective tissue cells, and 
 renders it possible that the greater part of the tunic, which has 
 apparently the structure of ordinary connective tissue, is in 
 reality derived from the original germinal epithelium, a view 
 which tallies with the fact that in some instances the cells of 
 the tunic appear as if about to assist in forming the follicular 
 epithelium of some of the developing ova. In Raja, the 
 similarity of the pseudo-epithelium to the subjacent tissue is 
 very much more marked than in Scyllium. The pseudo- 
 epithelium appears merely as the superficial layer of the ovarian 
 tunic somewhat modified by its position on the surface. It 
 is formed of columnar cells with vertically arranged fibres which 
 pass into the subjacent layers, and chiefly differ from the 
 ordinary fibres in that they still form parts of the cell-proto- 
 plasm enclosing the nucleus. In PL 25, fig. 34, an attempt is 
 made to represent the relations of the pseudo-epithelium to 
 the subjacent tissue in Raja. Ludwig's figures of the pseudo- 
 epithelium of the ovary, in the regular form of its constituent 
 cells, and its sharp separation by a basement membrane from 
 the tissue below, are quite unlike anything which I have met 
 with in my sections either of Raja or Scyllium. 
 
 Close to the dorsal border of the ovary the epithelial cells of 
 the non-ovarian region have very conspicuous tails, extending 
 into a more or less homogeneous substance below, which con- 
 stitutes a peculiar form of tunic for this part of the ovarian 
 ridge. 
 
 In the full-grown female the stroma of the ovarian region is 
 denser and has a more fibrous aspect than in the younger 
 animal. Below the pseudo-epithelium it is arranged in two or 
 three more or less definite layers, in which the fibres run at 
 right angles. It forms a definite ovarian tunic. The pseudo- 
 epithelium is much more distinct, and the tails of its cells, so 
 conspicuous in previous stages, can no longer be made out. 
 
 Formation of the permanent ova and the follicular epithelium. 
 In my monograph on the development of Elasmobranch Fishes 
 an account was given of the earliest stages in the development 
 of the primitive ova, and I now take up their development from 
 
OF THE VERTEBRATE OVARY. 559 
 
 the point at which it was left off in that work. From their first 
 formation till the stage spoken of in my monograph as P, 
 their size remains fairly constant. The larger examples have 
 a diameter of about 0*035 mm., and the medium-sized examples 
 of about O'O3 mm. The larger nuclei have a diameter of ftbout 
 O'i6 mm., but their variations in size are considerable. If the 
 above figures be compared with those on page 350 of my 
 monograph on Elasmobranch Fishes, it will be seen that the 
 size of the primitive ova during these stages is not greater than 
 it was at the period of their very first appearance. 
 
 The ova (PI. 24, fig. i) are usually aggregated in masses, 
 which might have resulted from division of a single ovum. The 
 outlines of the individual ova are always distinct. Their proto- 
 plasm is clear, and their nuclei, which are somewhat passive 
 towards staining reagents, are granular, with one to three 
 nucleoli. I have noticed, up to stage P, the occasional presence 
 of highly refractive spherules in the protoplasm of the primitive 
 ova already described in my monograph (pp. 353, 354, PI. 12, 
 fig. 15). They seem to occur up to a later period than I at first 
 imagined. Their want of constancy probably indicates that 
 they have no special importance. Professor Semper has de- 
 scribed similar appearances in the male primitive ova of a later 
 period. 
 
 As to the distribution of the primitive ova in the germinal 
 epithelium, Professor Semper's statement that the larger primi- 
 tive ova are found in masses in the centre, and that the smaller 
 ova are more peripherally situated is on the whole true, though 
 I do not find this distribution sufficiently constant to lay so 
 much stress on it as he does. 
 
 The passive condition of the primitive ova becomes suddenly 
 broken during stage Q, and is succeeded by a period of remark- 
 able changes. It has only been by the expenditure of much 
 care and trouble that I have been able to elucidate to my own 
 satisfaction what takes place, and there are still points which I 
 do not understand. 
 
 Very shortly after stage Q, in addition to primitive ova with 
 a perfectly normal nucleus, others may be seen in which the 
 nucleus is apparently replaced by a deeply stained irregular 
 body, smaller than the ordinary nuclei (PI. 24, fig. 11, d. ;/.). 
 
560 THE STRUCTURE AND DEVELOPMENT 
 
 This body, by the use of high objectives, is seen to be composed 
 of a number of deeply stained granules, and around it may be 
 noticed a clear space, bounded by a very delicate membrane. 
 The granular body usually lies close to one side of this mem- 
 brane, and occasionally sends a few fine processes to the 
 opposite side. 
 
 The whole body, i.e. all within the delicate membrane is, 
 according to my view, a modified nucleus ; as appears to me 
 very clearly to be shewn by the /act that it occupies the normal 
 position of a nucleus within a cell body. Semper, on the other 
 hand, regards the contained granular body as the nucleus, which 
 he compares with the spindles of BUtschli, Auerbach, &c.\ This 
 interpretation appears to me, however, to be negatived by the 
 position of these bodies. The manner in which Semper may, 
 perhaps, have been led to his views will be obvious when the 
 later changes of the primitive ova are described. The formation 
 of these nvuclei would seem to be due to a segregation of the 
 constituents of the original nuclei ; the solid parts becoming 
 separated from the more fluid. As a rule, the modified nuclei 
 are slightly larger than the original ones. In stage Q the fol- 
 lowing two tables shew the dimensions of the parts of three 
 unmodified and of three modified nuclei taken at random. 
 
 Primitive ova with unmodified nuclei 
 
 Nuclei 
 
 o'oi 4 mm. 
 o - oi 2 mm. 
 o'oi mm. 
 
 Primitive ova tvitk modified nuclei 
 
 Granular 
 Nuclei. Bodies in Nuclei. 
 
 O'oi 8 mm O'oo6 mm. 
 
 o'oiS mm 0-006 mm. 
 
 O'oi 2 mm 0*009 mm. 
 
 For a slightly older stage than Q, the two annexed tables 
 also shew the comparative size of the modified and unmodified 
 nuclei : 
 
 1 Loc. cit. p. 361. 
 
OF THE VERTEBRATE OVARY. 561 
 
 Unmodified nuclei of normal primitive ova 
 
 o'oi 4 mm. 
 o'oi 6 mm. 
 o - oi4 mm. 
 o'oi6 mm. 
 o'oi 6 mm. 
 
 Nuclei of primitive ova ivitJi modified nuclei 
 
 Granular 
 Nuclei. Bodies in Nuclei. 
 
 o'oiS mm o'ooS mm. 
 
 ox>i6 mm O'ooS mm. 
 
 o'oi 6 mm o'oi mm. 
 
 ox>i6 mm. ...... 
 
 o'oi 8 mm 
 
 These figures bring out with clearness the following points : 
 (i) that the modified nuclei are slightly but decidedly larger on 
 the average than the unmodified nuclei ; (2) that the contained 
 granular bodies are very considerably smaller than ordinary 
 nuclei. 
 
 Soon after the appearance of the modified nuclei, remarkable 
 changes take place in the cells containing them. Up to the 
 time such nuclei first make their appearance the outlines of the 
 individual ova are very clearly defined, but subsequently, 
 although numerous ova with but slightly modified nuclei are 
 still to be seen, yet on the whole the outlines of all the primitive 
 ova are much less distinct than before ; and this is especially 
 the case with the primitive ova containing modified nuclei. 
 
 From cases in which three or four ova are found in a mass 
 with modified nuclei, but in which the outline of each ovum 
 is fairly distinct, it is possible to pass by insensible gradations 
 to other cases in which two or three or more modified nuclei are 
 found embedded in a mass of protoplasm in which no division 
 into separate cells can be made out (fig. 14). For these masses 
 I propose to employ the term nests. They correspond in part 
 with the Ureierncster of Professor Semper. 
 
 Frequently they are found in hardened specimens to be 
 enclosed in a membrane-like tunic which appears to be of the 
 nature of coagulated fluid. These membranes closely resemble 
 and sometimes are even continuous with trabeculae which tra- 
 verse the germinal epithelium. Ovaries differ considerably as 
 
562 THE STRUCTURE AND DEVELOPMENT 
 
 to the time and completeness of the disappearance, of the out- 
 lines marking the separate cells, and although, so far as can be 
 gathered from my specimens, the rule is that the outlines of 
 the primitive ova with modified nuclei soon become indistinct, 
 yet in one of my best preserved ovaries very large nests 
 with modified nuclei are present in which the outline of each 
 ovum is as distinct as during the period before the nuclei 
 undergo these peculiar changes (PI. 24, fig. 12). In the same 
 ovary other nests are present in which the outlines of the indi- 
 vidual ova are no longer visible. The section represented on 
 PI. 24, fig. 2, is fairly average as to the disappearance of the 
 outlines of the individual ova. 
 
 It is clear from the above statements, that in the first in- 
 stance the nests are produced by the coalescence of several 
 primitive ova into a single mass or syncytium ; though of course, 
 the several separate ova of a nest may originally, as Semper 
 believes, have arisen from the division of a single ovum. In any 
 case there can be no doubt that the nests of separate ova in- 
 crease in size as development proceeds ; a phenomenon which 
 is more reasonably explained on the view that the ova divide, 
 than on the view that they continue to be freshly formed. The 
 same holds true for the nests of nuclei and this, as well as other 
 facts, appears to me to render it probable that the nests grow 
 by division of the nuclei without corresponding division of the 
 protoplasmic matrix. 1 cannot, however, definitely prove this 
 point owing to 'my having found nests, with distinct outlines to 
 the ova, as large as any without such outlines. 
 
 The nests are situated for the most part near the surface of 
 the germinal epithelium. The smaller ones are frequently 
 spherical, but the larger are irregular in form. The former are 
 about O'O5 mm. in diameter; the latter reach O'l mm. Scat- 
 tered generally, and especially in the deeper layers, and at the 
 edges of the germinal epithelium, are still unmodified or only 
 slightly modified primitive ova. These unmodified primitive 
 ova are aggregated in masses, but in these masses the outlines 
 of each ovum, though perhaps less clear than in the earlier 
 period, are still distinct. 
 
 When the embryo reaches a length of seven centimetres, and 
 even in still younger embryos, further changes are observable. 
 
OF THE VERTEBRATE OVARY. 563 
 
 In the first place many of the modified nuclei acquire fresh 
 characters, and it becomes necessary to divide the modified 
 nuclei into two categories. In both of these the outer boundary 
 of the nucleus is formed by a very delicate membrane, the space 
 within which is perfectly clear except for the granular -body. 
 In the variety which now appears in considerable numbers the 
 granular body has an irregular star-like form. The rays of the 
 star are formed of fibres frequently knobbed at their extremi- 
 ties, and the centre of the star usually occupies an eccentric 
 position. Typical examples of this form of modified nucleus, 
 which may be spoken of as the stellate variety, are represented 
 on PI. 25, fig. 17; between it and the older granular variety 
 there is an infinite series of gradations, many of which are repre- 
 sented on PI. 24, figs. 12, 14, 15, 1 6. Certain of the stellate 
 nuclei exhibit two centres instead of one, and in some cases, 
 like that represented on PI. 25, fig. 19, the stellate body of two 
 nuclei is found united. Both of these forms are possibly modi- 
 fications of the spindle-like form assumed by nuclei in the act 
 of dividing, and may be used in proving that the nests increase 
 in size by the division of the contained nuclei. In addition to 
 the normal primitive ova, a few of which are still present, there 
 are to be found, chiefly in the deeper layers of the germinal 
 epithelium, larger ova differing considerably from the primitive 
 ova. They form the permanent ova (PI. 24, fig. 3 o}. Their 
 average diameter is 0x34 mm., compared with 003 mm., the 
 diameter of original primitive ova. The protoplasm of which 
 they are composed is granular, but at first a membrane can 
 hardly be distinguished around them ; their nucleus is rela- 
 tively large, O'O2 0x327 mm. in diameter. It presents the 
 characters ascribed by Eimer 1 , and many other recent authors 2 , 
 to typical nuclei (vide PI. 24, fig. 3, and PI. 24, 25, figs. 13, 14, 15, 
 1 6, 17, 1 8). It is bounded by a distinct membrane, within which 
 is a more or less central nucleolus from which a number of radial 
 fibres which stain very deeply pass to the surface ; here they 
 form immediately internal to the membrane a network with 
 granules at the nodal points. In some instances the regularity 
 of the arrangement of these fibres is very great, in other in- 
 
 1 Archiv f. micr. Anat. Vol. xiv. 
 
 " Vide especially Klein. Quart. Joiir/i. of Mic. S<i. July 1878. 
 
564 THE STRUCTURE AND DEVELOPMENT 
 
 stances two central nucleoli are present, in which case the regu- 
 larity is considerably interfered with. The points in which the 
 youngest permanent ova differ from the primitive may be 
 summed up as follows : 
 
 (i) The permanent ova are larger, the smallest of them 
 being larger than the average primitive ova in the proportion of 
 four to three. (2) They have less protoplasm as compared to 
 the size of the nucleus. (3) Their protoplasm is granular instead 
 of being clear. (4) Their nucleus is clear with exception of a 
 network of fibres instead of being granular as in the primitive 
 ova. It thus appears that the primitive ova and permanent ova 
 are very different in constitution, though genetically related in 
 a way to be directly narrated. 
 
 The formation of permanent ova is at its height in embryos 
 of about seven centimetres or slightly larger. The nests at this 
 stage are for the most part of a very considerable size and 
 contain a large number of nuclei, which have probably, as before 
 insisted, originated from a division of the smaller number of 
 nuclei present in the nests at an earlier stage. Figs. 14 18 are 
 representations of nests at this period. The diameter of the 
 nuclei is, on the whole, slightly greater than at an earlier stage. 
 A series of measurements gave the following results : 
 
 o - oi6 mm. 
 ox>i6 mm. 
 o'oiS mm. 
 ox>2 mm. 
 o'O2 mm. 
 
 Both varieties of modified nuclei are common enough, though 
 the stellate variety predominates. The nuclei are sometimes in 
 very close contact, and sometimes separated by protoplasm, 
 which in many instances is very slightly granular. In a large 
 number of the nests nothing further is apparent than what 
 has just been described, but in a very considerable number one 
 or more nuclei are present, which exhibit a transitional character 
 between the ordinary stellate nuclei of my second category, and 
 the nuclei of permanent ova as above described ; and in these 
 nests the formation of permanent ova is taking place. Perma- 
 nent ova in the act of development are indicated in my figures 
 by the letters d o. Many of the intermediate nuclei are more 
 
OF THE VERTEBRATE OVARY. 565 
 
 definitely surrounded by granular protoplasm than the other 
 nuclei of the nests, and accordingly have their outlines more 
 sharply defined. Between nuclei of this kind, and others as 
 large as those of the permanent ova, there are numerous transi- 
 tional forms. The larger ones frequently lie in a mass ~of 
 granular protoplasm projecting from the nest, and only united 
 with it by a neck (PI. 24, figs. 14 and 16). For prominences of 
 this kind to become independent ova, it is only necessary for 
 the neck to become broken through. Nests in which such 
 changes are taking place present various characters. In some 
 cases several nuclei belonging to a nest appear to be undergoing 
 conversion into permanent ova at the same time. Such a case 
 is figured on PI. 25, figs. 17 and 18. In these cases the amount 
 of granular protoplasm in the nest and around each freshly 
 formed ovum is small. In the more usual cases only one or 
 two permanent ova at the utmost are formed at the same time, 
 and in these instances a considerable amount of granular proto- 
 plasm is present around the nucleus of the developing perma- 
 nent ovum. In such instances it frequently happens several of 
 the nuclei not undergoing conversion appear to be in the process 
 of absorption, and give to the part of the nest in which they are 
 contained a very hazy and indistinct aspect (PI. 24, fig. 15). 
 Their appearance leads me to adopt the view that ivhile some 
 of the nuclei of each nest are converted into the nuclei of t/ie 
 permanent ova, others break down and are used as the pabti- 
 !um, at the expense of which the protoplasm of the young ovum 
 grows. 
 
 It should, however, be stated, that after the outlines of the 
 permanent ova have become definitely established, I have only 
 observed in a single instance the inclusion of a nucleus within 
 an ovum (PI. 25, fig. 24). In many instances normal nuclei of 
 the germinal epithelium may be so observed within the ovum. 
 
 The nuclei which are becoming converted into the nuclei of 
 permanent ova gradually increase in size. The following table 
 gives the diameter of four such nuclei : 
 
 0*022 mm. 
 ox>22 mm. 
 0-024 mm. 
 0*032 mm. 
 
566 THE STRUCTURE AND DEVELOPMENT 
 
 These figures should be compared with those of the table on 
 page 564. 
 
 The ova when first formed are situated either at the surface 
 or in the deeper layers of the germinal epithelium. Though to a 
 great extent surrounded by the ordinary cells of the germinal 
 epithelium, they are not at first enclosed in a definite follicular 
 epithelium. The follicle is, however, very early formed. 
 
 My observations lead me then to the conclusion that in 
 a general way the permanent ova are formed by the increase of 
 protoplasm round some of the nuclei of a nest, and the subse- 
 quent separation of the nuclei with their protoplasm from the 
 nest as distinct cells a mode of formation exactly comparable 
 with that which so often takes place in invertebrate egg tubes. 
 
 Besides the mode of formation of permanent ova just de- 
 scribed, a second one also seems probably to occur. In ovaries 
 just younger than those in which permanent ova are distinctly 
 formed, there are present primitive ova, with modified nuclei of 
 the stellate variety, or nuclei sometimes even approaching in 
 character those of permanent ova, which are quite isolated and 
 not enclosed in a definite nest. The body of these ova is formed 
 of granular protoplasm, but their outlines are very indistinct. 
 Such ova are considerably larger than the normal primitive ova. 
 They may measure 0^04 mm. In a slightly later stage, when 
 fully formed permanent ova are present, isolated ones are not 
 infrequent, and it seems natural to conclude that these isolated 
 ova are the direct descendants of the primitive ova of the earlier 
 stage. It seems a fair deduction that in some cases primitive 
 ova undergo a direct metamorphosis into permanent ova by a 
 modification of their nucleus, and the assumption of a granular 
 character in their protoplasm, without ever forming the con- 
 stituent part of a nest. 
 
 It is not quite clear to me that in all nests the coalescence 
 of the protoplasm of the ova necessarily takes place, since some 
 nests are to be found at all stages in which the ova are distinct. 
 Nevertheless, I am inclined to believe that the fusion of the ova 
 is the normal occurrence. 
 
 The mode of formation of the permanent ova may then, 
 according to my observations, take place in two ways : I. By 
 the formation of granular protoplasm round the nucleus in a 
 
OF THE VERTEBRATE OVARY. 567 
 
 nest, and the separation of the nucleus with its protoplasm as 
 a distinct ovum. 2. By the direct metamorphosis of an isolated 
 primitive ovum into a permanent ovum. The difference between 
 these two modes of formation does not, from a morphological 
 point of view, appear to be of great importance. 
 
 The above results appear clearly to shew that the primitive 
 ova in the female are not to be regarded as true ova, but as the 
 parent sexual cells which give rise to the ova : a conclusion which 
 completely fits in with the fact that cells exactly similar to the 
 primitive ova in the female give rise to the spermatic cells in the 
 male. 
 
 Slightly after the period of their first formation the permanent 
 ova become invested by a very distinct and well-marked, some- 
 what flattened, follicular epithelium (PI. 24, fig. 3). Where the 
 ova lie in the deeper layers of the germinal epithelium, the 
 follicular epithelium soon becomes far more columnar on the 
 side turned inwards, than on that towards the surface, especially 
 when the inner side is in contact with the stroma (PI. 24, fig. 7, 
 and PI. 25, figs. 24 and 26). This is probably a special provision 
 for the growth and nutrition of the ovum. 
 
 There cannot be the smallest doubt that the follicular epithe- 
 lium is derived from the general cells of the germinal epithelium 
 a point on which my results fully bear out the conclusions of 
 Ludwig and Semper. 
 
 The larger ova themselves have a diameter of about O'o6 mm., 
 and their nucleus of about 0^04 mm. The vitellus is granular, 
 and provided with a distinct, though delicate membrane, which 
 has every appearance of being a product of the ovum itself 
 rather than of the follicular epithelium. The membrane would 
 seem indeed to be formed in some instances even before the 
 ovum has a definite investment of follicle cells. The vitellus is 
 frequently vacuolated, but occasionally the vacuoles appear to 
 be caused by a shrinking due to the hardening reagent The 
 nucleus has the same peculiar reticulate character as at first. 
 Its large size, as compared with the ovum, is very noticeable. 
 
 With this stage the embryonic development of the ova comes 
 to a close, though the formation of fresh ova continues till com- 
 paratively late in life. I have, however, two series of sections of 
 ovaries preserved in osmic acid, from slightly larger embryos 
 
568 THE STRUCTURE AND DEVELOPMENT 
 
 than the one last described, about which it may be well to say a 
 few words before proceeding to the further development of the 
 permanent ova. 
 
 The younger of these ovaries was from a Scyllium embryo 10 
 centimetres long, preserved in osmic acid. 
 
 A considerable number of nests were present (PI. 24, fig. 13), 
 exhibiting, on the whole, similar characters to those just 
 described. 
 
 A series of measurements of the nuclei in them were made, 
 leading to the following results : 
 
 0*014 mm. 
 0*014 mm. 
 o - oi6 mm. 
 o'oi6 mm. 
 o'oiS mm. 
 o'oiS mm. 
 
 Thus, if anything, the nuclei were slightly smaller than in the 
 younger embryo, ft is very difficult in the osmic specimens to 
 make out clearly the exact outlines of the various structures, the 
 nuclei in many instances being hardly more deeply stained than 
 in the protoplasm around them. The network in the nuclei is 
 also far less obvious than after treatment with picric acid. The 
 permanent ova were hardly so numerous as in the younger ovary 
 before described. A number of these were measured with the 
 following results : 
 
 Ovum. Nucleus. 
 
 ox>3 mm 0*014 mm - 
 
 ox>34 mm o'oiS mm. 
 
 0-028 mm o % oi6 mm. 
 
 ox>3 mm o'o2 mm. 
 
 0-04 mm o'02 mm. 
 
 0*04 mm 0-02 mm. 
 
 ox>48 mm ox>2 mm. 
 
 These figures shew that the nuclei of the permanent ova are 
 smaller than in the younger embryo, and it may therefore be 
 safely concluded that, in spite of the greater size of the embryo 
 from which it is taken, the ovary now being described is in a 
 more embryonic condition than the one last dealt with. 
 
 Though the permanent ova appeared to be formed from the 
 nests in the manner already described, it was fairly clear from 
 
OF THE VERTEBRATE OVARY. 569 
 
 the sections of this ovary that many of the original primitive ova, 
 after a metamorphosis of the nucleus and without coalescing with 
 other primitive ova to form nests, become converted directly into 
 the permanent ova. Many large masses of primitive ova, or at 
 least of ova with the individual outlines of each ovum distinct, 
 were present. The average size of ova composing these was how- 
 ever small, the body measuring about o - oi6 mm., and the nucleus 
 O'OI2 mm. Isolated ova with metamorphosed nuclei could 
 also be found measuring O'O22, and their nuclei about 0*014 mm. 
 The second of the two ovaries, hardened in osmic acid, was 
 somewhat more advanced than the ovary in which the formation 
 of permanent ova was at its height. Fewer permanent ova were 
 in the act of being formed, and many of these present had reached 
 a considerable size, measuring as much as O'O/ mm. Nests 
 of the typical forms were present as before, but the nuclei in them 
 were more granular than at the earlier period, and on the average 
 slightly smaller. A series measured had the following diameters : 
 
 o'oi mm. 
 o'oi2 mm. 
 0*014 mm. 
 O'oi6 mm. 
 
 One of these nests is represented on PI. 25. fig. 20. Many 
 nests with the outlines of the individual ova distinct were also 
 present. 
 
 On the whole it appeared to me, that the second mode of 
 formation of permanent ova, viz. that in which the nest does not 
 come into the cycle of development, preponderated to a greater 
 extent than in the earlier embryonic period. 
 
 POST-EMBRYONIC DEVELOPMENT OF THE OVA. My investi- 
 gations upon the post-embryonic growth and development of 
 the ova, have for the most part been conducted upon preserved 
 ova, and it has been impossible for me, on this account, to work 
 out, as completely as I should have wished, certain points, more 
 especially those connected with the development of the yolk. 
 
 Although my ovaries have been carefully preserved in a large 
 number of reagents, including osmic acid, picric acid, chromic 
 acid, spirit, bichromate of potash, and Miiller's fluid, none of 
 these have proved universally successful, and bichromate of potash 
 
 B. 37 
 
570 THE STRUCTURE AND DEVELOPMENT 
 
 and Muller's fluid are useless. Great difficulties have been ex- 
 perienced in distinguishing the artificial products of these 
 reagents. My investigations have led me to the result, that in 
 the gradual growth of the ova with the age of the individual 
 the changes are not quite identical with those during the rapid 
 growth which takes place at periods of sexual activity, after 
 the adult condition has been reached a result to which His 
 has also arrived, with reference to the ova of Osseous Fish. I 
 propose dealing separately with the several constituents of the 
 egg-follicle. 
 
 Egg membranes. A vitelline membrane has been described 
 by Leydig 1 in Raja, and an albuminous layer of the nature of a 
 chorion * by Gegenbaur 3 in Acanthias the membranes described 
 in these two ways being no doubt equivalent. 
 
 Dr Alex. Schultz 4 has more recently investigated a consider- 
 able variety of genera and finds three conditions of the egg 
 membranes, (i) In Torpedo, a homogeneous membrane, which 
 is of the nature of a chorion. (2) In Raja, a homogeneous 
 membrane which is, however, perforated. (3) In Squalidae, a 
 thick homogeneous membrane, internal to which is a thinner 
 perforated membrane. He apparently regards the perforated 
 inner membrane as a specialised part of the simple membrane 
 found in Torpedo, and states that this membrane is of the nature 
 of a chorion. 
 
 My own investigations have led me to the conclusion that 
 though the egg-membranes can probably be reduced to single 
 type for Elasmobranchs, yet that they vary with the stage of 
 development of the ovum. Scyllium (stellare and canicula) and 
 Raja have formed the objects of my investigation. I commence 
 with the two former. 
 
 It has already been stated that in Scyllium, even before the 
 follicular epithelium becomes formed, a delicate membrane round 
 
 1 Rochen u. Haie. 
 
 By chorion I mean, following E. van Beneden's nomenclature, a membrane 
 formed by the follicular epithelium, and, by vitelline membrane, one formed by the 
 vitellus or body of the ovum. 
 
 3 "Ban und Entwicklung d. Wirbelthiereier," &c., Mull. Archiv, i86r. 
 
 4 " Zur Entwicklungsgeschichte d. Selachier," Arrh. f. mikr. Anat. Vol. XI. 
 
OF TIIK VKRTEHKATK OVAI<\. 571 
 
 the ovum can be demonstrated, which appears to me to be 
 derived from the vitellus or body of the ovum, and is therefore of 
 the nature of a vitelline membrane. It becomes the vitelline 
 membrane of Leydig, the albuminous membrane of Gegenbaur, 
 and homogeneous membrane of Schultz. 
 
 In a young fish (not long hatched) with ova of not more than 
 O'I2 mm., this membrane, though considerably thicker than in 
 the embryo, is not thick enough to be accurately measured. In 
 ova of o - 5 mm. from a young female (PI. 25, fig. 21) the vitelline 
 membrane has a thickness of O'OO2 mm. and is quite -homo- 
 geneous 1 . Internally to it may be observed very faint indications 
 of the differentiation of the outermost layer of the vitellus into 
 the perforated or radially striated membrane of Schultz, which 
 will be spoken of as zona radiata. 
 
 In an ovum of I mm. from the nearly full grown though not 
 sexually mature female, the zona radiata has increased in thick- 
 ness and definiteness, and may measure as much as 0*004 mm. 
 It is always very sharply separated from the vitelline membrane, 
 but appears to be more or less continuous on its inner border 
 with the body of the ovum, at the expense of which it no doubt 
 grows in thickness. 
 
 In ova above I mm. in diameter, both vitelline membrane and 
 zona radiata, but especially the latter, increase in thickness. 
 The zona becomes marked off" from the yolk, and its radial striae 
 become easy to see even with comparatively low powers. In 
 many specimens it appears to be formed of a number of small 
 columns, as described by Gegenbaur and others. The stage of 
 about the greatest development of both the vitelline membrane 
 and zona radiata is represented on PI. 25, fig. 22. 
 
 At this time the vitelline membrane appears frequently to 
 exhibit a distinct stratification, dividing it into two or more suc- 
 cessive layers. It is not, however, acted on in the same manner 
 by all reagents, and with absolute alcohol appears at times longi- 
 tudinally striated. 
 
 From this stage onwards, both vitelline membrane and zona 
 gradually atrophy, simultaneously with a series of remarkable 
 
 1 The apparent structure in the vitelline membrane in my figure is merely in- 
 tended to represent the dark colour assumed by it on being stained. The zona 
 radiata has been made rather too thick by the artist. 
 
 372 
 
5/2 THE STRUCTURE AND DEVELOPMENT 
 
 changes which take place in the follicular epithelium. The zona 
 is the first to disappear, and the vitelline membrane next be- 
 comes gradually thinner. Finally, when the egg is nearly ripe, 
 the follicular epithelium is separated from the yolk by an im- 
 measurably thin membrane the remnant of the vitelline 
 membrane only visible in the most favourable sections (PI. 25, 
 fig. 23 v /.). When the egg becomes detached from the ovary 
 even this membrane is no longer to be seen. 
 
 Both the vitelline membrane and the zona radiata are found 
 in Raja, but in a much less developed condition than in Scyllium. 
 The vitelline membrane is for a long time the only membrane 
 present, but is never very thick (PI. 25, fig. 31). The zona is not 
 formed till a relatively much later period than in Scyllium, and 
 is always delicate and difficult to see (PI. 25, fig. 32). Both 
 membranes atrophy before the egg is quite ripe ; and an ap- 
 parently fluid layer between the follicular epithelium and the 
 vitellus, which coagulates in hardened specimens, is probably the 
 last remnant of the vitelline membrane. It is, however, much 
 thicker than the corresponding remnant in Scyllium. 
 
 Though I find the same membranes in Scyllium as Alexander 
 Schultz did in other Squalidae, my results do not agree with his 
 as to Raja. Torpedo I have not investigated. 
 
 It appears to me probable that the ova in all Elasmobranch 
 Fishes have at some period of their development the two mem- 
 branes described at length for Scyllium. Of these the inner one, 
 or zona radiata, will probably be admitted on all hands to be a 
 product of the peripheral protoplasm of the egg. 
 
 The outer one corresponds with the membrane usually 
 regarded in other Vertebrates as a chorion or product of the 
 follicular epithelium, but, by tracing it back to its first origin, I 
 have been led to reject this view of its nature. 
 
 The follicular epithelium. The follicular epithelium in the 
 eggs of Raja and Acanthias has been described by Gegenbaur 1 . 
 He finds it flat in young eggs, but in the larger eggs of Acanthias 
 more columnar, and with the cells wedged in so as to form a 
 double layer. These observations are confirmed by Ludwig 2 . 
 
 Alexander Schultz 3 states that in Torpedo, the eggs are at 
 first enclosed in a simple epithelium, but that in follicles of 
 
 1 Loc. n't. * Loc. at. 3 Loc. cit. 
 
OF THE VERTEBRATE OVARY. 573 
 
 008 mm. there appear between the original large cells of the 
 follicle (which he describes as granulosa cells and derives from 
 the germinal epithelium) a number of peculiar small cells. He 
 states that these are of the same nature as the general stroma 
 cells of the ovary, and believes that they originate in the stroma. 
 When the eggs have reached err 0^5 mm., he finds that the 
 small and large cells have a very regular alternating arrange- 
 ment. 
 
 Semper records but few observations on the follicular epithe- 
 lium, but describes in Raja the presence of a certain number of 
 large cells amongst smaller cells. He believes that they may 
 develope into ova, and considers them identical with the larger 
 cells described by Schultz, whose interpretations he does not, 
 however, accept. 
 
 My own results accord to a great extent with those of Dr 
 Schultz, as far as the structure of the follicular epithelium is 
 concerned, but I am at one with Semper in rejecting Schultz's 
 interpretations. 
 
 . In Scyllium, as has already been mentioned, the follicular 
 epithelium is at first flat and formed of a single layer of uniform 
 cells, each with a considerable amount of clear protoplasm and a 
 granular nucleus. It is bounded externally by a delicate mem- 
 brane the membrana propria folliculi of Waldeyer and in- 
 ternally by the vitelline membrane. In the ovaries of very 
 young animals the cells of the follicular epithelium are more 
 columnar on the side towards the stroma than on the opposite 
 side, but this irregularity soon ceases to exist- 
 In many cases the nuclei of the cells of the follicular epithe- 
 lium exhibit a spindle modification, which shews that the growth 
 of the follicular epithelium takes place by the division of its cells. 
 No changes of importance are observable in the follicular epithe- 
 lium till the egg has reached a diameter of more than i mm. 
 
 It should here be stated that I have some doubts respecting 
 the completeness of the history of the epithelium recorded in 
 the sequel. Difficulties have been met with in completely eluci- 
 dating the chronological order of the occurrences, and it is 
 possible that some points have escaped my observation. 
 
 The first important change is the assumption of a palisade- 
 like character by the follicle cells, each cell becoming very narrow 
 
5/4 TFiE STRUCTURE AND DEVELOPMENT 
 
 and columnar and the nucleus oval (PI. 25, fig. 28). In this 
 condition the thickness of the epithelium is about 0^025 mm. 
 The epithelium does not, however, become uniformly thick over 
 the whole ovum, but in the neighbourhood of the germinal 
 vesicle it is very flat and formed of granular cells with indistinct 
 outlines, rather like the hypodermis cells of many Annelida. 
 Coincidently with this change in the follicular epithelium the 
 commencement of the atrophy of the membranes of the ovum, 
 described in the last section, becomes apparent. 
 
 The original membrana propria folliculi is still present round 
 the follicular epithelium, but is closely associated with a fibrous 
 layer with elongated nuclei. Outside this there is now a layer 
 of cells, very much like an ordinary epithelial layer, which may 
 possibly be formed of cells of the true germinal epithelium (fig. 
 28, ft''). This layer, which will be spoken of as the secondary 
 follicle layer, might easily be mistaken for the follicular epithe- 
 lium, and it is possible that it has actually been so mistaken by 
 Eimer, Clark, and Klebs, in Reptilia, and that the true follicular 
 epithelium (in a flattened condition) has been then spoken of as 
 the Binnenepithel. 
 
 In slightly older eggs the epithelial cells are no longer uni- 
 form or arranged as a single layer. The general arrangement of 
 these cells is shewn in PI. 25, fig. 29. A considerable number of 
 them are more or less flask-shaped, with bulky protoplasm pro- 
 longed into a thin stem directed towards the vitelline membrane, 
 with which, in many instances if not all, it comes in contact. 
 These larger cells are arranged in several tiers. Intercalated 
 between them are a number of elongated small cells with scanty 
 protoplasm and a deeply staining nucleus, not very dissimilar 
 to, though somewhat smaller than, the columnar cells of the 
 previous stage. There is present a complete -series of cells 
 intermediate between the larger cells and those with a deeply 
 stained nucleus, and were it not for the condition of the epithe- 
 lium in Raja, to be spoken of directly, I should not sharply 
 divide the cells into two categories. In surface views of the 
 epithelium the division into two kinds of cells would not be 
 suspected. There can, it appears to me, be no question that 
 both varieties of cell are derived from the primitive uniform 
 follicle cells. 
 
OF THE VERTEBRATE OVARY. 575 
 
 The fibrous layer bounding the membrana propria folliculi is 
 thicker than in the last stage, and the epithelial-like layer (/<?') 
 which bounds it externally is more conspicuous than before. 
 Immediately adjoining it are vascular and lymph sinuses. The 
 thickness of the follicular epithelium at this stage may reach as 
 much as 0*04 mm., though I have found it sometimes consider- 
 ably flatter. The cells composing it are, however, so delicate 
 that it is not easy to feel certain that the peculiarities of any 
 individual ovum are not due to handling. The absence of the 
 peculiar columnar epithelium on the part of the surface adjoin- 
 ing the germinal vesicle is as marked a feature as in the earlier 
 stage. When the egg is nearly ripe, and the vitelline membrane 
 has been reduced to a mere remnant, the follicular epithelium is 
 still very columnar (PI. 25, fig. 23). The thickness is greater 
 than in the last stage, being now about 0*045 mm., but the cells 
 appear only to form a single definite layer. From the character 
 of their nuclei, I feel inclined to regard them as belonging to 
 the category of the smaller cells of the previous stage, and feel 
 confirmed in this view by finding certain bodies in the epithelium, 
 which have the appearance of degenerating cells with granular 
 nuclei, which I take to be the flask-shaped cells which were 
 present in the earlier stage. 
 
 I have not investigated the character of the follicular epithe- 
 lium in the perfectly ripe ovum ready to become detached from 
 the ovary. Nor can I state for the last-described stage anything 
 about the character of the follicular epithelium in the neighbour- 
 hood of the germinal vesicle. 
 
 As to the relation of the follicular epithelium to the vitelline 
 membrane, and the possible processes of its cells continued into 
 the yolk, I can say very little. I find in specimens teased out 
 after treatment with osmic acid, that the cells of the follicular 
 epithelium are occasionally provided with short processes, which 
 might possibly have perforated the vitelline membrane, but have 
 met with nothing so clear as the teased out specimens figured 
 by Eimer. Nothing resembling the cells within the vitelline 
 membrane, as described by His 1 in Osseous Fish, and Lindgren 
 in Mammalia, has been met with 2 . 
 
 1 Das Ei bei Knochcnfischcn. 
 3 Airk.f. Anal. Pliys. 1877. 
 
5/6 THE STRUCTURE AND DEVELOPMENT 
 
 My observations in Raja are not so full as those upon Scyllium, 
 but they serve to complete and reconcile the observations of 
 Semper and Schultz, and also to shew that the general mode of 
 growth of the follicular epithelium is fundamentally the same 
 in my representatives of the two divisions of the Elasmobranchii. 
 In very young eggs, in conformity with the results of all previous 
 observers, I find the follicular epithelium approximately uniform. 
 The cells are flat, but extended so as to appear of an unexpected 
 size in views of the surface of the follicle. This condition does 
 not, however, last very long. A certain number of the cells 
 enlarge considerably, others remaining smaller and flat. The 
 differences between the larger and the smaller cells are more 
 conspicuous in sections than in surface views, and though the 
 distribution of the cells is somewhat irregular, it may still be 
 predicted as an almost invariable rule that the smaller cells of 
 the follicle will line that part of the surface of the ovum, near to 
 which the germinal vesicle is situated. On PI. 25, fig. 30, is 
 shewn in section a fairly average arrangement of the follicle 
 cells. Semper considers the larger cells of such a follicle to be 
 probably primitive ova destined to become permanent ova. This 
 view I cannot accept : firstly, because these cells only agree with 
 primitive ova in being exceptionally large the character of 
 their nucleus, with its large nucleolus, being not very like that of 
 a primitive ovum. Secondly, because they shade into ordinary 
 cells of the follicle ; and thirdly, because no evidence of their 
 becoming ova has come before me, but rather the reverse, in 
 that it seems probable that they have a definite function con- 
 nected with the nutrition of the egg. To this point I shall 
 return. 
 
 In the next stage the small cells have become still smaller. 
 They are columnar, and are wedged in between the larger ones. 
 No great regularity in distribution is as yet attained (PI. 25, 
 fig- 3 1 )- Such a regularity appears in a later stage (PI. 25, fig. 
 32), which clearly corresponds with fig. 8 on PI. 34 of Schultz's 
 paper, and also with the stage of Scyllium in PI. 25, fig. 29, 
 though the distinction between the two kinds of cells is here far 
 better marked than in Scyllium. The big cells have now be- 
 come flask-shaped like those in Scyllium, and send a process 
 down to the vitelline membrane. The smaller cells are arranged 
 
OF THE VERTEBRATE OVARY. 577 
 
 in two or three tiers, but the larger cells in a single layer. The 
 distribution of the larger and smaller cells is in some instances 
 very regular, as shewn in the surface view on PI. 25, fig. 33. 
 There can, it appears to me, be no doubt that Schultz's view of 
 the smaller cells being lymph-cells which have migrated into the 
 follicle cannot be maintained. 
 
 The thickness of the epithelium at this stage is about 0*04 mm. 
 In the succeeding stages, during which the egg is rapidly grow- 
 ing to the colossal size which it eventually attains, the follicular 
 epithelium does not to any great extent alter in constitution. 
 It gro\vs thicker on the whole, and as the vitelline membrane 
 gradually atrophies, its lower surface becomes irregular, exhibit- 
 ing somewhat flattened prominences, which project into the 
 yolk. At the greatest height of the prominences the epithelium 
 may reach a thickness of ox)6 mm., or even more. The arrange- 
 ment of the tissues external to the follicular epithelium is the 
 same in Raja as in Scyllium. 
 
 The most interesting point connected with the follicle, both 
 in Scyllium and Raja and presumably in other Elasmobranchs 
 is that its epithelium at the time when the egg is rapidly ap- 
 proaching maturity is composed with more or less of distinctness 
 of two forms of cells. One of these is large flask-shaped and rich 
 in protoplasm, the other is small, consisting of a mere film of 
 protoplasm round a nucleus. Considering that the larger cells 
 appear at the time of rapid growth, it is natural to interpret 
 their presence as connected with the nutrition of the ovum. 
 This view is supported by the observations of Eimer and Braun, 
 on the development of Reptilian ova. In many Reptilian ova 
 it appears from Eimer's 1 observations, that the follicular epi- 
 thelium becomes several layers thick, and that a differentiation 
 of the cells, similar to that in Elasmobranchs, takes place. The 
 flask-shaped cells eventually undergo peculiar changes, becoming 
 converted into a kind of beaker-cell, with prolongations through 
 the egg membranes, which take the place of canals leading to 
 the interior of the egg. Braun also expresses himself strongly 
 in favour of the flask-shaped cells functioning in the nutrition of 
 the egg 3 . That these cells in the Reptilian ova really corrc- 
 
 1 Archir f. inikr. Anal. Vol. vni. 
 
 - I'.iaun, " Urogeuitalsystem d. Amphibien," Arbciten a. d. zool.-zoot. [nstilitt 
 
5/8 THE STRUCTURE AND DEVELOPMENT 
 
 spond with those in Elasmobranchs appears to me clear' from 
 Eimer's figures, but I have not myself studied any Reptilian 
 ovum. My reasons for dissenting from both Semper's and 
 Schultz's views on the nature of the two forms of follicular cells 
 have already been stated. 
 
 The Vitellus and the development of the yolk spherules. 
 Leydig, Gegenbaur, and Schultz, have recorded important ob- 
 servations on this head. Leydig 1 chiefly describes the peculiar 
 characters of the yolk spherules. 
 
 Gegenbaur 2 finds in the youngest eggs fine granules, which 
 subsequently develop into vesicles, in the interior of which the 
 solid oval spheres, so characteristic of Elasmobranchs, are de- 
 veloped. 
 
 Schultz describes in the youngest ova of Torpedo the minute 
 yolk spherules arranged in a semi-lunar form around the ec- 
 centric germinal vesicle. In older ova they spread through the 
 whole. He also gives a description of their arrangement in the 
 ripe ovum. Dr Schultz further finds in the body of the ovum 
 peculiar protoplastic striae, arranged as a series of pyramids, 
 with the bases directed outwards. In the periphery of the ovum 
 a protoplastic network is also present, which is continuous with 
 the above-mentioned pyramidal structures. 
 
 My observations do not very greatly extend those of Gegen- 
 baur and Schultz with reference to the development of the yolk, 
 and closely agree with what Gegenbaur has given in the paper 
 above quoted more fully for Aves and Reptilia than for Elasmo- 
 branchii. 
 
 In very young ova the body of the ovum is simply granular, 
 but when it has reached about O'5 mm. the granules are seen to be 
 arranged in a kind of network, or spongework (PL 25, fig. 21), 
 already spoken of in my monograph on Elasmobranch Fishes. 
 
 This network becomes more distinct in succeeding stages, 
 especially in chromic acid specimens (PL 25, fig. 22), probably 
 in part owing to a granular precipitation of the protoplasm. In 
 
 Witrzburg, Bd. iv. He says, in reference to the flask-shaped cell, p. 166, " Hochstens 
 wiirde ich die P'unktion der grossen Follikelzellen als eiuzellige Driisen mehr be- 
 tonen." 
 
 1 Loc. cit. ' 2 Loc. cit. 
 
OF THE VERTEBRATE OVARY. 579 
 
 the late stages, when the yolk spherules are fully developed, it 
 is difficult to observe this network, but, as has been shewn in my 
 monograph above quoted, it is still present after the commence- 
 ment of embryonic development. An arrangement of the proto- 
 plasmic striae like that described by Schultz has not come" under 
 my notice. 
 
 The development of the yolk appears to me to present spe- 
 cial difficulties, owing to the fact pointed out by His 1 that the 
 conditions of development vary greatly according to whether 
 the ovary is in a state of repose or of active development. I do 
 not feel satisfied with my results on this subject, but believe 
 there is still much to be made out. Observations on the yolk 
 spherules may be made either in living ova, in ova hardened in 
 osmic acid, or in ova hardened in picric or chromic acids. The 
 two latter reagents, as well as alcohol, are however unfavourable 
 for the purpose of this study, since by their action the yolk 
 spherules appear frequently to be broken up and otherwise 
 altered. This has to some extent occurred in PI. 25, fig. 21, and 
 the peculiar appearance of the yolk of this ovum is in part due 
 to the action of the reagent. On the whole I have found osmic 
 acid the most suitable reagent for the study of the yolk, since 
 without breaking up the developing spherules, it stains them 
 of a deep black colour. The yolk spherules commence to be 
 formed in ova, of not more than 0*06 mm. in the ovaries of 
 moderately old females. In young females they are apparently 
 not formed in such small ova. They arise as extremely minute, 
 highly refracting particles, in a stratum of protoplasm some little 
 way bclou 1 the surface, and are always most numerous at the pole 
 opposite the germinal vesicle. Their general arrangement is very 
 much that figured and described by Allen Thomson in Gaster- 
 osteus 2 , and by Gegenbaur and Eimer in young Reptilian ova. 
 In section they naturally appear as a ring, their general mode of 
 distribution being fairly typically represented on PI. 25, fig. 27. 
 The ovum represented in fig. 27 was o - 5 mm. in diameter, and 
 the yolk spherules were already largely developed ; in smaller 
 ova they are far less numerous, though arranged in a similar 
 fashion. The developing yolk spherules are not uniformly dis- 
 
 1 Das Ei bd Knochenfischen. 
 
 z "Ovum" in Todd's Encyclopedia, fig. 69. 
 
580 THE STRUCTURE AND DEVELOPMENT 
 
 tributed but are collected in peculiar little masses or aggrega- 
 tions (PI. 25, fig. 21). These resemble the granular masses, 
 figured by His (loc. cit. PL 4, fig. 33) in the Salmon, and may be 
 compared with the aggregations figured by Gotte in his mono- 
 graph on Bombinator igneus (PI. I, fig. 9). It deserves to be 
 especially noted, that when the yolk spherules are first formed, 
 the peripheral layer of tlie ovum is entirely free from them, a 
 feature which is however apt to be lost in ova hardened in picric 
 acid (PL 25, fig. 21). Two points about the spherules appear 
 clearly to point to their being developed in the protoplasm of 
 the ovum, and not in the follicular epithelium, (i) That they 
 do not make their appearance in the superficial stratum of the 
 ovum. (2) That no yolk spherules are present in the cells of 
 the follicular epithelium, in which they could not fail to be 
 detected, owing to the deep colour they assume on being treated 
 with osmic acid. 
 
 It need scarcely be said that the yolk spherules at this stage 
 are not cells, and have indeed no resemblance to cells. They 
 would probably be regarded by His as spherules of fatty mate- 
 rial, unrelated to the true food yolk. 
 
 As the ova become larger the granules of the peripheral 
 layer before mentioned gradually assume the character of the 
 yolk spheres of the adult, and at the same time spread towards 
 the centre of the egg. Not having worked at fresh specimens, 
 I cannot give a full account of the growth of the spherules ; but 
 am of opinion that Gegenbaur's account is probably correct, 
 according to which the spheres at first present gradually grow 
 and develop into vesicles, in the interior of which solid bodies 
 (nuclei of His ?) appear and form the permanent yolk spheres. 
 When the yolk spheres are still very small they have the typical 
 oblong form 1 of the ripe ovum, and this form is acquired while 
 the centre of the ovum is still free from them. 
 
 The growth of the yolk appears mainly due to the increase 
 in size and number of the individual yolk spheres. Even when 
 the ovum is quite filled with large yolk spheres, the granular 
 
 1 The peculiar oval, or at times slightly rectangular and striated yolk spherules of 
 Klasmobranchs are mentioned by Leydig and Gegenbaur"(Pl. u, fig. 20), and myself, 
 Preliminary Account of Dci'dopinent of Elasmobranch Fishes, and by Filippi and His 
 in Osseous Fishes. 
 
OF THE VERTEBRATE OVARY. 581 
 
 protoplastic network of the earlier stages is still present, and 
 serves to hold together the constituents of the yolk. In the 
 cortical layer of nearly ripe ova, the yolk has a somewhat differ- 
 ent character to that which it exhibits in the deeper layers, chiefly 
 owing to the presence of certain delicate granular (in hardened 
 specimens) bodies, whose nature I do not understand, and to 
 special yolk spheres rather larger than the ordinary, provided 
 with numerous smaller spherules in their interior, which are 
 probably destined in the course of time to become free and to 
 form ordinary yolk spheres. 
 
 The mode of formation of the yolk spheres above described 
 appears to me to be the normal, and possibly the only one. 
 Certain peculiar structures have, however, come under my notice, 
 which may perhaps be connected with the formation of the yolk. 
 One of these resembles the bodies described by Eimer 1 as 
 " Dotterschorfe." I have only met these bodies in a single 
 instance in ova of O'6 mm., from the ovary (in active growth) 
 of a specimen of Scy. canicnla 23 inches in length. In this 
 instance they consisted of homogeneous clear bodies (not bounded 
 by any membrane) of somewhat irregular shape, though usually 
 more or less oval, and rarely more than o - O2 mm. in their longest 
 diameter. They were very numerous in the peripheral layer of 
 the ovum, but quite absent in the centre, and also not found 
 outside the ovum (as they appear to be in Reptilia). Yolk 
 granules formed in the normal way, and staining deeply by 
 osmic acid, were present, but the " Dotterschorfe " presented 
 a marked contrast to the remainder of the ovum, in being 
 absolutely unstained by osmic acid, and indeed they appeared 
 more like a modified form of vacuole than any definite body. 
 Their general appearance in Scyllium may be gathered from 
 Eimer's figure 8, PI. H, though they were much more numerous 
 than represented in that figure, and confined to the periphery of 
 the ovum. 
 
 Dr Eimer describes a much earlier condition of these 
 structures, in which they form a clear shell enclosing a 
 central dark nucleus. This stage I have not met with, nor can 
 I see any grounds for connecting these bodies with the formation 
 
 1 " Untersuchung viher die Eier d. Rcptilien," Archiv f. mikros. Anat. Vol. vin. 
 
582 THE STRUCTURE AND DEVELOPMENT 
 
 of the yolk, and the fact of their not staining with osmic acid 
 is strongly opposed to this view of their function. Dr Eimer 
 does not appear to me to bring forward any satisfactory proof 
 that they are in any way related to the formation of the yolk, 
 but wishes to connect them with the peculiar body, well known 
 as the yolk nucleus, which is found in the Amphibian ovum 1 . 
 
 Another peculiar body found in the ova may be mentioned 
 here, though it more probably belongs to the germinal vesicle 
 than to the yolk. It has only been met with in the vitellus 
 of some of the medium sized ova of a young female. Examples 
 of this body are represented on PL 25, fig. 25 A, x. As a rule 
 there is only one in each of the ova in which they are present, 
 but there may be as many as four. They consist of small vesicles 
 with a very thick doubly contoured membrane, which are filled 
 with numerous deeply staining spherical granules. At times 
 they contain a vacuole. Some of the larger of them are not 
 very much smaller than the germinal vesicle of their ovum, 
 while the smallest of them present a striking resemblance to 
 the nucleoli (fig. 25 'B), which makes me think that they may 
 possibly be nucleoli which have made their way out of the 
 germinal vesicle. I have not found them in the late stages or 
 large ova. 
 
 The following measurements shew the size of some of these 
 bodies in relation to the germinal vesicle and ovum : 
 
 Diameter of Germinal Diameter of Body in 
 
 Diameter of Ovum. Vesicle. Vitellus. 
 
 0-096 mm. . . O'O3 mm. . . o'oog mm. 
 0*064 mm ' 0-025 mm - 0*012 mm. 
 
 fo-oio mm. 
 o'oo6 mm. . . 0*03 mm. . . \ 
 
 (p'oo3 mm. 
 
 Germinal vesicle. Gegenbaur 2 finds the germinal vesicle 
 completely homogeneous and without the trace of a germinal 
 spot. In Raja granules or vesicles may appear as artificial pro- 
 ducts, and in Acanthias even in the fresh condition isolated 
 vesicles or masses of such may be present. To these structures 
 he attributes no importance. 
 
 Alexander Schultz 3 states that there is nothing remarkable 
 in the germinal vesicle of the Torpedo egg, but that till the egg 
 
 1 Vide Allen Thomson, article "Ovum," Todd's Encyclopedia, p. 95. 
 ' 2 Loc. cit. 3 Loc. cit. 
 
OF THE VERTEBRATE OVARY. 583 
 
 reaches O'5 mm., a single germinal spot is always present-(mea- 
 suring about O'oi mm.), which is absent in larger ova. 
 
 The bodies described by Gegenbaur are now generally recog- 
 nised as germinal spots, and will be described as such in the 
 sequel. I have very rarely met with the condition with the 
 single nucleolus described by Schultz in Torpedo. 
 
 My own observations are confined to Scyllium. In very 
 young females, with ova not larger than 0*09 mm., the germinal 
 vesicle has the same characters as during the embryonic periods. 
 The contents are clear but traversed by a very distinct and 
 deeply staining reticulum of fibres connected with the several 
 nucleoli which are usually present and situated close to the 
 membrane. 
 
 In a somewhat older- female in the largest ova of about OT2 
 mm., the germinal vesicle measures about O'o6 mm., and usually 
 occupies an eccentric position. It is provided with a distinct 
 though delicate membrane. The network, so conspicuous during 
 the embryonic period, is not so clear as it was, and has the 
 appearance of being formed of lines of granules rather than of 
 fibres. The fluid contents of the nucleus remain as a rule, even 
 in the hardened specimens, perfectly clear, though they become 
 in some instances slightly granular. There are usually two, 
 three, or more nucleoli generally situated, as described by Eimer, 
 close to the membrane of the vesicle, the largest of which may 
 measure as much as 0*006 mm. They are highly refracting 
 bodies, containing in most instances a vacuole, and very frequently 
 a smaller spherical body of a similar nature to themselves 1 . 
 Granules are sometimes also present in the germinal vesicle, but 
 are probably only extremely minute nucleoli. 
 
 In ova of o - 5 mm. the germinal vesicle has a diameter of 0*12 
 mm. (PI. 25, fig. 21). It is usually shrunk in hardened specimens 
 though nearly spherical in the living ovum. Its contents are 
 rendered granular by reagents though quite clear when fresh, 
 and the reticulum of the earlier stages is sometimes with difficulty 
 to be made out, though in other instances fairly clear. In all 
 cases the fibres composing it are very granular. The membrane 
 
 1 Compare, with reference to several points, the germinal vesicle at this stage 
 with the germinal vesicle of the frog's ovum figured by O. Hertwig, Morphologisches 
 Jahrbuch, Vol. in. pi. 4, fig. r. 
 
584 THE STRUCTURE AND DEVELOPMENT 
 
 is thick. Peculiar highly refracting nucleoli, usually enclosing a 
 large vacuole, are present in considerable numbers, and are either 
 arranged in a circle round the periphery, or sometimes aggre- 
 gated towards one side of the vesicle ; and in addition, numerous 
 deeply staining smaller granular aggregations, probably belong- 
 ing to the same category as the nucleoli (from which in the 
 living ovum they can only be distinguished by their size), are 
 scattered close to the inner side of the membrane over the whole 
 or only a part of the surface of the germinal vesicle. In a fair 
 number of instances bodies like that figured on PI. 25, fig. 27, 
 are to be found in the germinal vesicle. They appear to be 
 nucleoli in which a number of smaller nucleoli are originating by 
 a process of endogenous growth, analogous perhaps to endogenous 
 cell-formation. The nucleoli thus formed are, no doubt, destined 
 to become free. The above mode of increase for the nucleoli 
 appears to be exceptional. The ordinary mode is, no doubt, 
 that by simple division into two, as was long ago shewn by 
 Auerbach. 
 
 Of the later stages of the germinal vesicle and its final fate, I 
 can give no account beyond the very fragmentary statements 
 which have already appeared in my monograph on Elasmobranch 
 Fishes. 
 
 Formation of fresh ova and ovarian nests in the post-embryonic 
 stages. Ludwig 1 was the first to describe the formation of ova in 
 the post-embryonic periods. His views will be best explained 
 by quoting the following passage : 
 
 " The follicle of Skates and Dog fish, with the ovum it con- 
 tains, is to be considered as an aggregation of the cells of the 
 single-layered ovarian epithelium which have grown into the 
 stroma, and of which one cell has become the ovum and the 
 others the follicular epithelium. The follicle, however, draws in 
 with it into the stroma a number of additional epithelial cells 
 in the form of a stalk connecting the follicle with the superficial 
 epithelium. At a later period the lower part of the stalk at 
 its junction with the follicle becomes continuously narrowed, 
 and at the same time a rupture takes place in the cells which 
 form it. In this manner the follicle becomes at last constricted 
 
 1 I.oc. cit. 
 
OF THE VERTEBRATE OVARY. 585 
 
 off from the stalk, and so from its place of origin in the super- 
 ficial epithelium, and subsequently lies freely in the stroma of 
 the ovary." 
 
 He further explains that the separation of the follicles from 
 the epithelium takes place much earlier in Acanthias than in 
 Raja, and that the sinkings of the epithelium into the stroma 
 may have two or three branches each with a follicle. 
 
 Semper gives very little information with reference to the 
 post-embryonic formation of ova. He expresses his agreement 
 on the whole with Ludwig, but, amongst points not mentioned 
 by Ludwig, calls attention to peculiar aggregations of primitive 
 ova in the superficial epithelium, which he regards as either 
 rudimentary testicular follicles or as nests similar to those in the 
 embryo. 
 
 My observations on this subject do not agree very closely 
 with those either of Ludwig or Semper. The differences between 
 us partly, though not entirely, depend upon the fundamentally 
 different view we hold about the constitution of the ovary and 
 the nature of the epithelium covering it (vide pp. 555 and 556). 
 
 In very young ovaries (PI. 24, fig. 8) nests of ova (in my 
 sense of the term) are very numerous, but though usually super- 
 ficial in position are also found in the deeper layers of the ovary. 
 They are especially concentrated in their old position, close to 
 the dorsal edge of the organ. In some instances they do not 
 present quite the same appearance as in the embryo, owing to 
 the outlines of the ova composing them being distinct, and to 
 the presence between the ova of numerous interstitial cells 
 derived from the germinal epithelium, and destined to become 
 follicular epithelium. These latter cells at first form a much 
 flatter follicular epithelium than in the embryonic periods, so 
 that the smaller adult ova have a much less columnar investment 
 than ova of the same size in the embryo. A few primitive ova 
 may still be found in a very superficial position, but occasionally 
 also in the deeper layers. I am inclined to agree with Semper 
 that some of these are freshly formed from the cells of the 
 germinal epithelium. 
 
 In the young female with ova of about O'5 mm. nests of ova 
 are still fairly numerous. The nests are characteristic, and 
 present the various remarkable peculiarities already described 
 B. 38 
 
586 THE STRUCTURE AND DEVELOPMENT 
 
 in the embryo. In many instances they form polynuclear 
 masses, not divided into separate cells, generally, however, the 
 individual ova are distinct. The ova in these nests are on the 
 average rather smaller than during the embryonic periods. The 
 nests are frequently quite superficial and at times continuous 
 with the pseudo-epithelium, and individual ova also occasionally 
 occupy a position in the superficial epithelium. Some of the 
 appearances presented by separate ova are not unlike the figures 
 of Ludwig, but a growth such as' he describes has, according to 
 my observations, no existence. The columns which he believes 
 to have grown into the stroma are merely trabeculae connecting 
 the deeper and more superficial parts of the germinal epithelium ; 
 and his whole view about the formation of the follicular epithe- 
 lium round separate ova certainly does not apply, except in rare 
 cases, to Scyllium. It is, indeed, very easy to see that most 
 freshly formed ova are derived from nests, as in the embryo ; 
 and the formation of a follicular epithelium round these ova 
 takes place as they become separated from the nests. A few 
 solitary ova, which have never formed part of a nest, seem to be 
 formed in this stage as in the embryo ; but they do not grow 
 into the stroma surrounded by the cells of the pseudo-epithelium, 
 and only as they reach a not inconsiderable size is a definite 
 follicular epithelium formed around them. The follicular epi- 
 thelium, though not always formed from the pseudo-epithelium, 
 is of course always composed of cells derived from the germinal 
 epithelium. 
 
 In all the ova formed at this stage the nucleus would seem 
 to pass through the same metamorphosis as in the embryo. 
 
 In the later stages, and even in the full-grown female of 
 Scyllium, fresh ova seemed to be formed and nests also to be 
 present. In Raja I have not found freshly formed ova or nests 
 in the adult, and have had no opportunity of studying the young 
 forms. 
 
 Summary of observations on the development of the ovary in 
 Scyllium and Raja. 
 
 (i) The ovary in the embryo is a ridge, triangular in sec- 
 tion, attached along the base. It is formed of a core of stroma 
 and a covering of epithelium. A special thickening of the epi- 
 
OF THE VERTEBRATE OVARY. 587 
 
 thelium on the outer side forms the true germinal epithelium, to 
 which the ova are confined (PI. 24, fig. i). In the development 
 of the ovary the stroma becomes differentiated into an external 
 vascular layer, especially developed in the neighbourhood of the 
 germinal epithelium, and an internal lymphatic portion, which 
 forms the main mass of the ovarian ridge (PI. 24, figs. 2, 3, and 6). 
 
 (2) At first the thickened germinal epithelium is sharply 
 separated by a membrane from the subjacent stroma (PI. 24, 
 figs, i, 2, and 3), but at about the time when the follicular epi- 
 thelium commences to be formed round the ova, numerous 
 strands of stroma grow into the epithelium, and form a regular 
 network of vascular channels throughout it, and partially isolate 
 individual ova (PI. 24, figs. 7 and 8). At the same time the 
 surface of the epithelium turned towards the stroma becomes 
 irregular (PI. 24, fig. 9), owing to the development of individual 
 ova. In still later stages the stroma ingrowths form a more or 
 less definite tunic close to the surface of the ovary. External 
 to this tunic is the superficial layer of the germinal epithelium, 
 which forms what has been spoken of as the pseudo-epithelium. 
 In many instances the protoplasm of its cells is produced into 
 peculiar fibrous tails which pass into the tunic below. 
 
 (3) Primitive ova. Certain cells in the epithelium lining 
 the dorsal angle of the body cavity become distinguished as 
 primitive ova by their abundant protoplasm and granular nuclei, 
 at a very early period in development, even before the forma- 
 tion of the genital ridges. Subsequently on the formation of 
 the genital ridges these ova become confined to the thickened 
 germinal epithelium on the outer aspect of the ridges (PL 24, 
 fig. i). 
 
 (4) Conversion of primitive ova into permanent ova. 
 Primitive ova may in Scyllium become transformed into perma- 
 nent ova in two ways the difference between the two ways 
 being, however, of secondary importance. 
 
 (a) A nest of primitive ova makes its appearance, either by 
 continued division of a single primitive ovum or otherwise. The 
 bodies of all the ova of the nest fuse together, and a polynuclear 
 mass is formed, which increases in size concomitantly with the 
 division of its nuclei. The nuclei, moreover, pass through a 
 series of transformations. They increase in size and form deli- 
 
 38-2 
 
588 THE STRUCTURE AND DEVELOPMENT 
 
 cate vesicles filled with a clear fluid, but contain close to one 
 side a granular mass which stains very deeply with colouring 
 reagents. The granular mass becomes somewhat stellate, and 
 finally assumes a reticulate form with one more highly refracting 
 nucleoli at the nodal points of the reticulum. When a nucleus 
 has reached this condition the protoplasm around it has become 
 slightly granular, and with the enclosed nucleus is segmented 
 off from the nest as a special cell a permanent ovum (figs. 13, 
 14, 15, 16). Not all the nuclei in a nest undergo the whole of 
 the above changes ; certain of them, on the contrary, stop short 
 in their development, atrophy, and become employed as a kind 
 of pabulum for the remainder. Thus it happens that out of 
 a large nest perhaps only two or three permanent ova become 
 developed. 
 
 (b] In the second mode of development of ova the nuclei 
 and protoplasm undergo the same changes as in the first mode ; 
 but the ova either remain isolated and never form part of a nest, 
 or form part of a nest in which no fusion of the protoplasm takes 
 place, and all the primitive ova develop into permanent ova. 
 Both the above modes of the formation continue through a great 
 part of life. 
 
 (5) The follicle. The cells of the germinal epithelium 
 arrange themselves as a layer around each ovum, almost imme- 
 diately after its separation from a nest, and so constitute a fol- 
 licle. They are at first flat, but soon become more columnar. 
 In Scyllium they remain for a long time uniform, but in large 
 eggs they become arranged in two or three layers, while at the 
 same time some of them become large and flask-shaped, and 
 others small and oval (fig. 29). The flask-shaped cells have 
 probably an important function in the nutrition of the egg, and 
 are arranged in a fairly regular order amongst the smaller cells. 
 Before the egg is quite ripe both kinds of follicle cells undergo 
 retrogressive changes (PI. 25, fig. 23). 
 
 In Raja a great irregularity in the follicle cells is observable 
 at an early stage, but as the ovum grows larger the cells 
 gradually assume a regular arrangement more or less similar to 
 that in Scyllium (PI. 25, figs. 30 33). 
 
 (6) The egg membranes. Two membranes are probably 
 always present in Klasmobranchs during some period of their 
 
OF THE VERTEBRATE OVARY. 589 
 
 growth. The first formed and outer of these arises in -some 
 instances before the formation of the follicular epithelium, and 
 would seem to be of the nature of a vitelline membrane. The 
 inner one is the zona radiata with a typical radiately striated 
 structure. It is formed from the vitellus at a much later period 
 than the proper vitelline membrane. It is more developed in 
 Scyllium than in Raja, but atrophies early in both genera. By 
 the time the ovum is nearly ripe both membranes are very much 
 reduced, and when the egg (in Scyllium and Pristiurus) is laid, 
 no trace of any membrane is visible. 
 
 (7) The vitellus. The vitellus is at first faintly granular, 
 but at a later period exhibits a very distinct (protoplasmic) 
 network of fibres, which is still present after the ovum has been 
 laid. 
 
 The yolk arises, in the manner described by Gegenbaur, in 
 ova of about o - o6 mm. as a layer of fine granules, which stain 
 deeply with osmic acid. They are at first confined to a stratum 
 of protoplasm slightly below the surface of the ovum, and are 
 most numerous at the pole furthest removed from the germinal 
 vesicle. They are not regularly distributed, but are aggregated 
 in small masses. They gradually grow into vesicles, in the inte- 
 rior of which oval solid bodies are developed, which form the 
 permanent yolk-spheres. These oval bodies in the later stages 
 exhibit a remarkable segmentation into plates, which gives them 
 a peculiar appearance of transverse striation. 
 
 Certain bodies of unknown function are occasionally met 
 with in the vitellus, of which the most remarkable are those 
 figured at x on PI. 25, fig. 25 A. 
 
 (8) The germinal vesicle. A reticulum is very conspicuous 
 in the germinal vesicle in the freshly formed ova, but becomes 
 much less so in older ova, and assumes, moreover, a granular 
 appearance. At first one to three nucleoli are present, but they 
 gradually increase in number as the germinal vesicle grows 
 older, and are frequently situated in close proximity to the 
 membrane. 
 
59O THE STRUCTURE AND DEVELOPMENT 
 
 THE MAMMALIAN OVARY (PL 26). 
 
 The literature of the mammalian ovary has been so often 
 dealt with that it may be passed over with only a few words. 
 The papers which especially call for notice are those of Pfliiger 1 , 
 Ed. van Beneden 2 , and especially Waldeyer 3 , as inaugurating the 
 newer view on the nature of the ovary, and development of the 
 ova ; and of Foulis 4 and Kolliker 5 , as representing the most 
 recent utterances on the subject. There are, of course, many 
 points in these papers which are touched on in the sequel, but 
 I may more especially here call attention to the fact that I have 
 been able to confirm van Beneden's statement as to the existence 
 of polynuclear protoplasmic masses. I have found them, how- 
 ever, by no means universal or primitive ; and I cannot agree in 
 a general way with van Beneden's account of their occurrence. 
 I have found no trace of a germogene (Keimfache) in the sense 
 of Pfluger and Ed. van Beneden. My own results are most in 
 accordance with those of Waldeyer, with whom I agree in the 
 fundamental propositions that both ovum and follicular epithe- 
 lium are derived from the germinal epithelium, but I cannot 
 accept his views of the relation of the stroma to the germinal 
 epithelium. 
 
 In the very interesting paper of Foulis, the conclusion is 
 arrived at, that while the ova are derived from the germinal 
 epithelium, the cells of the follicle originate from the ordinary 
 connective tissue cells of the stroma. Foulis regards the zona 
 pellucida as a product of the ovum and not of the follicle. To 
 both of these views I shall return, and hope to be able to shew 
 that Foulis has not traced back the formation of the follicle 
 through a sufficient number of the earlier stages. It thus comes 
 about that though I fully recognise the accuracy of his figures, 
 I am unable to admit his conclusions. Kolliker's statements 
 
 1 Die Eierstocke d. Siiugethiere u. d. Menschen, Leipzig, 1863. 
 
 2 "Composition et Signification de 1'oeuf," Acad. r. de Belgique, 1868. 
 
 3 Eierstock u. Ei. Leipzig, 1870. 
 
 4 Trans, of Royal Society^ Edinburgh, Vol. XXVII. 1875, and Quarterly Journal 
 of Microscopical Science ) Vol. xvi. 
 
 ~ Vcrhandliing d. Phys. Med. Gesellschaft, Wiirzburg, 1875, N. F. Bel. vm. 
 
OF THE VERTEBRATE OVARY. 591 
 
 are again very different from those of Foulis. He finds certain 
 cords of cells in the hilus of the ovary, which he believes to be 
 derived from the Wolman body, and has satisfied himself that 
 they are continuous with Pfliiger's egg-tubes, and that they 
 supply the follicular epithelium. To the general accuracy of 
 Kolliker's statements with reference to the relations of these 
 cords in the hilus of the ovary I can fully testify, but am of 
 opinion that he is entirely mistaken as to their giving rise to the 
 follicular epithelium, or having anything to do with the ova. 
 I hope to be able to give a fuller account of their origin than he 
 or other observers have done. 
 
 My investigations on the mammalian ovary have been made 
 almost entirely on the rabbit the type of which it is most 
 easy to procure a continuous series of successive stages ; but 
 in a general way my conclusions have been controlled and 
 confirmed by observations on the cat, the dog, and the sheep. 
 My observations commence with an embryo of eighteen days. 
 A transverse section, slightly magnified, through the ovary at 
 this stage, is represented on PI. 26, fig. 35, and a more highly 
 magnified portion of the same in fig. 35 A. The ovary is a cylin- 
 drical ridge on the inner side of the Wolffian body, composed 
 of a superficial epithelium, the germinal epithelium (g.e.\ and 
 of a tissue internal to this, which forms the main mass of 
 it. In the latter two constituents have to be distinguished 
 (i) an epithelial-like tissue ((), coloured brown, which forms 
 the most important element, and (2) vascular and stroma ele- 
 ments in this. 
 
 The germinal epithelium is a layer about 0*03 0*04 mm. in 
 thickness. It is (vide fig. 35 A, g.e.} composed of two or three 
 layers of cells, with granular nuclei, of which the outermost 
 layer is more columnar than the remainder, and has elongated 
 rather than rounded nuclei. Its cells, though they vary slightly 
 in size, are all provided with a fair amount of protoplasm, and 
 cannot be divided (as in the case of the germinal epithelium of 
 Birds, Elasmobranchii, &c.), into primitive ova, and normal 
 epithelial cells. Very occasionally, however, a specially large 
 cell, which, perhaps, deserves the appellation primitive ovum, 
 may be seen. From the subjacent tissue the germinal epithe- 
 lium is in most parts separated by a membrane-like structure 
 
592 THE STRUCTURE AND DEVELOPMENT 
 
 (fluid coagulum) ; but this is sometimes absent, and it is then 
 very difficult to determine with exactness the inner border of 
 the epithelium. The tissue (t), which forms the greater mass 
 of the ovary at this stage, is formed of solid columns or trabe- 
 culae of epithelial-like cells, which present a very striking re- 
 semblance in size and character to the cells of the germinal 
 epithelium. The protoplasm of these cells stains slightly more 
 deeply with osmic acid than does that of the cells of the germinal 
 epithelium, so that it is rather easier to note a difference between 
 the two tissues in osmic acid than in picric acid specimens. 
 This tissue approaches very closely, and is in many parts in 
 actual contact with the germinal epithelium. Between the 
 columns of it are numerous vascular channels (shewn diagram- 
 matically in my figures) and a few normal stroma cells. This 
 remarkable tissue continues visible through the whole course of 
 the development of the ovary, till comparatively late in life, and 
 during all the earlier stages might easily be supposed to be 
 about to play some part in the development of the ova, or 
 even to be part of the germinal epithelium. It really, however, 
 has nothing to do with the development of the ova, as is 
 easily demonstrated when the true ova begin to be formed. 
 In the later stages, as will be mentioned in the description of 
 those stages, it is separated from the germinal epithelium by 
 a layer of stroma ; though at the two sides of the ovary it 
 is, even in later stages, sometimes in contact with the germinal 
 epithelium. 
 
 In most parts this tissue is definitely confined within the 
 limits of the ovary, and does not extend into the mesentery 
 by which the ovary is attached. It may, however, be traced at 
 the anterior end of the ovary into connection with the walls of 
 the Malpighian bodies, which lie on the inner side of the Wolffian 
 body (vide fig. 35 B), and I have no doubt that it grows out 
 from the walls of these bodies into the ovary. In the male it 
 appears to me to assist in forming, together with cells derived 
 from the germinal epithelium, the seminiferous tubules, the 
 development of which is already fairly advanced by this stage. 
 I shall .speak of it in the sequel as tubuliferous tissue. The 
 points of interest in connection with it concern the male sex, 
 which I hope to deal with in a future paper, but I have no 
 
OF THE VERTEBRATE OVARY. 593 
 
 hesitation in identifying it with the segmental cords (segwent- 
 alstrdnge] discovered by Braun in Reptilia, and described at 
 length in his valuable memoir on their urogenital system *. Ac- 
 cording to Braun the segmental cords in Reptilia are buds from 
 the outer walls of the Malpighian bodies. The bud from each 
 Malpighian body grows into the genital ridge before the period 
 of sexual differentiation, and sends out processes backwards 
 and forwards, which unite with the buds from the other Mal- 
 pighian bodies. There is thus formed a kind of trabecular 
 work of tissue in the stroma of the ovary, which in the Lacertilia 
 comes into connection with the germinal epithelium in both 
 sexes, but in Ophidia in the male only. In the female, in all 
 cases, it gradually atrophies and finally vanishes, but in the 
 male there pass into it the primitive ova, and it eventually forms, 
 with the enclosed primitive ova, the tubuli seminiferi. From 
 my own observations in Reptilia I can fully confirm Braun's 
 statements as to the entrance of the primitive ova into this 
 tissue in the male, and the conversion of it into the tubuli 
 seminiferi. The chief difference between Reptilia and Mammalia, 
 in reference to this tissue, appears to be that in Mammalia 
 it arises only from a few of the Malpighian bodies at the 
 anterior extremity of the ovary, but in Reptilia from all the 
 Malpighian bodies adjoining the genital ridge. More extended 
 observations on Mammalia will perhaps shew that even this 
 difference does not hold good. 
 
 It is hardly to be supposed that this tissue, which is so con- 
 spicuous in all young ovaries, has not been noticed before ; but 
 the notices of it are not so numerous as I should have antici- 
 pated. His 2 states that the parenchyma of the "sexual glands 
 undoubtedly arises from the Wolffian canals, and adds that 
 while the cortical layer (Hulle) represents the earlier covering 
 of a part of the Wolffian body, the stroma of the hilus, with 
 its vessels, arises from a Malpighian body. In spite of these 
 statements of His, I still doubt very much whether he has 
 really observed either the tissue I allude to or its mode of 
 development. In any case he gives no recognisable description 
 or figure of it. 
 
 1 Arbeitcn a. d. Zool.-zoot. Tnstitiit Wurzburg, Bd. iv. 
 a Archivf. mikros. Anat. Vol. I. p. 160. 
 
594 THE STRUCTURE AND DEVELOPMENT 
 
 Waldeyer 1 notices this tissue in the dog, cat, and calf. The 
 following is a free translation of what he says, (p. 141) : 
 " In a full grown but young dog, with numerous ripe follicles, 
 there were present in the vascular zone of the ovary numerous 
 branched elongated small columns (Schlauche) of epithelial cells, 
 between which ran blood-vessels. They were only separated 
 from the egg columns of the cortical layer by a row of large 
 follicles. There can be no doubt that we have here remains 
 of the sexual part of the Wolffian body the canals of the 
 parovarium which in the female sex have developed themselves 
 to an extraordinary extent into the stroma of the sexual gland, 
 and perhaps are even to be regarded as homolognes of the 
 seminiferous tubules (the italics are my own). I have almost 
 always found the above condition in the dog, only in old animals 
 these seminiferous canals seem gradually to atrophy. Similar 
 columns are present in the cat, only they do not appear to grow 
 so far into the stroma." Identical structures are also described 
 in the calf. 
 
 Romiti gives a very similar description to Waldeyer of these 
 bodies in the dog 2 . Born also describes this tissue in young 
 and embryonic ovaries of the horse as the Keimlager*. The 
 columns described by Kolliker 4 and believed by him to furnish 
 the follicular epithelium, are undoubtedly my tubuliferous tissue, 
 and, as Kolliker himself points out, are formed of the same 
 tissue as that described by Waldeyer. 
 
 Egli gives a very clear and accurate description of this 
 tissue, though he apparently denies its relation with the Wolffian 
 body. 
 
 My own interpretation of the tissue accords with that of 
 Waldeyer. In addition to the rabbit, I have observed it in the 
 dog, cat, and sheep. In all these forms I find that close to the 
 attachment of the ovary, and sometimes well within it, a fair 
 number of distinct canals with a large lumen are present, which 
 are probably to be distinguished from the solid epithelial columns. 
 Such large canals are not as a rule present in the rabbit. In the 
 
 1 Loc. cit. 
 
 - Archivf. mikr. Anat. Vol. x. 
 
 3 Archivf. Anatomic, Pliysiologie, u. Ifiss. Median. 1874. 
 
 4 Loc. cit. 
 
OF THE VERTEBRATE OVARY. 595 
 
 dog solid columns are present in the embryo, but later they 
 appear frequently to acquire a tubular form, and a lumen. Pro- 
 bably there are great variations in the development of the tissue, 
 since in the cat (not as Waldeyer did in the dog) I have found it 
 most developed. 
 
 In the very young embryonic ovary of the cat the columns 
 are very small and much branched. In later embryonic stages 
 they are frequently elongated, sometimes convoluted, and are 
 very similar to the embryonic tubuli seminiferi. In the young 
 stages these columns are so similar to the egg tubes (which 
 agree more closely with Pfltiger's type in the cat than in other 
 forms I have worked at) that to any one who had not studied 
 the development of the tissue an embryo cat's ovary at certain 
 stages would be a very puzzling object. I have, however, met 
 with nothing in the cat or any other form which supports 
 Kolliker's views. 
 
 My next stage is that of a twenty-two days' embryo. Of this 
 stage I have given two figures corresponding to those of the 
 earlier stage (figs. 36 and 36 A). 
 
 From these figures it is at once obvious that the germinal 
 epithelium has very much increased in bulk. It has a thickness 
 O'l oxx) mm. as compared to 0*03 mm. in the earlier stage. 
 Its inner outline is somewhat irregular, and it is imperfectly 
 divided into lobes, which form the commencement of structures 
 nearly equivalent to the nests of the Elasmobranch ovary. The 
 lobes arc not separated from each other by connective tissue 
 prolongations ; the epithelium being at this stage perfectly free 
 from any ingrowths of stroma. The cells constituting the ger- 
 minal epithelium have much the same character as in the previous 
 stage. They form an outer row of columnar cells internal to 
 which the cells are more rounded. Amongst them a few large 
 cells with granular nuclei, which are clearly primitive ova, may 
 now be seen, but by far the majority of the cells are fairly 
 uniform in size, and measure from O'Oi O'O2 mm. in diameter, 
 and their nuclei from 0004 o - oo6 mm. The nuclei of the 
 columnar outer cells measure about o - oo8 mm. They are what 
 would ordinarily be called granular, though high powers shew 
 that they have the usual nuclear network. There is no special 
 nucleolus. The rapid growth of the germinal epithelium is due 
 
596 THE STRUCTURE AND DEVELOPMENT 
 
 to the division of its cells, and great masses of these may 
 frequently be seen to be undergoing division at the same time. 
 Of the tissue of the ovary internal to the germinal epithelium, it 
 may be noticed that the tubuliferous tissue derived from the 
 Malpighian bodies is no longer in contact with the germinal 
 epithelium, but that a layer of vascular stroma is to a great 
 extent interposed between the two. The vascular stroma of the 
 hilus has, moreover, greatly increased in quantity. 
 
 My next stage is that of a twenty-six days' embryo, but the 
 characters of the ovary at this stage so closely correspond with 
 those of the succeeding one at twenty-eight days that, for the 
 sake of brevity, I pass over this stage in silence. 
 
 Fig 8 - 37 an d 37 A are representative sections of the ovary 
 of the twenty-eighth day corresponding with those of the earlier 
 stages. 
 
 Great changes have become apparent in the constitution of 
 the germinal epithelium. The vascular stroma of the ovary has 
 grown into the germinal epithelium precisely as in Elasmobranchs. 
 It appears to me clear that the change in the relations between 
 the stroma and epithelium is not due to a mutual growth, but 
 entirely to the stroma, so that, as in the case of Elasmobranchs, 
 the result of the ingrowth is that the germinal epithelium is 
 honeycombed by vascular stroma. The vascular growths 
 generally take the paths of the lines which separated the nests 
 in an earlier condition, and cause these nests to become the egg 
 tubes of Pfliager. It is obvious in figure 37 that the vascular 
 ingrowths are so arranged as imperfectly to divide the germinal 
 epithelium into two layers separated by a space with connective 
 tissue and blood-vessels. The outer part is relatively thin, and 
 formed of a superficial row of columnar cells, and one or two 
 rows of more rounded cells ; the inner layer is much thicker, and 
 formed of large masses of rounded cells. The two layers are 
 connected together by numerous trabeculas, the stroma between 
 which eventually gives rise to the connective tissue capsule, or 
 tunica albuginea, of the adult ovary. 
 
 The germinal epithelium is now about 0*19 o - 22 mm. in 
 thickness. Its cells have undergone considerable changes. A 
 fair number of them (fig. 37 A, />.#.), especially in the outer layer 
 of the epithelium, have become larger than the cells around 
 
OF THE VERTEBRATE OVARY. 597 
 
 them, from which they are distinguished, not only by their size, 
 but by their granular nucleus and abundant protoplasm. They 
 are in fact undoubted primitive ova with all the characters which 
 primitive ova present in Elasmobranchs, Aves, &c. In a fairly 
 typical primitive ovum of this stage the body measures O'O2 mm. 
 and the nucleus 0*014 mm. In the inner part of the germinal 
 epithelium there are very few or no cells which can be dis- 
 tinguished by their size as primitive ova, and the cells them- 
 selves are of a fairly uniform size, though in this respect there is 
 perhaps a greater variation than might be gathered from fig. 3/A. 
 The cells are on the average about O'Oi6 mm. in diameter, and 
 their nuclei about o - oo8 0*001 mm., considerably larger, in fact, 
 than in the earlier stage. The nuclei are moreover more granular, 
 and make in this respect an approach to the character of the 
 nuclei of primitive ova. 
 
 The germinal epithelium is still rapidly increasing by the 
 division of its cells, and in fig 37 A there are shewn two or three 
 nuclei in the act of dividing. I have represented fairly accurately 
 the appearance they present when examined with a moderately 
 high magnifying power. With reference to the stroma of the 
 ovary, internal to the germinal epithelium, it is only necessary 
 to refer to fig. 37 to observe that the tubuliferous tissue (/) 
 forms a relatively smaller part of the stroma than in the previous 
 stage, and is also further removed from the germinal epithelium. 
 
 My next stage is that of a young rabbit two days after birth, 
 but to economise space I pass on at once to the following stage 
 five days after birth. This stage is in many respects a critical 
 one for the ovary, and therefore of great interest. Figure 38 
 represents a transverse section through the ovary (on rather a 
 smaller scale than the previous figures) and shews the general 
 relations of the tissues. 
 
 The germinal epithelium is very much thicker than before 
 about 0-38 mm. as compared with O'22 mm. It is divided 
 into three obvious layers: (i) an outer epithelial layer which 
 corresponds with the pseudo-epithelial layer of the Elasmobranch 
 ovary, average thickness 0x33 mm. (2) A middle layer of small 
 nests, which corresponds with the middle vascular layer of the 
 previous stage; average thickness O'l mm. (3) An inner layer 
 of larger nests; average thickness 0^23 mm. 
 
598 THE STRUCTURE AND DEVELOPMENT 
 
 The general appearance of the germinal epithelium at this 
 stage certainly appears to me to lend support to my view that 
 the whole of it simply constitutes a thickened epithelium inter- 
 penetrated with ingrowths of stroma. 
 
 The cells of the germinal epithelium, which form the various 
 layers, have undergone important modifications. In the first 
 place a large number of the nuclei at any rate of those cells 
 which are about to become ova have undergone a change 
 identical with that which takes place in the conversion of the 
 primitive into the permanent ova in Elasmobranchs. The 
 greater part of the contents of the nucleus becomes clear. The 
 remaining contents arrange themselves as a deeply staining 
 granular mass on one side of the membrane, and later on as 
 a somewhat stellate figure : the two stages forming what were 
 spoken of as the granular and stellate varieties of nucleus. To 
 avoid further circumlocution I shall speak of the nucleus under- 
 going the granular and the stellate modifications. At a still 
 later period the granular contents form a beautiful network 
 in the nucleus. 
 
 The pseudo-epithelium (fig. 38 A) is formed of several tiers of 
 cells, the outermost of which are very columnar and have less 
 protoplasm than in an earlier stage. In the lower tiers of cells 
 there are many primitive ova with granular nuclei, and others 
 in which the nuclei have undergone the granular modification. 
 The primitive ova are almost all of the same size as in the 
 earlier stage. The pseudo-epithelium is separated from the 
 middle layer by a more or less complete stratum of connective 
 tissue, which, however, is traversed by trabeculae connecting the 
 two layers of the epithelium. In the middle layer there are 
 comparatively few modified nuclei, and the cells still retain for 
 the most part their earlier characters. The diameter of the cells 
 is about O'OI2 mm., and that of the nucleus about O'ooS mm. 
 In the innermost layer (fig. 38 B), which is not sharply separated 
 from the middle layer, the majority of the cells, which in the 
 previous stage were ordinary cells of the epithelium, have com- 
 menced to acquire modified nuclei. This change, which first 
 became apparent to a small extent in the young two days after 
 birth, is very conspicuous at this stage. In some of the cells the 
 nucleus is modified in the granular manner, in others in the 
 
OF THE VERTEBRATE OVARY. 599 
 
 stellate, and in a certain number the nucleus has assumed a 
 reticular structure characteristic of the young permanent ovum. 
 
 In addition, however, to the cells which are becoming con- 
 verted into ova, a not inconsiderable number may be observed, 
 if carefully looked for, which are for the most part smaller than 
 the others, generally somewhat oval, and in which the nucleus 
 retains its primitive characters. A fair number of such cells are 
 represented in fig. 3811 In the larger ones the nucleus will 
 perhaps eventually become modified ; but the smaller cells 
 clearly correspond with the interstitial cells of the Elasmobranch 
 germinal epithelium, and are destined to become converted into 
 the epithelium of the Graafian follicle. In some few instances 
 indeed (at this stage very few), in the deeper part of the germinal 
 epithelium, these cells commence to arrange themselves round 
 the just formed permanent ova as a follicular epithelium. An 
 instance of this kind is shewn in fig. 38 B, o. The cells with 
 modified nuclei, which are becoming permanent ova, usually 
 present one point of contrast to the homologous cells in Elas- 
 mobranchs, in that they are quite distinct from each other, 
 and not fused into a polynuclear mass. They have around 
 them a dark contour line, which I can only interpret as the 
 commencement of the membrane (zona radiata ?), which after- 
 wards becomes distinct, and which would thus seem, as Foulis 
 has already insisted, to be of the nature of a vitelline mem- 
 brane. 
 
 In a certain number of instances the protoplasm of the cells 
 which are becoming permanent ova appears, however, actually to 
 fuse, and polynuclear masses identical with those in Elasmo- 
 branchs are thus formed (cf. E. van Beneden 1 ). These masses 
 become slightly more numerous in the succeeding stages. In- 
 dications of a fusion of this kind are shewn in fig. 38 B. That 
 the polynuclear masses really arise from a fusion of primitively 
 distinct cells is clear from the description of the previous stages. 
 The ova in the deeper layers, with modified granular nuclei, 
 measure about O'Oi6 cx>2 mm., and their nuclei from O'Oi 
 o - oi2 mm. 
 
 With reference to the tissue of the hilus of the ovary, it 
 may be noticed that the tubuliferous tissue (t} is relatively 
 
 1 /<><-. tit. 
 
6OO THE STRUCTURE AND DEVELOPMENT 
 
 reduced in quantity. Its cells retain precisely their previous 
 characters. 
 
 The chief difference between the stage of five days and that 
 of two days after birth consists in the fact that during the 
 earlier stage comparatively few modified nuclei were present, 
 but the nuclei then presented the character of the nuclei of 
 primitive ova. 
 
 I have ovaries both of the dog and cat of an equivalent stage, 
 and in both of these the cells of the nests or egg tubes may be 
 divided into two categories, destined respectively to become ova 
 and follicle cells. Nothing which has come under my notice 
 tends to shew that the tubuliferous tissue is in any way concerned 
 in supplying the latter form of cell. 
 
 In a stage, seven days after birth, the same layers in the 
 germinal epithelium may be noticed as in the last described 
 stage. The outermost layer or pseudo-epithelium contains nu- 
 merous developing ova, for the most part with modified nuclei. 
 It is separated by a well marked layer of connective tissue from 
 the middle layer of the germinal epithelium. The outer part of 
 the middle layer contains more connective tissue and smaller 
 nests than in the earlier stage, and most of the cells of this layer 
 contain modified nuclei. In a few nests the protoplasm of the 
 developing ova forms a continuous mass, not divided into dis- 
 tinct cells, but in the majority of instances the outline of each 
 ovum can be distinctly traced. In addition to the cells destined 
 to become ova, there are present in these nests other cells, which 
 will clearly form the follicular epithelium. A typical nest from 
 the middle layer is represented on PI. 26, fig. 39 A. 
 
 The nests or masses of ova in the innermost layer are for the 
 most part still very large, but, in addition to the nests, a few 
 isolated oVa, enclosed in follicles, are to be seen. 
 
 A fairly typical nest, selected to shew the formation of the 
 follicle, is represented on PI. 26, fig. 39 B. 
 
 The nest contains (i) fully formed permanent ova, com- 
 pletely or wholly enclosed in a follicle. (2) Smaller ova, not 
 enclosed in a follicle. (3) Smallish cells with modified nuclei of 
 doubtful destination. (4) Small cells obviously about to form 
 follicular epithelium. 
 
 The inspection of a single such nest is to my mind a satis- 
 
OF THE VERTEBRATE OVARY. 6OI 
 
 factory proof that the follicular epithelium takes its origin-from 
 the germinal epithelium and not from the stroma or tubuliferous 
 tissue. The several categories of elements observable in such a 
 nest deserve a careful description. 
 
 (1) The large ova in their follicles. These ova have 
 precisely the character of the young ova in Elasmobranchs. 
 They are provided with a granular body invested by a delicate, 
 though distinct membrane. Their nucleus is large and clear, 
 but traversed by the network so fully described for Elasmo- 
 branchs. The cells of their follicular epithelium have obviously 
 the same character as many other small cells of the nest. Two 
 points about them deserve notice (a) that many of them 
 are fairly columnar. This is characteristic only of the first 
 formed follicles. In the later formed follicles the cells are 
 always flat and spindle-shaped in section. In this difference 
 between the early and late formed follicles Mammals agree with 
 Elasmobranchs. (b] The cells of the follicle are much more 
 columnar towards the inner side than towards the outer. This 
 point also is common to Mammals and Elasmobranchs. 
 
 Round the completed follicle a very delicate membrana pro- 
 pria folliculi appears to be present 1 . 
 
 The larger ova, with follicular epithelium, measure about 
 0*04 mm., and their nucleus about O'O2 mm., the smaller ones 
 about o - O22 mm., and their nucleus about 0*014 mm. 
 
 (2) Medium sized ova. They are still without a trace of a 
 follicular epithelium, and present no special peculiarities. 
 
 (3) The smaller cells with modified nuclei. I have great 
 doubt as to what is the eventual fate of these cells. There ap- 
 pear to be three possibilities. 
 
 (a) That they become cells of the follicular epithelium ; (ft) 
 that they develop into ova ; (c) that they are absorbed as a kind 
 of food by the developing ova. I am inclined to think that 
 some of these cells may have each of the above-mentioned des- 
 tinations. 
 
 (4) The cells which form the follicle. The only point to be 
 noticed about these is that they are smaller than the indifferent 
 
 1 Loc. cit., Waldeyer, p. 23, denies the existence of this membrane for Mam- 
 malia. It certainly is not so conspicuous as in some other types, but appears to me 
 
 nevertheless to be always present. 
 
 !' 39 
 
6O2 THE STRUCTURE AND DEVELOPMENT 
 
 cells of the germinal epithelium, from which they no doubt 
 originate by. division. This fact has already been noticed by 
 Waldeyer. 
 
 The isolated follicles at this stage are formed by ingrowths 
 of connective tissue cutting off fully formed follicles from a nest. 
 They only occur at the very innermost border of the germinal 
 epithelium. This is in accordance with what has so often been 
 noticed about the mammalian ovary, viz. that the more ad- 
 vanced ova are to be met with in passing from without inwards. 
 
 By the stage seven days after birth the ovary has reached 
 a sufficiently advanced stage to answer the more important 
 question I set myself to solve, nevertheless, partly to reconcile 
 the apparent discrepancy between my account and that of Dr 
 Foulis, and partly to bring my description up to a better known 
 condition of the ovary, I shall make a few remarks about some 
 of the succeeding stages. 
 
 In a young rabbit about four weeks old the ovary is a very 
 beautiful object for the study of the nuclei, &c. 
 
 The pseudo-epithelium is now formed of a single layer of 
 columnar cells, with comparatively scanty protoplasm. In it 
 there are present a not inconsiderable number of developing 
 ova. 
 
 A layer of connective tissue the albuginea is now present 
 below the pseudo-epithelium, which contains a few small nests 
 with very young permanent ova. The layer of medium sized 
 nests internal to the albuginea forms a very pretty object in well 
 stained sections, hardened in Kleinenberg's picric acid. The 
 ova in it have all assumed the permanent form, and are provided 
 with beautiful reticulate nuclei, with, as a rule, one more espe- 
 cially developed nucleolus, and smaller granular bodies. Their 
 diameter varies from about 0*028 to 0*04 mm. and that of their 
 nucleus from o - oi6 to o - O2 mm. The majority of these ova are 
 not provided with a follicular investment, but amongst them are 
 numerous small cells, clearly derived from the germinal epithe- 
 lium, which are destined to form the follicle (vide fig. 40 A and B). 
 In a few cases the follicles are completed, and are then formed 
 of very flattened spindle-shaped (in section) cells. In the ma- 
 jority of cases all the ova of each nest are quite distinct, and 
 each provided with a delicate vitelline membrane (fig. 40 A). 
 
OF THE VERTEBRATE OVARY. 603 
 
 In other instances, which, so far as I can judge, are more 
 common than in the previous stages, the protoplasm of two or 
 more ova is fused together. 
 
 Examples of this are represented in PI. 26, fig. 40 A. In 
 some of these the nuclei in the undivided protoplasm are all of 
 about the same size and distinctness, and probably the proto- 
 plasm eventually becomes divided up into as many ova as 
 nuclei ; in other cases, however, one or two nuclei clearly pre- 
 ponderate over the others, and the smaller nuclei are indistinct 
 and hazy in outline. In these latter cases I have satisfied my- 
 self as completely as in the case of Elasmobranchs, that only 
 one or two ova (according to the number of distinct nuclei) will 
 develop out of the polynuclear mass, and that the other nuclei 
 atrophy, and the material of which they were composed serves 
 as the nutriment for the ova which complete their development. 
 This does not, of course, imply that the ova so formed have 
 a value other than that of a single cell, any more than the 
 development of a single embryo out of the many in one egg 
 capsule implies that the embryo so developing is a compound 
 organism. 
 
 In the innermost layer of the germinal epithelium the out- 
 lines of the original large nests are still visible, but many of the 
 follicles have been cut off by ingrowths of stroma. In the still 
 intact nests the formation of the follicles out of the cells of the 
 germinal epithelium may be followed with great advantage. 
 The cells of the follicle, though less columnar than was the case 
 at an earlier period, are more so than in the case of follicles 
 formed in the succeeding stages. The previous inequality in 
 the cells of the follicles is no longer present. 
 
 The tubuliferous tissue in the zona vasculosa appears to me 
 to have rather increased in quantity than the reverse; and is 
 formed of numerous solid columns or oval masses of cells, 
 separated by strands of connective tissue, with typical spindle 
 nuclei. 
 
 It is partially intelligible to me how Dr Foulis might from 
 an examination of the stages similar to this, conclude that the 
 follicle cells were derived from the stroma ; but even at this 
 stage the position of the cells which will form the follicular epi- 
 thelium, their passage by a series of gradations into obvious 
 
 392 
 
604 THE STRUCTURE AND DEVELOPMENT 
 
 cells of the germinal epithelium and the peculiarities of their 
 nuclei, so different from those of the stroma cells, supply a suffi- 
 cient series of characters to remove all doubt as to the deriva- 
 tion of the follicle cells. Apart from these more obvious points, 
 an examination of the follicle cells from the surface, and not in 
 section, demonstrates that the general resemblance in shape of 
 follicle cells to the stroma cells is quite delusory. They are in 
 fact flat, circular, or oval, plates not really spindle-shaped, but 
 only apparently so in section. While I thus fundamentally 
 differ from Foulis as to the nature of the follicle cells, I am on 
 this point in complete accordance with Waldeyer, and my own 
 results with reference to the follicle cannot be better stated than 
 in his own words (pp. 43, 44). 
 
 At six weeks after birth the ovary of the rabbit corresponds 
 very much more with the stages in the development of the 
 ovary, which Foulis has more especially studied, for the forma- 
 tion of the follicular epithelium, than during the earlier stages. 
 His figure (Quart. Journ. Mic. Set., Vol. XVI., PL 17, fig. 6) of the 
 ovary of a seven and a half months' human foetus is about the 
 corresponding age. Different animals vary greatly in respect to 
 the relative development of the ovary. For example, the ovary 
 of a lamb at birth about corresponds with that of a rabbit six 
 weeks after birth. The points which may be noticed about the 
 ovary at this age are first that the surface of the ovary begins to 
 be somewhat folded. The appearances of these folds in section 
 have given rise, as has already been pointed out by Foulis, to the 
 erroneous view that the germinal epithelium (pseudo-epithelium) 
 became involuted in the form of tubular open pits. The folds 
 appear to me to have no connection with the formation of ova, 
 but to be of the same nature as the somewhat similar folds in 
 Elasmobranchs. A follicular epithelium is present around the 
 majority of the ova of the middle layer, and around all those of 
 the inner layer of the germinal epithelium. The nests are, more- 
 over, much more cut up by connective tissue ingrowths than in 
 the previous stages. 
 
 The follicle cells of the middle layers are very flat, and 
 spindle-shaped in section, and though they stain more deeply 
 than the stroma cells, and have other not easily characterised 
 peculiarities, they nevertheless do undoubtedly closely resemble 
 
OF THE VERTEBRATE OVARY. 605 
 
 the stroma cells when viewed (as is ordinarily the case) in optical 
 section. 
 
 In the innermost layer many of the follicles with the enclosed 
 ova have advanced considerably in development and are formed 
 of columnar cells. The somewhat heterodox view of these cells 
 propounded by Foulis I cannot quite agree to. He says (Quart. 
 J. Mic. Sd., Vol. xvi., p. 210): "The protoplasm which sur- 
 rounds the vesicular nuclei acts as a sort of cement substance, 
 holding them together in the form of a capsular membrane 
 round the young ovum. This capsular membrane is the first 
 appearance of the membrana granulosa." I must admit that I 
 find nothing similar to this, nor have I met with any special 
 peculiarities (as Foulis would seem to indicate) in the cells of the 
 germinal epithelium or other cells of the ovary. 
 
 Figure 41 is a representation of an advanced follicle of a six 
 weeks' rabbit, containing two ova, which is obviously in the act 
 of dividing into two. Follicles of this kind with more than one 
 ovum are not very uncommon. It appears to me probable that 
 follicles, such as that I have figured, were originally formed of 
 a single mass of protoplasm with two nuclei ; but that instead 
 of one of the nuclei atrophying, both of them eventually de- 
 veloped and the protoplasm subsequently divided into two 
 masses. In other cases it is quite possible that follicles with 
 two ova should rather be regarded as two follicles not separated 
 by a septum of stroma. 
 
 On the later stages of development of the ovary I have no 
 complete series of observations. The yolk spherules I find to 
 be first developed in a peripheral layer of the vitellus. I have 
 not been able definitely to decide the relation of the zona radiata 
 to the first formed vitelline membrane. Externally to the zona 
 radiata there may generally be observed a somewhat granular 
 structure, against which the follicle cells abut, and I cannot 
 agree with Waldeyer (loc cit., p. 40) that this structure is con- 
 tinuous with the cells of the discus, or with the zona radiata. 
 Is it the remains of the first formed vitelline membrane ? I have 
 obtained some evidence in favour of this view, but have not been 
 successful in making observations to satisfy me on the point, 
 and must leave open the question whether my vitelline mem- 
 brane becomes the zona radiata or whether the zona is not a 
 
606 THE STRUCTURE AND DEVELOPMENT 
 
 later and independent formation, but am inclined myself to 
 adopt the latter view. The first formed membrane, whether or 
 no it becomes the zona radiata, is very similar to the vitelline 
 membrane of Elasmobranchs and arises at a corresponding stage. 
 
 Summary of observations on the mammalian ovary. The 
 general results of my observations on the mammalian ovary are 
 the following : 
 
 (1) The ovary in an eighteen days' embryo consists of a 
 cylindrical ridge attached along the inner side of the Wolffian 
 body, which is formed of two parts ; (a] an external epithelium 
 two or three cells deep (the germinal epithelium); (&) a hilus 
 or part forming in the adult the vascular zone, at this stage 
 composed of branched masses of epithelial tissue (tubuliferous 
 tissue) derived from the walls of the anterior Malpighian bodies, 
 and numerous blood-vessels, and some stroma cells. 
 
 (2) The germinal epithelium gradually becomes thicker, 
 and after a certain stage (twenty-three days) there grow into it 
 numerous stroma ingrowths, accompanied by blood-vessels. The 
 germinal epithelium thus becomes honeycombed by strands of 
 stroma. Part of the stroma eventually forms a layer close below 
 the surface, which becomes in the adult the tunica albuginea. 
 The part of the germinal epithelium external to this layer be- 
 comes reduced to a single row of cells, and forms what has been 
 spoken of in this paper as the pseudo-epithelium of the ovary. 
 The greater part of the germinal epithelium is situated internal 
 to the tunica albuginea, and this part is at first divided up by 
 strands of stroma into smaller divisions externally, and larger 
 ones internally. These masses of germinal epithelium (probably 
 sections of branched trabeculae) may be spoken of as nests. In 
 the course of the development of the ova they are broken up by 
 stroma ingrowths, and each follicle with its enclosed ovum is 
 eventually isolated by a layer of stroma. 
 
 (3) The cells of the germinal epithelium give rise both to 
 the permanent ova and to the cells of the follicular epithelium. 
 For a long time, however, the cells remain indifferent, so that 
 the stages, like those in Elasmobranchs, Osseous Fish, Birds, 
 Reptiles, &c., with numerous primitive ova embedded amongst 
 the small cells of the germinal epithelium, are not found. 
 
OF THE VERTEBRATE OVARY. 607 
 
 (4) The conversion of the cells of the germinal epithelium 
 into permanent ova commences in an embryo of about twenty- 
 two days. All the cells of the germinal epithelium appear to 
 be capable of becoming ova : the following are the stages in 
 the process, which are almost identical with those in Elasmo- 
 branchs : 
 
 (a) The nucleus of the cells loses its more or less distinct 
 network, and becomes very granular, with a few specially large 
 granules (nucleoli). The protoplasm around it becomes clear 
 and abundant primitive ovum stage. It may be noted that 
 the largest primitive ova are very often situated in the pseudo- 
 epithelium, (b) A segregation takes place in the contents of 
 the nucleus within the membrane, and the granular contents 
 pass to one side, where they form an irregular mass, while the 
 remaining space within the membrane is perfectly clear. The 
 granular mass gradually develops itself into a beautiful reticu- 
 lum, with two or three highly refracting nucleoli, one of which 
 eventually becomes the largest and forms the germinal spot par 
 excellence. At the same time the body of the ovum becomes 
 slightly granular. While the above changes, more especially 
 those in the nucleus, have been taking place, the protoplasm of 
 two or more ova may fuse together, and polynuclear masses be 
 so formed. In some cases the whole of such a polynuclear mass 
 gives rise to only a single ovum, owing to the atrophy of all the 
 nuclei but one, in others it gives rise by subsequent division to 
 two or more ova, each with a single germinal vesicle. 
 
 (5) All the cells of a nest do not undergo the above changes, 
 but some of them become smaller (by division) than the indif- 
 ferent cells of the germinal epithelium, arrange themselves round 
 the ova, and form the follicular epithelium. 
 
 (6) The first membrane formed round the ovum arises in 
 some cases even before the appearance of the follicular epithe- 
 lium, and is of the nature of a vitelline membrane. It seems 
 probable, although not definitely established by observation, 
 that the zona radiata is formed internally to the vitelline mem- 
 brane, and that the latter remains as a membrane, somewhat 
 irregular on its outer border, against which the ends of the follicle 
 cells abut. 
 
608 THE STRUCTURE AND DEVELOPMENT 
 
 GENERAL OBSERVATIONS ON THE STRUCTURE AND 
 DEVELOPMENT OF THE OVARY. 
 
 In selecting Mammalia and Elasmobranchii as my two 
 types for investigation, I had in view the consideration that 
 what held good for such dissimilar forms might probably be 
 accepted as true for all Vertebrata with the exception of Am- 
 phioxus. 
 
 The strticture of the ovary. From my study of these two 
 types, I have been led to a view of the structure of the ovary, 
 which differs to a not inconsiderable extent from that usually 
 entertained. For both types the conclusion has been arrived at 
 that the whole egg-containing part of the ovary is really the 
 thickened germinal epithelium, and that it differs from the original 
 thickened patch or layer of germinal epithelium, mainly in the 
 fact that it is broken up into a kind of meshwork by growths of 
 vascular stroma. If the above view be accepted for Elasmo- 
 branchii and Mammalia, it will hardly be disputed for the 
 ovaries of Reptilia and Aves. In the case also of Osseous Fish 
 and Amphibia, this view of the ovary appears to be very tenable, 
 but the central core of stroma present in the other types is 
 nearly or quite absent, and the ovary is entirely formed of the 
 germinal epithelium with the usual strands of vascular stroma 1 . 
 It is obvious that according to the above view Pfliiger's egg- 
 tubes are merely trabeculae of germinal epithelium, and have no 
 such importance as has been attributed to them. They are 
 present in a more or less modified form in all types of ovaries. 
 Even in the adult Amphibian ovary, columns of cells of the 
 germinal epithelium, some indifferent, others already converted 
 into ova, are present, and, as has been pointed out by Hertwig 2 , 
 represent Pfliiger's egg-tubes. 
 
 The formation of the permanent ova. The passage of primi- 
 tive ova into permanent ova is the part of my investigation to 
 which the greatest attention was paid, and the results arrived at 
 for Mammalia and Elasmobranchii are almost identical. Al- 
 
 1 My view of the structure of the ovary would seem to be that held by Gotte, 
 Ent-wicklungsgeschichte d. Unke, pp. 14 and 15. 
 
 2 Loc. cit. 36. 
 
OF THE VERTEBRATE OVARY 609 
 
 though there are no investigations as to the changes undergone 
 by the nucleus in other types, still it appears to me safe to con- 
 clude that the results arrived at hold good for Vertebrates 
 generally 1 . As has already been pointed out the transformation 
 which the so-called primitive ova undergo is sufficient to shew 
 that they are not to be regarded as ova but merely as embryonic 
 sexual cells. A feature in the transformation, which appears to 
 be fairly constant in Scyllium, and not uncommon in the rabbit, 
 is the fusion of the protoplasm of several ova into a syncytium, 
 the subsequent increase in the number of nuclei in the syncy- 
 tium, the atrophy and absorption of a portion of the nuclei, and 
 the development of the remainder into the germinal vesicles of 
 ova ; the vitellus of each ovum being formed by a portion of the 
 protoplasm of the syncytium. 
 
 As to the occurrence of similar phenomena in the Vertebrata 
 generally, it has already been pointed out that Ed. van Beneden 
 has described the polynuclear masses in Mammalia, though he 
 does not appear to me to have given a complete account of their 
 history. Gotte 2 describes a fusion of primitive ova in Amphibia, 
 but he believes that the nuclei fuse as well as the bodies of the 
 ova, so that one ovum (according to his view no longer a cell) 
 is formed by the fusion of several primitive ova with their 
 nuclei. I have observed nothing which tends to support Gotte's 
 view about the fusion of the nuclei, and regard it as very im- 
 probable. As regards the interpretation to be placed upon the 
 nests formed of fused primitive ova, Ed. van Beneden maintains 
 that they are to be compared with the upper ends of the egg 
 tubes of Insects, Nematodes, Trematodes, &c. There is no 
 doubt a certain analogy between the two, in that in both cases 
 certain nuclei of a polynuclear mass increase in size, and with 
 the protoplasm around them become segmented off from the 
 remainder of the mass as ova, but the analogy cannot be pressed. 
 The primitive ova, or even the general germinal epithelium, 
 rather than these nests, must be regarded as giving origin to the 
 ova, and the nests should be looked on, in my opinion, as con- 
 
 1 Since writing the above I have made out that in the Reptilia the formation of 
 the permanent ova takes place in the same fashion as in Elasmobranchii and Mam- 
 malia. 
 
 " EntwickltatgtgticMckU d. L Xv. 
 
6 10 THE STRUCTURE AND DEVELOPMENT 
 
 nected more with the nutrition than with the origin of the ova. 
 In favour of this view is the fact that as a rule comparatively 
 few ova are developed from the many nuclei of a nest ; while 
 against the comparison with the egg tubes of the Invertebrata 
 it is to be borne in mind that many ova appear to develop inde- 
 pendently of the nests. 
 
 In support of my view about the nests there may be cited 
 many analogous instances from the Invertebrata. In none of 
 them, however, are the phenomena exactly identical with those 
 in Vertebrata. In the ovary of many Hydrozoa (e.g. Tubularia 
 mesembryanthemum), out of a large number of ova which develop 
 up to a certain point, a comparatively very small number survive, 
 and these regularly feed upon the other ova. During this 
 process the boundary between a large ovum and the smaller ova 
 is indistinct : in the outermost layer of a large ovum a number 
 of small ova are embedded, the outlines of the majority of which 
 have become obscure, although they can still be distinguished. 
 Just beyond the edge of a large ovum the small ova have begun 
 to undergo retrogressive changes ; while at a little distance from 
 the ovum they are quite normal. An analogous phenomenon 
 has been very fully described by Weismann 1 in the case of 
 Leptodera, the ovary of which consists of a germogene, in which 
 the ova develop in groups of four. Each group of four occupies 
 a separate chamber of the ovary, but in summer only one of the 
 four eggs (the third from the germogene) develops into an 
 ovum, the other three are used as pabulum. In the case of the 
 winter eggs the process is carried still further, in that the contents 
 of the alternate chambers, instead of developing into ova, are 
 entirely converted, by a series of remarkable changes, into 
 nutritive reservoirs. Fundamentally similar occurrences to the 
 above are also well known in Insects. Phenomena of this nature 
 are obviously in no way opposed to the view of the ovum being 
 a single cell. 
 
 With reference to the origin of the primitive ova, it appears 
 to me that their mode of development in Mammals proves beyond 
 a doubt that they are modified cells of the germinal epithelium. 
 In Elasmobranchii their very early appearance, and the difficulty 
 
 1 Zeit. fiir wiss. Zool. Bd. xxvn. 
 
OF THE VERTEBRATE OVARY. 6ll 
 
 of finding transitional forms between them and ordinary "CeHs of 
 the germinal epithelium, caused me at one time to seek (un- 
 successfully) for a different origin for them. Any such attempts 
 appear to me, however, out of the question in the case of 
 Mammals. 
 
 The egg membranes. The homologies of the egg membranes 
 in the Vertebrata are still involved in some obscurity. In 
 Elasmobranchii there are undoubtedly two membranes present. 
 (i) An outer and first formed membrane the albuminous 
 membrane of Gegenbaur which, in opposition to previous ob- 
 servers, I have been led to regard as a vitelline membrane. (2) 
 An inner radiately striated membrane, formed as a differentiation 
 of the surface of the yolk at a later period. Both these mem- 
 branes usually atrophy before the ovum leaves the follicle. In 
 Reptilia 1 precisely the same arrangement is found as in Elasmo- 
 branchii, except that as a rule the zona radiata is relatively 
 more important. The vitelline membrane external to this (or as 
 it is usually named the chorion) is, as a rule, thin in Reptilia ; 
 but in Crocodilia is thick (Gegenbaur), and approaches the 
 condition found in Scyllium and other Squalidae. It appears, as 
 in Elasmobranchs, to be formed before the zona radiata. A 
 special internal differentiation of the zona radiata is apparently 
 found (Eimer) in many Reptilia. No satisfactory observations 
 appear to be recorded with reference to the behaviour of the two 
 reptilian membranes as the egg approaches maturity. In Birds 2 
 the same two membranes are again found. The first formed 
 and outer one is, according to Gegenbaur and E. van Beneden, 
 a vitelline membrane ; and from the analogy of Elasmobranchii 
 I feel inclined to accept their view. The inner one is the zona 
 radiata, which disappears comparatively early, leaving the ovum 
 enclosed only by the vitelline membrane, when it leaves the 
 follicle. All the large-yolked vertebrate ova appear then to 
 agree very well with Elasmobranchs in presenting during 
 some period of their development the two membranes above 
 mentioned. 
 
 Osseous fish have almost always a zona radiata, which it 
 seems best to assume to be equivalent to that in Elasmobranchs. 
 
 1 Gegenbaur, loc. cit.; Waldeyer, loc. tit.; Eimer, loc. cit.; and Ludwig, loc. fit. 
 - Gegenbaur, Waldeyer, E. van JJeiieden. Eimer. 
 
6l2 THE STRUCTURE AND DEVELOPMENT 
 
 Internal to this is a thin membrane, the equivalent, according to 
 Eimer, of the membrane found by the same author within the 
 zona in Reptilia. A membrane equivalent to the thick vitelline 
 membrane of Elasmobranchii would seem to be absent in most 
 instances, though a delicate membrane, external to the zona, has 
 not infrequently been described ; Eimer more especially asserts 
 that such a membrane exists in the perch within the peculiar 
 mucous covering of the egg of that fish. 
 
 In Petromyzon, a zona radiata appears to be present 1 , which 
 is divided in the adult into two layers, both of them perforated. 
 The inner of the two perhaps corresponds with the membrane 
 internal to the zona radiata in other types. In Amphibia the 
 single late formed and radiately striated (Waldeyer) membrane 
 would appear to be a zona radiata. If the suggestion on page 
 605 turns out to be correct the ova of Mammalia possess both a 
 vitelline membrane and zona radiata. E. van Beneden 2 has, 
 moreover, shewn that they are also provided at a certain period 
 with a delicate membrane within the zona. 
 
 TJu reticuhtm of the germinal vesicle. In the course of 
 description of the ovary it has been necessary for me to enter 
 with some detail into the structure of the nucleus, and I have 
 had occasion to figure and describe a reticulum identical with 
 that recently described by so many observers. The very interest- 
 ing observations of Dr Klein in the last number of this Journal 3 
 have induced me to say one or two words in defence of some 
 points in my description of the reticulum. Dr Klein says, on 
 page 323, " I have distinctly seen that when nucleoli are present 
 the instances are fewer than is generally supposed ; they are 
 accumulations of the fibrils of the network." I have no doubt 
 that Klein is correct in asserting that nucleoli are fewer than is 
 generally supposed ; and that in many of these instances what 
 are called nucleoli are accumulations, " natural or artificial," of 
 the fibrils of the network ; but I cannot accept the universality 
 of the latter statement, which appears to me most certainly not 
 to hold good in the case of ova, in which nucleoli frequently 
 exist in the absence of the network. 
 
 Again, I find that at the point of intersection of two or more 
 
 1 Carlberla, Zeit. /. wiss. Zoo/. Bd. xxx. a Loc. dt. 
 
 3 [Quarterly Journal Microscopical Science, July 1878.] 
 
OF THE VERTEBRATE OVARY. 613 
 
 fibrils there is, as a rule, a distinct thickening of the matter of 
 the fibrils, and that many of the dots seen are not merely, as Dr 
 Klein would maintain, optical sections of fibrils. 
 
 It appears to me probable that both the network and the 
 nucleoli are composed of the same material what Hertwig 
 calls nuclear substance and if Dr Klein merely wishes to assert 
 this identity in the passage above quoted, I am at one with 
 him. 
 
 Although a more or less distinct network is present in most 
 nuclei (I have found it in almost all embryonic nuclei) it is not 
 universally so. In the nuclei of primitive ova I have no doubt 
 that it is absent, though present in the unmodified nuclei of the 
 germinal epithelium ; and it is present only in a very modified 
 form in the nuclei of primitive ova undergoing a transformation 
 into permanent ova. The absence of the reticulum does not, 
 of course, mean that the substance capable of forming a reti- 
 culum is absent, but merely that it does not assume a particular 
 arrangement. 
 
 One of the most interesting points in Klein's paper, as well 
 as in those of Heitzmann and Eimer, is the demonstration of a 
 connection between the reticulum of the nucleus and fibres 
 in the body of the cell. Such a connection I have not found 
 in ova, but may point out that it appears to exist between the 
 subgerminal nuclei in Elasmobranchs and the protoplasmic net- 
 work in the yolk in which they lie. This point is called attention 
 to in my Monograph on Elasmobranch Fishes, page 39 1 , where it is 
 stated that " the network in favourable cases may be observed to 
 be in connection with the nuclei just described. Its meshes are 
 finer in the vicinity of the nuclei, and the fibres in some cases 
 appear almost to start from them." The nuclei in the yolk are 
 knobbed bodies divided by a sponge work of septa into a number 
 of areas each with a nucleolar body. 
 
 1 [This Edition, p. 252.] 
 
614 THE STRUCTURE AND DEVELOPMENT 
 
 EXPLANATION OF PLATES 24, 25, 26. 
 
 PLATE 24. 
 
 LIST OF REFERENCE LETTERS. 
 
 d n. Modified nucleus of primitive ovum, d o. Permanent ovum in the act of 
 being formed, dv. Developing blood-vessels, d yk. Developing yolk, e p. Non- 
 ovarian epithelium of ovarian ridge, f e. Follicular epithelium, g v. Germinal 
 vesicle. / sir. Lymphatic region of stroma. n n. Nests of nuclei of ovarian region. 
 o. Permanent ovum. ov r. Ovarian portion of ovarian ridge. / o. Primitive ovum. 
 ps e. Pseudo-epithelium of ovarian ridge, sir. Stroma ingrowths into ovarian epithe- 
 lium, v. Blood-vessel, v sir. Vascular region of stroma adjoining ovarian ridge. 
 vt. Vitelline membrane, x. Modified nucleus, yk. Yolk, z n. Zona radiata. 
 
 Fig. r. Transverse section of the ovarian ridge of an embryo of Scy. canicula, 
 belonging to stage P, shewing the ovarian region with thickened epithelium and 
 numerous primitive ova. Zeiss C, ocul. 2. Picric acid. 
 
 Fig. i. Transverse section of the ovarian ridge of an embryo of Scyllitim cani- 
 cula, considerably older than stage Q. Zeiss C, ocul. 2. Picric add. Several nests, 
 some with distinct ova, and others with the ova fused together, are present in the sec- 
 tion (n. n.), and several examples of modified nuclei in still distinct ova are also repre- 
 sented. One of these is marked x. The stroma of the ovarian ridge is exceptionally 
 scanty. 
 
 Fig. 3. Transverse section through part of the ovarian ridge, including the ovarian 
 region of an almost ripe embryo of Scy Ilium canicula. Zeiss C, ocul. 2. Picric acid. 
 Nuclear nests (n. n.), developing ova (d. o.), and ova (o.), with completely formed 
 follicular epithelium, are now present. The ovarian region is still well separated from 
 the subjacent stroma, and does not appear to contain any cells except those of the 
 original germinal epithelium. 
 
 Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to illustrate 
 the relation of the stroma (sir.) and ovarian region. Zeiss a a, ocul. 2. Picric acid. 
 
 Fig. 5. Section through the ovarian ridge of an embryo of Scy Ilium canicula, 
 to cm. long, in which the ovary was slightly less advanced than in fig. 3. To illus- 
 trate the relation of the ovarian epithelium to the subjacent vascula stroma. Zeiss A, 
 ocul. 2. Osmic acid. y. points to a small separated portion of the germinal epithe- 
 lium. 
 
 Fig. 6. Section through the ovarian ridge of an embryo of Scyllinm canicula, 
 slightly older than fig. 5. To illustrate the relation of the ovarian epithelium to the 
 subjacent vascular stroma. Zeiss A, ocul. 2. Osmic acid. 
 
 Fig. 7. More highly magnified portion of the same ovary as fig. 6. To illustrate 
 the same points. Zeiss C, ocul. 2 . Osmic acid. 
 
OF THE VERTEBRATE OVARY. 615 
 
 Fig. 8. Section through the ovarian region (close to one extremity, where it is 
 very small) from a young female of Scy. caniaila. Zeiss C, ocul. 2. Picric acid. It 
 shews the vascular ingrowths amongst the original epithelial cells of the ovarian 
 region. 
 
 Fig. 9. Section through the ovarian region of the same embryo as fig. 8, at its 
 point of maximum development. Zeiss A, ocul. 2. Picric acid, 
 
 Fig. 10. Section through superficial part of the ovary of an embryo, shewing 
 the pseudo-epithelium ; the cells of which are provided with tails prolonged into the 
 general tissue of the ovary. At/, e. is seen a surface view of the follicular epithelium 
 of an ovum. Zeiss C, ocul. 2. Picric acid. 
 
 Fig. it. Section through part of an ovary of Scy 'Ilium canicula of stage Q, with 
 three primitive ova, the most superficial one containing a modified nucleus. 
 
 Fig. 12. Section through part of an ovary of an example of Scyllmm canicula, 
 8 cm. long. The section passes through a nest of ova with modified nuclei, in which 
 the outlines of the individual ova are quite distinct. Zeiss E, ocul. 2. Picric acid. 
 
 Fig. \j ) . Section through part of ovary of the same embryo as in fig. 5. The 
 section passes through a nest of nuclei, with at the least two developing ova, and also 
 through one already formed permanent ovum. Zeiss E,'ocul. 2. Osmic acid. 
 
 Figs. 14, 15, 16, 17, 18 [Figs. 17 and 18 are on PI. 25]. Sections through parts 
 of the ovary of the same embryo as fig. 3, with nests of nuclei and a permanent ova 
 in the act of formation. Fig. 14 is drawn with Zeiss D D, ocul. 2. Figs. 15, 16, 
 17, with Zeiss E, ocul. 2. Picric acid. 
 
 PLATE 25. 
 
 LIST OF REFERENCE LETTERS. 
 
 do. Permanent ovum in the act of being formed, dyk. Developing yolk, j e. 
 Follicular epithelium, fe'. Secondary follicular epithelium, g v. Germinal vesicle. 
 nn. Nests of nuclei of ovarian region, o. Permanent ovum. pse. Pseudo epithelium. 
 str. Stroma ingrowths into ovarian epithelium, -vt. Vitelline membrane, x. Modified 
 nucleus, yk. Yolk (vitellus). z n. Zona radiata. 
 
 [Figs. 17 and 18. Vide description of Plate 24.] 
 
 Fig. 19. Two nuclei from a nest which appear to be in the act of division. From 
 ovary of the same embryo as fig. 3. 
 
 Fig. 20. Section through part of an ovary of the same embryo as fig. 6, contain- 
 ing a nest of nuclei. Zeiss F, ocul. 2. Osmic acid. 
 
 Fig. 21. Ovum from the ovary of a half-grown female, containing isolated deeply 
 stained patches of developing yolk granules. Zeiss B, ocul. 2. Picric acid. 
 
 Fig. 22. Section through a small part of the ovum of an immature female of 
 Scyllium canicula, to shew the constitution of the yolk, the follicular epithelium, and 
 the egg membranes. Zeiss E, ocul. 2. Chromic acid. 
 
 Fig. 23. Section through part of the periphery of a nearly ripe ovum of Scy. 
 canicida. Zeiss C, ocul. 2. It shews the remnant of the vitelline membrane (v. t.) 
 separating the columnar but delicate cells of the follicular epithelium (/ e.) from the 
 yolk (yk.). In the yolk are seen yolk-spherules in a protoplasmic network. The 
 transverse markings in the yolk-spherules have been made oblique by the artist. 
 
6l6 THE STRUCTURE AND DEVELOPMENT 
 
 Fig. 24. Fully formed ovum containing a second nucleus (x), probably about to 
 be employed as pabulum; from the same ovary as fig. 5. The follicular epithelium is 
 much thicker on the side adjoining the stroma than on the upper side of the ovum. 
 Zeiss F, ocul. 2. Osmic acid. 
 
 Fig. 25. A. Ovum from the same ovary as fig. 21, containing in the yolk three 
 peculiar bodies, similar in appearance to the two small bodies in the germinal vesicle. 
 B. Germinal vesicle of a large ovum from the same ovary, containing a body of a 
 strikingly similar appearance to those in the body of the ovum in A. Zeiss E, ocul. 2. 
 Picric acid. 
 
 Fig. 26. Section of the ovary of a young female of Scyllium stellare 163 centime- 
 tres in length. The ovary is exceptional, on account of the large size of the stroma 
 ingrowths into the epithelium. Zeiss C, ocul. 2. Osmic acid. 
 
 Fig.. 27. Ovum of Scyllium canicula, 5 mm. in diameter, treated with osmic acid. 
 The figure illustrates the development of the yolk and a peculiar mode of prolifera- 
 tion of the germinal spots. Zeiss A, ocul. 2. 
 
 Fig. 28. Small part of the follicular epithelium and egg membranes of a some- 
 what larger ovum of Scyllium canicula than fig. 22. Zeiss D D, ocul. i. 
 
 Fig. 29. The same parts as in fig. 28, from a still larger ovum. Zeiss D D, 
 ocul. 2. 
 
 Fig. 30. Ovum of Raja with follicular epithelium. Zeiss C, ocul. 2. 
 
 Fig. 31. Small portion of a larger ovum of Raja than fig. 30. Zeiss D D, 
 ocul. 2. 
 
 Fig. 32. Follicular epithelium, c., from an ovum of Raja still larger than fig. 31. 
 Zeiss D D, ocul. 2. 
 
 Fig. 33. Surface view of follicular epithelium from an ovum of Raja of about the 
 same age as fig. 33. 
 
 Fig. 34. Vertical section through the superficial part of an ovary of an adult Raja 
 to shew the relation of the pseudo-epithelium to the subjacent stroma. Zeiss D D, 
 ocul. 2. 
 
 PLATE 26. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 do. Developing ovum, f c. Cells which will form the follicular epithelium, f e. 
 Follicular epithelium, g e. Germinal epithelium, mg. Malpighian body. . Nest of 
 cells of the germinal epithelium, n d. Nuclei in the act of dividing, o. Permanent 
 ovum, o v. Ovary, p o. Primitive ovum. f. Tubuliferous tissue, derived from Mal- 
 pighian bodies. 
 
 Fig- 35- Transverse section through the ovary of an embryo rabbit of eighteen 
 days, hardened in osmic acid. The colours employed are intended to render clear 
 the distinction between the germinal epithelium (g e.) and the tubuliferous tissue (/.), 
 which has grown in from the Wolfnan body, and which gives rise in the male to parts 
 of the tubuli seminiferi. Zeiss A, ocul. 2. 
 
OF THE VERTEBRATE OVARY. 617 
 
 Fig- 35 A. Transverse section through a small part of the ovary of an embryo 
 from the same female as fig. 35, hardened in picric acid, shewing the relation of the 
 germinal epithelium to the subjacent tissue. Zeiss D D, ocul. 2. 
 
 Fig- 35 B. Longitudinal section through part of the Wolffian body and the ante- 
 rior end of the ovary of an eighteen days' embryo, to shew the derivation of tubu- 
 liferous tissue (/.) from the Malpighian bodies, close to the anterior extremity of the 
 ovary. Zeiss A, ocul. i. 
 
 Fig. 36. Transverse section through the ovary of an embryo rabbit of twenty- 
 two days, hardened in osmic acid. It is coloured in the same manner as fig. 35. 
 Zeiss A, ocul. 2. ' 
 
 Fig. 36 A. Transverse section through a small part of the ovary of an embryo, 
 from the same female as fig. 36, hardened in picric acid, shewing the relation of the 
 germinal epithelium to the stroma of the ovary. Zeiss D D, ocul. 2. 
 
 Figs. 37 and 37 A. The same parts of an ovary of a twenty-eight days' embryo as 
 figs. 36 and 36 A of a twenty-two days' embryo. 
 
 Fig. 38. Ovary of a rabbit five days after birth, coloured in the same manner as 
 figs- 35> 36 and 37, but represented on a somewhat smaller scale. Picric acid. 
 
 Fig. 38 A. Vertical section through a small part of the surface of the same ovary 
 as fig. 38. Zeiss D D, ocul. 2. 
 
 Fig. 38 B. Small portion of the deeper layer of the germinal epithelium of the 
 same ovary as fig. 38. The figure shews the commencing differentiation of the cells 
 of the germinal epithelium into true ova and follicle cells. Zeiss D D, ocul. 2. 
 
 Fig- 39 A. Section through a small part of the middle region of the germinal 
 epithelium of a rabbit seven days after birth. Zeiss D D, ocul. 2. 
 
 Fig- 39 B. Section through a small part of the innermost layer of the germinal 
 epithelium of a rabbit seven days after birth, shewing the formation of Graafian folli- 
 cles. Zeiss D D, ocul. 2. 
 
 Figs. 40 A and 40 B. Small portions of the middle region of the germinal epithe- 
 lium of a rabbit four weeks after birth. Zeiss D D, ocul. 2. 
 
 Fig. 41. Graafian follicle with two ova, about to divide into two follicles, from a 
 rabbit six weeks after birth. Zeiss D D, ocul. 2. 
 
 B. 40 
 
XIII. ON THE EXISTENCE OF A HEAD-KIDNEY IN THE 
 EMBRYO CHICK, AND ON CERTAIN POINTS IN THE DE- 
 VELOPMENT OF THE MtJLLERIAN 'DUCT \ By F. M. BAL- 
 
 FOUR and A. SEDGWICK. 
 
 (With Plates 27 and 28.) 
 
 THE following paper is divided into three sections. The 
 first of these records the existence of certain structures in the 
 embryo chick, which eventually become in part the abdominal 
 opening of the Miillerian duct, and which, we believe, corre- 
 spond with the head-kidney, or " Vorniere " of German authors. 
 The second deals with the growth and development of the Miil- 
 lerian duct. With reference to this we have come to the con- 
 clusion that the Miillerian duct does not develop entirely 
 independently of the Wolffian duct. The third section of our 
 paper is of a more general character, and contains a discussion of 
 the rectifications in the views of the homologies of the parts of 
 the excretory system in Aves, necessitated by the results of our 
 investigations. 
 
 We have, as far as possible, avoided entering into the ex- 
 tended literature of the excretory system, since this has been 
 very fully given in three general papers which have recently 
 appeared by Semper 2 , Fiirbinger 3 , and by one of us 4 . 
 
 All recent observers, including Braun 5 for Reptilia, and Egli 6 
 for Mammalia, have stated that the Miillerian duct develops as 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xix. 1879. 
 
 2 "Das Urogenital-System der Plagiostomen," Arbeiten a. d. zool.-zoot. Institut. 
 Wiirzburg. 
 
 3 " Zur vergl. Anat. u. Entwick. d. Excretionsorgane d. Vertebraten," Morpho- 
 logisches Jahrbuch, Vol. IV. 
 
 4 " On the Origin and History of the Urinogenital Organs of Vertebrates," 
 Journal of Anat. and Pkys., Vol. x. [This Edition No. vn.] 
 
 5 Arbeiten a. d. zool.-zoot. Institut. Wiirzburg, Vol. IV. 
 
 6 Beitr. zur Anat. u. Entwick. d, Geschlechtsorgane, Inaug. Diss., Zurich, 1876. 
 
EXISTENCE OF A HEAD-KIDNEY. 619 
 
 a groove in the peritoneal epithelium, which is continuecLback- 
 ward as a primitively solid rod in the space between the Wolf- 
 fian duct and peritoneal epithelium. 
 
 In our preliminary account we stated 1 , in accordance with 
 the general view, that the Miillerian duct was formed as a groove, 
 or elongated involution of the peritoneal epithelium adjoining 
 the Wolffian duct. We have now reason to believe that this is 
 not the case. In the earliest condition of the Miillerian duct 
 which we have been able to observe, it consists of three succes- 
 sive open involutions of the peritoneal epithelium, connected 
 together by more or less well-defined ridge-like thickenings of 
 the epithelium. We believe, on grounds hereafter to be stated, 
 that the'whole of this formation is equivalent to the head-kidney 
 of the Ichthyopsida. The head-kidney, as we shall continue to 
 call it, takes its origin from the layer of thickened epithelium 
 situated near the dorsal angle of the body-cavity, close to the 
 Wolffian duct, which has been known since the publication of 
 Waldeyer's important researches as the germinal epithelium. 
 The anterior of the three open involutions or grooves is situated 
 some little distance behind the front end of the Wolffian duct. 
 It is simply a shallow groove in the thickest part of the germinal 
 epithelium, and forms a corresponding projection into the ad- 
 jacent stroma. In front the projection is separated by a con- 
 siderable interval from the Wolffian duct ; but near its hinder- 
 most part it almost comes into contact with the Wolffian duct. 
 The groove extends in all for about five of our sections, and 
 then terminates by its walls becoming gradually continued into 
 a slight ridge-like thickening of the germinal epithelium. The 
 groove arises as a simple depression in a linear area of thick- 
 ened germinal epithelium. The linear area is, however, con- 
 tinued very considerably further forward than the groove, and 
 sometimes exhibits a slight central depression, which might be 
 regarded as a forward continuation of the groove. The passage 
 from the groove to the ridge may best be conceived by sup- 
 posing the groove to be suddenly filled up, so as to form a solid 
 ridge pointing inwards towards the Wolffian duct. 
 
 The ridge succeeding the first groove is continued for about 
 six sections, and is considerably more prominent at its posterior 
 
 1 /'i-ore citings of Royal Society, 1878. 
 
 40 2 
 
620 EXISTENCE OF A HEAD-KIDNEY 
 
 extremity than in front. It is replaced by groove number two, 
 which appears as if formed by the reverse process to that by 
 which the ridge arose, viz., by a hollowing out of the ridge on 
 the side towards the body-cavity. The wall of the second 
 groove is, after a few sections, continued into a second ridge or 
 thickening of the germinal epithelium, which, however, is so 
 faintly marked as to be hardly visible in its middle part. In its 
 turn this ridge is replaced by the third and last groove. This 
 vanishes after one or two sections, and behind the point of its 
 disappearance we have failed to find any further traces of the 
 head-kidney. The whole formation extends through about 
 twenty-four of our sections and one and a half segments (muscle- 
 plates). 
 
 We have represented (Plate 27, Series A, Nos. I 10) a fairly 
 complete series of sections through part of the head-kidney of 
 an embryo slightly older than that last described, containing 
 the second and third grooves and accessory parts. The connec- 
 tion between the grooves and the ridges is very well illustrated 
 in Nos. 3, 4, and 5 of this series. In No. 3 we have a pro- 
 minent ridge, in the interior of which there appears in No. 4 
 a groove, which becomes gradually wider in Nos. 5 and 6. 
 Both the grooves and ridges are better marked in this than in 
 the younger stage ; but the chief difference between the two 
 stages consists in the third groove no longer forming the hinder- 
 most limit of the head-kidney. Instead of this, the last groove 
 (No. 7) terminates by the upper part of its walls becoming con- 
 stricted off as a separate rod, which appears at first to contain 
 a lumen continuous with the open groove. This rod (Nos. 7, 8, 
 9, I o) situated between the germinal epithelium and Wolffian 
 duct is continued backward for some sections. It finally termi- 
 nates by a pointed extremity, composed of not more than two 
 cells abreast (Nos. 8 10). 
 
 Our third stage, sections of which are represented in series B 
 (Plate 27), is considerably advanced beyond that last described. 
 The most important change which has been effected concerns 
 the ridges connecting the successive grooves. A lumen has 
 appeared in each of these, which seems to open at both ends 
 into the adjacent grooves. At the same time the cells, which 
 previously constituted the ridge, have become (except where 
 
IN THE EMBRYO CHICK. 621 
 
 they are continuous with the walls of the grooves) partially con- 
 stricted off from the germinal epithelium. The ridges, in fact, 
 now form ducts situated in the stroma of the ovarian ridge, in 
 the space between the Wolffian duct and the germinal epithe- 
 lium. The duct continuous with the last groove is somewhat 
 longer than before. In a general way, the head-kidney may now 
 be described as a duct opening into the body-cavity by three 
 groove-like apertures, and continuous behind with the rudiment 
 of the true Miillerian duct. Although the general constitution 
 of the head-kidney at this stage is fairly simple, there are a few 
 features in our sections which we do not fully understand, and 
 a few points about the organ which deserve a rather fuller 
 description than we have given in this general sketch. 
 
 The anterior groove (Nos. i 3, series B, PI. 27) is at first 
 somewhat separated from the Woiffian duct, but approaches 
 close to it in No. 3. In Nos. 2 and 3 there appears a rod-like 
 body on the outer side of the walls of the groove. In No. 2 
 this body is disconnected with the walls of the groove, and even 
 appears as if formed by a second invagination of the germinal 
 epithelium. In No. 3 this body becomes partially continuous 
 with the walls of the groove, and finally in No. 4 it becomes 
 completely continuous with the walls of the groove, and its 
 lumen communicates freely with the groove 1 . 
 
 The last trace of this body is seen on the upper wall of the 
 groove in No. 5. We believe that the body (rj represents the 
 ridge between the first and second grooves of the earlier stage ; 
 so that in passing from No. 3 to No. 5 we pass from the first to 
 the second groove. The meaning of the features of the body r t 
 in No. 2 we do not fully understand, but cannot regard them as 
 purely accidental, since we have met with more or less similar 
 features in other series of sections. The second groove becomes 
 gradually narrower, and finally is continued into the second ridge 
 (No. 8). The ridge contains a lumen, and is only connected 
 with the germinal epithelium by a narrow wall of cells. A 
 narrow passage from the body-cavity leads into that wall for a 
 short distance in No. 8, but it is probably merely the hinder end 
 of the groove of No. 7. The third groove appears in No. 11, 
 
 1 A deep focus of the rather thick section represented in No. 3 shewed the body 
 much more nearly in the position it occupies in No. 4. 
 
622 EXISTENCE OF A HEAD-KIDNEY 
 
 and opens into the lumen of the second ridge (r. 2 ) in No. 12. In 
 No. 13 the groove is closed, and there is present in its place 
 a duct (r s ) connected with the germinal epithelium by a wall of 
 cells. This duct is the further development of the third ridge 
 of the last stage ; its lumen opens into the body-cavity through 
 the third and last groove (gr^). In the next section this duct 
 (r 3 ) is entirely separated from the germinal epithelium, and it 
 may be traced backwards through several sections until it term- 
 inates by a solid point, very much as in the last stage. 
 
 In the figures of this series (B) there may be noticed on the 
 outer side of the Mullerian duct a fold of the germinal epithe- 
 lium (x) forming a second groove. It is especially conspicuous 
 in the first six sections of the series. This fold sometimes 
 becomes much deeper, and then forms a groove, the upper end 
 of which is close to the grooves of the head-kidney. It is very 
 often much deeper than these are, and without careful study 
 might easily be mistaken for one of these grooves. Fig. C, 
 taken from a series slightly younger than B, shews this groove 
 (x) in its most exaggerated form. 
 
 The stage we have just described is that of the fullest de- 
 velopment of the head-kidney. In it, as in all the previous 
 stages, there appear to be only three main openings into the 
 body-cavity ; but we have met in some of our sections with 
 indications of the possible presence of one or two extra rudi- 
 mentary grooves. 
 
 In an embryo not very much older than the one last de- 
 scribed the atrophy of the head-kidney is nearly completed, 
 and there is present but a single groove opening into the body- 
 cavity. 
 
 In series D (PL 28) are represented a number of sections 
 from an embryo at this stage. Nos. I and 2 are sections through 
 the hind end of the single groove now present. Its walls are 
 widely separated from the Wolffian duct in front, but approach 
 close to it at the hinder termination of the groove (No. 2). 
 The features of the single groove present at this stage agree 
 closely with those of the anterior groove of the previous stages. 
 The groove is continued into a duct the Mullerian duct (as it 
 may now be called, but in a previous stage the hollow ridge 
 connecting the first and second grooves of the head -kidney) 
 
IN THE EMBRYO CHICK. 623 
 
 which, after becoming nearly separated from the germinal 
 epithelium, is again connected to it by a mass of cells at two 
 points (Nos. 5, 6, and 8). The germinal epithelium is slightly 
 grooved and is much reduced in thickness at these points of 
 contact (gr^ and gr^), and we believe that they are the remnants 
 of the posterior grooves of the head-kidney present at an earlier 
 stage. 
 
 The Mullerian duct has by this stage grown much further 
 backwards, but the peculiarities of this part of it are treated in 
 a subsequent section. 
 
 We consider that, taking into account the rudiments we have 
 just described, as well as the fact that the features of the single 
 groove at this stage correspond with those of the anterior groove 
 at an earlier stage, we are fully justified in concluding that the 
 permanent abdominal opening of the Mullerian duct corresponds 
 with the anterior of our three grooves. 
 
 Although we have, on account of their indefiniteness, avoided 
 giving the ages of the chicks in which the successive changes of 
 the head-kidney may be observed, we may, perhaps, state that 
 all the changes we have described are usually completed between 
 the QOth and i2oth hour of incubation. 
 
 The Glomerulus of the Head-Kidney. 
 
 In connection with the head-kidney in Amphibians there is 
 present, as is well known, a peculiar vascular body usually de- 
 scribed as the glomerulus of the head-kidney. We have found 
 in the chick a body so completely answering to this glomerulus 
 that we have hardly any hesitation in identifying it as such. 
 
 In the chick the glomerulus is paired, and consists of a vas- 
 cular outgrowth or ridge projecting into the body-cavity on each 
 side at the root of the mesentery. It extends from the anterior 
 end of the Wolffian body to the point where the foremost open- 
 ing of the head-kidney commences. We have found it at a 
 period slightly earlier than that of the first development of the 
 head-kidney. It is represented in figs. E and F, PI. 28 gl, and is 
 seen to form a somewhat irregular projection into the body- 
 cavity, covered by a continuation of the peritoneal epithelium, 
 
624 EXISTENCE OF A HEAD-KIDNEY 
 
 and attached by a narrow stalk to the insertion of the embryonic 
 mesentery (me). 
 
 In the interior of this body is seen a stroma with numerous 
 vascular channels and blood corpuscles, and a vascular connec- 
 tion is apparently becoming established, if it is not so already, 
 between the glomerulus and the aorta. We have reason to 
 think that the corpuscles and vascular channels in the glome- 
 rulus are developed in situ. The stalk connecting the glome- 
 rulus with the attachment of the mesentery varies in thickness 
 in different sections, but we believe that the glomerulus is 
 continued unbroken throughout the very considerable region 
 through which it extends. This point is, however, difficult to 
 make sure of owing to the facility with which the glomerulus 
 breaks away. 
 
 At the stage we are describing, no true Malpighian bodies 
 are present in the part of the Wolffian body on the same level 
 with the anterior end of the glomerulus, but the Wolffian body 
 merely consists of the Wolffian duct. At the level of the pos- 
 terior part of the glomerulus this is no longer the case, but here 
 a regular series of primary Malpighian bodies is present (using 
 the term "primary" to denote the Malpighian bodies developed 
 directly out of part of the primary segmental tubes), and the 
 glomerulus of the head-kidney may frequently be seen in the 
 same section as a Malpighian body. In most sections the two 
 bodies appear quite disconnected, but in those sections in which 
 the glomerulus of the Malpighian body comes into view it is 
 seen to be derived from the same formation as the glomerulus 
 of the head-kidney (PI. 28, fig. F). It would seem, in fact, that 
 the vascular tissue of the glomerulus of the head-kidney grows 
 into the concavity of the Malpighian bodies. Owing to the 
 stage we are now describing, in which we have found the glome- 
 rulus most fully developed, being prior to that in which the 
 head-kidney appears, it is not possible to determine with cer- 
 tainty the position of the glomerulus in relation to the head- 
 kidney. After the development of the head-kidney it is found, 
 however, as we have already stated, that the glomerulus termi- 
 nates at a point just in front of the anterior opening of the 
 head-kidney. It is less developed than before, but is still pre- 
 sent up to the period of the atrophy of the head-kidney. It 
 
IN THE EMBRYO CHICK. 625 
 
 does not apparently alter in constitution, and we have not 
 thought it worth while giving any further representations of it 
 during the later stages of its existence. 
 
 Summary of the development of tJie head-kidney and glome- 
 rulus. The first rudiment of the head-kidney arises as three 
 successive grooves in the thickened germinal epithelium, con- 
 nected by ridges, and situated some way behind the front end 
 of the Wolffian duct. In the next stage the three ridges con- 
 necting the grooves have become more marked, and in each of 
 them a lumen has appeared, opening at both extremities into 
 the adjoining grooves. Still later the ridges become more or 
 less completely detached from the peritoneal epithelium, and 
 the whole head-kidney then consists of a slightly convoluted 
 duct, with, at the least, three peritoneal openings, which is pos- 
 teriorly continued into the Mullerian duct. Still later the head- 
 kidney atrophies, its two posterior openings vanishing, and its 
 anterior opening remaining as the permanent opening of the 
 Mullerian duct. The glomerulus arises as a vascular prominence 
 at the root of the mesentery, slightly prior in point of time to 
 the head-kidney, and slightly more forward than it in position. 
 We have not traced its atrophy. 
 
 We stated in our preliminary paper that the peculiar struc- 
 tures we had interpreted as the head-kidney had completely 
 escaped the attention of previous observers, though we called 
 attention to a well-known figure of Waldeyer's (copied in the 
 Elements of Embryology, fig. 51). In this figure a connection 
 between the germinal epithelium and the Mullerian duct is 
 drawn, which is probably part of the head-kidney, and may be 
 compared with our figures (Series B, No. 8, and Series D, No. 4). 
 Since we made the above statement, Dr Gasser has called .our 
 attention to a passage in his valuable memoir on " The Develop- 
 ment of the Allantois 1 ," in which certain structures are described 
 which are, perhaps, identical with our head-kidney. The fol- 
 lowing is a translation of the passage : 
 
 "In the upper region of M tiller's duct I have often observed 
 small canals, especially in the later stages of development, which 
 appear as a kind of doubling of the duct, and run for a short 
 
 1 Beitrdge zur Entwickclnngsgeschichte d. Allantois dcr Mitllcr'schen Gange H. dcs 
 Afters. Frankfurt, 1874. 
 
626 EXISTENCE OF A HEAD-KIDNEY 
 
 distance close to Miiller's duct and in the same direction, open- 
 ing, however, into the body-cavity posterior to the main duct. 
 Further, one may often observe diverticula from the extreme 
 anterior end of the oviduct of the bird, which form blind pouches 
 and give one the impression of being receptacula seminis. Both 
 these appearances can quite well be accounted for on the suppo- 
 sition that an abnormal communication is effected between the 
 germinal epithelium and Miiller's duct at unusual places ; or 
 else that an attempt at such a communication is made, resulting, 
 however, only in the formation of a diverticulum of the wall of 
 the oviduct." 
 
 The statement that these accessory canals are late in de- 
 veloping, prevents us from feeling quite confident that they 
 really correspond with our head-kidney. 
 
 Before passing on to the other parts of this paper it is neces- 
 sary to say a few words in justification of the comparison we 
 have made between the modified abdominal extremity of the 
 Mullerian duct in the chick and the head-kidney of the Ichthy- 
 opsida. 
 
 For the fullest statement of what is known with reference to 
 the anatomy and development of the head-kidney in the lower 
 types we may refer to Spengel and Furbringer 1 . We propose our- 
 selves merely giving a sufficient account of the head-kidney in 
 Amphibia (which appears to be the type in which the head- 
 kidney can be most advantageously compared with that in the 
 bird) to bring out the grounds for our determination of the 
 homologies. 
 
 The development of the head-kidney in Amphibia has been 
 fully elucidated by the researches of W. Miiller 2 , Gotte 3 , and 
 Fiirbringer 4 , while to the latter we are indebted for a knowledge 
 of the development of the Mullerian duct in Amphibians. The 
 first part of the urino-genital system to develop is the segmental 
 duct ( Vornieregang of Furbringer), which is formed by a groove- 
 like invagination of the peritoneal epithelium. It becomes con- 
 stricted into a duct first of all in the middle, but soon in the 
 
 1 Loc, cit. 
 
 2 Jenaische Zeitschrift, Vol. ix. 1875. 
 ' A Entwickelungsgeschichte d. Unkt. 
 
 4 Loc. at. 
 
IN THE EMBRYO CHICK. 627 
 
 posterior part also. It then forms a duct, ending in frorrhby a 
 groove in free communication with the body-cavity, and term- 
 inating blindly behind. The open groove in front at first 
 deepens, and then becomes partially constricted into a duct, 
 which elongates and becomes convoluted, but remains in com- 
 munication with the body-cavity by from two to four (according 
 to the species) separate openings. The manner in which the 
 primitive single opening is related to the secondary openings is 
 not fully understood. By these changes there is formed out of 
 the primitive groove an anterior glandular body, communicating 
 with the body-cavity by several apertures, and a posterior duct, 
 which carries off the secretion of the gland, and which, though 
 blind at first, eventually opens into the cloaca. In addition to 
 these parts there is also formed on each side of the mesentery, 
 opposite the peritoneal openings, a very vascular projection into 
 this part of the body-cavity, which is known as the glomerulus of 
 the head-kidney, and which very closely resembles in structure 
 and position the body to which we have assigned the same name 
 in the chick. 
 
 The primitive segmental duct is at first only the duct for 
 the head-kidney, but on the formation of the posterior parts of 
 the kidney (Wolffian body, &c.) it becomes the duct for these 
 also. 
 
 After the Wolffian bodies have attained to a considerable 
 development, the head-kidney undergoes atrophy, and its peri- 
 toneal openings become successively closed from before back- 
 wards. At this period the formation of the Miillerian duct takes 
 place. It is a solid constriction of the ventral or lateral wall of 
 the segmental duct, which subsequently becomes hollow, and 
 acquires an opening into the body-cavity quite independent of the 
 openings of the head- kidney. 
 
 The similarity in development and structure between the 
 head-kidney in Amphibia and the body we have identified as 
 such in Aves, is to our minds too striking to be denied. Both 
 consist of two parts (i) a somewhat convoluted longitudinal 
 canal, with a certain number of peritoneal openings; (2) a vascu- 
 lar prominence at the root of the mesentery, which forms a 
 glomerulus. As to the identity in position of the two organs we 
 hope to deal with that more fully in speaking of the general 
 
628 EXISTENCE OF A HEAD-KIDNEY 
 
 structure of the excretory system, but may say that one of 
 us 1 has already, on other grounds, attempted to shew that the 
 abdominal opening of the Mullerian duct in the bird is the 
 homologue of the abdominal opening of the segmental duct in 
 Amphibia, Elasmobranchii, &c., and that we believe that this 
 homology will be admitted by most anatomists. If this homo- 
 logy is admitted, the identity in position of this organ in Aves 
 and Amphibia necessarily follows. The most striking difference 
 between Aves and Amphibia in relation to these structures 
 is the fact that in Aves the anterior pore of the head-kidney 
 remains as the permanent opening of the Mullerian duct, while 
 in Amphibia, the pores of the head-kidney atrophy, and an 
 entirely fresh abdominal opening is formed for the Mullerian 
 duct. 
 
 II. 
 
 The Growth of the Mullerian Duct. 
 
 Although a great variety of views have been expressed by 
 different observers on the growth of the Mullerian duct, it is 
 now fairly generally admitted that it grows in the space between 
 a portion of the thickened germinal epithelium and the Wolffian 
 duct, but quite independently of both of them. Both Braun 
 and Egli, who have specially directed their attention to this 
 point, have for Reptilia and Mammalia fully confirmed the views 
 of previous observers. We were, nevertheless, induced, partly 
 on account of the a priori difficulties of this view, and partly by 
 certain peculiar appearances which we observed, to undertake 
 the re-examination of this point, and have found ourselves un- 
 able altogether to accept the general account. We propose first 
 describing, in as matter-of-fact a way as possible, the actual 
 observations we have made, and then stating what conclusions 
 we think may be drawn from these observations. 
 
 We have found it necessary to distinguish three stages in the 
 growth of the Mullerian duct. Our first stage embraces the 
 
 1 Balfour, "Origin and History of Urinogenital Organs of Vertebrates," Journal 
 of Anat. and Phys. Vol. x., and Monograph on Elasmobranch Fishes. [This edition 
 Nos. vn. and x.] 
 
IN THE EMBRYO CHICK. 629 
 
 period prior to the disappearance of the head-kidney. At this 
 stage the structure we have already spoken of as the rudiment 
 of the Mullerian duct consists of a solid rod of cells, continuous 
 with the third groove of the head-kidney. It extends through 
 a very few sections, and terminates by a fine point of about two 
 cells, wedged in between the Wolffian duct and germinal epithe- 
 lium (described above, Nos. 7 10, series A, Plate 27). 
 
 In an embryo slightly older than the above, such as that 
 from which series B was taken, but still belonging to our first 
 stage, a definite lumen appears in the anterior part of the 
 Mullerian duct, which vanishes after a few sections. The duct 
 terminates in a'point which lies in a concavity of the wall of the 
 Wolffian duct (Plate 27, Nos. I and 2, series G). The limits of 
 the Wolffian wall and the pointed termination of the Mullerian 
 duct are in many instances quite distinct ; but the outline of the 
 Wolffian duct appears to be carried round the Mullerian duct, 
 and in some instances the terminal point of the Mullerian duct 
 seems almost to form an integral part of the wall of the Wolffian 
 duct. 
 
 The second of our stages corresponds with that in which the 
 atrophy of the head-kidney is nearly complete (series D and H, 
 Plate 28). 
 
 The Mullerian duct has by this stage made a very marked 
 progress in its growth towards the cloaca, and, in contradistinc- 
 tion to the earlier stage, a lumen is now continued close up to 
 the terminal point of the duct. In the two or three sections 
 before it ends it appears as a distinct oval mass of cells (No. 10, 
 series D, and No. I, series H), without a lumen, lying between 
 and touching the external wall of the Wolffian duct on the one 
 hand, and the germinal epithelium on the other. It may either 
 lie on the ventral side of the Wolffian duct (series D), or on the 
 outer side (series H), but in either case is opposite the maximum 
 thickening of that part of the germinal epithelium which always 
 accompanies the Mullerian duct in its backward growth. 
 
 In the last section in which any trace of the Mullerian duct 
 can be made out (series D, No. 1 1, and series H, No. 2), it has no 
 longer an oval, well-defined contour, but appears to have com- 
 pletely fused with the wall of the Wolffian duct, which is accord- 
 ingly very thick, and occupies the space which in the previous 
 
630 EXISTENCE OF A HEAD-KIDNEY 
 
 section was filled by its own wall and the Miillerian duct. In 
 the following section the thickening in the wall of the Wolffian 
 duct has disappeared (Plate 28, series H, No. 3), and every trace 
 of the Miillerian duct has vanished from view. The Wolffian 
 duct is on one side in contact with the germinal epithelium. 
 
 The stage during which the condition above described lasts 
 is not of long duration, but is soon succeeded by our third stage, 
 in which a fresh mode of termination of the Mullerian duct is 
 found. (Plate 28, series I.) This last stage remains up to about 
 the close of the sixth day, beyond which our investigations do 
 not extend. 
 
 A typical series of sections through the terminal part of the 
 MUllerian duct at this stage presents the following features: 
 
 A few sections before its termination the Mullerian duct 
 appears as a well-defined oval duct lying in contact with the 
 wall of the Wolffian duct on the one hand and the germinal 
 epithelium on the other (series I, No. i). Gradually, however, 
 as we pass backwards, the Mullerian cluct dilates ; the external 
 wall of the Wolffian duct adjoining it becomes greatly thickened 
 and pushed in in its middle part, so as almost to touch the 
 opposite wall of the duct, and so form a bay in which the 
 Mullerian duct lies (Plate 28, series I, Nos. 2 and 3). As soon 
 as the Mullerian duct has come to lie in this bay its walls lose 
 their previous distinctness of outline, and the cells composing 
 them assume a curious vacuolated appearance. No well-defined 
 line of separation can any longer be traced between the walls of 
 the Wolffian duct and those of the MUllerian, but between the 
 two is a narrow clear space traversed by an irregular network of 
 fibres, in some of the meshes of which nuclei are present. 
 
 The Mullerian duct may be traced in this condition for a 
 considerable number of sections, the peculiar features above 
 described becoming more and more marked as its termination is 
 approached. It continues to dilate and attains a maximum size 
 in the section or so before it disappears. A lumen may be ob- 
 served in it up to its very end, but is usually irregular in outline 
 and frequently traversed by strands of protoplasm. The MUller- 
 ian duct finally terminates quite suddenly (Plate 28, series I, No. 
 4), and in the section immediately behind its termination the 
 Wolffian duct assumes its normal appearance, and the part of 
 
IN THE EMBRYO CHICK. 631 
 
 its outer wall on the level of the Mtillerian duct comes into con- 
 tact with the germinal epithelium (Plate 28, series I, No. 5). 
 
 We have traced the growing point of the Miillerian duct with 
 the above features till not far from the cloaca, but we have not 
 followed the last phases of its growth and its final opening into 
 the cloaca. 
 
 In some of our embryos we have noticed certain rather pecu- 
 liar structures, an example of which is represented at y in fig. K, 
 taken from an embryo of 123 hours, in which all traces of the 
 head-kidney had disappeared. It consists of a cord of cells, 
 connecting the Wolffian duct and the hind end of the abdominal 
 opening of the Miillerian duct. At the least one similar cord 
 was met with in the same embryo, situated just behind the 
 abdominal opening of the Miillerian duct. We have found simi- 
 lar structures in other embryos of about the same age, though 
 never so well marked as in the embryo from which fig. K is 
 taken. We have quite failed to make out the meaning, if any, 
 of them. 
 
 Our interpretation of the appearances we have described in 
 connection with the growth of the Miillerian duct can be stated 
 in a very few words. Our second stage, where the solid point 
 of the Miillerian duct terminates by fusing with the walls of the 
 Wolflfian duct, we interpret as meaning that the Miillerian is 
 growing backwards as a solid rod of cells, split off from the 
 outer wall of the Wolffian duct; in the same manner, in fact, as 
 in Amphibia and Elasmobranchii. The condition of the terminal 
 part of the Miillerian duct during our third stage cannot, we 
 think, be interpreted in the same way, but the peculiarities of the 
 cells of both Miillerian and Wolffian ducts, and the indistinctness 
 of the outlines between them, appear to indicate that the Miiller- 
 ian duct grows by cells passing from the Wolffian duct to it. In 
 fact, although in a certain sense the growth of the two ducts is 
 independent, yet the actual cells which assist in the growth of 
 the Mullerian duct are, we believe, derived from the walls of the 
 Wolffian duct. 
 
632 EXISTENCE OF A HEAD-KIDNEY 
 
 III. 
 
 General considerations. 
 
 The excretory system of a typical Vertebrate consists of the 
 following parts: 
 
 1. A head-kidney with the characters already described. 
 
 2. A duct for the head-kidney the segmental duct. 
 
 3. A posterior kidney (Wolffian body, permanent kidney, 
 &c. The nature and relation of these parts we leave out of con- 
 sideration, as they have no bearing upon our present investiga- 
 tions). The primitive duct for the Wolffian body is the segmental 
 duct. 
 
 4. The segmental duct may become split into (a) a dorsal 
 or inner duct, which serves as ureter (in the widest sense of the 
 word); and (ft) a ventral or outer duct, which has an opening 
 into the body-cavity, and serves as the generative duct for the 
 female, or for both sexes. 
 
 These parts exhibit considerable variations both in their 
 structure and development, into some of which it is necessary 
 for us to enter. 
 
 The head-kidney 1 attains to its highest development in the 
 Marsipobranchii (Myxine, Bdellostoma). It consists of a longi- 
 tudinal canal, from the ventral side of which numerous tubules 
 pass. These tubules, after considerable subdivision, open by a 
 large number of apertures into the pericardial cavity. From 
 the longitudinal canal a few dorsal diverticula, provided with 
 glomeruli, are given off. In the young the longitudinal canal is 
 continued into the segmental duct ; but this connection becomes 
 
 1 I am inclined to give up the view I formerly expressed with reference to the 
 head-kidney and segmental duct, viz. " that they were to be regarded as the most 
 anterior segmental tube, the peritoneal opening of which had become divided, and 
 which had become prolonged backwards so as to serve as the duct for the posterior 
 segmental tubes," and provisionally to accept the Gegenbaur-Fiirbringer view which 
 has been fully worked out and ably argued for by Fiirbringer (loc. cit. p. 96). 
 According to this view the head-kidney and its duct are to be looked on as the pri- 
 mitive and unsegmented part of the excretory system, more or less similar to the 
 excretory system of many Trematodes and unsegmented Vermes. The segmental 
 tubes I regard as a truly segmental part of the excretory system acquired subse- 
 quently. F. M. B. 
 
IN THE EMBRYO CHICK. 633 
 
 lost in the adult. The head-kid ne}^ remains, however, through 
 life. In Teleostei and Ganoidei (?) the head-kidney is generally 
 believed to remain through life, as the dilated cephalic portion of 
 the kidneys when such is present. In Petromyzon and Amphi- 
 bia the head-kidney atrophies. In Elasmobranchii the head- 
 kidney, so far as is known, is absent. 
 
 The development of the segmental duct and head-kidney 
 (when present) is still more important for our purpose than their 
 adult structure. 
 
 In Myxine the development of these structures is not known. 
 In Amphibia and Teleostei it takes place upon the same type, 
 viz. by the conversion of a groove-like invagination of the peri- 
 toneal epithelium into a canal open in front. The head-kidney 
 is developed from the anterior end of this canal, the opening of 
 which remains in Teleostei single and closes early in embryonic 
 life, but becomes in Amphibia divided into two, three, or four 
 openings. In Elasmobranchii the development is very different. 
 
 :< The first trace of the urinary system makes its appearance 
 as a knob springing from the intermediate cell-mass opposite the 
 fifth proto-vertebra. This knob is the rudiment of the abdominal 
 opening of the segmental duct, and from it there grows back- 
 wards to the level of the anus a solid column of cells, which 
 constitutes the rudiment of the segmental duct itself. The knob 
 projects towards the epiblast, and the column connected with it 
 lies between the mesoblast and epiblast. The knob and column 
 do not long remain solid, but the former acquires an opening 
 into the body-cavity continuous with a lumen, which makes its 
 appearance in the latter." 
 
 The difference in the development of the segmental duct in 
 the two types (Amphibia and Elasmobranchii) is very im- 
 portant. In the one case a continuous groove of the peritoneal 
 epithelium becomes constricted into a canal, in the other a solid 
 knob of cells is continued into a rod, at first solid, which grows 
 backwards without any apparent relation to the peritoneal epi- 
 thelium 1 . 
 
 1 In a note on p. 50 of his memoir Fiirbringer criticises my description of the 
 mode of growth of the segmental duct. The following is a free translation of what 
 he says : "In Halfour's, as in other descriptions, an account is given of a backward 
 
 B. A\ 
 
634 EXISTENCE OF A HEAD-KIDNEY 
 
 The abdominal aperture of the segmental duct in Elasmo- 
 branchii, in that it becomes the permanent abdominal opening 
 of the oviduct, corresponds physiologically rather with the 
 abdominal opening of the Miillerian duct than with that of the 
 segmental duct of Amphibia, which, after becoming divided up 
 to form the pores of the head-kidney, undergoes atrophy. Mor- 
 phologically, however, it appears to correspond with the opening 
 of the segmental duct in Amphibia. We shall allude to this 
 point more than once again, and give our grounds for the above 
 view on p. 640. 
 
 The development of the segmental duct in Elasmobranchii 
 as a solid rod is, we hope to shew, of special importance for the 
 elucidation of the excretory system of Aves. 
 
 The development of these parts of Petromyzon is not fully 
 known, but from W. Miiller's account (Jcnaische Zeitschrift, 
 1875) it would seem that an anterior invagination of the peri- 
 toneal epithelium is continued backwards as a duct (segmental 
 duct), and that the anterior opening subsequently becomes 
 divided up into the various apertures of the head-kidney. If 
 this account is correct, Petromyzon presents a type intermediate 
 between Amphibia and Elasmobranchii. In certain types, viz. 
 Marsipobranchii and Teleostei, the segmental duct becomes the 
 duct for the posterior kidney (segmental tubes), but otherwise 
 undergoes no further differentiation. In the majority of types, 
 
 growth, which easily leads to the supposition of a structure formed anteriorly forcing 
 its way through the tissues behind. This is, however, not the case, since, to my 
 knowledge, no author has ever detected a sharp boundary between the growing point 
 of the segmental duct (or Miillerian duct) and the surrounding tissues." He goes on 
 to say that " the growth in these cases really takes place by a differentiation of tissue 
 along a line in the region of the peritoneal cavity." Although I fully admit that it 
 would be far easier to homologise the development of the segmental duct in Amphibia 
 and Elasmobranchii according to this view, I must nevertheless vindicate the accuracy 
 of my original account. I have looked over my specimens again, since the appear- 
 ance of Dr Furbringer's paper, and can find no evidence of the end of the duct 
 becoming continuous with the adjoining mesoblastic tissues. In the section, before 
 its disappearance, the segmental duct may, so far as I can make out, be seen as a 
 very small but distinct rod, which is much more closely connected with the epiblast 
 than with any other layer. From Gasser's observations on the Wolffian duct in the 
 bird, I am led to conclude that it behaves in the same way as the segmental duct in 
 the Elasmobranchii. I will not deny that it is possible that the growth of the duct 
 takes place by wandering cells, but on this point I have no evidence, and must there- 
 fore leave the question an open one. F. M. B. 
 
IN THE EMBRYO CHICK. 635 
 
 however, the case is different. In Amphibia 1 , as has already 
 been mentioned, a solid rod of cells is split off from its ventral 
 wall, which afterwards becomes hollow, acquires an opening into 
 the body-cavity, and forms the Mullerian duct. 
 
 In Elasmobranchii the segmental duct undergoes a more or 
 less similar division. " It becomes longitudinally split into two 
 complete ducts in the female, and one complete duct and parts 
 of a second in the male. The resulting ducts are (i) the Wolf- 
 fian duct dorsally, which remains continuous with the excretory 
 tubules of the kidney, and ventrally (2) the oviduct or Mullerian 
 duct in the female, and the rudiments of this duct in the male. 
 In the female the formation of these ducts takes place by a 
 nearly solid rod of cells, being gradually split off from the ventral 
 side of all but the foremost part of the original segmental' duct, 
 with the short undivided anterior part of which duct it is con- 
 tinuous in front. Into it a very small portion of the lumen of 
 the original segmental duct is perhaps continued. The re- 
 mainder of the segmental duct (after the loss of its anterior 
 section and the part split off from its ventral side) forms the 
 Wolffian duct. The process of formation of the ducts in the 
 male chiefly differs from that in the female, in the fact of the 
 anterior undivided part of the segmental duct, which forms the 
 front end of the Mullerian duct, being shorter, and in the column 
 of cells with which it is continuous being from the first incom- 
 plete." 
 
 It will be seen from the above that the Mullerian duct con- 
 sists of two distinct parts an anterior part with the abdominal 
 opening, and a posterior part split off from the segmental duct. 
 This double constitution of the Mullerian duct is of great im- 
 portance for a proper understanding of what takes place in the 
 Bird. 
 
 The Mullerian duct appears therefore to develop in nearly 
 the same manner in the Amphibian and Elasmobranch type, as 
 a solid or nearly solid rod split off from the ventral wall of the 
 segmental duct. But there is one important difference concern- 
 ing the abdominal opening of the duct. In Amphibia this is 
 a new formation, but in Elasmobranchii it is the original opening 
 of the segmental duct. Although we admit that in a large 
 
 1 Fiirbringer, loc. fit. 
 
 412 
 
636 EXISTENCE OF A HEAD-KIDNEY 
 
 number of points, including the presence of a head-kidney, the 
 urino-genital organs of Amphibia are formed on a lower type 
 than those of the Elasmobranchii, yet it appears to us that this 
 does not hold good for the development of the Miillerian duct. 
 
 The above description will, we trust, be sufficient to render 
 clear our views upon the development of the excretory system 
 in Aves. 
 
 In the bird the excretory system consists of the following 
 parts (using the ordinary nomenclature) which are developed in 
 the order below. 
 
 I. Wolffian duct. 2. Wolffian body. 3. Head-kidney. 4. 
 Miillerian duct. 5. Permanent kidney and ureter. 
 
 About 2 and 5 we shall have nothing to say in the sequel. 
 
 We have already in the early part of the paper given an 
 account of the head-kidney and Miillerian duct, but it will 
 be necessary for us to say a few words about the development 
 of the Wolffian duct (so called). Without entering into the 
 somewhat extended literature on the subject, we may state that 
 we consider that the recent paper of Dr Gasser 1 supplies us with 
 the best extant account of the development of the Wolffian duct. 
 
 The first trace of it, which he finds, is visible in an embryo 
 with eight proto-vertebrae as a slight projection from the inter- 
 mediate cell mass towards the epiblast in the region of the three 
 hindermost proto-vertebrse. In the next stage, with eleven 
 proto-vertebrae, the solid rudiment of the duct extends from 
 the fifth to the eleventh proto-vertebra, from the eighth to the 
 eleventh proto-vertebra it lies between the epiblast and meso- 
 blast, and is quite distinct from both, and Dr Gasser distinctly 
 states that in its growth backwards from the eighth proto- 
 vertebra the Wolffian duct never comes into continuity with the 
 adjacent layers. 
 
 In the region of the fifth proto-vertebra, where the duct was 
 originally continuous with the mesoblast, it has now become 
 free, but is still attached in the region of the sixth and to the 
 eighth proto-vertebra. In an embryo with fourteen proto-ver- 
 tebrae the duct extends from the fourth to the fourteenth proto- 
 vertebra, and is now free between epiblast and mesoblast for its 
 whole extent. It is still for the most part solid though perhaps 
 
 1 Arch, filr Mic. Anat. Vol. Xiv. 
 
IN THE EMBRYO CHICK. 637 
 
 a small lumen is present in its middle part In the succeeding 
 stages the lumen of the duct gradually extends backwards 
 and forwards, the duct itself also passes inwards till it acquires 
 its final position close to the peritoneal epithelium ; at the same 
 time its hind end elongates till it comes into connection with 
 the cloacal section of the hind-gut. It should be noted that the 
 duct in its backward growth does not appear to come into con- 
 tinuity with the subjacent mesoblast, but behaves in this respect 
 exactly as does the segmental duct in Elasmobranchii (vide note 
 on p. 634). 
 
 The question which we propose to ourselves is the follow- 
 ing : What are the homologies of the parts of the Avian urino- 
 genital system above enumerated ? The Wolffian duct appears 
 to us morphologically to correspond in part to the segmental 
 duct 1 , or what Furbringer would call the duct of the head-kidney. 
 This may seem a paradox, since in birds it never comes into 
 relation with the head-kidney. Nevertheless we consider that 
 this homology is morphologically established, for the following 
 reasons : 
 
 (1) That the Wolffian duct gives rise (vide supra, p. 631) to 
 the Mullerian duct as well as to the duct of the Wolffian body. 
 In this respect it behaves precisely as does the segmental duct 
 of Elasmobranchii and Amphibia. That it serves as the duct for 
 the Wolffian body, before the Mullerian duct originates from it, 
 is also in accordance with what takes place in other types. 
 
 (2) That it develops in a strikingly similar manner to the 
 segmental duct of Elasmobranchii. 
 
 We stated expressly that the Wolffian duct corresponded 
 only in part to the segmental duct. It does not, in fact, in our 
 opinion, correspond to the whole segmental duct, but to the 
 segmental duct minus the anterior abdominal opening in Elas- 
 mobranchii, which becomes the head-kidney in other types. In 
 fact, we suppose that the segmental duct and head-kidney, which 
 
 1 The views here expressed about the Wolffian duct are nearly though not exactly 
 those which one of us previously put forward (" Urinogenital Organs of Vertebrates," 
 &c., pp. 45 46) [This edition, pp. 164, 165], and with which Fiirb ringer appears exactly 
 to agree. Possibly Dr Furbringer would alter his view on this point were he to accept 
 the facts we believe ourselves to have discovered. Semper's view also differs from 
 ours, in that he believes the Wolffian duct to correspond in its entirety with the 
 segmental duct. 
 
638 EXISTENCE OF A HEAD-KIDNEY 
 
 in the Ichthyopsida develop as a single formation, develop in the 
 Bird as two distinct structures one of these known as the 
 Wolffian duct, and the other the head-kidney. If our view about 
 the head-kidney is accepted the above position will hardly 
 require to be disputed, but we may point out that the only 
 feature in which the Wolffian duct of the Bird differs in de- 
 velopment from the segmental duct of Elasmobranchii is in 
 the absence of the knob, which forms the commencement of the 
 segmental duct, and in which the abdominal opening is formed ; 
 so that the comparison of the development of the duct in the two 
 types confirms the view arrived at from other considerations. 
 
 The head-kidney and Mullerian duct in the Bird must be 
 considered together. The parts which they eventually give rise 
 to after the atrophy of the head-kidney have almost universally 
 been regarded as equivalent to the Mullerian duct of the Ichthy- 
 opsida. By Braun 1 , however, who from his researches on the 
 Lizard satisfied himself of the entire independence of the Mul- 
 lerian and Wolffian ducts in the Amniota, the Mullerian duct of 
 these forms is regarded as a completely new structure with no 
 genetic relations to the Mullerian duct of the Ichthyopsida. 
 Semper 2 , on the other hand, though he accepts the homology 
 of the Mullerian duct in the Ichthyopsida and Amniota, is of 
 opinion that the anterior part of the Mullerian duct in the 
 Amniota is really derived from the Wolfifian duct, though he 
 apparently admits the independent growth of the posterior part 
 of the Mullerian duct. We have been led by our observations, 
 as well as by our theoretical deductions, to adopt a view exactly 
 the reverse of that of Professor Semper. We believe that the 
 anterior part of the Mullerian duct of Aves, which is at first the 
 head-kidney, and subsequently becomes the abdominal opening 
 of the duct, is developed from the peritoneal epithelium inde- 
 pendently of all other parts of the excretory system ; but that 
 the posterior part of the duct is more or less completely derived 
 from the walls of the Wolffian duct. This view is clearly in 
 accordance with our account of the facts of development in Aves, 
 and it fits in very well with the development of the Mullerian 
 
 1 " Urogcnital-System d. Reptilien," .-/;/>. cms d. zool.-zoot, Inst. Wiirzburg, 
 Vol. iv. 
 
 2 Lot: fit.. 
 
IN THE EMBRYO CHICK. 639 
 
 duct in Elasmobranchii. We have already pointed out that jn 
 Elasmobranchii the Mullerian duct is formed of two factors 
 (i) of the whole anterior extremity of the segmental duct, in- 
 cluding its abdominal opening ; (2) of a rod split off from the 
 ventral side of the segmental duct. In Birds the anterior part 
 (corresponding to factor No. i) of the Mullerian duct has a 
 different origin from the remainder ; so that if the development 
 of the posterior part of the duct (factor No. 2) were to proceed 
 in the same manner in Birds and Elasmobranchii, it ought to be 
 formed at the expense of the Wolffian (i.e. segmental) duct, 
 though in connection anteriorly with the head-kidney. And 
 this is what actually appears to take place. 
 
 So far the homologies of the avian excretory system are 
 fairly clear; but there are still some points which have to be 
 dealt with in connection with the permanent opening of the 
 Mullerian duct, and the relatively posterior position of the head- 
 kidney. With reference to the first of these points the facts of 
 the case are the following : 
 
 In Amphibia the permanent opening of the Mullerian duct 
 is formed as an independent opening after the atrophy of the 
 head-kidney. 
 
 In Elasmobranchii the original opening of the segmental 
 duct forms the permanent opening of the Mullerian duct and no 
 head-kidney appears to be formed. 
 
 In Birds the anterior of the three openings of the head-kidney 
 remains as the permanent opening of the Mullerian duct. 
 
 With reference to the difficulties involved in there being 
 apparently three different modes in which the permanent opening 
 of the Mullerian duct is formed, we would suggest the following 
 considerations: 
 
 The history of the development of the excretory system 
 teaches us that primitively the segmental duct must have served 
 as efferent duct both for the generative products and kidney 
 secretion (just as the Wolffian duct still does for the testicular 
 products and secretion of the Wolffian body in Elasmobranchii 
 and Amphibia) ; and further, that at first the generative products 
 entered the segmental duct from the abdominal cavity by one 
 or more of the abdominal openings of the kidney (almost cer- 
 tainly of the head-kidney). That the generative products did 
 
640 EXISTENCE OF A HEAD-KIDNEY 
 
 not enter the segmental duct at first by an opening specially 
 developed for them appears to us to follow from Dohrn's princi- 
 ple of the transmutation of function (FunctionswecJisel}. As a 
 consequence (by a process of natural selection) of the segmental 
 duct having both a generative and a urinary function, a further 
 differentiation took place, by which that duct became split into 
 two a ventral Miillerian duct and dorsal Wolffian duct. 
 
 The Miillerian duct without doubt was continuous with the 
 head-kidney, and so with the abdominal opening or openings of 
 the head-kidney which served as generative pores. At first the 
 segmental duct was probably split longitudinally into two equal 
 portions, but the generative function of the Miillerian duct gra- 
 dually impressed itself more and more upon the embryonic 
 development, so that, in the course of time, the Mullerian duct 
 developed less and less at the expense of the Wolffian duct. 
 This process appears partly to have taken place in Elasmo- 
 branchii, and still more in Amphibia ; the Amphibia offering in 
 this respect a less primitive condition than Elasmobranchii ; 
 while in Aves it has been carried even further. The abdominal 
 opening no doubt also became specialised. At first it is quite 
 possible that more than one abdominal pore may have served for 
 the generative products ; one of which, no doubt, eventually came 
 to function alone. In Amphibia the specialisation of the open- 
 ing appears to have gone so far that it no longer has any 
 relation to the head-kidney, and even develops after the atrophy 
 of the head-kidney. In Elasmobranchii, on the other hand, the 
 functional opening appears at a period when we should expect 
 the head-kidney to develop. This state is very possibly the 
 result of a differentiation (along a different line to that in Am- 
 phibia) by which the head-kidney gradually ceased to become 
 developed, but by which the primitive opening (which in the 
 development of the head-kidney used to be divided into several 
 pores leading into the body-cavity) remained undivided and 
 served as the abdominal aperture of the Mullerian duct. Aves,. 
 finally, appear to have become differentiated along a third line ; 
 since in their ancestors the anterior pore of the head-kidney 
 appears to have become specialised as the permanent opening 
 of the Mullerian duct. 
 
 With reference to the posterior position of the head-kidney 
 
IN THE EMBRYO CHICK. 641 
 
 in Aves we have only to remark, that a change in position of 
 the head-kidney might easily take place after it acquired an 
 independent development. The fact that it is slightly behind 
 the glomerulus would seem to indicate, on the one hand, that it 
 has already ceased to be of any functional importance ; and, on 
 the other, that the shifting has been due to its having a connec- 
 tion with the Mullerian duct. 
 
 We have made a few observations on the development of the 
 Mullerian duct in Lacerta muralis, which have unfortunately 
 led us to no decided conclusions. In a fairly young stage in 
 the development of the Miillerian duct (the youngest we have 
 met with), no trace of a head-kidney could be observed, but the 
 character of the abdominal opening of the Mullerian duct Was 
 very similar to that figured by Braun 1 . As to the backward 
 growth of the Mullerian duct, we can only state that the solid 
 point of the duct in the young stages is in contact with the 
 wall of the Wolffian duct, and the relation between the two is 
 rather like that figured by Fiirbringer (PI. I, figs. 14 15) in 
 Amphibia. 
 
 DESCRIPTION OF PLATES 27 AND 28. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 ao. Aorta, c v. Cardinal vein. gl. Glomerulus. gr r First groove of head- 
 kidney, gr^ Second groove of head-kidney. gr y Third groove of head-kidney. 
 ge. Germinal epithelium, mrb. Malpighian body. me. Mesentery, md. Mullerian 
 duct. r r First ridge of head-kidney, r^. Second ridge of head-kidney. r y Third 
 ridge of head-kidney. W ' d. Wolffian duct. x. Fold in germinal epithelium. 
 
 PLATE 27. 
 
 SERIES A. Sections through the head-kidney at our second stage. Zeiss 2, ocul. 
 3 (reduced one-third). The second and third grooves are represented with the ridge 
 connecting them, and the rod of cells running backwards for a short distance. 
 
 No. r. Section through the second groove. 
 
 No. 2. Section through the ridge connecting the second and third grooves. 
 No. 3. Section passing through the same ridge at a point nearer the third groove. 
 Nos. 4, 5, 6. Sections through the third groove. 
 
 No. 7. Section through the point where the third groove passes into the solid 
 rod of cells. 
 
 1 /<><., //. 
 
642 EXISTENCE OF HEAD-KIDNEY IN EMBRYO-CHICK. 
 
 No. 8. Section through the rod when quite separated from the germinal epi- 
 thelium. 
 
 No. 9. Section very near the termination of the rod. 
 
 No. 10. Last section in which any trace of the rod is seen. 
 
 SERIES B. Sections passing through the head-kidney at our third stage. Zeiss C, 
 ocul. 2. Our figures are representations of the following sections of the series, section 
 i being the first which passes through the anterior groove of the head-kidney. 
 
 No. i SECTION 
 
 2 
 
 3 
 
 4 
 
 5 
 
 3- 
 4- 
 5- 
 6. 
 8. 
 
 IO. 
 
 n. 
 
 No. 8 SECTION 13. 
 
 15- 
 16. 
 
 17- 
 18. 
 19. 
 20. 
 
 The Miillerian duct extends through eleven more sections. 
 The first groove (gr^.) extends to No. 3. 
 The second groove (gr^.) extends from No. 4 to No, 7. 
 The third groove (gr 3 .) extends from No. 11 to No. 13. 
 The first ridge (r r ) extends from No. 2 to No. 5. 
 The second ridge (r 2 .) extends from No. 8 to No. n. 
 
 The third ridge (r y ) extends from No. 13 backwards through twelve sections, 
 when it terminates by a pointed extremity. 
 
 FlG. C. Section through the ridge connecting the second and third grooves of 
 the head-kidney of an embryo slightly younger than that from which Series B was 
 taken. Zeiss C, ocul. 3 (reduced one-third). 
 
 The fold of the germinal epithelium, which gives rise to a deep groove (x.) 
 external to the head-kidney is well marked. 
 
 SERIES G. Sections through the rod of cells constituting the termination of the 
 Miillerian duct at a stage in which the head-kidney is still present. Zeiss C, ocul. 2. 
 
 PLATE 28. 
 
 SERIES D. Sections chosen at intervals from a complete series traversing the 
 peritoneal opening of the Miillerian duct, the remnant of the head-kidney, and the 
 termination of the Miillerian duct. Zeiss C, ocul. 3 (reduced one-third). 
 
 Nos. i and 2. Sections through the persistent anterior opening of the head- 
 kidney (abdominal opening of Miillerian duct). The approach of the Wolffian duct 
 to the groove may be seen by a comparison of these two figures. In the sections in 
 front of these (not figured) the two are much more widely separated than in No. i. 
 
 No. 3. Section through the Miillerian duct, just posterior to the persistent 
 opening. 
 
 Nos. 4 and 5. Remains of the ridges, which at an earlier stage connected the 
 first and second grooves, are seen passing from the Miillerian duct to the peritoneal 
 epithelium. 
 
 No. 6. Rudiment of the second groove (gr z .) of the head-kidney. 
 
 Between 6 and 7 is a considerable interval. 
 
 No. 7. All traces of this groove (gr v ) have vanished, and the Miillerian duct is 
 quite disconnected from the epithelium. 
 
DESCRIPTION OF PLATES 27 AND 28. 643 
 
 No. 8. Rudiment of the third groove (gr a .). 
 
 No. 9. Miillerian duct quite free in the space between the peritoneal epithelium 
 and the Wolffian duct, in which condition it extends until near its termination. 
 
 Between Nos. 9 and 10 is an interval of eight sections. 
 
 No. 10. The penultimate section, in which the Miillerian duct is seen. A lumen 
 cannot be clearly made out. 
 
 No. n. The last section in which any trace of the Miillerian duct is visible. No 
 line of demarcation can be seen separating the solid end of the Miillerian duct from 
 the ventral wall of the Wolffian duct. 
 
 FIGS. E. and F. Sections through the glomerulus of the head-kidney from an 
 embryo prior to the appearance of the head-kidney. Zeiss B, ocul. i. A comparison 
 of the two figures shows the variation in the thickness of the stalk of the glomerulus. 
 E. Section anterior to the foremost Malpighian body. F. Section through both the 
 glomerulus of the head-kidney and that of a Malpighian body. The two are seen to 
 be connected. 
 
 SERIES H. Consecutive sections through the hind end of the Miillerian duct, 
 from an embryo in which the head-kidney was only represented by a rudiment. (The 
 embryo was, perhaps, very slightly older than that from which Series D was taken.) 
 Zeiss C, ocul. 3 (reduced one-third). 
 
 No. i. Miillerian duct is without a lumen, and quite distinct from the Wolffian 
 wall. 
 
 No. 2. The solid end of the Miillerian duct is no longer distinct from the internal 
 wall of the Wolffian duct. 
 
 No. 3. All trace of the Miillerian duct has vanished. 
 
 SERIES i. Sections through the hinder end of the Miillerian duct from an embryo 
 of about the middle of the sixth day. Zeiss C, ocul. 2 (reduced one-third). 
 
 No. i. The Miillerian duct is distinct and small. 
 
 No. 2. Is posterior by twelve sections to No. i. The Miillerian duct is dilated, 
 and its cells are vacuolated. 
 
 No. 3. Penultimate section, in which the Miillerian duct is visible ; it is separated 
 by three sections from No. 2. 
 
 No. 4. Last section in which any trace of the Miillerian duct is visible ; the 
 lumen, which was visible in the previous section, is now absent. 
 
 No. 5. No trace of Miillerian duct. Nos. 3, 4, and 5 are consecutive sections. 
 
 FIG. K. Section through the hind end of the abdominal opening of the Miillerian 
 duct of a chick of 123 hours. Zeiss C, ocul. i (reduced one-third). It illustrates the 
 peculiar cord connecting the Miillerian and Wolffian ducts. 
 
XIV. ON THE EARLY DEVELOPMENT OF THE LACERTILIA, 
 TOGETHER WITH SOME OBSERVATIONS ON THE NATURE 
 AND RELATIONS OF THE PRIMITIVE STREAK*. 
 
 (With Plate 29.) 
 
 TILL quite recently no observations were recorded on the 
 early developmental changes of the reptilian ovum. Not long 
 ago Professors Kupffer and Benecke published a preliminary 
 note on the early development of Lacerta agilis and Emys 
 Europea?. I have myself also been able to make some observa- 
 tions on the embryo of Lacerta mnralis. The number of my 
 embryos has been, somewhat limited, and most of those which I 
 have had have been preserved in bichromate of potash, which 
 has turned out a far from satisfactory hardening reagent In 
 spite of these difficulties I have been led on some points to very 
 different results from those of the German investigators, and to 
 results which are more in accordance with what we know of 
 other Sauropsidan types. I commence with a short account of 
 the results of Kupffer and Benecke. 
 
 Segmentation takes place exactly as in birds, and the result- 
 ing blastoderm, which is thickened at its edge, spreads rapidly 
 over the yolk. Shortly before the yolk is half enclosed a small 
 embryonic shield (area pellucida) makes its appearance in the 
 centre of the blastoderm, which has, in the meantime, become 
 divided into two layers. The upper of these is the epiblast, and 
 the lower the hypoblast. The embryonic shield is mainly dis- 
 tinguished from the remainder of the blastoderm by the more 
 columnar character of its constituent epiblast cells. It is some- 
 what pyriform in shape, the narrower end corresponding with 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xix. 1879. 
 z Die Erste Enlwickluiigsvorgd/igc am Eider Rcplilien, Konigsberg, 1878. 
 
EARLY DEVELOPMENT OF THE LACERTILIA. 645 
 
 the future posterior end of the embryo. At the narrow end an 
 invagination takes place, which gives rise to an open sac, the 
 blind end of which is directed forwards. The opening of this 
 sac is regarded by the authors as the blastopore. A linear 
 thickening of epiblast arises in front of the blastopore, along 
 the median line of which the medullary groove soon appears. 
 In the caudal region the medullary folds spread out and enclose 
 between them the blastopore, behind which they soon meet 
 again. On the conversion of the medullary groove into a closed 
 canal the blastopore becomes obliterated. The mesoblast grows 
 out from the lip of the blastopore as four masses. Two of these 
 are lateral: a third is anterior and median, and, although at first 
 independent of the epiblast, soon attaches itself to it, and forms 
 with it a kind of axis-cord. A fourth mass applied itself to the 
 walls of the sac formed by invagination. 
 
 With reference to the very first developmental phenomena 
 my observations are confined to two stages during the segmenta- 
 tion 1 . In the earliest of these the segmentation was about half 
 completed, in the later one it was nearly over. My observations 
 on these stages bear out generally the statements of Kupffer and 
 Benecke. In the second of them the blastoderm was already 
 imperfectly divided into two layers a superficial epiblastic layer 
 formed of a single row of cells, and a layer below this several 
 rows deep. Below this layer fresh segments were obviously 
 being added to the blastoderm from the subjacent yolk. 
 
 Between the second of these blastoderms and my next stage 
 there is a considerable gap. The medullary plate is just estab- 
 lished, and is marked by a shallow groove which becomes deeper 
 in front. A section through the embryo is represented in PL 29, 
 Series A, fig. I. In this figure there may be seen the thickened 
 medullary plate with a shallow medullary groove, below which 
 are two independent plates of mesoblast (me. p.}, one on each 
 side of the middle line, very imperfectly divided into somato- 
 pleuric and splanchnopleuric layers. Below the mesoblast is a 
 continuous layer of hypoblast (/ty.\ which -develops a rod-like 
 thickening along the axial line (c/i.}. This rod becomes in the 
 next stage the notochord. Although this embryo is not well 
 
 1 For these two specimens, which were hardened in picric acid, I am indebted to 
 Dr Kleinenberg. 
 
646 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 preserved I feel very confident in asserting the continuity of the 
 notochord with the hypoblast at this stage. 
 
 At the hind end of the embryo is placed a thickened ridge of 
 tissue which continues the embryonic axis. In this ridge all the 
 layers coalesce, and I therefore take it to be equivalent to the 
 primitive streak of the avian blastoderm. It is somewhat triangu- 
 lar in shape, with the apex directed backward, the broad base 
 placed in front. 
 
 At the junction between the primitive streak and the blasto- 
 derm is situated a passage, open at both extremities, leading 
 from the upper surface of the blastoderm obliquely forwards to 
 the lower. 
 
 The dorsal and anterior wall of this passage is formed of a 
 distinct epithelial layer, continuous at its upper extremity with 
 the epiblast, and at its lower with the notochordal plate, so that 
 it forms a layer of cells connecting together the epiblast and 
 hypoblast. The hinder and lower wall of the passage is formed 
 by the cells of the primitive streak, which only assume a colum- 
 nar form near the dorsal opening of the passage (vide fig. 4). 
 This passage is clearly the blind sac of Kupffer and Benecke, 
 who, if I am not mistaken, have overlooked its lower opening. 
 As I hope to show in the sequel, it is also the equivalent of the 
 neurenteric passage, which connects the neural and alimentary 
 canals in the Ichthyopsida, and therefore represents the blasto- 
 pore of Amphioxus, Amphibians, &c. 
 
 Series A, figs. 2, 3, 4, 5, illustrate the features of the passage 
 and its relation to the embryo. 
 
 Fig. 2 passes through the ventral opening of the passage. 
 The notochordal plate (cJi^] is vaulted over the opening, and on 
 the left side is continuous with the mesoblast as well as the 
 hypoblast. Figs. 3 and 4 are taken through the middle part of 
 the passage (ne.), which is bounded above by a continuation of 
 the notochordal plate, and below by the tissue of the primitive 
 streak. The hypoblast (/y.)> in the middle line, is imperfectly 
 fused with the mesoblast of the primitive streak, which is now 
 continuous across the middle line. The medullary groove has 
 disappeared, but the medullary plate (in p.) is quite distinct. 
 
 In fig. 5 is seen the dorsal opening of the passage (tie.). If 
 a section behind this had been figured, as is done for the next 
 
EARLY DEVELOPMENT OF THE LACERTILIA. 647 
 
 series (B), it would have passed through the primitive streak, 
 and, as in the chick, all the layers would have been fused to- 
 gether. The epiblast in the primitive streak completely coales- 
 ces with the mesoblast; but the hypoblast, though attached to 
 the other layers in the middle line,can always be traced as a 
 distinct stratum. 
 
 Fig. B is a surface view of my next oldest embryo. The 
 medullary groove has become much deeper, especially in front. 
 Behind it widens out to form a space equivalent to the sinus 
 rhomboidalis of the embryo bird. The amnion forms a small 
 fold covering over the cephalic extremity of the embryo, which 
 is deeply embedded in the yolk. Some somites (protovertebrae) 
 were probably present, but this could not be made out in the 
 opaque embryo. 
 
 The woodcut (fig. i) represents a diagrammatic longitudinal 
 section through this embryo, and the sections belonging to 
 
 FIG. i. Diagrammatic longitudinal section of an embryo of Lacerta. pp. Body 
 cavity, am. Amnion. ne. Neurenteric canal, ch. Notochord. hy. Hypoblast. 
 ep. Epiblast. pr. Primitive streak. 
 
 Series B illustrate the features of the hind end of the embryo 
 and of the primitive streak. 
 
 As is shown in fig. i, the notochord (c/i.) has now throughout 
 the region of the embryo become separated from the subjacent 
 hypoblast, and the lateral plates of mesoblast are distinctly 
 divided into somatic and splanchnic layers. The medullary 
 groove is continued as a deepish groove up to the opening of the 
 neurenteric passage, which thus forms a perforation in the floor 
 of the hinder end of the medullary groove (vide Series B, figs. 2, 
 3, and 4). 
 
 The passage itself is somewhat shorter than in the previous 
 stage, and the whole of it is shown in a single section (fig. 4). 
 This section must either have been taken somewhat obliquely, 
 
648 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 or else the passage have been exceptionally short in this embryo, 
 since in an older embryo it could not all be seen in one section. 
 
 The front wall of the passage is continuous with the noto- 
 chord, which for two sections or so in front remains attached to 
 the hypoblast (figs. 2 and 3). Behind the perforation in the floor 
 of the medullary groove is placed the primitive streak (fig. 5), 
 where all the layers become fused together, as in the earlier 
 stage. Into this part a narrow diverticulum from the end of the 
 medullary groove is continued for a very short distance (vide 
 fig. 5, me.). 
 
 The general features of the stage will best be understood by 
 an examination of the diagrammatic longitudinal section, repre- 
 sented in woodcut, fig. I. In front is shown the amnion (<7w.), 
 growing over the head of the embryo. The notochord (c/i.) is 
 seen as an independent cord for the greater part of the length of 
 the embryo, but falls into the hypoblast shortly in front of the 
 neurenteric passage. The neurenteric passage is shown at ne., 
 and behind it is shown the primitive streak. 
 
 In a still older stage, represented in surface view on PL 29, 
 fig. C, the medullary folds have nearly met above, but have not yet 
 united. The features of the passage from the neural groove to 
 the hypoblast are precisely the same in the embryo just described, 
 although the lumen of the passage has become somewhat nar- 
 rower. There is still a short primitive streak behind the embryo. 
 
 The neurenteric passage persists but a very short time after 
 the complete closure of the medullary canal. It is in no way 
 connected with the allantois, as conjectured by Kupffer and 
 Benecke, but the allantois is formed, as I have satisfied myself 
 by longitudinal sections of a later stage, in the manner already 
 described by Dobrynin, Gasser, and Kolliker for the bird and 
 mammal. 
 
 The general results of Kupffer's and Benecke's observations, 
 with the modifications introduced by my own observations, are 
 as follows : After the segmentation and the formation of the 
 embryonic shield (area pellucida) the blastoderm becomes dis- 
 tinctly divided into epiblast and hypoblast 1 . At the hind end of 
 the shield a somewhat triangular primitive streak is formed by 
 
 1 This appears to me to take place before the formation of the embryonic shield. 
 
EARLY DEVELOPMENT OF THE LACERTILIA. 649 
 
 the fusion of the epiblast and hypoblast with a number of cells 
 between them, which are probably derived from the lower rows 
 of the segmentation cells. At the front end of the streak a 
 passage arises, open at both extremities, leading obliquely for- 
 wards through the epiblast to the space below the hypoblast. 
 The walls of the passage are formed of a layer of columnar cells 
 continuous both with epiblast and hypoblast. In front of 
 the primitive streak the body of the embryo becomes first 
 differentiated by the formation of a medullary plate, and at 
 the same time there grows out from the primitive streak a layer 
 of mesoblast, which spreads out in all directions between the 
 epiblast and hypoblast. In the axis of the embryo the meso- 
 blast plate is stated by Kupffer and Benecke to be continuous 
 across the middle line, but this appears very improbable. In 
 a slightly later stage the medullary plate becomes marked by 
 a shallow groove, and the mesoblast of the embryo is then un- 
 doubtedly constituted of two lateral plates, one on each side of 
 the median line. In the median line the notochord arises as a 
 ridge-like thickening of the hypoblast, which becomes very soon 
 quite separated from the hypoblast, except at the hind end, 
 where it is continued into the front wall of the neurenteric pas- 
 sage. It is interesting to notice the remarkable relation of the 
 notochord to the walls of the neurenteric passage. More or less 
 similar relations are also well marked in the case of the goose 
 and the fowl (Gasser) 1 , and support the conclusion deducible 
 from the lower forms of vertebrata, that the notochord is essenti- 
 ally hypoblastic. 
 
 The passage at the front end of the primitive streak forms the 
 posterior boundary of the medullary plate, though the medullary 
 groove is not at first continued back to it. The anterior wall of 
 this passage connects together the medullary plate and the noto- 
 chordal ridge of the hypoblast. In the succeeding stages the 
 medullary groove becomes continued back to the opening of the 
 passage, which then becomes enclosed in the medullary folds, 
 and forms a true neurenteric passage. It becomes narrowed as 
 the medullary folds finally unite to form the medullary canal, 
 and eventually disappears. 
 
 1 Gasser, Der Primitivstreifen bd Vogelembryonen, Marburg, 1878. 
 B. 4 2 
 
650 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 I conclude this paper with a concise statement of what 
 appears to me the probable nature of the much-disputed organ, 
 the primitive streak, and of the arguments in support of my 
 view. 
 
 In a paper on the primitive streak in the Quart. Journ, of Mic. 
 Sci., in 1873 (p. 280) [This edition, p. 45], I made the following 
 statement with reference to this subject : "It is clear, therefore, 
 that the primitive groove must be the rudiment of some ancestral 
 
 feature It is just possible that it is the last trace of that 
 
 involution of the epiblast by which the hypoblast is formed in 
 most of the lower animals." 
 
 At a later period, in July, 1876, after studying the develop- 
 ment of Elasmobranch fishes. I enlarged the hypothesis in a 
 review of the first part of Prof. Kolliker's Entwicklungs- 
 geschicJite. The following is the passage in which I speak 
 of it 1 : 
 
 " In treating of the exact relation of the primitive groove to 
 the formation of the embryo, Professor Kolliker gives it as his 
 view that though the head of the embryo is formed independently 
 of the primitive groove, and only secondarily unites with this, 
 yet that the remainder of the body is without doubt derived 
 from the primitive groove. With this conclusion we cannot 
 agree, and the very descriptions of Professor Kolliker appear to 
 us to demonstrate the untenable nature of his results. We be- 
 lieve that the front end of the primitive groove at first occupies 
 the position eventually filled by about the third pair of proto- 
 vertebrae, but that as the protovertebne are successively formed, 
 and the body of the embryo grows in length, the primitive groove 
 is carried further and further back, so as always to be situated 
 immediately behind the embryo. As Professor Kolliker himself 
 has shewn it may still be seen in this position even later than 
 the fortieth hour of incubation. 
 
 "Throughout the whole period of its existence it retains a 
 character which at once distinguishes it in sections from the 
 medullary groove. 
 
 " Beneath it the epiblast and mesoblast are always fused, 
 though they are always separate elsewhere ; this fact, which was 
 
 1 Journal of Anat. and Phys., Vol. X. pp. 790 and 791. Compare also my 
 Monograph on Elasmobranch Fishes, note on p. 68 [This edition, p. 281]. 
 
EARLY DEVELOPMENT OF THE LACERTILIA. 651 
 
 originally shewn by ourselves, has been very clearly brought out 
 by Professor Kolliker's observations. 
 
 " The features of the primitive groove which throw special 
 light on its meaning are the following : 
 
 "(i) It does not enter directly into the formation of the 
 embryo. 
 
 " (2) The epiblast and mesoblast always become fused be- 
 neath it. 
 
 " (3) It is situated immediately behind the embryo. 
 
 " Professor Kolliker does not enter into any speculations as to 
 the meaning of the primitive groove, but the above-mentioned 
 facts appear to us clearly to prove that the primitive groove is a 
 rudimentary structure, the origin of which can only be com- 
 pletely elucidated by a knowledge of the development of the 
 Avian ancestors. 
 
 " In comparing the blastoderm of a bird with that of any 
 anamniotic vertebrate, we are met at the threshold of our in- 
 vestigations by a remarkable difference between the two. 
 Whereas in all the lower vertebrates the embryo is situated at 
 the edge of the blastoderm, it is in birds and mammals situated 
 in the centre. This difference of position at once suggests the 
 view that the primitive groove may be in some way connected 
 with the change of position in the blastoderm which the ancestors 
 of birds must have undergone. If we carry our investigations 
 amongst the lower vertebrates a little further, we find that the 
 Elasmobranch embryo occupies at first the normal position at 
 the edge of the blastoderm, but that in the course of develop- 
 ment the blastoderm grows round the yolk far more slowly in 
 the region of the embryo than elsewhere. Owing to this, the 
 embryo becomes left in a bay, the two sides of which eventually 
 meet and coalesce in a linear fashion immediately behind the 
 embryo, thus removing the embryo from the edge of the blasto- 
 derm and forming behind it a linear streak not unlike the primi- 
 tive streak. We would suggest the hypothesis that the primitive 
 groove is a rudiment which gives the last indication of a change 
 made by the Avian ancestors in their position in the blastoderm, 
 like that made by Elasmobranch embryos when removed from 
 the edge of the blastoderm and placed in a central situation 
 similar to that of the embryo bird. On this hypothesis the 
 
 42 2 
 
652 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 situation of the primitive groove immediately behind the em- 
 bryo, as well as the fact of its not becoming converted into any 
 embryonic organ would be explained. The central groove might 
 probably also be viewed as the groove naturally left between 
 the coalescing edges of the blastoderm. 
 
 "Would the fusion of epiblast and mesoblast also receive its 
 explanation on this hypothesis ? We are of opinion that it 
 would. At the edge of the blastoderm which represents the 
 blastopore mouth of Amphioxus all the layers become fused 
 together in the anamniotic vertebrates. So that if the primitive 
 groove is in reality a rudiment of the coalesced edges of the 
 blastoderm, we might naturally expect the layers to be fused 
 there, and the difficulty presented by the present condition of 
 the primitive groove would rather be that the hypoblast is not 
 fused with the other layers than that the mesoblast is indis- 
 solubly united with the epiblast. The fact that the hypoblast is 
 not fused with the other layers does not appear to us to be fatal 
 to our hypothesis, and in Mammalia, where the primitive and 
 medullary grooves present precisely the same relations as in 
 birds, all three layers are, according to Hensen's account, fused 
 together. This, however, is denied by Kolliker, who states "that 
 in Mamrfials, as in Birds, only the epiblast and mesoblast fuse 
 together. Our hypothesis as to the origin of the primitive 
 groove appears to explain in a fairly satisfactory manner all the 
 peculiarities of this very enigmatical organ ; it also relieves us 
 from the necessity of accepting Professor Kolliker's explanation 
 of the development of the mesoblast, though it does not, of 
 course, render that explanation in any way untenable." 
 
 At a somewhat later period Rauber arrived at a more or less 
 similar conclusion, which, however, he mixes up with a number 
 of opinions from which I am compelled altogether to dissent 1 . 
 
 The general correctness of my view, as explained in my 
 second quotation, appears to me completely established by 
 Gasser's beautiful researches on the early development of the 
 chick and goose 2 , and by my own observations just recorded on 
 the lizard. While at the same time the parallel between the 
 blastopore of Elasmobranchii and of the Sauropsida, is rendered 
 
 1 " Primitivrinne u. Urmund," Morphologisches Jahrbuch, Band n. p. 551. 
 
 2 Gasser, Der Primitivstreifen bei Vogelembryonen, Marburg, 1878. 
 
EARLY DEVELOPMENT OF THE LACERTILIA. 653 
 
 more complete by the discovery of the neurenteric passage^in 
 the latter group, which was first of all made by Gasser. 
 
 The following paragraphs contain a detailed attempt to 
 establish the above view by a careful comparison of the primi- 
 tive streak and its adjuncts in the amniotic vertebrates with the 
 blastopore in Elasmobranchii. 
 
 In Elasmobranchii the blastopore consists of the following 
 parts: (i), a section at the end of the medullary plate, which 
 becomes converted into the neurenteric canal 1 ; (2), a section 
 forming what may be called the yolk blastopore, which even- 
 tually constitutes a linear streak connecting the embryo with 
 the edge of the blastoderm (vide monograph on Elasmobranch 
 fishes, pp. 281 and 296). In order to establish my hypothesis 
 on the nature of the primitive streak, it is necessary to find the 
 representatives of both these parts in the primitive streak of the 
 amniotic vertebrates. The first section ought to appear as a 
 passage from the neural to the enteric side of the blastoderm 
 at the posterior end of the medullary plate. At its front edge 
 the epiblast and hypoblast should be continuous, as they are 
 at the hind end of the embryo in Elasmobranchii, and, finally, 
 the passage should, on the closure of the medullary groove, 
 become converted into the neurenteric canal. All these con- 
 ditions are exactly fulfilled by the opening at the front end of 
 the primitive streak of the lizard (vide woodcut, fig. I, p. 647). 
 In the chick there is at first no such opening, but, as I hope to 
 shew in a future paper, it is replaced by the epiblast and hypo- 
 blast falling into one another at the front end of the primitive 
 streak. At a later period, as has been shewn by Gasser 2 , there 
 is a distinct rudiment of the neurenteric canal in the chick, and a 
 complete canal in the goose. Finally, in mammals, as has been 
 shewn by Schaffer 3 for the guinea-pig, there is at the front end 
 of the primitive streak a complete continuity between epiblast 
 and hypoblast. The continuity of the epiblast and hypoblast at 
 the hind end of the embryo in the bird and the mammal is a 
 
 1 I use this term for the canal connecting the neural and alimentary tract, which 
 was first discovered by Kowalevsky. 
 
 2 Loc. cit. 
 
 3 "A contribution to the history of the development in the Guinea-pig," Journal 
 of Anat. and Phys. Vol. XI. pp. 332 336. 
 
654 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 rudiment of the continuity of these layers at the dorsal lip of 
 the blastopore in Elasmobranchii, Amphibia, &c. The second 
 section of the blastopore in Elasmobranchii or yolk blastopore 
 is, I believe, partly represented by the primitive streak. The 
 yolk blastopore in Elasmobranchii is the part of the blastopore 
 belonging to the yolk sac as opposed to that belonging to the 
 embryo, and it is clear that the primitive streak cannot cor- 
 respond to the whole of this, since the primitive streak is far 
 removed from the edge of the blastoderm long before the yolk 
 is completely enclosed. Leaving this out of consideration the 
 primitive streak, in order that the above comparison may hold 
 good, should satisfy the following conditions : 
 
 1. It should connect the embryo with the edge of the blasto- 
 derm. 
 
 2. It should be constituted as if formed of the fused edges of 
 the blastoderm. 
 
 3. The epiblast of it should eventually not form part of the 
 medullary plate of the embryo, but be folded over on to the 
 ventral side. 
 
 The first of these conditions is only partially fulfilled, but, 
 considering the rudimentary condition of the whole structure, no 
 great stress can, it seems to me, be laid on this fact. 
 
 The second condition seems to me very completely satisfied. 
 Where the two edges of the blastoderm become united we should 
 expect to find a complete fusion of the layers such as takes 
 place in the primitive streak ; and the fact that in the primitive 
 streak the hypoblast does not so distinctly coalesce with the me- 
 soblast as the mesoblast with the epiblast cannot be urged as a 
 serious argument against me. 
 
 The growth outwards of the mesoblast from the axis of the 
 primitive streak is probably a remnant of the invagination of the 
 hypoblast and mesoblast from the lip of the blastopore in Am- 
 phibia, &c. 
 
 The groove in the primitive streak may with great plausi- 
 bility be regarded as the indication of a depression which would 
 naturally be found along the line where the thickened edges of 
 the blastoderm became united. 
 
 With reference to the third condition, I will make the follow- 
 ing observations. The neurenteric canal, as it is placed at the 
 
EARLY DEVELOPMENT OF THE LACERTILTA. 655 
 
 extreme end of the embryo, must necessarily, with reference to 
 the embryo, be the hindermost section of the blastopore, and 
 therefore the part of the blastopore apparently behind this can 
 only be so owing to the embryo not being folded off from the 
 yolk sac ; and as the yolk sac is in reality a specialised part of 
 the ventral wall of the body, the yolk blastopore must also be 
 situated on the ventral side of the embryo. 
 
 Kolliker and other distinguished embryologists have believed 
 that the epiblast of the whole of the primitive streak became 
 part of the neural plate. If this view were correct, which is 
 accepted even by Rauber, the hypothesis I am attempting to 
 establish would fall to the ground. I have, however, no doubt 
 that these embryologists are mistaken. The very careful ob- 
 servations of Gasser shew that the part of the primitive streak 
 adjoining the embryo becomes converted into the tail-swelling, 
 and that the posterior part is folded in on the ventral side of the 
 embryo, and, losing its characteristic structure, forms part of the 
 ventral wall of the body. On this point my own observations 
 confirm those of Gasser. In the lizard the early appearance of 
 the neurenteric canal at the front end of the primitive streak 
 clearly shews that here also the primitive streak can take no 
 share in forming the neural plate. 
 
 The above considerations appear to me sufficient to establish 
 my hypothesis with reference to the nature of the primitive 
 streak, which has the merit of explaining, not only the structural 
 peculiarities of the primitive streak, but also the otherwise inex- 
 plicable position of the embryo of the amniotic vertebrates in 
 the centre of the blastoderm. 
 
656 EARLY DEVELOPMENT OF THE LACERTILIA. 
 
 DESCRIPTION OF PLATE 29. 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 am. Amnion. ch. Notochord. ch '. Notochordal thickening of hypoblast. 
 ep. Epiblast. hy. Hypoblast. m.g. Medullary groove, me. p. Mesoblastic plate. 
 ne. Neurenteric canal (blastopore). pr. Primitive streak. 
 
 SERIES A. Sections through an embryo shortly after the formation of the medul- 
 lary groove. X I2O 1 . 
 
 Fig. i. Section through the trunk of the embryo. 
 Figs. 2 5. Sections through the neurenteric canal. 
 
 Fig. B. Surface view of a somewhat older embryo than that from which Series A 
 is taken, x 30. 
 
 SERIES B. Sections through the embryo represented in Fig. B. x 120. 
 
 Fig. i. Section through the trunk of the embryo. 
 
 Figs. 2, 3. Sections through the hind end of the medullary groove. 
 
 Fig. 4. Section through the neurenteric canal. 
 
 Fig. 5. Section through the primitive streak. 
 
 Fig. C. Surface view of a somewhat older embryo than that represented in 
 Fig. B. x 30. 
 
 1 The spaces between the layers in these sections are due to the action of the 
 hardening re-agent. 
 
XV. ON CERTAIN POINTS IN THE ANATOMY OF 
 PERIPATUS CAPENSIS'. 
 
 THE discovery by Mr Moseley 2 of a tracheal system in Peri- 
 patus must be reckoned as one of the most interesting results 
 obtained by the naturalists of the " Challenger." The discovery 
 clearly proves that the genus Peripatus, which is widely dis- 
 tributed over the globe, is the persisting remnant of what was 
 probably a large group of forms, from which the present tracheate 
 Arthropoda are descended. 
 
 The affinities of Peripatus render any further light on its 
 anatomy a matter of some interest ; and through the kindness 
 of Mr Moseley I have had an opportunity of making investiga- 
 tions on some well preserved examples of Peripatus capensis, 
 a few of the results of which I propose to lay before the Society. 
 
 I shall confine my observations to three organs, (i) The 
 segmental organs, (2) the nervous system, (3) the so-called fat 
 bodies of Mr Moseley. 
 
 In all the segments of the body, with the exception of the 
 first two or three postoral ones, there are present glandular 
 bodies, apparently equivalent to the segmental organs of An- 
 nelids. 
 
 These organs have not completely escaped the attention of 
 previous observers. The anterior of them were noticed by 
 Grube 3 , but their relations were not made out. By Saenger 4 , as 
 I gather from Leuckart's Bericht for the years 1868 9, these 
 structures were also noticed, and they were interpreted as seg- 
 
 1 From the Proceedings of the Cambridge Philosophical Society, Vol. III. 1879. 
 
 2 "On the Structure and Development of Peripatus Capensis," Phil. Trans.. 
 Vol. CLXIV. 1874. 
 
 3 "Bau von Perip. Edwardsii" Archiv f. Aiiat. it. Phys. 1853. 
 
 4 Moskaner Naturforsclu-r Saniviliing, Abth. Zool. 1869. 
 
658 POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS. 
 
 mental organs. Their external openings were correctly identi- 
 fied. They are not mentioned by Moseley, and no notice of 
 them is to be found in the text-books. The observations of 
 Grube and Saenger seem, in fact, to have been completely for- 
 gotten. 
 
 The organs are placed at the bases of the feet in two lateral 
 divisions of the body-cavity shut off from the main central 
 median division of the body-cavity by longitudinal septa of 
 transverse muscles. 
 
 Each fully developed organ consists of three parts : 
 
 (1) A dilated vesicle opening externally at the base of a 
 foot. 
 
 (2) A coiled glandular tube connected with this and subdi- 
 vided again into several minor divisions. 
 
 (3) A short terminal portion opening at one extremity into 
 the coiled tube (2) and at the other, as I believe, into the body- 
 cavity. This section becomes very conspicuous in stained 
 preparations by the intensity with which the nuclei of its walls 
 absorb the colouring matter. 
 
 The segmental organs of Peripatus, though formed on a type 
 of their own, more nearly resemble those of the Leech than of 
 any other form with which I am acquainted. The annelidan 
 affinities shewn by their presence are of some interest. Around 
 the segmental organs in the feet are peculiar cells richly supplied 
 with tracheae, which appear to me to be similar to the fat bodies 
 in insects. There are two glandular bodies in the feet in addi- 
 tion to the segmental organs. 
 
 The more obvious features of the nervous system have been 
 fully made out by previous observers, who have shewn that it 
 consists of large paired supraoesophageal ganglia connected with 
 two widely separated ventral cords stated by them not to be 
 ganglionated. Grube describes the two cords as falling into one 
 another behind the anus a feature the presence of which is 
 erroneously denied by Saenger. The lateral cords are united by 
 numerous (5 or 6 for each segment) transverse cords. 
 
 The nervous system would appear at first sight to be very 
 lowly organised, but the new points I believe myself to have 
 made out, as well as certain previously known features in it 
 appear to me to shew that this is not the case. 
 
POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS. 659 
 
 The following is a summary of the fresh points "I have 
 observed in the nervous system : 
 
 (1) Immediately underneath the oesophagus the oesophageal 
 commissures dilate and form a pair of ganglia equivalent to the 
 annelidan and arthropodan suboesophageal ganglia. These 
 ganglia are closely approximated and united by 5 or 6 com- 
 missures. They give off large nerves to the oral papillae. 
 
 (2) The ventral nerve cords are covered on their ventral 
 side by a thick ganglionic layer 1 , and at each pair of feet they 
 dilate into a small but distinct ganglionic swelling. From each 
 ganglionic swelling are given off a pair of large nerves 2 to the 
 feet; and the ganglionic swellings of the two cords are connected 
 together by a pair of commissures containing ganglion cells*. 
 The other commissures connecting the two cords together do 
 not contain ganglion cells. 
 
 The chief feature in which Peripatus was supposed to differ 
 from normal Arthropoda and Annelida, viz. the absence of 
 ganglia on the ventral cords, does not really exist. In other 
 particulars, as in the amount of nerve cells in the ventral cords 
 and the completeness of the commissural connections between 
 the two cords, &c., the organisation of the nervous system of 
 Peripatus ranks distinctly high. The nervous system lies within 
 the circular and longitudinal muscles, and is thus not in 
 proximity with the skin. In this respect also Peripatus shews 
 no signs of a primitive condition of the nervous system. 
 
 A median nerve is given off from the posterior border of the 
 supracesophageal ganglion to the oesophagus, which probably 
 forms a rudimentary sympathetic system. I believe also that I 
 have found traces of a paired sympathetic system. 
 
 The organ doubtfully spoken of by Mr Moseley as a fat body, 
 and by Grube as a lateral canal, is in reality a glandular tube, 
 lined by beautiful columnar cells containing secretion globules, 
 which opens by means of a non-glandular duct into the mouth. 
 It lies close above the ventral nerve cords in a lateral com- 
 
 1 This was known to Grube, loc. cit. 
 
 2 These nerves were noticed by Milne-Edwards, but Grube failed to observe that 
 they were much larger than the nerves given off between the feet. 
 
 3 These commissures were perhaps observed by Saenger, loc. cit. 
 
66o POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS. 
 
 partment of the body-cavity, and extends backwards for a 
 varying distance. 
 
 This organ may perhaps be best compared with the simple 
 salivary gland of Julus. It is not to be confused with the slime 
 glands of Mr Moseley, which have their opening in the oral 
 papillae. If I am correct in regarding it as homologous with 
 the salivary glands so widely distributed amongst the Tracheata, 
 its presence indicates a hitherto unnoticed arthropodan affinity 
 in Peripatus. 
 
XVI. ON THE MORPHOLOGY AND SYSTEMATIC POSITION OF 
 
 THE SPONGIDA 1 . 
 
 PROFESSOR SCHULZE'S* last memoir on the development 
 of Calcareous Sponges, confirms and enlarges MetschnikoflTs 8 
 earlier observations, and gives us at last a fairly complete history 
 of the development of one form of Calcareous Sponge. The 
 facts which have been thus established have suggested to me a 
 view of the morphology and systematic position of the Spongida, 
 somewhat different to that now usually entertained. In bringing 
 forward this view, I would have it understood that it does not 
 claim to be more than a mere suggestion, which if it serves no 
 other function may, perhaps, be of use in stimulating research. 
 
 To render clear what I have to say, I commence with a very 
 brief statement of the facts which may be considered as estab- 
 lished with reference to the development of Sycandra raphamts, 
 the form which was studied by both Metschnikoff and Schulze. 
 The segmentation of the ovum, though in many ways remark- 
 able, is of no importance for my present purpose, and I take up 
 the development at the close of the segmentation, while the 
 embryo is still encapsuled in the parental tissues. It is at this 
 stage lens-shaped, with a central segmentation cavity. An 
 equatorial plane divides it into two parts, which have equal 
 shares in bounding the segmentation cavity. One of these 
 halves is formed of about thirty-two large, round, granular cells, 
 the other of a larger number of ciliated clear columnar cells. 
 While the embryo is still encapsuled a partial invagination of the 
 
 1 From the Quarterly Journ. of Microscopical Science, Vol. xix, 1879. 
 
 2 " Untersuchungen iiber d. Bau u. d. Entwickelung der Spongien," Zeit. f. wiss. 
 Zool. Bd. xxxi. 1878. 
 
 3 " Zur Entwickelungsgeschichte der Kalkschwamme," Zeit. f. wiss. Zool. Bd. 
 xxiv. 1874. 
 
662 
 
 MORPHOLOGY AND SYSTEMATIC 
 
 granular cells takes place, reducing the segmentation cavity to a 
 mere slit; this invagination is, however, quite temporary and 
 unimportant, and on the embryo becoming free, which shortly 
 takes place, no trace of it is visible; but, on the contrary, the 
 segmentation cavity becomes larger, and the granular cells 
 project very much more prominently than in the encapsuled 
 state. 
 
 FIG. i. 
 
 en 
 
 en\ 
 
 e.g. 
 
 Two free stages in the development of Sycandra raphanus (copied from Schulze). 
 
 A. Amphiblastula stage; B, a later stage after the ciliated cells have commenced to 
 become invaginated ; cs. segmentation cavity ; ec. granular cells which will form 
 the ectoderm; en. ciliated cells which become invaginated to form the entoderm. 
 
 The larva, after it has left the parental tissues, has an oval 
 form and is transversely divided into two areas (fig. I, A). One of 
 these areas is formed of the elongated, clear, ciliated cells, with 
 a small amount of pigment near the inner ends (en}, and the 
 other and larger area of the thirty-two granular cells already 
 mentioned (ec). Fifteen or sixteen of these are arranged as a 
 special ring on the border of the clear cells. In the centre of 
 the embryo is a segmentation cavity (cs) which lies between the 
 granular and the clear cells, but is mainly bounded by the vaulted 
 inner surface of the latter. This stage is known as the amphi- 
 blastula stage. After the larva has for some time enjoyed a 
 free existence, a remarkable series of changes takes place, which 
 result in the invagination of the half of it formed of the clear 
 
POSITION OF THE SPONGIDA. 
 
 66 3 
 
 cells, and form a prelude to the permanent attachment-of-the 
 larva. The entire process of invagination is completed in about 
 half an hour. The whole embryo first becomes flattened, but 
 especially the ciliated half which gradually becomes less promi- 
 nent (fig. i, B), and still later the cells composing it undergo a 
 true process of invagination. As a result of this invagination 
 the segmentation cavity is obliterated and the larva assumes a 
 compressed plano-convex form with a central gastrula cavity, 
 and a blastopore in the middle of the flattened surface. The 
 two layers of the gastrula may now be spoken of as ectoderm 
 and entoderm. The blastopore becomes gradually narrowed by 
 the growth over it of the outer row of granular cells. When it 
 has become very small the attachment of the larva takes place 
 by the flat surface where the blastopore is situated. It is 
 effected by protoplasmic processes of the outer ring of ectoderm 
 cells, which, together with the other ectoderm cells, now become 
 amoeboid. At the same time they become clearer and permit a 
 view of the interior of the gastrula. Between the ectoderm cells 
 and the entoderm cells which line the gastrula cavity there arises 
 a hyaline structureless layer, which is more closely attached to 
 the ectoderm than to the entoderm, and is probably derived from 
 the former. A view of the gastrula stage after the larva has 
 become fixed is given in fig. 2. 
 
 FIG. 2. 
 
 ec 
 
 Fixed Gastrula stage of Sycandra raphanns (copied from Schulze). 
 
 The figure shews the amoeboid ectoderm cells (ec) derived from the granular cells of 
 the earlier stage, and the columnar entoderm cells, lining the gastrula cavity, 
 derived from the ciliated cells of the earlier stage. The larva is fixed by the 
 amoeboid cells on the side on which the blastopore is situated. 
 
664 
 
 MORPHOLOGY AND SYSTEMATIC 
 
 After imagination the cilia of the entoderm cells can no 
 longer be seen, and are probably absorbed, and their disap- 
 pearance is nearly coincident with the complete obliteration of 
 the blastopore, an event which takes place shortly after the 
 attachment of the larva. After the formation of the structureless 
 layer between the ectoderm and entoderm, calcareous spicules 
 make their appearance in it as delicate unbranched rods pointed 
 at both extremities. The larva when once fixed rapidly grows 
 in length and assumes a cylindrical form (fig. 3, A). The sides 
 
 The young of Sycandra raphanus shortly after the development of the spictila 
 (copied from Schulze). 
 
 A. View from the side ; B, view from the free extremity ; os. osculum ; cc. ectoderm ; 
 en. entoderm composed of collared ciliated cells. The terminal osculum and 
 lateral pores are represented as oval white spaces. 
 
 of the cylinder are beset with calcareous spicules which project 
 beyond the surface, and in addition to the unbranched forms, 
 spicules are developed with three and four rays as well as 
 some with a blunt extremity and serrated edge. The extremity 
 of the cylinder opposite the attached surface is flattened, and 
 
POSITION OF THE SPONGIDA. 665 
 
 though surrounded by a ring of four-rayed spicules is itself free 
 from them. At this extremity a small perforation is formed leading 
 into the gastric cavity which rapidly increases in size and forms 
 an exhalent osculum (os). A series of inhalent apertures are 
 also formed at the sides of the cylinder. The relative times of 
 appearance of the single osculum and smaller apertures is not 
 constant for the different larvae. On the central gastrula cavity 
 of the sponge becoming placed in communication with the ex- 
 ternal water, the entoderm cells lining it become ciliated afresh 
 (fig. 3, B, en] and develop the peculiar collar characteristic of the 
 entoderm cells of the Spongida. When this stage of develop- 
 ment is reached we have a fully developed sponge of the type 
 made known by Haeckel as Olynthus. 
 
 Till the complete development of other forms of Spongida 
 has been worked out it is not possible to feel sure how far the 
 phenomena observable in Sycandra hold good in all cases. 
 Quite recently the Russian embryologist, M. Ganin 1 , has given 
 an account, without illustrations, of the development of Spongilla 
 fluviatilis, which does not appear reconcileable with that of 
 Sycandra. Considering the difficulties of observation it appears 
 better to assume for this and some other descriptions that the 
 observations are in error rather than that there is a fundamental 
 want of uniformity in development amongst the Spongida. 
 
 The first point in the development of Sycandra which deserves 
 notice is the character of the free swimming larva. The peculiar 
 larval form, with one half of the body composed of amoeboid 
 granular cells and the other of clear ciliated cells is nearly con- 
 stant amongst the Calcispongise, and widely distributed in a 
 somewhat modified condition amongst the Fibrospongiae and 
 Myxospongiae. Does this larva retain the characters of an 
 ancestral type of the Spongida, and if so what does its form 
 mean ? It is, of course, possible that it has no ancestral meaning 
 but has been secondarily acquired ; I prefer myself to think 
 that this is not the case, more especially as it appears to me that 
 the characters of the larva may be plausibly explained by 
 regarding it as a transitional form between the Protozoa and 
 Metazoa. According to this view the larva is to be considered 
 
 1 " Zur Entwickclung d. Spongilla fluviatilis," Zoologischer Anzeiger, Vol. 'l. 
 No. 9, 1878. 
 
 B. 43 
 
666 MORPHOLOGY AND SYSTEMATIC 
 
 as a colony of Protozoa, one half of the individuals of which 
 have become differentiated into nutritive forms, and the other 
 half into locomotor and respiratory forms. The granular 
 amoeboid cells represent the nutritive forms, and the ciliated cells 
 represent the locomotor and respiratory forms. That the passage 
 from the Protozoa to the Metazoa may have been effected by 
 such a differentiation is not improbable on a priori grounds, and 
 fits in very well with the condition of the free swimming larva 
 of Spongida, though another and perhaps equally plausible 
 suggestion as to this passage has been put forward by my friend 
 Professor Lankester 1 . 
 
 While the above view seems fairly satisfactory for the free 
 swimming stage of the larval Sponge there arises in the subsequent 
 development a difficulty which appears at first sight fatal to it. 
 This difficulty is the invagination of the ciliated cells instead of 
 the granular ones. If the granular cells represent the nutritive 
 individuals of the colony, they and not the ciliated cells ought 
 most certainly to give rise to the lining of the gastrula cavity, 
 according to the generally accepted views of the morphology of 
 the Spongida. The -suggestion which I would venture to put 
 forward in explanation of this paradox involves a completely 
 new view of the nature and functions of the germinal layers of 
 adult Sponges. 
 
 It is as follows: When the free swimming ancestor of the 
 Spongida became fixed, the ciliated cells by which its move- 
 ments used to be effected must have to a great extent become 
 functionless. At the same time the amoeboid nutritive cells 
 would need to expose as large a surface as possible. In these 
 two considerations there may, perhaps, be found a sufficient 
 explanation of the invagination of the ciliated cells, and the 
 growth of the amoeboid cells over them. Though respiration 
 was, no doubt, mainly effected by the ciliated cells, it is im- 
 probable that it was completely localised in them, but the 
 continuation of their function was provided for by the formation 
 
 1 " Notes on P'mbryology and Classification." Quarterly Journal of Microscopical 
 Science, Vol. xvtl. 1877. It seems not impossible, if the speculations in this paper 
 have any foundation that while the views here put forward as to the passage from 
 the Protozoon to the Metazoon condition may hold true for the Spongida, some other 
 mode of passage may have taken place in the case of the other Metazoa. 
 
POSITION OF THE SPONGIDA. 667 
 
 of an osculum and pores. The ciliated collared cells which line 
 the ciliated chambers, or in some cases the radial tubes, are 
 undoubtedly derived from the invaginated cells, and if there is 
 any truth in the above suggestion, the collared cells in the adult 
 Sponge must be mainly respiratory and not digestive in function, 
 while the normal epithelial cells which cover the surface of the 
 sponge, and in most cases line the greater part of the passages 
 through its substance, must carry on the digestion 1 . If the 
 reverse is the case the whole theory falls to the ground. It has 
 not, so far as I know, been definitely made out where the 
 digestion is carried on. Lieberkuhn would appear to hold the 
 view that the amoeboid lining cells of the passages are mainly 
 concerned with digestion, while Carter holds that digestion is 
 carried on by the collared cells of the ciliated chambers. 
 
 If it is eventually proved by actual experiments on the nutri- 
 tion of Sponges, that digestion is carried on by the general cells 
 lining the passages, and not by the ciliated cells, it is clear that 
 neither the ectoderm nor entoderm of Sponges will correspond 
 with the similarly named layers in the Ccelenterata and the 
 Metozoa. The invaginated entoderm will be the respiratory layer 
 and the ectoderm the digestive and sensory layer ; the sensory 
 function being probably mainly localised in the epithelium on 
 the surface, and the digestive one in the epithelium lining the 
 passages. Such a fundamental difference in the germinal layers 
 between the Spongida and the other Metazoa, would necessarily 
 involve the creation of a special division of the Metazoa for the 
 reception of the former group. 
 
 1 That the flat cells which line the greater part of the passages of most Sponges 
 are really derived from ectodcnnic invaginations appears to me clearly proved by 
 Schulze's and Rarrois' observations on the young fixed stages of Halisarca. Ganin 
 appears, however, to maintain a contrary view for Spongilla. 
 
 432 
 
XVII. NOTES ON THE DEVELOPMENT OF THE ARANEINA*. 
 
 (With Plates 30, 31, 32.) 
 
 THE following observations do not profess to contain a 
 complete history of the development even of a single species 
 of spider. They are the result of investigations carried on at 
 intervals during rather more than two years, on the ova of 
 Agelena labyrinthica ; and I should not have published them 
 now, if I had any hope of being able to complete them before 
 the appearance of the work I am in the course of publishing 
 on Comparative Embryology. It appeared to me, however, 
 desirable to publish in full such parts of my observations as 
 are completed before the appearance of my treatise, since the 
 account of the development of the Araneina is mainly founded 
 upon them. 
 
 My investigations on the germinal layers and organs have 
 been chiefly conducted by means of sections. To prepare the 
 embryos for sections, I employed the valuable method first 
 made known by Bobretzky. I hardened the embryos in bichro- 
 mate of potash, after placing them for a short time in nearly 
 boiling water. They were stained as a whole with haematoxylin 
 after the removal of the membranes, and embedded for cutting 
 in coagulated albumen. 
 
 The number of investigators who have studied the develop- 
 ment of spiders is inconsiderable. A list of them is given at 
 the end of the paper. 
 
 The earliest writer on the subject is Herold (No. 4) ; he was 
 followed after a very considerable interval of time by Claparede 
 
 1 From the Quarterly Journ. of Microscopical Science^ Vol. xx. 1880. 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 669 
 
 (No. 3), whose memoir is illustrated by a series of beautiful 
 plates, and contains a very satisfactory account of the external 
 features of development. 
 
 Balbiani (No. i) has gone with some detail into the history 
 of the early stages; and Ludvvig (No. 5) has published some 
 very important observations on the development of the blasto- 
 derm. Finally, Barrois (No. 2) has quite recently taken up the 
 study of the group, and has added some valuable observations 
 on the development of the germinal layers. 
 
 In addition to these papers on the true spiders, important 
 investigations have been published by Metschnikoff on other 
 groups of the Arachnida, notably the scorpion. Metschnikoff's 
 observations on the formation of the germinal layers and organs 
 accord in most points with my own. 
 
 The development of the Araneina may be divided into four 
 periods : (i) the segmentation ; (2) the period from the close of 
 the segmentation up to the period when the segments commence 
 to be formed ; (3) the period from the commencing formation of 
 the segments to the development of the full number of limbs ; 
 (4) the subsequent stages up to the attainment of the adult 
 form. 
 
 In my earliest stage the segmentation was already completed, 
 and the embryo was formed of a single layer of large flattened 
 cells enveloping a central mass of polygonal yolk-segments. 
 
 Each yolk-segment is formed of a number of large clear 
 somewhat oval yolk-spherules. In hardened specimens the yolk- 
 spherules become polygonal, and in ova treated with hot water 
 prior to preservation are not unfrequently broken up. Amongst 
 the yolk-segments are placed a fair number of nucleated bodies 
 of a very characteristic appearance. Each of them is formed of 
 (i) a large, often angular, nucleus, filled with deeply staining 
 bodies (nucleoli ?). (2) Of a layer of protoplasm surrounding 
 the nucleus, prolonged into a protoplasmic reticulum. The 
 exact relation of these nucleated bodies to the yolk-segments is 
 not very easy to make out, but the general tendency of my 
 observations is to shew (i) that each nucleated body belongs to 
 a yolk-sphere, and (2) that it is generally placed not at the 
 centre, but to one side of a yolk-sphere. If the above conclusions 
 are correct each complete yolk-segment is a. cell, and each such 
 
6/O NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 cell consists of a normal nucleus, protoplasm, and yolk-spherules. 
 There is a special layer of protoplasm surrounding the nucleus, 
 while the remainder of the protoplasm consists of a reticulum 
 holding together the yolk-spherules. Yolk-cells of this character 
 are seen in Pis. 31 and 32, figs. 10 21. 
 
 The nuclei of the yolk-cells are probably derived by division 
 from the nuclei of the segmentation rosettes (vide Ludwig, No. 5), 
 and it is probable that they take their origin at the time when 
 the superficial layer of protoplasm separates from the yolk- 
 columns below to form the blastoderm. 
 
 The protoplasm of the yolk^cells undergoes rapid division, as 
 is shewn by the fact that there are often two nucleated bodies 
 close together, and sometimes two nuclei in a single mass of 
 protoplasm (fig. 10). It is probable that in some cases the yolk- 
 spheres divide at the same time as the protoplasm belonging to 
 them ; the division of the nucleated bodies is, however, in the 
 main destined to give rise to fresh cells which enter the blasto- 
 derm. 
 
 I have not elucidated to my complete satisfaction the next 
 stage or two in the development of the embryo ; and have not 
 succeeded in completely reconciling the results of my own 
 observations with those of Claparede and Balbiani. In order to 
 shew exactly where my difficulties lie it is necessary briefly to 
 state the results arrived at by the above authors. 
 
 According to Claparede the first differentiation in Pholcus 
 consists in the accumulation of the cells over a small area to 
 form a protuberance, which he calls the primitive cumulus. 
 Owing to its smaller specific gravity the part of the ovum with 
 the cumulus always turns upwards, like the blastodermic pole of 
 a fowl's egg. 
 
 After a short time the cumulus elongates itself on one side, 
 and becomes connected by a streak with a white patch, which 
 appears on the surface of the egg, about 90 from the cumulus. 
 This patch gradually enlarges, and soon covers the whole surface 
 of the ovum except the region where the cumulus is placed. 
 It becomes the ventral plate or germinal streak of the embryo, 
 its extremity adjoining the cumulus is the anal extremity, and 
 its opposite extremity the cephalic one. The cumulus itself is 
 placed in a depression on the dorsal surface of the ovum. 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/1 
 
 Claparede compares the cumulus to the dorsal organ of many 
 Crustacea. 
 
 Balbiani (No. i) describes the primitive cumulus in Tegenaria 
 domestica, Epcira diadcnia, and Agelena labyrinthica, as originating 
 as a protuberance at the centre of the ventral surface, surrounded 
 by a specialised portion of the blastoderm (p. 57), which I will 
 call the ventral plate. In Tegenaria domestica he finds that it 
 encloses the so-called yolk-nucleus, p. 62. By an unequal 
 growth of the ventral plate the primitive cumulus comes to be 
 placed at the cephalic pole of the ventral plate. The cumulus 
 now becomes less prominent, and in a few cases disappears. In 
 the next stage the central part of the ventral plate becomes 
 very prominent and forms the procephalic lobe, close to the 
 anterior border of which is usually placed the primitive cumulus 
 (p. 67). The space between the cumulus and the procephalic 
 lobe grows larger, so that the latter gradually travels towards 
 the dorsal surface and finally vanishes. Behind the procephalic 
 lobe the first traces of the segments make their appearance, 
 as three transverse bands, before a distinct anal lobe becomes 
 apparent. 
 
 The points which require to be cleared up are, (i) what is 
 the nature of the primitive cumulus ? (2) where is it situated 
 in relation to the embryo ? Before attempting to answer these 
 questions I will shortly describe the development, so far as 
 I have made it out, for the stages during which the cumulus is 
 visible. 
 
 The first change that I find in the embryo (when examined 
 after it has been hardened) 1 is the appearance of a small, whitish 
 spot, which is at first very indistinct. A section through such an 
 ovum (PI. 31, fig. 10) shews that the cells of about one half 
 of the ovum have become more columnar than those of the other 
 half, and that there is a point (pr. c.) near one end of the thick- 
 ened half where the cells are more columnar, and about two 
 layers or so deep. It appears to me probable that this point is 
 the whitish spot visible in the hardened ovum. In a somewhat 
 later stage (PI. 30, fig. i) the whitish spot becomes more con- 
 
 1 T was unfortunately too much engaged, at the time when the eggs were collected, 
 to study them in the fresh Condition ; a fact which has added not a little to my 
 difficulties in elucidating the obscure points in the early stages. 
 
6/2 NOTES ON THE DEVELOPMENT OF THE ARANETNA. 
 
 spicuous (pc.), and appears as a distinct prominence, which is, 
 without doubt, the primitive cumulus, and from it there proceeds 
 on one side a whitish streak. The prominence, as noticed by 
 Claparede and Balbiani, is situated on the flatter side of the 
 ovum. Sections at this stage shew the same features as the 
 previous stage, except that (i) the cells throughout are smaller, 
 (2) those of the thickened hemisphere of the ovum more columnar, 
 and (3) the cumulus is formed of several rows of cells, though not 
 divided into distinct layers. In the next stage the appearances 
 from the surface are rather more obscure, and in some of my 
 best specimens a coagulum, derived from the fluid surrounding 
 the ovum, covers the most important part of the blastoderm. 
 In PL 30, fig. 2, I have attempted to represent, as truly as I 
 could, the appearances presented by the ovum. There is a 
 well-marked whitish side of the ovum, near one end of which is 
 a prominence (/>.), which must, no doubt, be identified with the 
 cumulus of the earlier stages. Towards the opposite end, or 
 perhaps rather nearer the centre of the white side of the ovum, is 
 an imperfectly marked triangular white area. There can be no 
 doubt that the line connecting the cumulus with the triangular 
 area is the future long axis of the embryo, and the white area is, 
 without doubt, the procephalic lobe of Balbiani. 
 
 A section of the ovum at this stage is represented in PL 31, 
 fig. ii. It is not quite certain in what direction the section is 
 taken, but I think it probable it is somewhat oblique to the long 
 axis. However this may be, the section shews that the whitish 
 hemisphere of the blastoderm is formed of columnar cells, for 
 the most part two or so layers deep, but that there is, not very 
 far from the middle line, a wedge-shaped internal thickening of 
 the blastoderm where the cells are several rows deep. With 
 what part visible in surface view this thickened portion corre- 
 sponds is not clear. To my mind it most probably corresponds 
 to the larger white patch, in which case I have not got a section 
 through, the terminal prominence. In the other sections of the 
 same embryo the wedge-shaped thickening was not so marked, 
 but it, nevertheless, extended through all the sections. It 
 appears to me probable that it constitutes a longitudinal thick- 
 ened ridge of the blastoderm. In any case, it is clear that the 
 white hemisphere of the blastoderm is a thickened portion of the 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 673 
 
 blastoderm, and that the thickening is in part due to the. cells 
 being more columnar, and, in part, to their being more than one 
 row deep, though they have not become divided into two distinct 
 germinal layers. It is further clear that the increase in the 
 number of cells in the thickened part of the blastoderm is, in the 
 main, a result of the multiplication of tlie original single roiv of 
 cells, while a careful examination of my sections proves that it is 
 also partly due to cells, derived from the yolk, having been 
 added to the blastoderm. 
 
 In the following stage which I have obtained (which cannot 
 be very much older than the previous stage, because my speci- 
 mens of it come from the same batch of eggs), a distinct and 
 fairly circumscribed thickening forming the ventral surface of 
 the embryo has become established. Though its component 
 parts are somewhat indistinct, it appears to consist of a proce- 
 phalic lobe, a less prominent caudal lobe, and an intermediate 
 portion divided into about three segments ; but its constituents 
 cannot be clearly identified with the structures visible in the 
 previous stage. I am inclined, however, to identify the anterior 
 thickened area of the previous stage with the procephalic lobe, 
 and a slight protuberance of the caudal portion (visible from the 
 surface) with the primitive cumulus. I have, however, failed to 
 meet with any trace of the cumulus in my sections. 
 
 To this stage, which forms the first of the second period 
 of the larval history, I shall return, but it is necessary now to go 
 back to the observations of Claparede and Balbiani. 
 
 There can, in the first place, be but little doubt that what I 
 have called the primitive cumulus in my description is the struc- 
 ture so named by Claparede and Balbiani. 
 
 It is clear that Balbiani and Claparede have both failed to 
 appreciate the importance of the organ, which my observations 
 shew to be the part of the ventral thickening of the blastoderm 
 where two rows of cells are first established, and therefore the 
 point where the first traces of the future mesoblast becomes 
 visible. 
 
 Though Claparede and Balbiani differ somewhat as to the 
 position of the organ, they both make it last longer than I do : 
 I feel certainly inclined to doubt whether Claparede is right in 
 considering a body he figures after six segments are present, to 
 
674 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 be the same as the dorsal organ of the embryo before the form- 
 ation of any segments, especially as all the stages between the 
 two appear to have escaped him. In Agelena there is undoubt- 
 edly no organ in the position he gives when six segments are 
 found. 
 
 Balbiani's observations accord fairly with my own up to the 
 stage represented in fig. 2. Beyond this stage my own observa- 
 tions are not satisfactory, but I must state that I feel doubtful 
 whether Balbiani is correct in his description of the gradual 
 separation of the procephalic lobe and the cumulus, and the 
 passage of the latter to the dorsal surface, and think it possible 
 that he may have made a mistake as to which side of the pro- 
 cephalic lobe, in relation to the parts of the embryo, the cumulus 
 is placed. 
 
 Although there appear to be grounds for doubting whether 
 either Balbiani and Claparede are correct in the position they 
 assign to the cumulus, my observations scarcely warrant me in 
 being very definite in my statements on this head, but, as already 
 mentioned, I am inclined to place the organ near the posterior 
 end (and therefore, as will be afterwards shewn, in a somewhat 
 dorsal situation) of the ventral embryonic thickening. 
 
 In my earliest stage of the third period there is present, as 
 has already been stated, a procephalic lobe, and an indistinct 
 and not very prominent caudal portion, and about three segments 
 between the two. The definition of the parts of the blastoderm 
 at this stage is still very imperfect, but from subsequent stages it 
 appears to me probable that the first of the three segments is 
 that of the first pair of ambulatory limbs, and that the segments 
 of the chelicerai and pedipalpi are formed later than those of 
 the first three ambulatory appendages. 
 
 Balbiani believes that the segment of the chelicerje is formed 
 later than that of the six succeeding segments. He further 
 concludes, from the fact that this segment is cut off from the 
 procephalic portion in front, that it is really part of the pro- 
 cephalic lobe. I cannot accept the validity of this argument ; 
 though I am glad to find myself in, at any rate, partial harmony 
 with the distinguished French embryologist as to the facts. 
 Balbiani denies for this stage the existence of a caudal lobe. 
 There is certainly, as is very well shewn in my longitudinal 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 675 
 
 sections, a thickening of the blastoderm in the caudal region, 
 though it is not so prominent in surface views as the procephalic 
 lobe. 
 
 A transverse section through an embryo at this stage (PI. 31, 
 fig. 12) shews that there is a ventral plate of somewhat columnar 
 cells more than one row deep, and a dorsal portion of the blasto- 
 derm formed of a single row of flattened cells. Every section 
 at this stage shews that the inner layer of cells of the ventral 
 plate is receiving accessions of cells from the yolk, which has 
 not to any appreciable extent altered its constitution. A large 
 cell, passing from the yolk to the blastoderm, is shewn in fig. 12 
 at y. c. 
 
 The cells of the ventral plate are now divided into tivo distinct 
 layers. The outer of these is the epiblast, the inner the mcso- 
 blast. The cells of both layers are quite continuous across the 
 median line, and exhibit no trace of a bilateral arrangement. 
 
 This stage is an interesting one on account of the striking 
 similarity which (apart from the amnion.) exists between a sec- 
 tion through the blastoderm of a spider and that of an insect 
 immediately after the formation of the mesoblast. The reader 
 should compare Kowalevsky's (Mhn. Acad. Pttersbonrg, Vol. 
 XVI. 1871) fig. 26, PI. IX. with my fig. 12. The existence of a 
 continuous ventral plate of mesoblast has been noticed by 
 Barrois (p. 532), who states that the two mesoblastic bands 
 originate from the longitudinal division of a primitive single 
 band. 
 
 In a slightly later stage (PL 30, fig. 3^ and 3 b} six distinct 
 segments are interpolated between the procephalic and the 
 caudal lobes. The two foremost, c/i and pd (especially the first), 
 of these are far less distinct than the remainder, and the first 
 segment is very indistinctly separated from the procephalic lobe. 
 From the indistinctness of the first two somites, I conclude that 
 they are later formations than the four succeeding ones. -The 
 caudal and procephalic lobes are very similar in appearance, but 
 the procephalic lobe is slightly the wider of the two. There is 
 a slight protuberance on the caudal lobe, which is possibly the 
 remnant of the cumulus. The superficial appearance of seg- 
 mentation is produced by a series of transverse valleys, sepa- 
 rating raised intermediate portions which form the segments. 
 
6/6 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 The ventral thickening of the embryo now occupies rather more 
 than half the circumference of the ovum. 
 
 Transverse sections shew that considerable changes have 
 been effected in the constitution of the blastoderm. In the 
 previous stage, the ventral plate was formed of an uniform ex- 
 ternal layer of epiblast, and a continuous internal layer of meso- 
 blast. The mesoblast has now become divided along the whole 
 length of the embryo, except, perhaps, the procephalic lobes, 
 into two lateral bands which are not continuous across the 
 middle line (PI. 31, fig. 13 me). It has, moreover, become 
 a much more definite layer, closely attached to the epiblast. 
 Between each mesoblastic band and the adjoining yolk there are 
 placed a few scattered cells, which in a somewhat later stage 
 become the splanchnic mesoblast. These cells are derived from 
 the yolk-cells ; and almost every section contains examples of 
 such cells in the act of joining the mesoblast. 
 
 The epiblast of the ventral plate has not, to any great extent, 
 altered in constitution. It is, perhaps, a shade thinner in the 
 median line than it is laterally. The division of the mesoblast 
 plate into two bands, together, perhaps, with the slight reduc- 
 tion of the epiblast in the median ventral line, gives rise at this 
 stage to an imperfectly marked median groove. 
 
 The dorsal epiblast is still formed of a single layer of flat 
 cells. In the neighbourhood of this layer the yolk nuclei are 
 especially concentrated. The yolk itself remains as before. 
 
 The segments continue to increase regularly, each fresh seg- 
 ment being added in the usual way between the last formed 
 segment and the unsegmented caudal lobe. At the stage when 
 about nine or ten segments have become established, the first 
 rudiments of appendages become visible. At this period (PI. 
 30, fig. 4) there is a distinct median ventral groove, extending 
 through the whole length of the embryo, which becomes, how- 
 ever, considerably shallower behind. The procephalic region is 
 distinctly bilobed. The first segment (that of the chelicerae) is 
 better marked off from it than in the previous stage, but is with- 
 out a trace of an appendage, and exhibits therefore, in respect 
 to the development of its appendages, the same retardation that 
 characterised its first appearance. The next five segments, viz. 
 those of the pedipalpi and four ambulatory appendages, present 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/7 
 
 a very well-marked swelling at each extremity. These swejlings 
 are the earliest traces of the appendages. Of the three succeed- 
 ing segments, only the first is well differentiated. The caudal 
 lobe, though less broad than the procephalic lobe, is still a 
 widish structure. The most important internal changes con- 
 cern the mesoblast, which is now imperfectly though distinctly 
 divided into somites, corresponding with segments visible ex- 
 ternally. Each mesoblastic somite is formed of a distinct 
 somatic layer closely attached to the epiblast, and a thinner 
 and less well-marked splanchnic layer. In the appendage- 
 bearing segments the somatic layer is continued up into the 
 appendages. 
 
 The epiblast is distinctly thinner in the median line than at 
 the two sides. 
 
 The next stage figured (PI. 30, figs. 5 and 6) is an important 
 one, as it is characterized by the establishment of the full num- 
 ber of appendages. The whole length of the ventral plate has 
 greatly increased, so that it embraces nearly the circumference 
 of the ovum, and there is left uncovered but a very small arc 
 between the two extremities of the plate (PI. 30, fig. 6; PI. 31, 
 fig. 15). This arc is the future dorsal portion of the embryo, which 
 lags in its development immensely behind the ventral portion. 
 
 There is a very distinctly bilobed procephalic region (pr. 1} 
 well separated from the segment with the chelicerae (c/i). It is 
 marked by a shallow groove opening behind into a circular 
 depression (st.) -the earliest rudiment of the stomodaeum. The 
 six segments behind the procephalic lobes are the six largest, 
 and each of them bears two prominent appendages. They con- 
 stitute the six appendage-bearing segments of the adult. The 
 four future ambulatory appendages are equal in size : they are 
 slightly larger than the pedipalpi, and these again than the 
 chelicerae. Behind the six somites with prominent appendages 
 there are four well-marked somites, each with a small protuber- 
 ance. These four protuberances are provisional appendages. 
 They have been found in many other genera of Araneina (Clapa- 
 rede, Barrois). The segments behind these are rudimentary and 
 difficult to count, but there are, at any rate, five, and at a slightly 
 later stage probably six, including the anal lobe. These fresh 
 segments have been formed by the continued segmentation of 
 
678 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 the anal lobe, which has greatly altered its shape in the process. 
 The ventral groove of the earlier stage is still continued along 
 the whole length of the ventral plate. 
 
 By the close of this stage the full number of post-cephalic 
 segments has become established. They are best seen in the 
 longitudinal section (PI. 31, fig. 15). There are six anterior 
 appendage-bearing segments, followed by four with rudimentary 
 appendages (not seen in this figure), and six without appendages 
 behind. There are, therefore, sixteen in all. This number 
 accords with the result arrived at by Barrois, but is higher by 
 two than that given by Claparede. 
 
 The germinal layers (vide PL 31, fig. 14) have by this stage 
 undergone a further development. The mesoblastic somites are 
 more fully developed. The general relations of these somites 
 is shewn in longitudinal section in PL 31, fig. 15, and in trans- 
 verse section in PL 31, fig. 14. In the tail, where they are 
 simplest (shewn on the upper side in fig. 14), each mesoblastic 
 somite is formed of a somatic layer of more or less cubical cells 
 attached to the epiblast, and a splanchnic layer of flattened cells. 
 Between the two is placed a completely circumscribed cavity, 
 which constitutes part of the embryonic body-cavity. Between 
 the yolk and the splanchnic layer are placed a few scattered 
 cells, which form the latest derivatives of the yolk-cells, and are 
 to be reckoned as part of the splanchnic mesoblast. The meso- 
 blastic somites do not extend outwards beyond the edge of the 
 ventral plate, and the corresponding mesoblastic somites of the 
 two sides do not nearly meet in the middle line. In the limb- 
 bearing somites the mesoblast has the same general characters 
 as in the posterior somites, but the somatic layer is prolonged as 
 a hollow papilliform process into the limb, so that each limb 
 has an axial cavity continuous with the section of the body- 
 cavity of its somite. The description given by Metschnikoff 
 of the formation of the mesoblastic somites in the scorpion, 
 and their continuation into the limbs, closely corresponds with 
 the history of these parts in spiders. In the region of each 
 procephalic lobe the mesoblast is present as a continuous layer 
 underneath the epiblast, but in the earlier part of the stage, 
 at any rate, is not formed of two distinct layers with a cavity 
 between them. 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/9 
 
 The epiblast at this stage has also undergone important 
 changes. Along the median ventral groove it has become very 
 thin. On each side of this groove it exhibits in each append- 
 age-bearing somite a well-marked thickening, which gives in 
 surface views the appearance of a slightly raised area (PI. 30, 
 fig. 5), between each appendage and the median line. These 
 thickenings are the first rudiments of the ventral nerve gang- 
 lia. The ventral nerve cord at this stage is formed of two 
 ridge-like thickenings of the epiblast, widely separated in the 
 median line, each of which is constituted of a series of raised 
 divisions the ganglia united by shorter, less prominent divi- 
 sions (fig. 14, vg}. The nerve cords are formed from before 
 backwards, and are not at this stage found in the hinder seg- 
 ments. There is a distinct ganglionic thickening for tJie chelicercs 
 quite independent of the proccpJialic lobes. 
 
 In the procephalic lobes the epiblast is much thickened, 
 and is formed of several rows of cells. The greater part of 
 it is destined to give rise to the supra-cesophageal ganglia. 
 
 During the various changes which have been described the 
 blastoderm cells have been continually dividing, and, together 
 with their nuclei, have become considerably smaller than at 
 first. The yolk cells have in the meantime remained much as 
 before, and are, therefore, considerably larger than the nuclei 
 of the blastoderm cells. They are more numerous than in the 
 earlier stages, but are still surrounded by a protoplasmic body, 
 which is continued into a protoplasmic reticulum. The yolk is 
 still divided up into polygonal segments, but from sections it 
 would appear that the nuclei are more numerous than the seg- 
 ments, though I have failed to arrive at quite definite conclu- 
 sions on this point. 
 
 As development proceeds the appendages grow longer, and 
 gradually bend inwards. They become very soon divided by 
 a series of ring-like constrictions which constitute the first indi- 
 cations of the future joints (PI. 30, fig. 6). The full number of 
 joints are not at once reached, but in the ambulatory ap- 
 pendages five only appear at first to be formed. There are four 
 joints in the pedipalpi, while the chelicerae do not exhibit any 
 signs of becoming jointed till somewhat later. The primitive 
 presence of only five joints in the ambulatory appendages 
 
68o NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 is interesting, as this number is permanent in Insects and in 
 Peripatus. 
 
 The next stage figured forms the last of the third period 
 (PI. 30, figs. 7 and yd). The ventral plate is still rolled round the 
 egg (fig. 7), and the end of the tail and the procephalic lobes 
 nearly meet dorsally, so that there is but a very slight develop- 
 ment of the dorsal region. There are the same number of 
 segments as before, and the chief differences in appearance be- 
 tween the present and the previous stage depend upon the fact 
 (i) that the median ventral integument between the nerve 
 ganglia has become wider, and at the same time thinner ; (2) 
 that the limbs have become much more developed; (3) that 
 the stomodaeum is definitely established; (4) that the pro- 
 cephalic lobes have undergone considerable development. 
 
 Of these features, the three last require a fuller description. 
 The limbs of the two sides are directed towards each other, and 
 nearly, meet in the ventral line. The chelicerae are two-jointed, 
 and terminate in what appear like rudimentary chelae, a fact 
 which perhaps indicates that the spiders are descended from 
 ancestors with chelate chelicerae. The four embryonic post- 
 ambulatory appendages are now at the height of their develop- 
 ment. 
 
 The stomodaeum (PI. 30, fig. 7, and PI. 31, fig. 17, sf) is a 
 deepish pit between the two procephalic lobes, and distinctly in 
 front of the segment of the cheliceran. It is bordered in front by 
 a large, well-marked, bilobed upper lip, and behind by a smaller 
 lower lip. The large upper lip is a temporary structure, to be 
 compared, perhaps, with the gigantic upper lip of the embryo of 
 Chelifer (cf. Metschnikoff). On each side of and behind the 
 mouth two whitish masses are visible, which are the epiblastic 
 thickenings which constitute the ganglia of the chelicerae (PI. 30, 
 fig- 7> ch. g). 
 
 The procephalic lobes (pr. /) now form two distinct masses, 
 and each of them is marked by a semicircular groove, dividing 
 them into a narrower anterior and a broader posterior division. 
 
 In the region of the trunk the general arrangement of the 
 germinal layers has not altered to any great extent. The ven- 
 tral ganglionic thickenings are now developed in all the segments 
 in the abdominal as well as in the thoracic region. The individ- 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 68 1 
 
 ual thickenings themselves, though much more conspicuous than 
 in the previous stage (PI. 31, fig. 16, v. c], are still integral parts 
 of the epiblast. They are more widely separated than before in 
 the middle line. The mesoblastic somites retain their earlier 
 constitution (PI. 31, fig. 16). Beneath the procephalic lobes the 
 mesoblast has, in most respects, a constitution similar to that of 
 a mesoblastic somite in the trunk. It is formed of two bodies, 
 one on each side, each composed of a splanchnic and somatic 
 layer (PI. 31, fig. 17, sp. and so), enclosing between them a 
 section of the body-cavity. But the cephalic somites, unlike 
 those of the trunk, are united by a median bridge of mesoblast, 
 in which no division into two layers can be detected. This 
 bridge assists in forming a thick investment of mesoblast round 
 the stomodaeum (st). 
 
 The existence of a section of the body-cavity in the praeoral 
 region is a fact of some interest, especially when taken in con- 
 nection with the discovery, by Kleinenberg, of a similar structure 
 in the head of Lumbricus. The procephalic lobe represents the 
 praeoral lobe of Chaetopod larvae, but the prolongation of the 
 body-cavity into it does not, in my opinion, necessarily imply 
 that it is equivalent to a post-oral segment. 
 
 The epiblast of the procephalic lobes is a thick layer several 
 . cells deep, but without any trace of a separation of the ganglio- 
 nic portion from the epidermis. 
 
 The nuclei of the yolk have, increased in number, but the 
 yolk, in other respects, retains its earlier characters. 
 
 The next period in the development is that in which the 
 body of the embryo gradually acquires the adult form. The 
 most important event which takes place during this period is 
 the development of the dorsal region of the embryo, which, up 
 to its commencement, is practically non-existent. As a con- 
 sequence of the development of the dorsal region, the embryo, 
 which has hitherto had what may be called a dorsal flexure, 
 ^gradually unrolls itself, and acquires a ventral flexure. This 
 change in the flexure of the embryo is in appearance a rather 
 complicated phenomenon, and has been somewhat differently 
 described by the two naturalists who have studied it in recent 
 times. 
 
 !<\>r Claparedc the prime cause of the change of flexure is 
 B. 44 
 
682 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 the translation dorsalwards of the limbs. He compares the 
 dorsal region of the embryo to the arc of a circle, the two ends 
 of which are united by a cord formed by the line of insertion of 
 the limbs. He points out that if you bring the middle of the 
 cord, so stretched between the two ends of the arc, nearer to the 
 summit of the arc, you necessarily cause the two ends of the 
 arc to approach each other, or, in other words, if the insertion 
 of the limbs is drawn up dorsally, the head and tail must ap- 
 proach each other ventrally. 
 
 Barrois takes quite a different view to that of Claparede, 
 which will perhaps be best understood if I quote a translation 
 of his own words. He says : " At the period of the last stage 
 of the embryonic band (the stage represented in PI. 31, fig. 7, in 
 the present paper) this latter completely encircles the egg, and 
 its posterior extremity nearly approaches the cephalic region. 
 Finally, the germinal bands, where they unite at the anal lobe 
 (placed above on the dorsal surface), form between them a very 
 acute angle. During the following stages one observes the anal 
 segment separate further and further from the cephalic region, 
 and approach nearer and nearer to the ventral region. This 
 displacement of the anal segment determines, in its turn, a 
 modification in the divergence of the anal bands ; the angle 
 which they form at their junction tends to become more obtuse. 
 The same processes continue regularly till the anal segment 
 comes to occupy the opposite extremity to the cephalic region, 
 a period at which the two germinal bands are placed in the 
 same plane and the two sides of the obtuse angle end by 
 meeting in a straight line. If we suppose a continuation of the 
 same phenomenon it is clear that the anal segment will come to 
 occupy a position on the ventral surface, and the germinal bands 
 to approach, but in the inverse way, so as to form an angle 
 opposite to that which they formed at first. This condition 
 ends the process by which the posterior extremity of the em- 
 bryonic band, at first directed towards the dorsal side, comes to 
 bend in towards the ventral region." 
 
 Neither of the above explanations is to my mind perfectly 
 satisfactory. The whole phenomenon appears to me to be very 
 simple, and to be caused by the elongation of the dorsal region, 
 i.e. the region on the dorsal surface between the anal and pro- 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 683 
 
 cephalic lobes. Such an elongation necessarily separates^ the 
 anal and procephalic lobes ; but, since the ventral plate does 
 not become shortened in the process, and the embryo cannot 
 straighten itself on account of the egg-shell, it necessarily be- 
 comes flexed, and such flexure can only be what I have already 
 called a ventral flexure. If there were but little food yolk this 
 flexure would cause the whole embryo to be bent in, so as to 
 have the ventral surface concave, but instead of this the flexure 
 is confined at first to the two bands which form the ventral 
 plate. These bands are bent in the natural way (PL 30, fig. 8, B v , 
 but the yolk forms a projection, a kind of yolk-sack as Barrois 
 calls it, distending the thin integument between the two ventral 
 bands. This yolk-sack is shewn in surface view in PL 30, fig. 8, 
 and in section in PL 32, fig. 18. At a later period, when the 
 yolk has become largely absorbed in the formation of various 
 organs, the true nature of the ventral flexure becomes apparent, 
 and the abdomen of the young Spider is found to be bent over 
 so as to press against the ventral surface of the thorax (PL 30, 
 fig. 9). This flexure is shewn in section in PL 32, fig. 21. 
 
 At the earliest stage of this period of which I have ex- 
 amples, the dorsal region has somewhat increased, though not 
 very much. The limbs have grown very considerably and now 
 cross in the middle line. 
 
 The ventral ganglia, though not the supra-cesophageal, have 
 become separated from the epiblast. 
 
 The yolk nuclei, each surrounded by protoplasm as before, 
 are much more numerous. 
 
 In other respects there are no great changes in the internal 
 features. 
 
 In my next stage, represented in PL 30, figs. 8 a, and 8 b, a 
 very considerable advance has become effected. In the first 
 place the dorsal surface has increased in length to rather more 
 than one half the circumference of the ovum. The dorsal region 
 has, however, not only increased in length, but also in definite- 
 ness, and a series of transverse markings (figs. 8 a and b}, which 
 are very conspicuous in the case of the four anterior abdominal 
 segments (the segments with rudimentary appendages), have 
 appeared, indicating the limits of segments dorsally. The terga 
 of the somites may, in fact, be said to have become formed. 
 
 442 
 
684 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 The posterior terga (fig. 8 a) are very narrow compared to the 
 anterior. 
 
 The caudal protuberance is more prominent than it was, and 
 somewhat bilobed ; it is continued on each side into one of the 
 bands, into which the ventral plate is divided. These bands, as 
 is best seen in side view (fig. 8 &), have a ventral curvature, or, 
 perhaps more correctly, are formed of two parts, which meet at 
 a large angle open towards the ventral surface. The posterior 
 of these parts bears the four still very conspicuous provisional 
 appendages, and the anterior the six pairs of thoracic append- 
 ages. The four ambulatory appendages are now seven-jointed, 
 as in the adult, but though longer than in the previous stage 
 they do not any longer cross or even meet in the middle line, but 
 are, on the contrary, separated by a very considerable interval. 
 This is due to the great distension by the yolk of the ventral 
 part of the body, in the interval between the two parts of the 
 original ventral plate. The amount of this yolk may be gathered 
 from the section (PL 32, fig. 18). The pedipalpi carry a blade 
 on their basal joint. The chelicerae no longer appear to spring 
 from an independent postoral segment. 
 
 There is a conspicuous lower lip, but the upper is less 
 prominent than before. Sections at this stage shew that the 
 internal changes have been nearly as considerable as the ex- 
 ternal. 
 
 The dorsal region is now formed of a (i) flattened layer of 
 epiblast cells, and a (2) fairly thick layer of large and rather 
 characteristic cells which any one who has studied sections of 
 spider's embryos will recognize as derivatives of the yolk. 
 These cells are not, therefore, derived from prolongations of the 
 somatic and splanchnic layers of the already formed somites, 
 but are new formations derived from the yolk. They com- 
 menced to be formed at a much earlier period, and some of 
 them are shewn in the longitudinal section (PL 31, fig. 15). In 
 the next stage these cells become differentiated into the somatic 
 and splanchnic mesdblast layers of the dorsal region of the 
 embryo. 
 
 In the dorsal region of the abdomen the heart has already 
 become established. So far as I have been able to make out it 
 is formed from a solid cord of the cells of the dorsal region. 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 685 
 
 The peripheral layer of this cord gives rise to the walls of the 
 heart, while the central cells become converted into the cor- 
 puscles of the blood. 
 
 The rudiment of the heart is in contact with the epiblast 
 above, and there is no greater evidence of its being derived from 
 the splanchnic than from the somatic mesoblast; it is, in fact, 
 formed before the dorsal mesoblast has become differentiated 
 into two layers. 
 
 In the abdomen three or four transverse septa, derived from 
 the splanchnic mesoblast, grow a short way into the yolk. 
 They become more conspicuous during the succeeding stage, 
 and are spoken of in detail in the description of that stage. 
 In the anterior part of the thorax a longitudinal and vertical 
 septum is formed, which grows downwards from the median 
 dorsal line, and divides the yolk in this region into two parts. 
 In this septum there is formed at a later stage a vertical muscle 
 attached to the suctorial part of the stomodaeum. 
 
 The mesoblastic somites of the earlier stage are but little 
 modified ; and there are still prolongations of the body cavity 
 into the limbs (PL 32, fig. 18). 
 
 The lateral parts of the ventral nerve cords are now at their 
 maximum of separation (PI. 32, fig. 18, v. g.). Considerable 
 differentiation has already set in in the constitution of the 
 ganglia themselves, which are composed of an outer mass of 
 ganglion cells enclosing a kernel of nerve fibres, which lie on 
 the inner side and connect the successive ganglia. There are 
 still distinct thoracic and abdominal ganglia for each segment, 
 and there is also a pair of separate ganglion for the chelicerae, 
 which assists, however, in forming the oesophageal commissures. 
 
 The thickenings of the praeoral lobe which form the supra- 
 cesophageal ganglia are nearly though not quite separated from 
 the epiblast. The semicircular grooves of the earlier stages are 
 now deeper than before, and are well shewn in sections nearly 
 parallel to the outer anterior surface of the ganglion (PI. 32, 
 fig. 19). The supra-oesophageal ganglia are still entirely formed 
 of undifferentiated cells, and are without commissural tissue like 
 that present in the ventral ganglia. 
 
 The stomodaeum has considerably increased in length, and 
 the proctodaium has become formed as a short, posteriorly 
 
686 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 directed involution of the epiblast I have seen traces of what 
 I believe to be two outgrowths from it, which form the Mal- 
 pighian bodies. 
 
 The next stage constitutes (PL 30, fig. 9) the last which 
 requires to be dealt with so far as the external features are con- 
 cerned. The yolk has now mainly passed into the abdomen, 
 and the constriction separating the thorax and abdomen has 
 begun to appear. The yolk-sack has become absorbed, so that 
 the two halves of the ventral plate in the thorax are no longer 
 widely divaricated. The limbs have to a large extent acquired 
 their permanent structure, and the rings of which they are 
 formed in the earlier stages are now replaced by definite joints. 
 A delicate cuticle has become formed, which is not figured in 
 my sections. The four rudimentary appendages have dis- 
 appeared, unless, which seerns to me in the highest degree im- 
 probable, they remain as the spinning mammillae, two pairs of 
 which are now present. Behind is the anal lobe, which is much 
 smaller and less conspicuous than in the previous stage. The 
 spinnerets and anal lobe are shewn as five papillae in PL 30, fig. 9. 
 Dorsally the heart is now very conspicuous, and in front of the 
 chelicerae may be seen the supra-cesophageal ganglia. 
 
 The indifferent mesoblast has now to a great extent become 
 converted into the permanent tissues. On the dorsal surface 
 there was present in the last stage a great mass of unformed 
 mesoblast cells. This mass of cells has now become divided 
 into a somatic and splanchnic layer (PL 32, fig. 22). It has, 
 moreover, in the abdominal region at any rate, become divided 
 up into somites. At the junction between the successive somites 
 the splanchnic mesoblast on each side of the abdomen dips 
 down into the yolk and forms a septum (PL 32, fig. 22 s). 
 The septa so formed, which were first described by Barrois, 
 are not complete. The septa of the two sides do not, in the 
 first place, quite meet along the median dorsal or ventral lines, 
 and in the second place they only penetrate the yolk for a 
 certain distance. Internally they usually end in a thickened 
 border. 
 
 Along the line of insertion of each of these septa there is 
 developed a considerable space between the somatic and splanch- 
 nic layers of mesoblast. The parts of the body-cavity so estab- 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 68/ 
 
 lished are transversely directed channels passing from the Jieart 
 outwards. They probably constitute the venous spaces, and 
 perhaps also contain the transverse aortic branches. 
 
 In the intervals between these venous spaces the somatic and 
 splanchnic layers of mesoblast are in contact with each other. 
 
 I have not been able to work out satisfactorily the later 
 stages of development of the septa, but I have found that 
 they play an important part in the subsequent development 
 of the abdomen. In the first place they send off lateral off- 
 shoots, which unite the various septa together, and divide up 
 the cavity of the abdomen into a number of partially sepa- 
 rated compartments. There appears, however, to be left a 
 free axial space for the alimentary tract, the mesoblastic walls 
 of which are, I believe, formed from the septa. 
 
 At the present stage the splanchnic mesoblast, apart from 
 the septa, is a delicate membrane of flattened cells (fig. 22, sp}. 
 The somatic mesoblast is thicker, and is formed of scattered 
 cells (so). 
 
 The somatic layer is in part converted, in the posterior 
 region of the abdomen, into a delicate layer of longitudinal 
 muscles, the fibres of which are not continuous for the whole 
 length of the body, but are interrupted at the lines of junc- 
 tion of the successive segments. They are not present in the 
 anterior part of the abdomen. The longitudinal direction of 
 these fibres, and their division with myotomes, is interesting, 
 since both these characters, which are preserved in Scorpions, 
 are lost in the abdomen of the adult Spider. 
 
 The original mesoblastic somites have undergone quite as 
 important changes as the dorsal mesoblast In the abdominal 
 region the somatic layer constitutes two powerful bands of 
 longitudinal muscles, inserted anteriorly at the root of the 
 fourth ambulatory appendage, and posteriorly at the spinning 
 mammillae. Between these two bands are placed the nervous 
 bands. The relation of these parts are shewn in the section 
 in PL 32, fig. 20 d, which cuts the abdomen horizontally and 
 longitudinally. The mesoblastic bands are seen at ;., and the 
 nervous bands within them at ab. g. In the thoracic region 
 the part of the somatic layer in each limb is converted into 
 muscles, which are continued into dorsal and ventral muscles 
 
688 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 in the thorax (vide fig. 20 c)> There are, in addition to these, 
 intrinsic transverse fibres on the ventral side of the thorax. 
 Besides these muscles there are in the thorax, attached to the 
 suctorial extremity of the stomodaeum, three powerful muscles, 
 which I believe to be derived from the somatic mesoblast. One 
 of these passes vertically down from the dorsal surface, in the 
 septum the commencement of which was described in the last 
 stage. The two other muscles are lateral, one on each side (PI. 
 31, fig. 20 c.). 
 
 The heart has now, in most respects, reached its full de- 
 velopment. It is formed of an outer muscular layer, within 
 which is a doubly-contoured lining, containing nuclei at inter- 
 vals, which is probably of the nature of an epithelioid lining 
 (PI. 32, fig. 22 hf}. In its lumen are numerous blood-corpuscles 
 (not represented in my figure). The heart lies in a space bound 
 below by the splanchnic mesoblast, and to the sides by the 
 somatic mesoblast. This space forms a kind of pericardium 
 (fig. 22 pc\ but dorsally the heart is in contact with the epi- 
 blast. The arterial trunks connected with it are fully established. 
 
 The nervous system has undergone very important changes. 
 
 In the abdominal region the ganglia of each side have fused 
 together into a continuous cord (fig. 21 ab. g.}. In fig. 20, in 
 which the abdomen is cut horizontally and longitudinally, there 
 are seen the two abdominal cords (ab. g^} united by two trans- 
 verse commissures; and I believe that there are at this stage 
 three or four transverse commissures at any rate, which remain 
 as indications of the separate ganglia, from the coalescence of 
 which the abdominal cords are formed. The two abdominal 
 cords are parallel and in close contact. 
 
 In the thoracic region changes of not less importance have 
 taken place. The ganglia are still distinct. The two cords 
 formed of these ganglia are no longer widely separated in 
 median line, but meet, in the usual way, in the ventral line. 
 Transverse commissures have become established (fig. 20 c) be- 
 tween the ganglia of the two sides. There is as little trace at 
 this, as at the previous stages, of an ingrowth of epiblast, to 
 form a median portion of the central nervous system. Such 
 a median structure has been described by Hatschek for Lepi- 
 doptera, and he states that it gives rise to the transverse com- 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 689 
 
 missures between the ganglia. My observations shew that for 
 the spider, at any rate, nothing of the kind is present. 
 
 As shewn in the longitudinal section (PI. 32, fig. 21), the 
 ganglion of the chelicerae has now united with the supra-ceso- 
 phageal ganglion. It forms, as is shewn in fig. 20 b (ch. g.}, 
 a part of the oesophageal commissure, and there is no sub- 
 cesophageal commissure uniting the ganglia of the chelicerae, 
 but the oesophageal ring is completed below by the ganglia of 
 the pedipalpi (fig. 20 c,pd. g.}. 
 
 The supra-oesophageal ganglia have become completely sepa- 
 rated from the epiblast. 
 
 I have unfortunately not studied their constitution in the 
 adult, so that I cannot satisfactorily identify the parts which can 
 be made out at this stage. 
 
 I distinguish, however, the following regions: 
 
 (1) A central region containing the commissural part, and 
 continuous below with the ganglia of the chelicerae. 
 
 (2) A dorsal region formed of two hemispherical lobes. 
 
 (3) A ventral anterior region. 
 
 The central region contains in its interior the commissural 
 portion, forming a punctiform, rounded mass in each ganglion. 
 A transverse commissure connects the two (vide fig. 20 b}. 
 
 The dorsal hemispherical lobes are derived from the part 
 which, at the earlier stage, contained the semicircular grooves. 
 When the supra-oesophageal ganglia become separated from the 
 epidermis the cells lining these grooves become constricted off 
 with them, and form part of these ganglia. Two cavities are 
 thus formed in this part of the supra oesophageal ganglia. 
 These cavities become, for the most part, obliterated, but persist 
 at the outer side of the hemispherical lobes (figs. 20 a and 21). 
 
 The ventral lobe of the brain is a large mass shewn in 
 longitudinal section in fig. 21. It lies immediately in front of 
 and almost in contact with the ganglia of the chelicerae. 
 
 The two hemispherical lobes agree in position with the fungi- 
 form body (pilshittformige Korpeni), which has attracted so much 
 the attention of anatomists, in the supra-oesophageal ganglia of 
 Insects and Crustacea; but till the adult brain of Spiders has 
 been more fully studied it is not possible to state whether the 
 hemispherical lobes become fungiform bodies. 
 
690 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 Hatschek 1 has described a special epiblastic invagination in 
 the supra-cesophageal ganglion of Bombyx, which is probably 
 identical with the semicircular groove of Spiders and Scorpions, 
 but in the figure he gives the groove does not resemble that in 
 the Arachnida. A similar groove is found in Peripat i<--, and 
 there forms, as I have found, a large part of the supra-ceso- 
 phageal ganglia. It is figured by Moseley, Phil. Trans., Vol. 
 CLXIV. pi. Ixxv, fig. 9. 
 
 The stomodseum is considerably larger than in the last stage, 
 and is lined by a cuticle; it is a blind tube, the blind end of 
 which is the suctorial pouch of the adult. To this pouch are 
 attached the vertical dorsal, and two lateral muscles spoken of 
 above. 
 
 The protodaeum (/r.) has also grown in length, and the two 
 Malpighian vessels which grow out from its blind extremity 
 (fig. 20 e. nip. g.} have become quite distinct. The part now 
 formed is the rectum of the adult. The proctodasum is sur- 
 rounded by a great mass of splanchnic mesoblast. The mesen- 
 teron has as yet hardly commenced to be developed. There 
 is, however, a short tube close to the proctodaeum (fig. 20 e. 
 mes], which would seem to be the commencement of it. It 
 ends blindly on the side adjoining the rectum, but is open an- 
 teriorly towards the yolk, and there can be very little doubt that 
 it owes its origin to cells derived from the yolk. On its outer 
 surface is a layer of mesoblast. 
 
 From the condition of the mesenteron at this stage there 
 can be but little doubt that it will be formed, not on the surface, 
 but in the interior of the yolk. I failed to find any trace of an 
 anterior part of the mesenteron adjoining the stomodaeum. In 
 the posterior part of the thorax (vide fig. 20 d], there is un- 
 doubtedly no trace of the alimentary tract. 
 
 The presence of this rudiment shews that Barrois is mis- 
 taken in supposing that the alimentary canal is formed entirely 
 from the stomodaeum and proctodaeum, which are stated by him 
 to grow towards each other, and to meet at the junction of the 
 thorax and abdomen. My own impression is that the stomo- 
 daeum and proctodaeum have reached their full extension at the 
 
 1 " Bcitragc z. Entwick. d. Lepidopteren," Jenaische Zeit., Vol. XI. p. 124. 
 
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 69! 
 
 present stage, and that both the stomach in the thorax .and the 
 intestine in the abdomen are products of the mesenteron. 
 
 The yolk retains its earlier constitution, being divided into 
 polygonal segments, formed of large yolk vesicles. The nuclei 
 are more numerous than before. In the thorax the yolk is 
 anteriorly divided into two lobes by the vertical septum, which 
 contains the vertical muscle of the suctorial pouch. In the 
 posterior part of the thorax it is undivided. 
 
 I have not yet been able clearly to make out the eventual 
 fate of the yolk. At a subsequent stage, when the cavity of the 
 abdomen is cut up into a series of compartments by the growth 
 of the septa, described above, the yolk fills these compartments, 
 and there is undoubtedly a proliferation of yolk cells round the 
 walls of these compartments. It would not be unreasonable to 
 conclude from this that the compartments were destined to form 
 the hepatic caeca, each caecum being enclosed in a layer of 
 splanchnic mesoblast, and its hypoblastic wall being derived 
 from the yolk cells. I think that this hypothesis is probably 
 correct, but I have met with some facts which made me think it 
 possible that the thickenings at the ends of the septa, visible in 
 PL 32, fig. 22, were the commencing hepatic caeca. 
 
 I must, in fact, admit that I have hitherto failed to work 
 out satisfactorily the history of the mesenteron and its append- 
 ages. The firm cuticle of young spiders is an obstacle both in 
 the way of making sections and of staining, which I have not 
 yet overcome. 
 
 General Conclusions. 
 
 Without attempting to compare at length the development 
 of the spiders with that of other Arthropoda, I propose to point 
 out a few features in the development of spiders, which appear 
 to shew that the Arachnida are undoubtedly more closely re- 
 lated to the other Tracheata than to the Crustacea. 
 
 The whole history of the formation of the mesoblast is very 
 similar to that in insects. The mesoblast in both groups is 
 formed by a thickening of the median line of the ventral plate 
 (germinal streak). 
 
692 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 In insects there is usually formed a median groove, the walls 
 of which become converted into a plate of mesoblast. In spiders 
 there is no such groove, but a median keel- like thickening of the 
 ventral plate (PI. 31, fig. u), is very probably an homologous 
 structure. The unpaired plate of mesoblast formed in both 
 insects and Arachnida is exactly similar, and becomes divided, 
 in both groups, into two bands, one on each side of the middle 
 line. Such differences as there are between Insects and Arach- 
 nida sink into insignificance compared with the immense differ- 
 ences in the origin of the mesoblast between either group, and 
 that in the Isopoda, or, still more, the Malacostraca and most 
 Crustacea. In most Crustacea we find that the mesoblast is 
 budded off from the walls of an invagination, which gives rise to 
 the mesenteron. 
 
 In both spiders and Myriopoda, and probably insects, the 
 mesoblast is subsequently divided into somites, the lumen of 
 which is continued into the limbs. In Crustacea mesoblastic 
 somites have not usually been found, though they appear occa- 
 sionally to occur, e.g. Mysis, but they are in no case similar to 
 those in the Tracheata. 
 
 In the formation of the alimentary tract, again, the differ- 
 ences between the Crustacea and Tracheata are equally marked, 
 and the Arachnida agree with the Tracheata. There is gene- 
 rally in Crustacea an invagination, which gives rise' to the 
 mesenteron. In Tracheata this never occurs. The proctodaeum 
 is usually formed in Crustacea before or, at any rate, not later 
 than the stomodaeum 1 . The reverse is true for the Tracheata. 
 In Crustacea the proctodaeum and stomodaeum, especially the 
 former, are very long, and usually give rise to the greater part 
 of the alimentary tract, while the mesenteron is usually short. 
 
 In the Tracheata the mesenteron is always considerable, and 
 the proctodaeum is always short. The derivation of the Mal- 
 pighian bodies from the proctodaeum is common to most Tra- 
 cheata. Such organs are not found in the Crustacea. 
 
 With reference to other points in my investigations, the 
 evidence which I have got that the chelicerae are true postoral 
 appendages supplied in the embryo from a distinct postoral 
 
 1 If Grohben's account of the development of Moina is correct this statement must 
 be considered not to be universally true. 
 
NOTES ON THE DEVELOPMENT OE THE ARANEINA. 693 
 
 ganglion, confirms the conclusions of most previous investi- 
 gators, and shews that these appendages are equivalent to the 
 mandibles, or possibly the first pair of maxillae of other Tra- 
 cheata. The invagination, which I have found, of part of a 
 groove of epiblast in the formation of the supra-cesophageal 
 ganglia is of interest, owing to the wide extension of a similar 
 occurrence amongst the Tracheata. 
 
 The wide divarication of the ventral nerve cords in the em- 
 bryo renders it easy to prove that there is no median invagina- 
 tion of epiblast between them, and supports Kleinenberg's 
 observations on Lumbricus as to the absence of this invagina- 
 tion. I have further satisfied myself as to the absence of such 
 an invagination in Peripatus. It is probable that Hatschek and 
 other observers who have followed him are mistaken in affirming 
 the existence of such an invagination in either the Chaetopoda 
 or the Arthropoda. 
 
 The observations recorded in this paper on the yolk cells 
 and their derivations are, on the whole, in close harmony with 
 the observations of Dohrn, Bobretzky, and Graber, on Insects. 
 They shew, however, that the first formed mesoblastic plate 
 does not give rise to the whole of the mesoblast, but that during 
 the whole of embryonic life the mesoblast continues to receive 
 accessions of cells derived from the cells of the yolk. 
 
 Araneina. 
 
 1. Balbiani, " Mdmoire sur le DeVeloppement des Araneides," Ann. 
 Set. Nat., series v, Vol. xvn. 1873. 
 
 2. J. Barrois, " Recherches s. 1. DeVeloppement des Araigne"es," Journal 
 de PAnat. et de la Physiol., 1878. 
 
 3. E. Claparede, Recherches s. V Evolution des Aratgn&s, Utrecht, 
 1860. 
 
 4. Her old, De Generations Araniorum in Ovo, Marburg, 1824. 
 
 5. H. Ludwig, "Ueb. d. Bildung des Blastoderm bei d. Spinnen," 
 Zeit.f. iviss.Zool., Vol. XXVI. 1876. 
 
694 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 
 
 EXPLANATION OF PLATES 30, 31, AND 32. 
 
 PLATE 30. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 ch. Chelicerse. ch. g. Ganglion of chelicerae. c. 1. Caudal lobe. p. c. Primitive 
 cumulus, pd. Pedipalpi. pr. I. Praeoral lobe. pp 1 . // 2 . etc. Provisional ap- 
 pendages, sp. Spinnerets, st. Stomodaeum. 
 
 I IV. Ambulatory appendages, i 16. Postoral segments. 
 
 Fig. i. Ovum, with primitive cumulus and streak proceeding from it. 
 
 Fig. 2. Somewhat later stage, in which the primitive cumulus is still visible. 
 Near the opposite end of the blastoderm is a white area, which is probably the 
 rudiment of the procephalic lobe. 
 
 Fig. 30 and 3^. View of an embryo from the ventral surface and from the side 
 when six segments have become established. 
 
 Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of the 
 appendages become visible. 
 
 Fig. 5. Embryo ideally unrolled at the stage when all the appendages have 
 become established. 
 
 Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed 
 from the side. 
 
 Fig. 7. Later stage, viewed from the side. 
 
 Fig. la. 'Same embryo as fig. 7, ideally unrolled. 
 
 Figs. 8a and 8/>. View from the ventral surface and from the side of an embryo, 
 after the ventral flexure has considerably advanced. 
 
 Fig. 9. Somewhat older embryo, viewed from the ventral surface. 
 
 PLATES 31 AND 32. 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 ao. Aorta, ab. g. Abdominal nerve cord. ch. Chelicerae. ch. g. Ganglion of 
 chelicerse. ep. Epiblast. hs. Hemispherical lobe of supra-cesophageal ganglion. 
 At. Heart. /. /. Lower lip. m. Muscles, me. Mesoblast. mes. Mesenteron. mfi.g. 
 Malpighian tube. ms. Mesoblastic somite, a. CEsophagus. p. c. Pericardium. 
 pd. Pedipalpi. pd. g. Ganglion of pedipalpi. pr. Proctodwum (rectum), pr. c. 
 Primitive cumulus, s. Septum in abdomen, so. Somatopleure. sp. Splanchnopleure. 
 
EXPLANATION OF PLATES 30, 31, 32. 695 
 
 sf. Stomodaeum. su. Suctorial apparatus. su. g. Supra-cesophageal ganglion. 
 t&. g. Thoracic ganglion, v. g. Ventral nerve cord. y. c. Cells derived fromTyolk. 
 yk. Yolk. y. n. Nuclei of yolk cells. 
 
 I ^ IV^-. Ganglia of ambulatory limbs, i 16. Postoral segments. 
 
 Fig. 10. Section through an ovum, slightly younger than fig. i. Shewing 
 the primitive cumulus and the columnar character of the cells of one half of the 
 blastoderm. 
 
 Fig. n. Section through an embryo of the same age as fig. 2. Shewing the 
 median thickening of the blastoderm. 
 
 Fig. 12. Transverse section through the ventral plate of a somewhat older embryo. 
 Shewing the division of the ventral plate into epiblast and mesoblast. 
 
 Fig. 13. Section through the ventral plate of an embryo of the same age as 
 fig. 3, shewing the division of the mesoblast of the ventral plate into two mesoblastic 
 bands. 
 
 Fig. 14. Transverse section through an embryo of the same age as fig. 5, passing 
 through an abdominal segment above and a thoracic segment below. 
 
 Fig. 15. Longitudinal section slightly to one side of the middle line through an 
 embryo of the same age. 
 
 Fig. 1 6. Transverse section through the ventral plate in the thoracic region 
 of an embryo of the same age as fig. 7. 
 
 Fig. 17. Transverse section through the procephalic lobes of an embryo of the 
 same age. gr. Section of hemicircular groove in procephalic lobe. 
 
 Fig. 1 8. Transverse section through the thoracic region of an embryo of the 
 same age as fig. 8. 
 
 Fig. 19. Section through the procephalic lobes of an embryo of the same age. 
 
 Fig. 20 a, b, c, d, e. Five sections through an embryo of the same age as fig. 9. 
 a and b are sections through the procephalic lobes, c through the front part of the 
 thorax, d cuts transversely the posterior parts of the thorax, and longitudinally 
 and horizontally the ventral surface of the abdomen, e cuts the posterior part of the 
 abdomen longitudinally and horizontally, and shews the commencement of the 
 mesenteron. 
 
 Fig. 21. Longitudinal and vertical section of an embryo of the same age. The 
 section passes somewhat to one side of the middle line, and shews the structure of the 
 nervous system. 
 
 Fig. 22. Transverse section through the dorsal part of the abdomen of an embryo 
 of the same stage as fig. 9. 
 
XVIII. ON THE SPINAL NERVES OF AMPHIOXUS '. 
 
 IN an interesting memoir devoted to the elucidation of a 
 series of points in the anatomy and development of the Ver- 
 tebrata, Schneider 2 has described what he believes to be motor 
 nerves in Amphioxus, which spring from the anterior side of the 
 spinal cord. According to Schneider these nerves have been 
 overlooked by all previous observers except Stieda. 
 
 I 3 myself attempted to shew some time ago that anterior 
 roots were absent in Amphioxus ; and in some speculations on 
 the cranial nerves, I employed this peculiarity of the nervous 
 system of Amphioxus to support a view that Vertebrata were 
 primitively provided only with nerves of mixed function springing 
 from the posterior side of the spinal cord. Under these circum- 
 stances, Schneider's statement naturally attracted my attention, 
 and I have made some efforts to satisfy myself as to its accuracy. 
 The nerves, as he describes them, are very peculiar. They arise 
 from a number of distinct roots in the hinder third of each 
 segment. They form a flat bundle, of which part passes up- 
 wards and part downwards. When they meet the muscles they 
 bend backwards, and fuse with the free borders of the muscle- 
 plates. The fibres, which at first sight appear to form the nerve, 
 are, however, transversely striated, and are regarded by Schneider 
 as muscles ; and he holds that each muscle-plate sends a process 
 to the edge of the spinal cord, which there receives its innerva- 
 tion. A considerable body of evidence is requisite to justify a 
 belief in the existence of such very extraordinary and un- 
 paralleled motor nerves ; and for my part I cannot say that 
 Schneider's observations are convincing to me. I have attempted 
 to repeat his observations, employing the methods he describes. 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. XX. 1880. 
 
 2 Beitriige z. Anat. u. Entivick. d. Wirbdthiere, Berlin, 1879. 
 
 3 " On the Spinal Nerves of Amphioxus," Journ. of Anat. and Phys. Vol. X. 1876. 
 [This edition, No. IX. p. 197.] 
 
THE SPINAL NERVES OF AMPIIIOXDS. 697 
 
 In the first place, he states that by isolating the spinal cord 
 by boiling in acetic acid, the anterior roots may be brought into 
 view as numerous conical processes of the spinal cord in each 
 segment. I find by treating the spinal cord in this way, that 
 processes more or less similar, but more irregular than those 
 which he figures, are occasionally present ; but I cannot persuade 
 myself that they are anything but parts of the sheath of the 
 spinal cord which is not completely dissolved -by treatment with 
 acetic acid. By treatment with nitric acid no such processes are 
 to be seen, though the whole length and very finest branches of 
 the posterior nerves are preserved. 
 
 By treating with nitric acid and clarifying by oil of cloves, 
 and subsequently removing one half of the body so as to expose 
 the spinal cord in sit A, the origin and distribution of the posterior 
 nerves is very clearly exhibited. But I have failed to detect 
 any trace of the anterior nerve-roots. Horizontal section, which 
 ought also to bring them clearly into view, failed to shew me 
 anything which I could interpret as such. I agree with Schneider 
 that a process of each muscle-plate is prolonged up to the an- 
 terior border of the spinal cord, but I can find no trace of a con- 
 nection between it and the cord. 
 
 Schneider has represented a transverse section in which the 
 anterior nerves are figured. I am very familiar with an ap- 
 pearance in section such as that represented in his figure, but I 
 satisfied myself when I previously studied the nerves in Amphi- 
 oxus, that the body supposed to be a nerve by Schneider was 
 nothing else than part of the intermuscular septum, and after re- 
 examining my sections I see no reason to alter my view. 
 
 A very satisfactory proof that the ventral nerves do not exist 
 would be found, if it could be established that the dorsal nerves 
 contained both motor and sensory fibres. So far I have not 
 succeeded in proving this ; I have not, however, had fresh 
 specimens to assist me in the investigation. Langerhans 1 , whose 
 careful observations appear to me to have been undervalued by 
 Schneider, figures a branch distributed to the muscles, which 
 passes off from the dorsal roots. Till the inaccuracy of this 
 observation is demonstrated, the balance of evidence appears to 
 me to be opposed to Schneider's view. 
 
 1 Arehiv f. Alikros. Anatoiiiif, Vol. XII. 
 
 B. 45 
 
XIX. ADDRESS. TO THE DEPARTMENT OF ANATOMY AND 
 PHYSIOLOGY OF THE BRITISH ASSOCIATION, 1880. 
 
 IN the spring of the present year, Professor Huxley delivered 
 an address at the Royal Institution, to which he gave the felici- 
 tous title of ' The coming of age of the origin of species' It is, as 
 he pointed out, twenty-one years since Mr Darwin's great work 
 was published, and the present occasion is an appropriate one to 
 review the effect which it has had on the progress of biological 
 knowledge. 
 
 There is, I may venture to say, no department of biology the 
 growth of which has not been profoundly influenced by the 
 Darwinian theory. When Messrs Darwin and Wallace first 
 enunciated their views to the scientific world, the facts they 
 brought forward seemed to many naturalists insufficient to sub- 
 stantiate their far-reaching conclusions. Since that time an 
 overwhelming mass of evidence has, however, been rapidly accu- 
 mulating in their favour. Facts which at first appeared to be 
 opposed to their theories have one by one been shewn to afford 
 striking proofs of their truth. There are at the present time but 
 few naturalists who do not accept in the main the Darwinian 
 theory, and even some of those who reject many of Darwin's 
 explanations still accept the fundamental position that all ani- 
 mals are descended from a common stock. 
 
 To attempt in the brief time which I have at my disposal to 
 trace the influence of the Darwinian theory on all the branches 
 of anatomy and physiology would be wholly impossible, and I 
 shall confine myself to an attempt to do so for a small section 
 only. There is perhaps no department of Biology which has 
 been so revolutionised, if I may use the term, by the theory of 
 animal evolution, as that of Development or Embryology. The 
 reason of this is not far to seek. According to the Darwinian 
 
ADDRESS TO THE BRITISH ASSOCIATION. 699 
 
 theory, the present order of the organic world has been caused 
 by the action of two laws, known as the laws of heredity and of 
 variation. The law of heredity is familiarly exemplified by the 
 well-known fact that offspring resemble their parents. Not only, 
 however, do the offspring belong to the same species as their 
 parents, but they inherit the individual peculiarities of their 
 parents. It is on this that the breeders of cattle depend, and it 
 is a fact of every-day experience amongst ourselves. A further 
 point with reference to heredity to which I must call your atten- 
 tion is the fact that the characters, which display themselves at 
 some special period in the life of the parent, are acquired by the 
 offspring at a corresponding period. Thus, in many birds the 
 males have a special plumage in the adult state. The male 
 offspring is not, however, born with the adult plumage, but only 
 acquires it when it becomes adult. 
 
 The law of variation is in a certain sense opposed to the law 
 of heredity. It asserts that the resemblance which offspring 
 bear to their parents is never exact. The contradiction between 
 the two laws is only apparent. All variations and modifications 
 in an organism are directly or indirectly due to its environments; 
 that is to say, they are either produced by some direct influence 
 acting upon the organism itself, or by some more subtle and 
 mysterious action on its parents; and the law of heredity really 
 asserts that the offspring and parent would resemble each other 
 if their environments were the same. Since, however, this is 
 never the case, the offspring always differ to some extent from 
 the parents. Now, according to the law of heredity, every ac- 
 quired variation tends to be inherited, so that, by a summation 
 of small changes, the animals may come to differ from their 
 parent stock to an indefinite extent. 
 
 We are now in a position to follow out the consequences of 
 these two laws in their bearing on development. Their applica- 
 tion will best be made apparent by taking a concrete example. 
 Let us suppose a spot on the surface of some very simple organ- 
 ism to become, at a certain period of life, pigmented, and there- 
 fore to be especially sensitive to light. In the offspring of this 
 form, the pigment-spot will reappear at a corresponding period ; 
 and there will therefore be a period in the life of the offspring 
 during which there is no pigment-spot, and a second period in 
 
 452 
 
/OO ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 which there is one. If a naturalist were to study the life-history, 
 or, in other words, the embryology of this form, this fact about 
 the pigment-spot would come to his notice, and he would be 
 justified, from the laws of heredity, in concluding that the species 
 was descended from an ancestor without a pigment-spot, because 
 a pigment-spot was absent in the young. Now, we may suppose 
 the transparent layer of skin above the pigment-spot to become 
 thickened, so as gradually to form a kind of lens, which would 
 throw an image of external objects on the pigment-spot. In this 
 way a rudimentary eye might be evolved out of the pigment- 
 spot. A naturalist studying the embryology of the form with 
 this eye would find that the pigment-spot was formed before the 
 lens, and he would be justified in concluding, by the same pro- 
 cess of reasoning as before, that the ancestors of the form he 
 was studying first acquired a pigment-spot and then a lens. We 
 may picture to ourselves a series of steps by which the simple 
 eye, the origin of which I have traced, might become more com- 
 plicated ; and it is easy to see how an embryologist studying the 
 actual development of this complicated eye would be able to 
 unravel the process of its evolution. 
 
 The general nature of the methods of reasoning employed 
 by embryologists, who accept the Darwinian theory, is exempli- 
 fied by the instance just given. If this method is a legitimate 
 one, and there is no reason to doubt it, we ought to find that 
 animals, in the course of their development, pass through a series 
 of stages, in each of which they resemble one of their remote 
 ancestors; but it is to be remembered that, in accordance with 
 the law of variation, there is a continual tendency to change, and 
 that the longer this tendency acts the greater will be the total 
 effect. Owing to this tendency, we should not expect to find a 
 perfect resemblance between an animal, at different stages of its 
 growth, and its ancestors; and the remoter the ancestors, the 
 less close ought the resemblance to be. In spite, however, of 
 this limitation, it may be laid down as one of the consequences 
 of the law of inheritance that every animal ought, in the course 
 of its individual development, to repeat with more or less fidelity 
 the history of its ancestral evolution. 
 
 A direct verification of this proposition is scarcely possible. 
 There is ample ground for concluding that the forms from which 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 70 1 
 
 existing animals are descended have in most instances perished ; 
 and although there is no reason why they should not have been 
 preserved in a fossil state, yet, owing to the imperfection of the 
 geological record, palaeontology is not so often of service as 
 might have been hoped. 
 
 While, for the reasons just stated, it is not generally possible 
 to prove by direct observation that existing forms in their em- 
 bryonic state repeat the characters of their ancestors, there is 
 another method by which the truth of this proposition can be 
 approximately verified. 
 
 A comparison of recent and fossil forms shews that there 
 are actually living at the present day representatives of a con- 
 siderable proportion of the groups which have in previous times 
 existed on the globe, and there are therefore forms allied to the 
 ancestors of those living at the present day, though not actually 
 the same species. If therefore it can be shewn that the em- 
 bryos of existing forms pass through stages in which they have 
 the characters of more primitive groups, a sufficient proof of our 
 proposition will have been given. 
 
 That such is often the case is a well-known fact, and was 
 even known before the publication of Darwin's works. Von 
 Baer, the greatest embryologist of the century, who died 
 at an advanced age but a few years ago, discussed the pro- 
 position at considerable length in a work published between the 
 years 1830 and 1840. He came to the conclusion that the 
 embryos of higher forms never actually resemble lower forms, 
 but only the embryos of lower forms ; and he further main- 
 tained that such resemblances did not hold at all, or only to a 
 very small extent, beyond the limits of the larger groups. Thus 
 he believed that, though the embryos of Vertebrates might 
 agree amongst themselves, there was no resemblance between 
 them and the embryos of any invertebrate group. We now 
 know that these limitations of Von Baer do not hold good, but 
 it is to be remembered that the meaning now attached by em- 
 bryologists to such resemblances was quite unknown to him. 
 
 These preliminary remarks will, I trust, be sufficient to de- 
 monstrate how completely modern embryological reasoning is 
 dependent on the two laws of inheritance and variation, which 
 constitute the keystones of the Darwinian theory. 
 
702 ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 Before the appearance of the Origin of Species many very 
 valuable embryological investigations were made, but the facts 
 discovered were to their authors merely so many ultimate facts, 
 which admitted of being classified, but could not be explained. 
 No explanation could be offered of why it is that animals, in- 
 stead of developing in a simple and straightforward way, un- 
 dergo in the course of their growth a series of complicated 
 changes, during which they often acquire organs which have no 
 function, and which, after remaining visible for a short time, dis- 
 appear without leaving a trace. 
 
 No explanation, for instance, could be offered of why it is 
 that a frog in the course of its growth has a stage in which it 
 breathes like a fish, and then why it is like a newt with a long 
 tail, which gradually becomes absorbed, and finally disappears. 
 To the Darwinian the explanation of such facts is obvious. The 
 stage when the tadpole breathes by gills is a repetition of the 
 stage when the ancestors of the frog had not advanced in the 
 scale of development beyond a fish, while the newt-like stage 
 implies that the ancestors of the frog were at one time organized 
 very much like the newts of to-day. The explanation of such 
 facts has opened out to the embryologist quite a new series of 
 problems. These problems may be divided into two main 
 groups, technically known as those of phylogeny and those of 
 organogeny. The problems of phylogeny deal with the ge- 
 nealogy of the animal kingdom. A complete genealogy would 
 form what is known as a natural classification. To attempt to 
 form such a classification has long been the aim of a large 
 number of naturalists, and it has frequently been attempted 
 without the aid of embryology. The statements made in the 
 earlier part of my address clearly shew how great an assistance 
 embryology is capable of giving in phylogeny ; and as a matter 
 of fact embryology has been during the last few years very 
 widely employed in all phylogenetic questions, and the results 
 which have been arrived at have in many cases been very 
 striking. To deal with these results in detail would lead me 
 into too technical a department of my subject ; but I may point 
 out that amongst the more striking of the results obtained 
 entirely by embryological methods is the demonstration that the 
 Vertebrata are not, as was nearly universally believed by older 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 703 
 
 naturalists, separated by a wide gulf from the Invertebrate, but 
 that there is a group of animals, known as the Ascidians, formerly 
 united with the Invertebrata, which are now universally placed 
 with the Vertebrata. 
 
 The discoveries recently made in organogeny, or the genesis 
 of organs, have been quite as striking, and in many respects 
 even more interesting, than those in phylogeny, and I propose 
 devoting the remainder of my address to a history of results 
 which have been arrived at with reference to the origin of the 
 nervous system. 
 
 To render clear the nature of these results I must say a few 
 words as to the structure of the animal body. The body is 
 always built of certain pieces of protoplasm, which are technically 
 known to biologists as cells. The simplest organisms are com- 
 posed either of a single piece of this kind, or of several similar 
 pieces loosely aggregated together. Each of these pieces or 
 cells is capable of digesting and assimilating food, and of 
 respiring; it can execute movements, and is sensitive to ex- 
 ternal stimuli, and can reproduce itself. All the functions of 
 higher animals can, in fact, be carried on in this single cell. 
 Such lowly organized forms are known to naturalists as the 
 Protozoa. All other animals are also composed of cells, but 
 these cells are no longer complete organisms in themselves. 
 They exhibit a division of labour : some carrying on the work 
 of digestion ; some, which we call nerve-cells, receiving and 
 conducting stimuli ; some, which we call muscle-cells, altering 
 their form in fact, contracting in one direction under the 
 action of the stimuli brought to them by the nerve-cells. In 
 most cases a number of cells with the same function are united 
 together, and thus constitute a tissue. Thus the cells which 
 carry on the work of digestion form a lining membrane to a 
 tube or sack, and constitute a tissue known as a secretory epi- 
 thelium. The whole of the animals with bodies composed of 
 definite tissues of this kind are known as the Metazoa. 
 
 A considerable number of early developmental processes are 
 common to the whole of the Metazoa. 
 
 In the first place every Metazoon commences its existence 
 as a simple cell, in the sense above defined ; this cell is known 
 as the ovum. The first developmental process which takes 
 
704 ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 place consists in the division or segmentation of the single cell 
 into a number of smaller cells. The cells then arrange them- 
 selves into two groups or layers known to embryologists as the 
 primary germinal layers. These two layers are usually placed 
 one within the other round a central cavity, The inner of the 
 two is called the hypoblast, the outer the epiblast. The ex- 
 istence of these two layers in the embryos of vertebrated animals 
 was made out early in the present century by Pander, and his 
 observations were greatly extended by Von Baer and Remak. 
 But it was supposed that these layers were, confined to ver- 
 tebrated animals. In the year 1849, ar >d at greater length in 
 1859, Huxley demonstrated that the bodies of all the polype 
 tribe or Ccelenterata that is to say of the group to which the 
 common polype, jelly-fish and the sea-anemone belong were 
 composed of two layers of cells, and stated that in his opinion 
 these two layers were homologous with the epiblast and hypo- 
 blast of vertebrate embryos. This very brilliant discovery came 
 before its time. It fell upon barren ground, and for a long time 
 bore no fruit. In the year 1866 a young Russian naturalist 
 named Kowalevsky began to study by special histological 
 methods the development of a number of invertebrated forms 
 of animals, and discovered that at an early stage of develop- 
 ment the bodies of all these animals were divided into eer- 
 
 o 
 
 minal layers like those in vertebrates. Biologists were not 
 long in recognizing the importance of these discoveries, and 
 they formed the basis of two remarkable essays, one by 
 our own countryman, Professor Lankester, and the other 
 by a distinguished German naturalist, Professor Haeckel, of 
 Jena. 
 
 In these essays the attempt was made to shew that the 
 stage in development already spoken of, in which the cells are 
 arranged in the form of two layers enclosing a central cavity has 
 an ancestral meaning, and that it is to be interpreted to signify- 
 that all the Metazoa are descended from an ancestor which had 
 a more or less oval form, with a central digestive cavity pro- 
 vided with a single opening, serving both for the introduction of 
 food and for the ejection of indigestible substances. The body 
 of this ancestor was supposed to have been a double-walled sack 
 formed of an inner layer, the hypoblast, lining the digestive 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 70$ 
 
 cavity, and an outer layer, the epiblast. To this form H-aeckel 
 gave the name of gastrsa or gastrula. 
 
 There is every reason to think that Lankester and Haeckel 
 were quite justified in concluding that a form more or less like 
 that just described was the ancestor of the Metazoa; but the 
 further speculations contained in their essays as to the origin of 
 this form from the Protozoa can only be regarded as suggestive 
 feelers, which, however, have been of great importance in stimu- 
 lating and directing embryological research. It is, moreover, 
 very doubtful whether there are to be found in the develop- 
 mental histories of most animals any traces of this gastraea 
 ancestor, other than the fact of their passing through a stage in 
 which the cells are divided into two germinal layers. 
 
 The key to the nature of the two germinal layers is to be 
 found in Huxley's comparison between them, and the two layers 
 in the fresh-water polype and the sea-anemone. The epiblast is 
 the primitive skin, and the hypoblast is the primitive epithelial 
 wall of the alimentary tract. 
 
 In the whole of the polype group, or Ccelenterata, the body 
 remains through life composed of the two layers, which Huxley 
 recognized as homologous with the epiblast and hypoblast of the 
 Vertebrata ; but in all the higher Metazoa a third germinal 
 layer, known as the mesoblast, early makes its appearance 
 between the two primary layers. The mesoblast originates as 
 a differentiation of one or of both the primary germinal layers ; 
 but although the different views which have been held as to its 
 mode of origin form an important section of the history of recent 
 embryological investigations, I must for the moment confine 
 myself to saying that from this layer there take their origin the 
 whole of the muscular system, of the vascular system, and of 
 that connective-tissue system which forms the internal skeleton, 
 tendons, and other parts. 
 
 We have seen that the epiblast represents the skin or epider- 
 mis of the simple sack-like ancestor common to all the Metazoa. 
 In all the higher Metazoa it gives rise, as might be expected, 
 to the epidermis, but it gives rise at the same time to a number 
 of other organs ; and, in accordance with the principles laid 
 down in the earlier part of my address, it is to be concluded 
 that the organs so derived Jinrc been formed as differentiations of 
 
706 ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 the primitive epidermis. One of the most interesting of recent 
 embryological discoveries is the fact that the nervous system 
 is, in all but a very few doubtful cases, derived from the epiblast. 
 This fact was made out for vertebrate animals by the great 
 embryologist Von Baer; and the Russian naturalist Kowalevsky, 
 to whose researches I have already alluded, shewed that this was 
 true for a large number of invertebrate animals. The derivation 
 of the nervous system from the epiblast has since been made 
 out for a sufficient number of forms satisfactorily to establish 
 the generalization that it is all but universally derived from the 
 epiblast. 
 
 In any animal in which there is no distinct nervous system, 
 it is obvious that the general surface of the body must be sensitive 
 to the action of its surroundings, or to what are technically called 
 stimuli. We know experimentally that this is so in the case 
 of the Protozoa, and of some very simple Metazoa, such as the 
 freshwater Polype or Hydra, where there is no distinct nervous 
 system. The skin or epidermis of the ancestor of the Metazoa 
 was no doubt similarly sensitive ; and the fact of the nervous 
 system being derived from the epiblast implies that the functions 
 of the central nervous system, whiah were originally taken by the 
 whole skin, became gradually concentrated in a special part of 
 the skin which was step by step removed from the surface, and 
 finally became a well-defined organ in the interior of the body. 
 
 What were the steps by which this remarkable process took 
 place ? How has it come about that there are nerves passing 
 from the central nervous system to all parts of the skin, and 
 also to the muscles ? How have the arrangements for reflex 
 actions arisen by which stimuli received on the surface of the 
 body are carried to the central part of the nervous system, 
 and are thence transmitted to the appropriate muscles, and cause 
 them to contract ? All these questions require to be answered 
 before we can be said to possess a satisfactory knowledge of 
 the origin of the nervous system. As yet, however, the know- 
 ledge of these points derived from embryology is imperfect, 
 although there is every hope that further investigation will render 
 it less so? Fortunately, however, a study of comparative anatomy, 
 especially that of the Coelenterata, fills up some of the gaps left 
 from our study of embryology. 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 707 
 
 From embryology we learn that the ganglion-cells _of jthe 
 central part of the nervous system are originally derived from 
 the simple undifferentiated epithelial cells of the surface of the 
 body. We further learn that the nerves are out-growths of the 
 central nervous system. It was supposed till quite recently 
 that the nerves in Vertebrates were derived from parts of the 
 middle germinal layer or mesoblast, and that they only became 
 secondarily connected with the central nervous system. This is 
 now known not to be the case, but the nerves are formed as 
 processes growing out from the central part of the nervous 
 system. 
 
 Another important fact shewn by embryology is that the 
 central nervous system, and percipient portion of the organs 
 of special sense, are often formed from the . same part of the 
 primitive epidermis. Thus, in ourselves and in other vertebrate 
 animals the sensitive part of the eye, known as the retina, is 
 formed from two lateral lobes of the front part of the primitive 
 brain. The crystalline lens and cornea of the eye are, however, 
 subsequently formed from the skin. 
 
 The same is true for the peculiar compound eyes of crabs 
 or Crustacea. The most important part of the central nervous 
 system of these animals is the supra-cesophageal ganglia, often 
 known as the brain, and these are formed in the embryo from 
 two thickened patches of the skin at the front end of the body. 
 These thickened patches become gradually detached from the 
 surface, remaining covered over by a layer of skin. They then 
 constitute the supra-oesophageal ganglia ; but they form not only 
 the ganglia, but also the rhabdons or retinal elements of the 
 eye the parts in fact which correspond to the rods and cones 
 in our own retina. The layer of epidermis or skin which lies im- 
 mediately above the supra-cesophageal ganglia becomes gradually 
 converted into the refractive media of the crustacean eye. A 
 cuticle which lies on its surface forms the peculiar facets on the 
 surface of the eye, which are known as the corneal lenses, while 
 the cells of the epidermis give rise to lens-like bodies known as 
 the crystalline cones. 
 
 It would be easy to quote further instances of the same kind, 
 but I trust that the two which I have given will be sufficient to 
 shew the kind of relation which often exists between the organs 
 
708 ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 of special sense, especially those of vision, and the central 
 nervous system. It might have been anticipated a priori that 
 organs of special sense would only appear in animals provided 
 with a well-developed central nervous system. This, however, 
 is not the case. Special cells, with long delicate hairs, which 
 are undoubtedly highly sensitive structures, are present in animals 
 in which as yet nothing has been found which could be called a 
 central nervous system ; and there is every reason to think that 
 the organs of special sense originated part passn with the central 
 nervous system. It is probable that in the simplest organisms 
 the whole body is sensitive to light, but that with the appearance 
 of pigment-cells in certain parts of the body, the sensitiveness 
 to light became localised to the areas where the pigment-cells 
 were present. Since, however, it was necessary that stimuli 
 received by such organs should be communicated to other parts 
 of the body, some of the epidermic cells in the neighbourhood 
 of the pigment-spots, which were at first only sensitive, in the 
 same manner as other cells of the epidermis, became gradually 
 differentiated into special nerve-cells. As to the details of this 
 differentiation, embryology does not as yet throw any great 
 light ; but from the study of comparative anatomy there are 
 grounds for thinking that it was somewhat as follows : Cells 
 placed on the surface sent protoplasmic processes of a nervous 
 nature inwards, which came into connection with nervous pro- 
 cesses from similar cells placed in other parts of the body. The 
 cells with such processes then became removed from the surface, 
 forming a deeper layer of the epidermis below the sensitive cells 
 of the organ of vision. With these cells they remained connected 
 by protoplasmic filaments, and thus they came to form a thick- 
 ening of the epidermis underneath the organ of vision, the cells 
 of which received their stimuli from those of the organ of 
 vision, and transmitted the stimuli so received to other parts of 
 the body. Such a thickening would obviously be the rudiment 
 of a central nervous system, and it is easy to see by what steps 
 it might become gradually larger and more important, and might 
 gradually travel inwards, remaining connected with the sense 
 organ at the surface by protoplasmic filaments, which would then 
 constitute nerves. The rudimentary eye would at first merely con- 
 sist partly of cells sensitive to light, and partly of optical structures 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 709 
 
 constituting the lens, which would throw an image of external 
 objects upon it, and so convert the whole structure into a true 
 organ of vision. It has thus come about that, in the develop- 
 ment of the individual, the retina or sensitive part of the eye 
 is first formed in connection with the central nervous system, 
 while the lenses of the eye are independently evolved from the 
 epidermis at a later period. 
 
 The general features of the origin of the nervous system 
 which have so far been made out by means of the study of 
 embryology are the following : 
 
 (1) That the nervous system of the higher Metazoa has 
 been developed in the course of a long series of generations 
 by a gradual process of differentiation of parts of the epidermis. 
 
 (2) That part of the central nervous system of many forms 
 arose as a local collection of nerve-cells in the epidermis, in the 
 neighbourhood of rudimentary organs of vision. 
 
 (3) That ganglion cells have been evolved from simple 
 epithelial cells of the epidermis. 
 
 (4) That the primitive nerves were outgrowths of the original 
 ganglion cells ; and that the nerves of the higher forms are 
 formed as outgrowths of the central nervous system. 
 
 The points on which embryology has not yet thrown a satis- 
 factory light are : 
 
 (1) The steps by which the protoplasmic processes, from 
 the primitive epidermic cells, became united together so as to 
 form a network of nerve-fibres, placing the various parts of the 
 body in nervous communication. 
 
 (2) The process by which nerves became connected with 
 muscles, so that a stimulus received by a nerve-cell could be 
 communicated to and cause a contraction in a muscle. 
 
 Recent investigations on the anatomy of the Ccelenterata, 
 especially of jelly-fish and sea-anemones, have thrown some 
 light on these points, although there is left much that is still 
 obscure. 
 
 In our own country Mr Romaines has conducted some in- 
 teresting physiological experiments on these forms; and Professor 
 Schafer has made some important histological investigations 
 upon them. In Germany a series of interesting researches have 
 also been made on them by Professors Kleinenberg, Claus and 
 
7IO ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 Eimer, and more especially by the brothers Hertwig, of Jena. 
 Careful histological investigations, especially those of the last- 
 named authors, have made us acquainted with the forms of 
 some very primitive types of nervous system. In the common 
 sea-anemones there are, for instance, no organs of special sense, 
 and no definite central nervous system. There are, however, 
 scattered throughout the skin, and also throughout the lining of 
 the digestive tract, a number of specially modified epithelial 
 cells, which are no doubt delicate organs of sense. They are 
 provided at their free extremity with a long hair, and are pro- 
 longed on their inner side into a fine process which penetrates 
 the deeper part of the epithelial layer of the skin or digestive 
 wall. They eventually join a fine network of protoplasmic fibres 
 which forms a special layer immediately within the epithelium. 
 The fibres of this network are no doubt essentially nervous. In 
 addition to fibres there are, moreover, present in the network 
 cells of the same character as the multipolar ganglion-cells in 
 the nervous system of Vertebrates, and some of these cells are 
 characterized by sending a process into the superjacent epithelium. 
 Such cells are obviously epithelial cells in the act of becoming 
 nerve-cells ; and it is probable that the nerve-cells are, in 
 fact, sense-cells which have travelled inwards and lost their 
 epithelial character. 
 
 There is every reason to think that the network just described 
 is not only continuous with the sense-cells in the epithelium, but 
 that it is also continuous with epithelial cells which are provided 
 with muscular prolongations. The nervous system thus consists 
 of a network of protoplasmic fibres, continuous on the one hand 
 with sense-cells in the epithelium, and on the other with muscular 
 cells. The nervous network is generally distributed both beneath 
 the epithelium of the skin and that of the digestive tract, but is 
 especially concentrated in the disc-like region between the mouth 
 and tentacles. The above observations have thrown a very clear 
 light on the characters of the nervous system at an early stage 
 of its evolution, but they leave unanswered the questions (i) 
 how the nervous network first arose, and (2) how its fibres 
 became continuous with muscles. It is probable that the nervous 
 network took its origin from processes of the sense-cells. The 
 processes of the different cells probably first met and then fused 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 711 
 
 together, and, becoming more arborescent, finally gave rise to a 
 complicated network. 
 
 The connection between this network and the muscular cells 
 also probably took place by a process of contact and fusion. 
 
 Epithelial cells with muscular processes were discovered by 
 Kleinenberg before epithelial cells with nervous processes were 
 known, and he suggested that the epithelial part of such cells 
 was a sense-organ, and that the connecting part between this 
 and the contractile processes was a rudimentary nerve. This 
 ingenious theory explained completely the fact of nerves being 
 continuous with muscles ; but on the further discoveries being 
 made which I have just described, it became obvious that this 
 theory would have to be abandoned, and that some other expla- 
 nation would have to be given of the continuity between nerves 
 and muscles. The hypothetical explanation just offered is that 
 of fusion. 
 
 It seems very probable that many of the epithelial cells were 
 originally provided with processes the protoplasm of which, like 
 that of the Protozoa, carried on the functions of nerves and 
 muscles at the same time, and that these processes united 
 amongst themselves into a network. By a process of differentia- 
 tion parts of this network may have become specially contractile, 
 and other parts may have lost their contractility and become 
 solely nervous. In this way the connection between nerves and 
 muscles might be explained, and this hypothesis fits in very well 
 with the condition of the neuro-muscular system as we find it in 
 the Ccelenterata. 
 
 The nervous system of the higher Metazoa appears then to 
 have originated from a differentiation of some of the superficial 
 epithelial cells of the body, though it is possible that some parts 
 of the system may have been formed by a differentiation of the 
 alimentary epithelium. The cells of the epithelium were most 
 likely at the same time contractile and sensory, and the differ- 
 entiation of the nervous system may very probably have com- 
 menced, in the first instance, from a specialization in the function 
 of part of a network formed of neuro-muscular prolongations of 
 epithelial cells. A simultaneous differentiation of other parts of 
 the network into muscular fibres may have led to the continuity 
 at present obtaining between nerves and muscles. 
 
712 ADDRESS TO THE DEPARTMENT OF ANATOMY 
 
 Local differentiations of the nervous network, which was no 
 doubt distributed over the whole body, took place on the forma- 
 tion of organs of special sense, and such differentiations gave 
 rise to the formation of a central nervous system. The central 
 nervous system was at first continuous with the epidermis, but 
 became separated from it and travelled inwards. Ganglion-cells 
 took their origin from sensory epithelial cells, provided with 
 prolongations, continuous with the nervous network. Such 
 epithelial cells gradually lost their epithelial character, and finally 
 became completely detached from the epidermis. 
 
 Nerves, such as we find them in the higher types, originated 
 from special differentiations of the nervous network, radiating 
 from the parts of the central nervous system. 
 
 Such, briefly, is the present state of our knowledge as to the 
 genesis of the nervous system. I ought not, however, to leave 
 this subject without saying a few words as to the hypothetical 
 views which the distinguished evolutionist Mr Herbert Spencer 
 has put forward on this subject in his work on Psychology. 
 
 For Herbert Spencer nerves have originated, not as pro- 
 cesses of epithelial cells, but from the passage of motion along 
 the lines of least resistance. The nerves would seem, according 
 to this view, to have been formed in any tissue from the con- 
 tinuous passage of nervous impulses through it. " A wave of 
 molecular disturbance," he says, " passing along a tract of 
 mingled colloids closely allied in composition, and isomerically 
 transforming the molecules of one of them, will be apt at the 
 same time to form some new molecules of the same type," and 
 thus a nerve becomes established. 
 
 A nervous centre is formed, according to Herbert Spencer, at 
 the point in the colloid in which nerves are generated, where 
 a single nervous wave breaks up, and its parts diverge along 
 various lines of least resistance. At such points some of the 
 nerve-colloid will remain in an amorphous state, and as the wave 
 of molecular motion will there be checked, it will tend to cause 
 decompositions amongst the unarranged molecules. The de- 
 compositions must, he says, cause " additional molecular motion 
 to be disengaged ; so that along the outgoing lines there will be 
 discharged an augmented wave. Thus there will arise at this 
 point something having the character of a ganglion corpuscle." 
 
AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 713 
 
 These hypotheses of Herbert Spencer, which have been-widely 
 adopted in this country, are, it appears to me, not borne out by 
 the discoveries to which I have called your attention to-day. 
 The discovery that nerves have been developed from processes 
 of epithelial cells, gives a very different conception of their genesis 
 to that of Herbert Spencer, which makes them originate from 
 the passage of nervous impulses through a tract of mingled 
 colloids ; while the demonstration that ganglion-cells arose as 
 epithelial cells of special sense, which have travelled inwards 
 from the surface, admits still less of a reconciliation with Herbert 
 Spencer's view on the same subject. 
 
 Although the present state of our knowledge on the genesis 
 of the nervous system is a great advance on that of a few years 
 ago, there is still much remaining to be done to make it com- 
 plete. 
 
 The subject is well worth the attention of the morphologist, 
 the physiologist, or even of the psychologist, and we must not 
 remain satisfied by filling up the gaps in our knowledge by such 
 hypotheses as I have been compelled to frame. New methods 
 of research will probably be required to grapple with the pro- 
 blems that are still unsolved ; but when we look back and survey 
 what has been done in the past, there can be no reason for 
 mistrusting our advance in the future. 
 
 B. 46 
 
XX. ON THE DEVELOPMENT OF THE SKELETON OF THE 
 PAIRED FINS OF ELASMOBRANCHII, CONSIDERED IN RE- 
 LATION TO ITS BEARINGS ON THE NATURE OF THE 
 LIMBS OF THE VERTEBRATA 1 . 
 
 (With Plate 33.) 
 
 SOME years ago the study of the development of the soft 
 parts of the fins in several Elasmobranch types, more especially 
 in Torpedo, led me to the conclusion that the vertebrate limbs 
 were remnants of two continuous lateral fins 2 . More or less 
 similar views (which I was not at that time acquainted with) had 
 been previously held by Maclise, Humphrey, and other anato- 
 mists ; these views had not, however, met with much acceptance, 
 and diverge in very important points from those put forward by 
 me. Shortly after the appearance of my paper, J. Thacker pub- 
 lished two interesting memoirs comparing the skeletal parts of 
 the paired and unpaired fins 8 . 
 
 In these memoirs Thacker arrives at conclusions as to the 
 nature of the fins in the main similar to mine, but on entirely 
 independent grounds. He attempts to shew that the structure of 
 the skeleton of the paired fins is essentially the same as that of 
 the unpaired fins, and in this comparison lays special stress on 
 the very simple skeleton of the pelvic fin in the cartilaginous 
 Ganoids, more especially in Acipenser and Polyodon. He points 
 out that the skeleton of the pelvic fin of Polyodon consists essen- 
 tially of a series of nearly isolated rays, which have a strikingly 
 similar arrangement to that of the rays of the skeleton in 
 
 1 From the Proceedings of the Zoological Society of London, 1881. 
 
 2 "Monograph on the Development of Elasmobranch Fishes," pp. 319, 320. 
 
 3 J. K. Thacker, "Median and Paired Fins; a Contribution to the History of 
 the Vertebrate Limbs," Trans, of the Connecticut Acad. Vol. ill. 1877. "Ventral 
 Fins of Ganoids," Trans, of the Connecticut Acad. Vol. iv. 1877. 
 
SKELETON OF THE PAIRED FINS OF ELASMOBRANCHS. 715 
 
 many unpaired fins. He sums up his views in the following 
 way ' : 
 
 "As the dorsal and anal fins were specializations of the 
 median folds of Amp/iioxus, so the paired fins were specializa- 
 tions of the two lateral folds which are supplementary to the 
 median in completing the circuit of the body. These lateral 
 folds, then, are the homologues of Wolffian ridges, in embryos of 
 higher forms. Here, as in the median fins, there were formed 
 chondroid and finally cartilaginous rods. These became at 
 least twice segmented. The orad ones, with more or less con- 
 crescence proximally, were prolonged inwards. The cartilages 
 spreading met in the middle line ; and a later extension of the 
 cartilages dorsad completed the limb-girdle. 
 
 " The limbs of the Protognathostomi consisted of a series of 
 parallel articulated cartilaginous rays. They may have coalesced 
 somewhat proximally and orad. In the ventral pair they had 
 extended themselves mesiad until they had nearly or quite met 
 and formed the hip-girdle ; they had not here extended them- 
 selves dorsad. In the pectoral limb the same state of things 
 prevailed, but was carried a step further, namely, by the dorsal 
 extension of the cartilage constituting the scapular portion, thus 
 more nearly forming a ring or girdle." 
 
 The most important point in Thacker's theories which I can- 
 not accept is the derivation of the folds, of which the paired 
 fins of the Vertebrata are supposed to be specializations, from 
 the lateral folds of Amphioxus ; and Thacker himself recognizes 
 that this part of his theory stands on quite a different footing to 
 the remainder. 
 
 Not long after the publication of Thacker's paper, an im- 
 portant memoir was published by Mivart in the Transactions 
 of this Society 2 . The object of the researches recorded in this 
 paper was, as Mivart explains, to test how far the hard parts of 
 the limbs and of the azygos fins may have arisen through cen- 
 tripetal chondrifications or calcifications, and so be genetically 
 exoskeletal 3 . 
 
 1 Loc. cit. p. 298. 
 
 2 St George Mivart, "On the Fins of Elasmobranchii," Zoological Trans. Vol. x. 
 
 3 Mivart used the term exoskeletal in an unusual and (as it appears to me) incon- 
 venient manner. The term is usually applied to dermal skeletal structures ; but the 
 
 46 2 
 
716 DEVELOPMENT OF THE SKELETON 
 
 Mivart's investigations and the majority of his views were 
 independent of Thacker's memoir ; but he acknowledges that he 
 has derived from Thacker the view that pelvic and pectoral 
 girdles, as well as the skeleton of the limbs, may have arisen 
 independently of the axial skeleton. 
 
 The descriptive part of Mivart's paper contains an account 
 of the structure of a great variety of interesting and undescribed 
 types of paired and unpaired fins, mainly of Elasmobranchii. 
 The following is the summary given by Mivart of the conclu- 
 sions at which he has arrived 1 : 
 
 " i. Two continuous lateral longitudinal folds were deve- 
 loped, similar to dorsal and ventral median longitudinal folds. 
 
 " 2. Separate narrow solid supports (radials), in longitudinal 
 series, and with their long axes directed more or less outwards 
 at right angles with the long axis of the body, were developed 
 in varying extents in all these four longitudinal folds. 
 
 "3. The longitudinal folds became interrupted variously, 
 but so as to form two prominences on each side, i.e. the primi- 
 tive paired limbs. 
 
 " 4. Each anterior paired limb increased in size more rapidly 
 than the posterior limb. 
 
 " 5. The bases of the cartilaginous supports coalesced as 
 was needed, according to the respective practical needs of the 
 different separate portions of the longitudinal folds, i.e. the 
 respective needs of the several fins. 
 
 "6. Occasionally the dorsal radials coalesced (as in Noti- 
 danus, &c.) and sought centripetally (Pristis, &c.) adherence to 
 the skeletal axis. 
 
 "7. The radials of the hinder paired limb did so more con- 
 stantly, and ultimately prolonged themselves inwards by mesiad 
 growth from their coalesced base, till the piscine pelvic structure 
 arose, as, e.g., in Squatina. 
 
 " 8. The pectoral radials with increasing development also 
 coalesced proximally, and thence prolonging themselves inwards 
 to seek a point d'appui, shot dorsad and ventrad to obtain a 
 firm support, and at the same time to avoid the visceral cavity. 
 
 skeleton of the limbs, with which we are here concerned, is undoubtedly not of this 
 nature. 
 
 1 Loc. cit. p. 480. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 717 
 
 Thus they came to abut dorsally against the axial skeleton, and 
 to meet ventrally together in the middle line below. 
 
 " 9. The lateral fins, as they were applied to support the body 
 on the ground, became elongated, segmented, and narrowed, so 
 that probably the line of the propterygium, or possibly that of 
 the mesopterygium, became the cheiropterygial axis. 
 
 " 10. The distal end of the incipient cheiropterygium either 
 preserved and enlarged preexisting cartilages or developed fresh 
 .ones to serve fresh needs, and so grew into the developed cheir- 
 opterygium ; but there is not yet enough evidence to determine 
 what was the precise course of this transformation. 
 
 " 1 1. The pelvic limb acquired a solid connection with the 
 axial skeleton (a pelvic girdle) through its need of a point 
 d'appui as a locomotive organ on land. 
 
 " 12. The pelvic limb became also elongated ; and when its 
 function was quite similar to that of the pectoral limb, its struc- 
 ture became also quite similar (e.g. Ichthyosaurus, Plesiosaurus, 
 CJielydra, &c.) ; but for the ordinary quadrupedal mode of pro- 
 gression it became segmented and inflected in a way generally 
 parallel with, but (from its mode of use) in part inversely to, the 
 inflections of the pectoral limb." 
 
 Giinther 1 has propounded a theory on the primitive character 
 of the fins, which, on the whole, fits in with the view that the 
 paired fins are structures of the same nature as the unpaired 
 fins. The interest of Giinther's views on the nature of the 
 skeleton of the fins more especially depends upon the fact that 
 he attempts to evolve the fin of Ceratodus from the typical Sela- 
 chian type of pectoral fin. His own statement on this subject 
 is as follows 2 : 
 
 " On further inquiry into the more distant relations of the 
 Ceratodns-\\mb, we may perhaps be justified in recognizing in it 
 a modification of the typical form of the Selachian pectoral fin. 
 Leaving aside the usual treble division of the carpal cartilage 
 (which, indeed, is sometimes simple), we find that this shovel- 
 like carpal forms the base for a great number of phalanges, 
 which are arranged in more or less regular transverse rows (zones) 
 and in longitudinal rows (series). The number of phalanges of 
 
 1 " Description of Ceratodus" Phil. Trans. 1871. 
 - Loc. cit. p. 534. 
 
7l8 DEVELOPMENT OF THE SKELETON 
 
 the zones and series varies according to the species and the 
 form of the fin ; in Cestracion philippi the greater number of 
 phalanges is found in the proximal zones and middle series, all 
 the phalanges decreasing in size from the base of the fin towards 
 the margins. In a Selachian with a long, pointed, scythe-shaped 
 pectoral fin, like that of Ceratodus, we may, from analogy, pre- 
 sume that the arrangement of the cartilages might be somewhat 
 like that shewn in the accompanying diagram, which I have 
 divided into nine zones and fifteen series. 
 
 "When we now detach the outermost phalanx from each 
 side of the first horizontal zone, and with it the other phalanges 
 of the same series, when we allow the remaining phalanges of 
 this zone to coalesce into one piece (as, in nature, we find 
 coalesced the carpals of Ceratodus and many phalanges in 
 Selachian fins), and when we repeat this same process with the 
 following zones and outer series, we arrive at an arrangement 
 identical with what we actually find in Ceratodus" 
 
 While the researches of Thacker and Mivart are strongly 
 confirmatory of the view at which I had arrived with reference 
 to the nature of the paired fins, other hypotheses as to the 
 nature of the skeleton of the fins have been enunciated, both 
 before and after the publication of my memoir, which are either 
 directly or indirectly opposed to my view. 
 
 Huxley in his memoir on Ceratodus, which throws light on 
 so many important morphological problems, has dealt with the 
 nature of paired fins 1 . 
 
 He holds, in accordance with a view previously adopted by 
 Gegenbaur, that the limb of Ceratodus " presents us with the 
 nearest known approximation to the fundamental form of ver- 
 tebrate limb or archipterygium," and is of opinion that in a still 
 more archaic fish than Ceratodus the skeleton of the fin " would 
 be made up of homologous segments, which might be termed 
 pteromeres, each of which would consist of a mesomere with a 
 preaxial and a postaxial paramere." He considers that the 
 pectoral fins of Elasmobranchii, more especially the fin of Noti- 
 danus, which he holds to be the most primitive form of Elasmo- 
 branch fin, " results in the simplest possible manner from the 
 
 1 T. H. Huxley, "On Ceratodus Fosteri, with some Observations on the Classifi- 
 cation of Fishes," Proc. Zool. Soc. 1876. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 719 
 
 shortening of the axis of such a fin-skeleton as that of Cerajodus, 
 and the coalescence of some of its elements." Huxley does not 
 enter into the question of the origin of the skeleton of the pelvic 
 fin of Elasmobranchii. 
 
 It will be seen that Huxley's idea of the primitive structure 
 of the archipterygium is not easily reconcilable with the view 
 that the paired fins are parts of a once continuous lateral fin, in 
 that the skeleton of such a lateral fin, if it has existed, must 
 necessarily have consisted of a series of parallel rays. 
 
 Gegenbaur 1 has done more than any other living anatomist 
 to elucidate the nature of the fins ; and his views on this subject 
 have undergone considerable changes in the course of his in- 
 vestigations. After Giinther had worked out the structure of 
 the fin of Ceratodus, Gegenbaur suggested that it constituted the 
 most primitive persisting type of fin, and has moreover formed a 
 theory as to the origin of the fins founded on this view, to the 
 effect that the fins, together with their respective girdles, are to 
 be derived from visceral arches with their rays. 
 
 His views on this subject are clearly explained in the sub- 
 joined passages quoted from the English translation of his 
 Elements of Comparative Anatomy, pp. 473 and 477. 
 
 "The skeleton of the free appendage is attached to the 
 extremity of the girdle. When simplest, this is made up of car- 
 tilaginous rods (rays), which differ in their size, segmentation, 
 and relation to one another. One of these rays is larger than 
 the rest, and has a number of other rays attached to its sides. I 
 have given the name of archipterygium to the ground-form of 
 the skeleton which extends from the limb-bearing girdle into 
 the free appendage. The primary ray is the stem of this archip- 
 terygium, the characters of which enable us to follow out the 
 lines of development of the skeleton of the appendage. Carti- 
 laginous arches beset with the rays form the branchial skeleton. 
 The form of skeleton of the appendages may be compared with 
 
 1 C. Gegenbaur, Untersuchungen z. vergldch. Anat. d. Wirbeithierc (Leipzig 
 1864-5): erstes Heft, "Carpus u. Tarsus;" zweites Heft, " Brustflosse d. Fische." 
 " Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hinterglied- 
 maassen d. Selachier insbesondere," Jenaische Zeitschrift, Vol. V. 1870. "Ueb. d. 
 Archipterygium," Jtnaische Zeitschrift, Vol. vn. 1873. "Zur Morphologic d. Glied- 
 maassen d. Wirbelthiere," Morphologisches Jahrbuch, Vol. II. 1876. 
 
720 DEVELOPMENT OF THE SKELETON 
 
 them ; and we are led to the conclusion that it is possible that 
 they may have been derived from such forms. In the branchial 
 skeleton of the Selachii the cartilaginous bars are beset with 
 simple rays. In many a median one is developed to a greater 
 size. As the surrounding rays become smaller, and approach 
 the larger one, we get an intermediate step towards that arrange- 
 ment in which the larger median ray carries a few smaller ones. 
 This differentiation of one ray, which is thereby raised to a 
 higher grade, may be connected with the primitive form of the 
 appendicular skeleton ; and as we compare the girdle with a* 
 branchial arch, so we may compare the median ray and its 
 secondary investment of rays with the skeleton of the free 
 appendage. 
 
 "All the varied forms which the skeleton of the free ap- 
 pendages exhibits may be derived from a ground-form which 
 persists in a few cases only, and which represents the first, and 
 consequently the lowest, stage of the skeleton in the fin the 
 archipterygium. This is made up of a stem which consists of 
 jointed pieces of cartilage, which is articulated to the shoulder- 
 girdle and is beset on either side with rays which are likewise 
 jointed. In addition to the rays of the stem there are others 
 which are directly attached to the limb-girdle. 
 
 " Ceratodus has a fin-skeleton of this form ; in it there is a 
 stem beset with two rows of rays. But there are no rays in the 
 shoulder-girdle. This biserial investment of rays on the stem 
 of the fin may also undergo various kinds of modifications. 
 Among the Dipnoi, Protopterus retains the medial row of rays 
 only, which have the form of fine rods of cartilage; in the 
 Selachii, on the other hand, the lateral rays are considerably 
 developed. The remains of the medial row are ordinarily quite 
 small, but they are always sufficiently distinct to justify us in 
 supposing that in higher forms the two sets of rays might be 
 better developed. Rays are still attached to the stem and are 
 connected with the shoulder-girdle by means of larger plates. 
 The joints of the rays are sometimes broken up into polygonal 
 plates which may further fuse with one another ; concrescence of 
 this kind may also affect the pieces which form the base of the 
 fin. By regarding the free rays, which are attached to these 
 basal pieces, as belonging to these basal portions, we are able to 
 
OF THE PAIRED FINS OF F.LASMOBRANCHS. /2I 
 
 divide the entire skeleton of the fin into three segments pro-, 
 meso-, and metapterygium. 
 
 "The metapterygium represents the stem of the archiptery- 
 gium and the rays on it. The propterygium and the mesop- 
 terygium are evidently derived from the rays which still remain 
 attached to the shoulder-girdle." 
 
 Since the publication of the memoirs of Thacker, Mivart, and 
 myself, a pupil of Gegenbaur's, M. v. Davidoff 1 , has made a 
 series of very valuable observations, in part directed towards 
 demonstrating the incorrectness of our theoretical views, more 
 especially Thacker's and Mivart's view of the genesis of the 
 skeleton of the limbs. Gegenbaur 2 has also written a short 
 paper in connection with Davidoff's memoir, in support of his 
 own as against our views. 
 
 It would not be possible here to give an adequate account of 
 Davidoff's observations on the skeleton, muscular system, and 
 nerves of the pelvic fins. His main argument against the view 
 that the paired fins are the remains of a continuous lateral fin 
 is based on the fact that a variable but often considerable 
 number of the spinal nerves in front of the pelvic fin are united 
 by a longitudinal commissure with the true plexus of the nerves 
 supplying the fin. From this he concludes that the pelvic fin 
 has shifted its position, and that it may once therefore have been 
 situated close behind the visceral arches. Granting, however, 
 that Davidoff's deduction from the character of the pelvic 
 plexus is correct, there is, so far as I see, no reason in the nature 
 of the lateral-fin theory why the pelvic fins should not have 
 shifted ; and, on the other hand, the longitudinal cord connecting 
 some of the ventral roots in front of the pelvic fin may have 
 another explanation. It may, for instance, be a remnant of the 
 time when the pelvic fin had a more elongated form than at 
 present, and accordingly extended further forwards. 
 
 In any case our knowledge of the nature and origin of nervous 
 plexuses is far too imperfect to found upon their characters such 
 conclusions as those of Davidoff. 
 
 1 M. v. Davidoff, " Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. 
 Fische, I.," Morphol. Jahrbuch, Vol. V. 1879. 
 
 3 " Zur Gliedmaassenfrage. An die Untersuchungen von Davidoff's angekniipfte 
 Bemerkungen," JMorphol, Jahrbuch, Vol. v. 1879. 
 
722 DEVELOPMENT OF THE SKELETON 
 
 Gegenbaur, in his paper above quoted, further urges against 
 Thacker and Mivart's views the fact that there is no proof that 
 the fin of Polyodon is a primitive type ; and also suggests that 
 the epithelial line which I have found connecting the embryonic 
 pelvic and pectoral fins in Torpedo maybe a rudiment indicating 
 a migration backwards of the pelvic fin. 
 
 With reference to the development of the pectoral fin in 
 the Teleostei there are some observations of 'Swirski 1 , which 
 unfortunately do not throw very much light upon the nature of 
 the limb. 
 
 'Swirski finds that in the Pike the skeleton of the limb is 
 formed of a plate of cartilage continuous with the pectoral girdle, 
 which soon becomes divided into a proximal and a distal portion. 
 The former is subsequently segmented into five basal rays, and 
 the latter into twelve parts, the number of which subsequently 
 becomes reduced. 
 
 The observations which I have to lay before the Society 
 were made with the object of determining how far the develop- 
 ment of the skeleton of the limbs throws light on the points on 
 which the anatomists whose opinions have just been quoted are 
 at variance. 
 
 They were made, in the first instance, to complete a chapter 
 in my work on comparative embryology ; and, partly owing to 
 the press of other engagements, but still more to the difficulty of 
 procuring material, my observations are confined to the two 
 British species of the genus Scyllium, viz. Sc. stellare and Sc. 
 caniciila; yet I venture to believe that the results at which I 
 have arrived are not wholly without interest. 
 
 Before dealing with the development of the skeleton of the 
 fin, it will be convenient to describe with great brevity the 
 structure of the pectoral and pelvic fins of the adult. The 
 pectoral fins consist of broad plates inserted horizontally on 
 the sides of the body ; so that in each there may be distinguished 
 a dorsal and a ventral surface, and an anterior and a posterior 
 border. Their shape may best be gathered from the woodcut 
 (fig. i) ; and it is to be especially noted that the narrowest part 
 
 1 G. 'Swirski, Untersuch. ul>. d. Entwick. d. Schultergiirtels u. d. Skelets d. 
 Brustflosse d. Hechts, Inaug. Diss. Dorpat, 1 880. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 
 
 723 
 
 of the fin is the base, where is it attached to the side of the body. 
 The cartilaginous skeleton only occupies a small zone at the base 
 of the fin, the remainder being formed of a fringe supported by 
 radiately arranged horny fibres 1 . 
 
 FIG. i. 
 
 Pectoral fins and girdle of an adult of Scyllium canicula (natural size, 
 
 seen from behind and above). 
 
 co, Coracoid. sc. scapula. //. propterygium. me p. mesopterygium. mp. metap- 
 terygium. fn. part of fin supported by horny fibre. 
 
 FIG. 2. 
 
 Right pelvic fin and part of pelvic girdle of an adult female of Scyllium 
 
 canicula (natural size). 
 
 il. iliac process. /. pubic process, cut across below, bp. basipterygiiun. 
 af. anterior cartilaginous fin-ray articulated to pelvic girdle, fn. part of fin supported 
 by horny fibres. 
 
 1 The horny fibres are mesoblastic products ; they are formed, in the first 
 instance, as extremely delicate fibrils on the inner side of the membrane separating 
 the epiblast from the mesoblast. 
 
724 DEVELOPMENT OF THE SKELETON 
 
 The true skeleton consists of three basal pieces articulating 
 with the pectoral girdle ; on the outer side of which there is 
 a series of more or less segmented cartilaginous fin-rays. Of 
 the basal cartilages one (//) is anterior, a second (mep] is placed 
 in the middle, and a third is posterior (mp). They have 
 been named by Gegenbaur the propterygiwn, the mesopterygium, 
 and the metapterygium ; and these names are now generally 
 adopted. 
 
 The metapterygium is by far the most important of the three, 
 and in Scy Ilium canicula supports 12 or 13 rays 1 . It forms a 
 large part of the posterior boundary of the fin, and bears rays 
 only on its anterior border. 
 
 The mesopterygium supports 2 or 3 rays, in the basal parts 
 of which the segmentation into distinct rays is imperfect ; and 
 the propterygium supports only a single ray. 
 
 The pelvic fins are horizontally placed, like the pectoral fins, 
 but differ from the latter in nearly meeting each other along the 
 median ventral line of the body. They also differ from the 
 pectoral fins in having a relatively much broader base of attach- 
 ment to the sides of the body. Their cartilaginous skeleton 
 (woodcut, fig. 2) consists of a basal bar, placed parallel to the base 
 of the fin, and articulated in front with the pelvic girdle. 
 
 On its outer border it articulates with a series of cartilaginous 
 fin-rays. I shall call the basal bar the basipterygium. The 
 rays which it bears are most of them less segmented than those 
 of the pectoral fin, being only divided into two ; and the posterior 
 ray, which is placed in the free posterior border of the fin, con- 
 tinues the axis of the basipterygium. In the male it is modified 
 in connection with the so-called clasper. 
 
 The anterior fin-ray of the pelvic fin, which is broader than 
 the other rays, articulates directly with the pelvic girdle, instead 
 of with the basipterygium. This ray, in the female of Scy Ilium 
 canicula and in the male of Scy Ilium catulus (Gegenbaur), is 
 peculiar in the fact that its distal segment is longitudinally 
 divided into two or more pieces, instead of being single as is 
 the case with the remaining rays. It is probably equivalent to 
 two of the posterior rays. 
 
 1 In one example where the metapterygium had 13 rays the mesopterygium had 
 only i rays. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 725 
 
 Development of the paired Fins. The first rudiments- of the 
 limbs appear in Scy Ilium, as in other fishes, as slight longitudinal 
 ridge-like thickenings of the epiblast, which closely resemble the 
 first rudiments of the unpaired fins. 
 
 These ridges are two in number on each side an anterior 
 immediately behind the last visceral fold, and a posterior on the 
 level of the cloaca. In most Fishes they are in no way con- 
 nected ; but in some Elasmobranch embryos, more especially in 
 that of Torpedo, they are connected together at their first develop- 
 ment by a line of columnar-epiblast cells. This connecting line 
 of columnar epiblast, however, is a very transitory structure. 
 The rudimentary fins soon become more prominent, consisting 
 of a projecting ridge both of epiblast and mesoblast, at the outer 
 edge of which is a fold of epiblast only, which soon reaches con- 
 siderable dimensions. At a later stage the mesoblast penetrates 
 into this fold, and the fin becomes a simple ridge of mesoblast 
 covered by epiblast. The pectoral fins are at first considerably 
 ahead of the pelvic fins in development. 
 
 The direction of the original epithelial line which connected 
 the two fins of each side is nearly, though not quite, longitudinal, 
 sloping somewhat obliquely ventralwards. It thus comes about 
 that the attachment of each pair of limbs is somewhat on a slant, 
 and that the pelvic pair nearly meet each other in the median 
 ventral line shortly behind the anus. 
 
 The embryonic muscle-plates, as I have elsewhere shewn, 
 grow into the bases of the fins ; and the cells derived from these 
 ingrowths, which are placed on the dorsal and ventral surfaces 
 in immediate contact with the epiblast, probably give rise to the 
 dorsal and ventral muscular layers of the limb, which are shewn 
 in section in Plate 33, fig. i m, and in Plate 33, fig. 7 m. 
 
 The cartilaginous skeleton of the limbs is developed in the 
 indifferent mesoblast cells between the two layers of muscles. Its 
 early development in both the pectoral and the pelvic fins is 
 very similar. When first visible it differs histologically from the 
 adjacent mesoblast simply in the fact of its cells being more 
 concentrated ; while its boundary is not sharply marked. 
 
 At this stage it can only be studied by means of sections. 
 It arises simultaneously and continuously with the pectoral and 
 pelvic girdles, and consists, in both fins, of a bar springing at 
 
726 DEVELOPMENT OF THE SKELETON 
 
 right angles from the posterior side of the pectoral or pelvic 
 girdle, and running parallel to the long axis of the body along 
 the base of the fin. The outer side of this bar is continued into 
 a thin plate, which extends into the fin. 
 
 The structure of the skeleton of the fin slightly after its first 
 differentiation will be best understood from Plate 33, fig. I, and 
 Plate 33, fig. 7. These figures represent transverse sections 
 through the pelvic and pectoral fins of the same embryo on the 
 same scale. The basal bar is seen at bp, and the plate at this 
 stage (which is considerably later than the first differentiation) 
 already partially segmented into rays at br. Outside the region 
 of the cartilaginous plate is seen the fringe with the horny fibres 
 (Ji. f.} ; and dorsally and ventrally to the cartilaginous skeleton 
 are seen the already well-differentiated muscles (ra). 
 
 The pectoral fin is shewn in horizontal section in Plate 33, 
 fig. 6, at a somewhat earlier stage than that to which the trans- 
 verse sections belong. The pectoral girdle (p. g} is cut trans- 
 versely, and is seen to be perfectly continuous with the basal 
 bar (vp} of the fin. A similar continuity between the basal bar 
 of the pelvic fin and the pelvic girdle is shewn in Plate 33, fig. 2, 
 at a somewhat later stage. The plate continuous with the basal 
 bar of the fin is at first, to a considerable extent in the pectoral, 
 and to some extent in the pelvic fin, a continuous lamina, which 
 subsequently segments into rays. In the parts of the plate 
 which eventually form distinct rays, however, almost from the 
 first the cells are more concentrated than in those parts which 
 will form the tissue between the rays ; and I am not inclined to 
 lay any stress whatever upon the fact of the cartilaginous fin-rays 
 being primitively part of a continuous lamina, but regard it as a 
 secondary phenomenon, dependent on the mode of conversion of 
 embryonic mesoblast cells into cartilage. In all cases the sepa- 
 ration into distinct rays is to a large extent completed before 
 the tissue of which the plates are formed is sufficiently differ- 
 entiated to be called cartilage by an histologist. 
 
 The general position of the fins in relation to the body, and 
 their relative sizes, may be gathered from Plate 33, figs. 4 and 5 } 
 which represent transverse sections of the same embryo as that 
 from which the transverse sections shewing the fin on a larger 
 scale were taken. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 727 
 
 During the first stage of its development the skeleton of both 
 fins may thus be described as consisting of a longitudinal bar 
 running along the base of tJie fin, and giving off at right angles 
 series of rays which pass into the fin. The longitudinal bar 
 may be called the basipterygium ; and it is continuous in front 
 with the pectoral or pelvic girdle, as the case may be. 
 
 The further development of the primitive skeleton is different 
 in the case of the two fins. 
 
 The Pelvic Fin. The changes in the pelvic fin are compara- 
 tively slight. Plate 33, fig. 2, is a representation of the fin and 
 its skeleton in a female of Scy Ilium stellarc shortly after the 
 primitive tissue is converted into cartilage, but while it is still so 
 soft as to require the very greatest care in dissection. The fin 
 itself forms a simple projection of the side of the body. The 
 skeleton consists of a basipterygium (bp], continuous in front 
 with the pelvic girdle. To the outer side of the basipterygium 
 a series of cartilaginous fin-rays are attached the posterior ray 
 forming a direct prolongation of the basipterygium, while the 
 anterior ray is united rather with the pelvic girdle than with the 
 basipterygium. All the cartilaginous fin-rays except the first 
 are completely continuous with the basipterygium, their structure 
 in section being hardly different from that shewn in Plate 33, fig. i. 
 
 The external form of the fin does not change very greatly in 
 the course of the further development ; but the hinder part of 
 the attached border is, to some extent, separated off from the 
 wall of the body, and becomes the posterior border of the adult 
 fin. With the exception of a certain amount of segmentation in 
 the rays, the character of the skeleton remains almost as in the 
 embryo. The changes which take place are illustrated by Plate 
 33, fig. 3, shewing the fin of a young male of Scy Ilium stellare. 
 The basipterygium has become somewhat thicker, but is still 
 continuous in front with the pelvic girdle, and otherwise retains 
 its earlier characters. The cartilaginous fin-rays have now 
 become segmented off from it and from the pelvic girdle, the 
 posterior end of the basipterygial bar being segmented off as the 
 terminal ray. 
 
 The anterior ray is directly articulated with the pelvic 
 girdle, and the remaining* rays continue articulated with the 
 basipterygium. Some of the latter are partially segmented. 
 
728 DEVELOPMENT OF THE SKELETON 
 
 As may be gathered by comparing the figure of the fin at 
 the stage just described with that of the adult fin (woodcut, fig. 
 2), the remaining changes are very slight. The most important 
 is the segmentation of the basipterygial bar from the pelvic 
 girdle. 
 
 The pelvic fin thus retains in all essential points its primitive 
 structure. 
 
 The Pectoral Fin. The earliest stage of the pectoral fin dif- 
 fers, as I have shewn, from that of the pelvic fin only in minor 
 points (PI. 33, fig. 6). There is the same longitudinal or basip- 
 terygial bar (bp], to which the fin-rays are attached, which is 
 continuous in front with the pectoral girdle (p g). The changes 
 which take place in the course of the further development, how- 
 ever, are very much more considerable in the case of the pectoral 
 than in that of the pelvic fin. 
 
 The most important change in the external form of the fin is 
 caused by a reduction in the length of its attachment to the body. 
 At first (PI. 33, fig. 6), the base of the fin is as long as the great- 
 est breadth of the fin ; but it gradually becomes shortened by 
 being constricted off from the body at its hinder end. In con- 
 nection with this process the posterior end of the basipterygial 
 bar is gradually rotated outwards, its anterior end remaining 
 attached to the pectoral girdle. In this way this bar comes to 
 form the posterior border of the skeleton of the fin (PI. 33, figs. 
 8 and 9), constituting the metapterygium (inp). It becomes 
 eventually segmented off from the pectoral girdle, simply articu- 
 lating with its hinder edge. 
 
 The plate of cartilage, which is continued outwards from the 
 basipterygium, or, as we may now call it, the metapterygium, 
 into the fin, is not nearly so completely divided up into fin-rays 
 as the homologous part of the pelvic fin; and this is especially 
 the case with the basal part of the plate. This basal part be- 
 comes, in fact, at first only divided into two parts (PI. 33, fig. 8) 
 a small anterior part at the front end (me. p\ and a larger pos- 
 terior along the base of the metapterygium (mp) ; and these two 
 parts are not completely segmented from each other. The 
 anterior part directly joins the pectoral girdle at its base, re- 
 sembling in this respect the anterior fin-ray of the pelvic girdle. 
 It constitutes the (at this stage undivided) rudiment of the meso- 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 729 
 
 pterygium and propterygium of Gcgenbaur. It bears in^ my 
 specimen of this age four fin-rays at its extremity, the anterior 
 not being well marked. The remaining fin-rays are prolonga- 
 tions outwards of the edge of the plate continuous with the 
 metapterygium. These rays are at the stage figured more or 
 less transversely segmented; but at their outer edge they are 
 united together by a nearly continuous rim of cartilage. The 
 spaces between the fin-rays are relatively considerably larger 
 than in the adult. 
 
 The further changes in the cartilages of the pectoral limb are, 
 morphologically speaking, not important, and are easily under- 
 stood by reference to PI. 33, fig. 9 (representing the skeleton of 
 the limb of a nearly ripe embryo). The front end of the anterior 
 basal cartilage becomes segmented off as a propterygium (//), 
 bearing a single fin-ray, leaving the remainder of the cartilage as 
 a mesopterygium (mes). The remainder of the now considerably 
 segmented fin-rays are borne by the metapterygium. 
 
 General Conclusions. From the above observations, conclu- 
 sions of a positive kind may be drawn as to the primitive 
 structure of the skeleton ; and the observations have also, it 
 appears to me, important bearings on the theories of my pre- 
 decessors in this line of investigation. 
 
 The most obvious of the positive conclusions is to the effect 
 that the embryonic skeleton of the paired fins consists of a 
 series of parallel rays similar to those of the unpaired fins. 
 These rays support the soft parts of the fins, which have the 
 form of a longitudinal ridge ; and they are continuous at their 
 base with a longitudinal bar. This bar, from its position at 
 the base of the fin, can clearly never have been a median axis 
 with the rays on both sides. It becomes the basipterygium 
 in the pelvic fin, which retains its embryonic structure much 
 more completely than the pectoral fin ; and the metapterygium 
 in the pectoral fin. The metapterygium of the pectoral fin is 
 thus clearly homologous with the basipterygium of the pelvic 
 fin, as originally supposed by Gegenbaur, and as has since been 
 maintained by Mivart. The propterygium and mesopterygium 
 are obviously relatively unimportant parts of the skeleton as 
 compared with the metapterygium. 
 
 B. 47 
 
730 DEVELOPMENT OF THE SKELETON 
 
 My observations on the development of the skeleton of the 
 fins certainly do not of themselves demonstrate that the paired 
 fins are remnants of a once continuous lateral fin ; but they sup- 
 port this view in that they shew the primitive skeleton of the 
 fins to have exactly the character which might have been an- 
 ticipated if the paired fins had originated from a continuous 
 lateral fin. The longitudinal bar of the paired fins is believed 
 by both Thacker and Mivart to be due to the coalescence of the 
 bases of the primitively independent rays of which they believe 
 the fin to have been originally composed. This view is probable 
 enough in itself, and is rendered more so by the fact, pointed 
 out by Mivart, that a longitudinal bar supporting the cartilagin- 
 ous rays of unpaired fins is occasionally formed ; but there is no 
 trace in the embryo Scylliums of the bar in question being 
 formed by the coalescence of rays, though the fact of its being 
 perfectly continuous with the bases of the fin-rays is somewhat 
 in favour of such coalescence. 
 
 Thacker and Mivart both hold that the pectoral and pelvic 
 girdles are developed by ventral and dorsal growths of the ante- 
 rior end of the longitudinal bar supporting the fin-rays. 
 
 There is, so far as I see, no theoretical objection to be taken 
 to this view ; and the fact of the pectoral and pelvic girdles 
 originating continuously and long remaining united with the 
 longitudinal bars of their respective fins is in favour of it 
 rather than the reverse. The same may be said of the fact 
 that the first part of each girdle to be formed is that in the 
 neighbourhood of the longitudinal bar (basipterygium) of the 
 fin, the dorsal and ventral prolongations being subsequent 
 growths. 
 
 On the whole my observations do not throw much light on 
 the theories of Thacker and Mivart as to the genesis of the 
 skeleton of the paired fin ; but, so far as they bear on the sub- 
 ject, they are distinctly favourable to those theories. 
 
 The main results of my observations appear to me to be 
 decidedly adverse to the views recently put forward on the struc- 
 ture of the fin by Gegenbaur and Huxley, both of whom, as 
 stated above, consider the primitive type of fin to be most nearly 
 retained in Ceratodus, and to consist of a central multisegmented 
 axis with numerous lateral rays. 
 
OF THE PAIRED FINS OF ELASMOBRANCHS. 731 
 
 Gegenbaur derives the Elasmobranch pectoral fin from _a 
 form which he calls the archipterygium, nearly like that of 
 Ceratodus, with a median axis and two rows of rays -but holds 
 that in addition to the rays attached to the median axis, which 
 are alone found in Ceratodus, there were other rays directly 
 articulated to the shoulder-girdle. He considers that in the 
 Elasmobranch fin the majority of the lateral rays on the poste- 
 rior (or median according to his view of the position of the limb) 
 side have become aborted, and that the central axis is repre- 
 sented by the metapterygium ; while ( the pro- and mesoptery- 
 gium and their rays are, he believes, derived from those rays 
 of the archipterygium which originally articulated directly with 
 the shoulder-girdle. 
 
 This view appears to me to be absolutely negatived by the 
 facts of development of the pectoral fin in Scyllium not so 
 much because the pectoral fin in this form is necessarily to be 
 regarded as primitive, but because what Gegenbaur holds to be 
 the primitive axis of the biserial fin is demonstrated to be really 
 the base, and it is only in the adult that it is conceivable that 
 a second set of lateral rays could have existed on the posterior 
 side of the metapterygium. If Gegenbaur's view were correct, 
 we should expect to find in the embryo, if anywhere, traces of 
 the second set of lateral rays ; but the fact is that, as may easily 
 be seen by an inspection of figs. 6 and 7, such a second set of 
 lateral rays could not possibly have existed in a type of fin like 
 that found in the embryo. With this view of Gegenbaur's it 
 appears to me that the theory held by this anatomist to the 
 effect that the limbs are modified gill-arches also falls, in that 
 his method of deriving the limbs from gill-arches ceases to be 
 admissible, while it is not easy to see how a limb, formed on the 
 type of the embryonic limb of Elasmobranchs, could be derived 
 from a gill-arch with its branchial rays. 
 
 Gegenbaur's older view, that the Elasmobranch fin retains 
 a primitive uniserial type, appears to me to be nearer the truth 
 than his more recent view on this subject ; though I hold the 
 fundamental point established by the development of these 
 parts in Scyllium to be that the posterior border of the adult 
 Elasmobranch pectoral fin is the primitive base-line, i.e. line of 
 attachment of the fin to the side pf the body. 
 
 472 
 
732 DEVELOPMENT OF FINS OF ELASMOBRANCHS. 
 
 Huxley holds that the mesopterygium is the proximal piece 
 of the axial skeleton of the limb of Ceratodus, and derives the 
 Elasmobranch fin from that of Ceratodus by the shortening of 
 its axis and the coalescence of some of its elements. The en- 
 tirely secondary character of the mesopterygium, and its total 
 absence in the young embryo Scyllium, appear to me as con- 
 clusive against Huxley's view as the character of the embryonic 
 fin is against that of Gegenbaur ; and I should be much more 
 inclined to hold that the fin of Ceratodus has been derived from 
 a fin like that of the Elasmobranchs by a series of steps similar 
 to those which Huxley supposes to have led to the establishment 
 of the Elasmobranch fin, but in exactly the reverse order. 
 
 There is one statement of Davidoff's which I cannot allow to 
 pass without challenge. In comparing the skeletons of the 
 paired and unpaired fins he is anxious to prove that the former 
 are independent of the axial skeleton in their origin and that 
 the latter have been segmented from the axial skeleton, and 
 thus to shew that an homology between . the two is impossible. 
 In support of his view he states 1 that he has satisfied himself, 
 from embryos of Acanthias and Scy Ilium, that the rays of the 
 unpaired fins are undoubtedly products of the segmentation of tJie 
 dorsal and ventral spinous processes. 
 
 This statement is wholly unintelligible to me. From my 
 examination of the development of the first dorsal and the anal 
 fins of Scyllium I find that their rays develop at a considerable 
 distance from, and quite independently of, the neural and haemal 
 arches, and that they are at an early stage of development dis- 
 tinctly in a more advanced state of histological differentiation 
 than the neural and haemal arches of the same region. I have 
 also found exactly the same in the embryos of Lepidostetis. 
 
 I have, in fact, no doubt that the skeleton of both the paired 
 and the unpaired fins of Elasmobranchs and Lepidosteus is in 
 its development independent of the axial skeleton. The phylo- 
 genetic mode of origin of the skeleton both of the paired and of 
 the unpaired fins cannot, however, be made out without further 
 investigation. 
 
 1 Loc. cit. p. 514. 
 
EXPLANATION OF PLATE 33. 733 
 
 EXPLANATION OF PLATE 33. 
 
 Fig. i. Transverse section through the pelvic fin of an embryo of Scy Ilium 
 belonging to stage P 1 , magnified 50 diameters, bp. basipterygium. br. fin ray. 
 ///. muscle, hf. horny fibres supporting the peripheral part of the fin. 
 
 Fig. 2. Pelvic fin of a very young female embryo of Scy Ilium stellare, magnified 
 1 6 diameters, bp. basipterygium. pit. pubic process of pelvic girdle (cut across 
 below), il. iliac process of pelvic girdle, fo. foramen. 
 
 Fig. 3. Pelvic fin of a young male embryo of Scy Ilium stellare, magnified 16 
 diameters, bp. basipterygium. mo. process of basipterygium continued into clasper. 
 il. iliac process of pelvic girdle, pit. pubic section of pelvic girdle. 
 
 Fig. 4. Transverse section through the ventral part of the trunk of an embryo 
 Scyllium of stage P, in the region of the pectoral fins, to shew how the fins are 
 attached to the body, magnified 18 diameters, br. cartilaginous fin-ray, bp. basi- 
 pterygium. m. muscle of fin. nip. muscle-plate. 
 
 Fig. 5. Transverse section through the ventral part of the trunk of an embryo 
 Scyllium of stage P, in the region of the pelvic fin, on the same scale as fig. 4. 
 bp. basipterygium. br. cartilaginous fin-rays, m. muscle of the fins. mp. muscle- 
 plate. 
 
 Fig. 6. Pectoral fin of an embryo of Scyllium canicula, of a stage between O and 
 P, in longitudinal and horizontal section (the skeleton of the fin was still in the condi- 
 tion of embryonic cartilage), magnified 36 diameters, bp. basipterygium (eventual 
 metapterygium). fr. cartilaginous fin-rays, pg. pectoral girdle in transverse section. 
 fo. foramen in pectoral girdle, pe. epithelium of peritoneal cavity. 
 
 Fig. 7. Transverse section through the pectoral fin of a Scyllium embryo of stage 
 P, magnified 50 diameters, bp. basipterygium. br. cartilaginous fin-ray, m. muscle. 
 h f. horny fibres. 
 
 Fig. 8. Pectoral fin of an embryo of Scyllium stellare, magnified 16 diameters. 
 mp. metapterygium (basipterygium of earlier stage), me.p. rudiment of future pro- 
 and mesopterygium. sc. cut surface of a scapular process, cr. coracoid process. 
 fr. foramen, hf. horny fibres. 
 
 Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe 
 embryo of Scyllittm stellare, magnified 10 diameters, mp. metapterygium. t/ies. 
 mesopterygium. //. propterygium. cr. coracoid process. 
 
 1 I employ here the same letters to indicate the stages as in my "Monograph on 
 Elasmobranch Fishes." 
 
XXI. ON THE EVOLUTION OF THE PLACENTA, AND ON THE 
 POSSIBILITY OF EMPLOYING THE CHARACTERS OF THE 
 PLACENTA IN THE CLASSIFICATION OF THE MAMMALIA'. 
 
 FROM Owen's observations on the Marsupials it is clear that 
 the yolk-sack in this group plays an important (if not the most 
 important) part, in absorbing the maternal nutriment destined 
 for the foetus. The fact that in Marsupials both the yolk-sack 
 and the allantois are concerned in rendering the chorion vascular, 
 makes it a priori probable that this was also the case in the 
 primitive types of the Placentalia ; and this deduction is sup- 
 ported by the fact that in the Rodentia, Insectivora, and Cheiro- 
 ptera this peculiarity of the fcetal membranes is actually found. 
 In the primitive Placentalia it is also probable that from the 
 discoidal allantoic region of the chorion simple fcetal villi, like 
 those of the Pig, projected into uterine crypts ; but it is not 
 certain how far the umbilical region of the chorion, which was 
 no doubt vascular, may also have been villous. From such a 
 primitive type of foetal membranes divergencies in various 
 directions have given rise to the types of fcetal membranes found 
 at the present day. 
 
 In a general way it may be laid down that variations in any 
 direction which tended to increase the absorbing capacities of 
 the chorion would be advantageous. There are two obvious 
 ways in which this might be done, viz. (i) by increasing the 
 complexity of the fetal villi and maternal crypts over a limited 
 area, (2) by increasing the area of the part of the chorion covered 
 by the placental villi. Various combinations of the two pro- 
 cesses would also, of course, be advantageous. 
 
 1 From the Proceedings of the Zoological Society of London, 1881. 
 
THE EVOLUTION OF THE PLACENTA. 735 
 
 The most fundamental change which has taken place Jn jail 
 the existing Placentalia is the exclusion of the umbilical vesicle 
 from any important function in the nutrition of the foetus. 
 
 The arrangement of the fcetal parts in the Rodentia, In- 
 sectivora, and Cheiroptera may be directly derived from the 
 primitive form by supposing the villi of the discoidal placental 
 area to have become more complex, so as to form a deciduate 
 discoidal placenta, while the yolk-sack still plays a part, though 
 physiologically an unimportant part, in rendering the chorion 
 vascular. 
 
 In the Carnivora, again, we have to start from the discoidal 
 placenta, as evinced by the fact that in the growth of the pla- 
 centa the allantoic region of the placenta is at first discoidal, 
 and only becomes zonary at a later stage. A zonary deciduate 
 placenta indicates an increase both in area and in complexity. 
 The relative diminution of the breadth of the placental zone in 
 late fcetal life in the zonary placenta of the Carnivora is probably 
 due to its being on the whole advantageous to secure the nutri- 
 tion of the foetus by insuring a more intimate relation between 
 the fcetal and maternal parts, than by increasing their area of 
 contact. The reason of this is not obvious, but, as shewn below, 
 there are other cases where it is clear that a diminution in the 
 area of the placenta has taken place, accompanied by an increase 
 in the complexity of its villi. 
 
 The second type of differentiation from the primitive form of 
 placenta is illustrated by the Lemuridae, the Suidae, and Manis. 
 In all these cases the area of the placental villi appears to have 
 increased so as to cover nearly the whole subzonal membrane, 
 without the villi increasing to any great extent in complexity. 
 From the diffused placenta covering the whole surface of the 
 chorion, differentiations appear to have taken place in various 
 directions. The placenta of Man and Apes, from its mode of 
 ontogeny, is clearly derived from a diffused placenta (very 
 probably similar to that of Lemurs) by a concentration of the 
 fcetal villi, which are originally spread over the whole chorion, to 
 a disk-shaped area, and by an increase in their arborescence. 
 Thus the discoidal placenta of Man has no connexion with, and 
 ought not to be placed in, the same class as those of the Ro- 
 dentia, Cheiroptera, and Insectivora. 
 
736 THE EVOLUTION OF THE PLACENTA. 
 
 The polycotyledonary forms of placenta are due to similar 
 concentrations of the fcetal villi of an originally diffused pla- 
 centa. . 
 
 In the Edentata we have a group with very varying types of 
 placenta. Very probably these may all be differentiations within 
 the group itself from a diffused placenta such as that found in 
 Manis. The zonary placenta of Orycteropus is capable of being 
 easily derived from that of Manis by the disappearance of the 
 fcetal villi at the two poles of the ovum. The small size of the 
 umbilical vesicle in Orycteropus indicates that its discoidal pla- 
 centa is not, like that of the Carnivora, directly derived from a 
 type with both allantoic and umbilical vascularization of the 
 chorion. The discoidal and dome-shaped placentae of the 
 Armadillos, Myrmecophaga, and the Sloths may easily have been 
 formed from a diffused placenta, just as the discoidal placenta of 
 the Simiidae and Hominidae appears to have been formed from a 
 diffused placenta like that of the Lemuridae. 
 
 The presence of zonary placentae in Hyrax and Elephas does 
 not necessarily afford any proof of affinity of these types with 
 the Carnivora. A zonary placenta may be quite as easily de- 
 rived from a diffused placenta as from a discoidal placenta ; and 
 the presence of two villous patches at the poles of the chorion in 
 Elephas very probably indicates that its placenta has been evolved 
 from a diffused placenta. 
 
 Although it would not be wise to attempt to found a classi- 
 fication upon the placental characters alone, it may be worth 
 while to make a few suggestions as to the affinities of the orders 
 of Mammalia indicated by the structure of the placenta. We 
 clearly, of course, have to start with forms which could not be 
 grouped with any of the existing orders, but which might be 
 called the Protoplacentalia. They probably had the primitive 
 type of placenta described above : the nearest living repre- 
 sentatives of the group are the Rodentia, Insectivora, and Chei- 
 roptera. Before, however, these three groups had become dis- 
 tinctly differentiated, there must have branched off from the 
 .primitive stock the ancestors of the Lemuridae, the Ungulata, 
 and the Edentata. 
 
 It is obvious on general anatomical grounds that the Monkeys 
 and Man are to be derived from a primitive Lemurian type ; and 
 
THE EVOLUTION OF THE PLACENTA. 737 
 
 with this conclusion the form of the placenta completely _tal]ies. 
 The primitive Edentata and Ungulata had no doubt a diffused 
 placenta which was probably not very different from that of the 
 primitive Lemurs ; but how far these groups arose quite in- 
 dependently from the primitive stock, or whether they may have 
 had a nearer common ancestor, cannot be decided from the 
 structure of the placenta. The Carnivora were certainly an 
 offshoot from the primitive placental type which was quite in- 
 dependent of the three groups just mentioned ; but the character 
 of the placenta of the Carnivora does not indicate at what stage 
 in the evolution of the placental Mammalia a primitive type of 
 Carnivora was first differentiated. 
 
 No important light is thrown by the placenta on the affinities 
 of the Proboscidea, the Cetacea, or the Sirenia ; but the character 
 of the placenta in the latter group favours the view of their being 
 related to the Ungulata. 
 
XXII. ON THE STRUCTURE AND DEVELOPMENT OF LEPI- 
 DOSTEUS 1 . By F. M. BALFOUR and W. N. PARKER. 
 
 (With Plates 3442.) 
 TABLE OF CONTENTS. 
 
 PAGE 
 
 INTRODUCTION 739 
 
 GENERAL DEVELOPMENT 740 
 
 BRAIN 
 
 Adult brain ........... 759 
 
 Development of the brain 764 
 
 Comparison of the larval and adult brain of Lepidosteiis, together with 
 some observations on the systematic value of the characters of the 
 
 Ganoid brain 767 
 
 SENSE ORGANS 
 
 Olfactory organ 771 
 
 Anatomy of the eye ib. 
 
 Development of the eye 772 
 
 SUCTORIAL Disc 774 
 
 MUSCULAR SYSTEM 775 
 
 SKELETON 
 
 Vertebral column and ribs of the adult ...... 776 
 
 Development of the vertebral column and ribs 778 
 
 Comparison of the vertebral column of Lepidosteus with that of other 
 
 forms 792 
 
 The ribs of Fishes 793 
 
 The skeleton of the ventral lobe of the tail fin, and its bearing on the 
 
 nature of the tail fin of the various types of Pisces . . . 80 1 
 
 EXCRETORY AND GENERATIVE ORGANS 
 
 Anatomy of the excretory and generative organs of the female . . 810 
 
 Anatomy of the excretory and generative organs of the male . . 813 
 
 Development of the excretory and generative organs . . . . 815 
 
 Theoretical considerations 822 
 
 1 From the Philosophical Transactions of the Royal Society, 1882. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 739 
 
 Tin, ALIMENTARY CANAL AND ITS APPENDAGES PAGE 
 
 Topographical anatomy of the alimentary canal 828 
 
 Development of the alimentary canal and its appendages . . . 831 
 
 THE GILL ON THE HYOID ARCH 835 
 
 THE SYSTEMATIC POSITION OF LEPIDOSTEUS ' 836 
 
 LIST OK MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF LEPIDOSTEUS 840 
 
 LIST OF REFERENCE LETTERS 841 
 
 EXPLANATION OF PLATES 842 
 
 INTRODUCTION. 
 
 THE following paper is the outcome of the very valuable gift 
 of a series of embryos and larvae of Lepidosteus by Professor Alex. 
 Agassiz, to whom we take this opportunity of expressing our 
 most sincere thanks. The skull of these embryos and larvae has 
 been studied by Professor Parker, and forms the subject of a 
 memoir already presented to the Royal Society. 
 
 Considering that Lepidosteus is one of the most interesting of 
 existing Ganoids, and that it is very closely related to species of 
 Ganoids which flourished during the Triassic period, we naturally 
 felt keenly anxious to make the most of the opportunity of 
 working at its development offered to us by Professor Agassiz' 
 gift. Professor Agassiz, moreover, most kindly furnished us with 
 four examples of the adult Fish, which have enabled us to make 
 this paper a study of the adult anatomy as well as of the develop- 
 ment. 
 
 The first part of our paper is devoted to the segmentation, 
 formation of the germinal layers, and general development of the 
 embryo and larva. The next part consists of a series of sections 
 on the organs, in which both their structure in the adult and 
 their development are dealt with. This part is not, however, in 
 any sense a monograph, and where already known, the anatomy 
 is described with the greatest possible brevity. In this part of 
 the paper considerable space is devoted to a comparison of the 
 organs of Lepidosteus with those of other Fishes, and to a state- 
 ment of the conclusions which follow from such comparison. 
 
 The last part of the paper deals with the systematic position 
 of Lepidosteus and of the Ganoids generally. 
 
740 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 GENERAL DEVELOPMENT. 
 
 The spawning of Lepidosteus takes place in the neighbour- 
 hood of New York about May 2Oth. Agassiz (No. i) 1 gives an 
 account of the process from Mr S. W. Carman's notes, which we 
 venture to quote in full. 
 
 " Black Lake is well stocked with Bill-fish. When they 
 appear, they are said to come in countless numbers. This is 
 only for a few days in the spring, in the spawning season, between 
 the 1 5th of May and the 8th of June. During the balance of the 
 season they are seldom seen. They remain in the deeper parts 
 of the lake, away from the shore, and, probably, are more or less 
 nocturnal in habits. Out of season, an occasional one is caught 
 on a hook baited with a minnow. Commencing with the 2Oth 
 of April, until the I4th of May we were unable to find the Fish, 
 or to find persons who had seen them during this time. Then a 
 fisherman reported having seen one rise to the surface. Later, 
 others were seen. On the afternoon of the i8th, a few were 
 found on the points, depositing the spawn. The temperature at 
 the time was 68 to 69 on the shoals, while out in the lake the 
 mercury stood at 62 to 63. The points on which the eggs were 
 laid were of naked granite, which had been broken by the frost 
 and heat into angular blocks of 3 to 8 inches in diameter. The 
 blocks were tumbled upon each other like loose heaps of brick- 
 bats, and upon and between them the eggs were dropped. The 
 points are the extremities of small capes that make out into the 
 lake. The eggs were laid in water varying in depth from 2 to 
 14 inches. At the time of approaching the shoals, the Fish 
 might be seen to rise quite often to the surface to take air. This 
 they did by thrusting the bill out of the water as far as the 
 corners of the mouth, which was then opened widely and closed 
 with a snap. After taking the air, they seemed more able to 
 remain at the surface. Out in the lake they are very timid, but 
 once buried upon the shoals they become quite reckless as to 
 what is going on about them. A few moments after being driven 
 
 1 The numbers refer to the list of memoirs of the anatomy and development given 
 at the end of this memoir. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 74! 
 
 off, one or more of the males would return as if scouting.. If 
 frightened, he would retire for some time ; then another scout 
 would appear. If all promised well, the females, with the atten- 
 dant males, would come back. Each female was accompanied 
 by from one to four males. Most often, a male rested against 
 each side, with their bills reaching up toward the back of her 
 head. Closely crowded together, the little party would pass 
 back and forth over the rocky bed they had selected, sometimes 
 passing the same spot half-a-dozen times without dropping an 
 egg, then suddenly would indulge in an orgasm ; and, lashing 
 and plashing the water in all directions with their convulsive 
 movements, would scatter at the same instant the eggs and the 
 sperm. This ended, another season of moving slowly back and 
 forth was observed, to be in turn followed by another of excite- 
 ment. The eggs were excessively sticky. To whatever they 
 happened to touch, they stuck, and so tenaciously that it was 
 next to impossible to release them without tearing away a 
 portion of their envelopes. It is doubtful whether the eggs 
 would hatch if removed. As far as could be seen at the time, 
 upon or under the rocks to which the eggs were fastened there 
 was an utter absence of anything that might serve as food for 
 the young Fishes. 
 
 " Other Fishes, Bull-heads, &c., are said to follow the Bill-fish 
 to eat the spawn. It may be so. It was not verified. Certainly 
 the points under observations were unmolested. During the 
 afternoon of the i8th of May a few eggs were scattered on 
 several of the beds. On the igth there were more. With the 
 spear and the snare, several dozens of both sexes of the Fish 
 were taken. Taking one out did not seem greatly to startle the 
 others. They returned very soon. The males are much smaller 
 than the average size of the females ; and, judging from those 
 taken, would seem to have as adults greater uniformity in size. 
 The largest taken was a female, of 4 feet ii inch in length. 
 Others of 2 feet 6 inches contained ripe ova. With the iQth of 
 May all disappeared, and for a time the weather being mean- 
 while cold and stormy there were no signs of their continued 
 existence to be met with. Nearly two weeks later, on the 3ist 
 of May, as stated by Mr Henry J. Perry, they again came up, 
 not in small detachments on scattered points as before, but in 
 
742 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 multitudes, on every shoal at all according with their ideas of 
 spawning beds. They remained but two days. During the 
 summer it happens now and then that one is seen to come up for 
 his mouthful of air ; beyond this there will be nothing to suggest 
 the ravenous masses hidden by the darkness of the waters." 
 
 Egg membranes. The ova of Lepidosteus are spherical bodies 
 of about 3 millims. in diameter. They have a double investment 
 consisting of (i) an outer covering formed of elongated, highly 
 refractive bodies, somewhat pyriform at their outer ends (Plate 
 34, fig. 17, f.e.}, which are probably metamorphosed follicular 
 cells 1 , and (2) of an inner membrane, divided into two zones, 
 viz. : an outer and thicker zone, which is radially striated, and 
 constitutes the zona radiata (2. r.}, and an inner and narrow 
 homogeneous zone (z. r'.\ 
 
 Segmentation. We have observed several stages in the seg- 
 mentation, which shew that it is complete, but that it approaches 
 the meroblastic type more nearly than in the case of any other 
 known holoblastic ovum. 
 
 Our earliest stage shewed a vertical furrow at the upper or 
 animal pole, extending through about one-fifth of the circum- 
 ference (Plate 34, fig. i), and in a slightly later stage we found a 
 second similar furrow at right angles to the first (Plate 34, fig. 2). 
 We have not been fortunate enough to observe the next phases 
 of the segmentation, but on the second day after impregnation 
 (Plate 34, fig. 3), the animal pole is completely divided into small 
 segments, which form a disc, homologous to the blastoderm of 
 meroblastic ova ; while the vegetative pole, which subsequently 
 forms a large yolk-sack, is divided by a few vertical furrows, four 
 of which nearly meet at the pole opposite the blastoderm (Plate 
 34, fig. 4). The majority of the vertical furrows extend only a 
 short way from the edge of the small spheres, and are partially 
 intercepted by imperfect equatorial furrows. 
 
 1 We have examined the structure of the ovarian ova in order to throw light on 
 the nature of these peculiar pyriform bodies. Unfortunately, the ovaries of our adult 
 examples of Lepidosteus were so badly preserved, that we could not ascertain any- 
 thing on this subject. The ripe ova in the ovary have an investment of pyriform 
 bodies similar to those of the just laid ova. With reference to the structure of the 
 ovarian ova we may state that the germinal vesicles are provided with numerous 
 nucleoli arranged in close proximity with the membrane of the vesicle. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 743 
 
 Development of the embryo. We have not been able to work 
 out the stages immediately following the segmentation, owing to 
 want of material ; and in the next stage satisfactorily observed, 
 on the third day after impregnation, the body of the embryo is 
 distinctly differentiated. The lower pole of the ovum is then 
 formed of a mass in which no traces of the previous segments or 
 segmentation furrows could any longer be detected. 
 
 Some of the dates of the specimens sent to us appear to have 
 been transposed ; so that our statements as to ages must only be 
 taken as approximately correct. 
 
 TJiird day after impregnation. In this stage the embryo is 
 about 3'5 millims. in length, and has a somewhat dumb-bell shaped 
 outline (Plate 34, fig. 5). It consists of (i) an outer area (/. z) 
 with some resemblance to the area pellucida of the Avian 
 embryo, forming the parietal part of the body ; and (2) a central 
 portion consisting of the vertebral and medullary plates and the 
 axial portions of the embryo. In hardened specimens the 
 peripheral part forms a shallow depression surrounding the 
 central part of the embryo. 
 
 The central part constitutes a somewhat prominent ridge, the 
 axial part of it being the medullary plate. Along the anterior 
 half of this part a dark line could be observed in all our speci- 
 mens, which we at first imagined to be caused by a shallow groove. 
 We have, however, failed to find in our sections a groove in this 
 situation except in a single instance (Plate 35, fig. 20, x), and are 
 inclined to attribute the appearance above-mentioned to the 
 presence of somewhat irregular ridges of the outer layer of the 
 epiblast, which have probably been artificially produced in the 
 process of hardening. 
 
 The anterior end of the central part is slightly dilated to form 
 the brain (b.) ; and there is present a pair of lateral swellings 
 near the anterior end of the brain which we believe to be the 
 commencing optic vesicles. We could not trace any other clear 
 indications of the differentiation of the brain into distinct lobes. 
 
 At the hinder end of the central part of the embryo a very 
 distinct dilatation may also be observed, which is probably homo- 
 logous with the tail swelling of Teleostei. Its structure is more 
 particularly dealt with in the description of our sections of this 
 stage. 
 
744 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 After the removal of the egg-membranes described above 
 we find that there remains a delicate membrane closely attached 
 to the epiblast. This membrane can be isolated in distinct 
 portions, and appears to be too definite to be regarded as an 
 artificial product. 
 
 We have been able to prepare several more or less complete 
 series of sections of embryos of this stage (Plate 35, figs. 18 22). 
 These sections present as a whole a most striking resemblance 
 to those of Teleostean embryos at a corresponding stage of 
 development. 
 
 Three germinal layers are already fully established. The 
 epiblast (ep.} is formed of the same parts as in Teleostei, viz. : - 
 of an outer epidermic and an inner nervous or mucous stratum. 
 In the parietal region of the embryo these strata are each 
 formed of a single row of cells only. The cells of both strata 
 are somewhat flattened, but those of the epidermic stratum are 
 decidedly the more flattened of the two. 
 
 Along the axial line there is placed, as we have stated 
 above, the medullary plate. The epidermic stratum passes over 
 this plate without undergoing any change of character, and 
 the plate is entirely constituted of the uervojts stratum of tlie 
 epidermis. 
 
 The medullary plate has, roughly speaking, the form of a 
 solid keel, projecting inwards towards the yolk. There is no 
 trace, at this stage at any rate, of a medullary groove ; and as 
 we shall afterwards shew, the central canal of the cerebro-spinal 
 cord is formed in the middle of the solid keel. The shape of 
 this keel varies according to the region of the body. In the 
 head (Plate 35, fig. 18, m.c.}, it is very prominent, and forming, 
 as it does, the major part of the axial tissue of the body, impresses 
 its own shape on the other parts of the head and gives rise to 
 a marked ridge on the surface of the head directed towards the 
 yolk. In the trunk (Plate 35, figs. 19, 20) the keel is much less 
 prominent, but still projects sufficiently to give a convex form 
 to the surface of the body turned towards the yolk. 
 
 In the head, and also near the hind end of the trunk, the 
 nervous layer of the epiblast continuous with the keel on each 
 side is considerably thicker than the lateral parts of the layer. 
 The thickening of the nervous layer in the head gives rise to 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 745 
 
 what has been called by Gotte l " the special sense plate," .owing 
 to its being subsequently concerned in the formation of parts of 
 the organs of special sense. We cannot agree with Gotte in 
 regarding it as part of the brain. 
 
 In the keel itself two parts may be distinguished, viz.: a 
 superficial part, best marked in the region of the brain, formed 
 of more or less irregularly arranged polygonal cells, and a deeper 
 part of horizontally placed flatter cells. The upper part is 
 mainly concerned in the formation of the cranial nerves, and of 
 the dorsal roots of the spinal nerves. 
 
 The mesoblast (uis.} in the trunk consists of a pair of inde- 
 pendent plates which are continued forwards into the head, 
 and in the prechordal region of the latter, unite below the 
 medullary keel. 
 
 The mesoblastic plates of the trunk are imperfectly divided 
 into vertebral and lateral regions. Neither longitudinal sections 
 nor surface views shew at this stage any trace of a division of 
 the mesoblast into somites. The mesoblast cells are polygonal, 
 and no indication is as yet present of a division into splanchnic 
 and somatic layers. 
 
 The notochord (nc] is well established, so that its origin 
 could not be made out. It is, however, much more sharply 
 separated from the mesoblastic plates than from the hypoblast, 
 though the ventral and inner corners of the mesoblastic plates 
 which run in underneath it on either side, are often imperfectly 
 separated from it. It is formed of polygonal cells, of which 
 between 40 and 50 may as a rule be seen in a single section. 
 No sheath is present around it. It has the usual extension in 
 front. 
 
 The hypoblast (///.) has the form of a membrane, composed of 
 a single row of oval cells, bounding the embryo on the side 
 adjoining the yolk. 
 
 In the region of the caudal swelling the relations of the 
 germinal layers undergo some changes. This region may, from 
 the analogy of other Vertebrates, be assumed to constitute the 
 lip of the blastopore. We find accordingly that the layers be- 
 come more or less fused. In the anterior part of the tail 
 
 1 " Ueh. d. Entwick. d. Central Nerven Systems <1. Teleostier," Archiv fiir mikr. 
 Auat. Vol. xv. 1878. 
 
 H. 48 
 
746 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 swelling, the boundary between the notochord and hypoblast 
 becomes indistinct A short way behind this point (Plate 35, 
 fig. 21), the notochord unites with the medullary keel, and a 
 neurenteric cord, homologous with the neurenteric canal of other 
 Ichthyopsida, is thus established. In the same region the boun- 
 dary between the lateral plates of mesoblast and the notochord, 
 and further back (Plate 35, fig. 22), that between the mesoblast 
 and the medullary keel, becomes obliterated. 
 
 Fifth day after impregnation. Between the stage last de- 
 scribed and the next stage of which we have specimens, a con- 
 siderable progress has been made. The embryo (Plate 34, figs. 
 6 and 7) has grown markedly in length and embraces more than 
 half the circumference of the ovum. Its general appearance is, 
 however, much the same as in the earlier stage, but in the 
 cephalic region the medullary plate is divided by constrictions 
 into three distinct lobes, constituting the regions of the fore- 
 brain, the mid-brain, and the hind-brain. The fore-brain (Plate 
 34, fig. 6,f.b.} is considerably the largest of the three lobes, and 
 a pair of lateral projections forming the optic vesicles are 
 decidedly more conspicuous than in the previous stage. The 
 mid-brain (m.b,} is the smallest of the three lobes, while the 
 hind-brain (/z..) is decidedly longer, and passes insensibly into 
 the spinal cord behind. 
 
 The medullary keel, though retaining to a great extent the 
 shape it had in the last stage, is no longer completely solid. 
 Throughout the whole region of the brain and in the anterior 
 part of the trunk (Plate 35, figs. 23, 24, 25) a slit-like lumen has 
 become formed. We are inclined to hold that this is due to the 
 appearance of a space between the cells, and not, as supposed by 
 Oellacher for Teleostei, to an actual absorption of cells, though 
 we must admit that our sections are hardly sufficiently well pre- 
 served to be conclusive in settling this point. Various stages in 
 its growth may be observed in different regions of the cerebro- 
 spinal cord. When first formed, it is a very imperfectly defined 
 cavity, and a few cells may be seen passing right across from 
 one side of it to the other. It gradually becomes more definite, 
 and its wall then acquires a regular outline. 
 
 The optic vesicles are now to be seen in section (Plate 35, 
 fig- 2 3 P-} as flattish outgrowths of the wall of the fore-brain, 
 
STRUCTURE: AND DKYKI.OI-MKNT OK LKITDOSTKUS. 747 
 
 into which the lumen of the third ventricle is prolonged -for a 
 short distance. 
 
 The brain has become to some extent separate from the 
 superjacent epiblast, but the exact mode in which this is effected 
 is not clear to us. In some sections it appears that the separation 
 takes place in such a way that the nervous keel is only covered 
 above by the epidermic layer of the epiblast, and that the 
 nervous layer, subsequently interposed between the two, grows 
 in from the two sides. Such a section is represented in Plate 35, 
 'fig. 24. Other sections again favour the view that in the isolation 
 of the nervous keel, a superficial layer of it remains attached to 
 the nervous layer of the epidermis at the two sides, and so, 
 from the first, forms a continuous layer between the nervous 
 keel and the epidermic layer of the epiblast (Plate 35, fig. 25). 
 In the absence of a better series of sections we do not feel able 
 to determine this point. The posterior part of the nervous keel 
 retains the characters of the previous stage. 
 
 At the sides of the hind-brain very distinct commencements 
 of the auditory vesicles are apparent. They form shallow pits 
 (Plate 35, fig. 24, an.} of the thickened part of the nervous 
 layer adjoining the brain in this region, Each pit is covered 
 over by the epidermic layer above, which has no share in its 
 formation. 
 
 In many parts of the lateral regions of the body the nervous 
 layer of the epidermis is more than one cell deep. 
 
 The mesoblastic plates are now divided in the anterior part 
 of the trunk into a somatic and a splanchnic layer (Plate 35, fig. 
 25, so., sp.\ though no distinct cavity is as yet present between 
 these two layers. Their vertebral extremities are somewhat 
 wedge-shaped in section, the base of the wedge being placed 
 at the sides of the medullary keel. The wedge-shaped portions 
 are formed of a superficial layer of palisade-like cells and an 
 inner kernel of polygonal cells. The superficial layer on the 
 dorsal side is continuous with the somatic mesoblast, while the 
 remainder pertains to the splanchnic layer. 
 
 The diameter of the notochord has diminished, and the cells 
 have assumed a flattened form, the protoplasm being confined to 
 an axial region. In consequence of this, the peripheral layer 
 appears clear in transverse sections. A delicate cuticular sheath 
 
 482 
 
748 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 is formed around it. This sheath is probably the commence- 
 ment of the permanent sheath of later stages, but at this 
 stage it cannot be distinguished in structure from a delicate 
 cuticle which surrounds the greater part of the medullary 
 cord. 
 
 The hypoblast has undergone no changes of importance. 
 
 The layers at the posterior end of the embryo retain the 
 characters of the last stage. 
 
 Sixth day after impregnation. At this stage (Plate 34, fig. 8) 
 the embryo is considerably more advanced than at the last stage. 
 The trunk has decidedly increased in length, and the head forms 
 a relatively smaller portion of the whole. The regions of the 
 brain are more distinct. The optic vesicles (op.} have grown 
 outwards so as to nearly reach the edges of the area which forms 
 the parietal part of the body. The fore-brain projects slightly 
 in front, and the mid-brain is seen as a distinct rounded promi- 
 nence. Behind the latter is placed the hind-brain, which passes 
 insensibly into the spinal cord. On either side of the mid- and 
 hind-brain a small region is slightly marked off from the rest of 
 the parietal part, and on this are seen two more or less trans- 
 versely directed streaks, which, by comparison with the Sturgeon 1 . 
 we are inclined to regard as the two first visceral clefts (br.c.}. 
 We have, however, failed to make them out in sections, and 
 owing to the insufficiency of our material, we have not even 
 studied them in surface views as completely as we could have 
 wished. 
 
 The body is now laterally compressed, and more decidedly 
 raised from the yolk than in the previous stages. In the lateral 
 regions of the trunk the two segmental or archinephric ducts 
 (sg.) are visible in surface views : the front end of each is placed 
 at the level of the hinder border of the head, and is marked by 
 a flexure inwards towards the middle line. The remainder of 
 each duct is straight, and extends backwards for about half the 
 length of the embryo. The tail has much the same appearance 
 as in the last stage. 
 
 The vertebral regions of the mesoblastic plates are now seg- 
 mented for the greater part of the length of the trunk, and the 
 
 1 Salensky, " Recherches s. le Developpement du Sterlet." Archives de Biol. 
 Vol. II. 1881, pi. xvn. fig. 27. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 749 
 
 somites of which they are composed (Plate 36, fig. 30, pr.} are 
 very conspicuous in surface views. 
 
 Our sections of this stage are not so complete as could be 
 desired : they shew, however, several points of interest. 
 
 The central canal of the nervous system is large, with well- 
 defined walls, and in hardened specimens is filled with a coagu- 
 lum. It extends nearly to the region of the tail. 
 
 The optic vesicles, which are so conspicuous in surface views, 
 appear in section (Plate 35, fig. 26, op.} as knob-like outgrowths 
 of the fore-brain, and very closely resemble the figures given by 
 Oellacher of these vesicles in Teleostei 1 . 
 
 From the analogy of the previous stage, we are inclined to 
 think that they have a lumen continuous with that of the fore- 
 brain. In our only section through them, however, they are 
 solid, but this is probably due to the section merely passing 
 through them to one side. 
 
 The auditory pits (Plate 35, fig. 27, au.) are now well marked, 
 and have the form of somewhat elongated grooves, the walls of 
 which are formed of a single layer of columnar cells belonging 
 to the nervous layer of the epidermis, and extending inwards so 
 as nearly to touch the brain. 
 
 In an earlier stage it was pointed out that the dorsal part of 
 the medullary keel was different in its structure from the re- 
 mainder, and that it was destined to give rise to the nerves. 
 The process of differentiation is now to a great extent com- 
 pleted, and may best be seen in the auditory region (Plate 35, fig. 
 27, VIII.). In this region there was present during the last stage 
 a great rhomboidal mass of cells at the dorsal region of the brain 
 (Plate 35, fig. 24, VIII.). In the present stage, this, which is the 
 rudiment of the seventh and auditory nerves, is seen growing 
 down on each side from the roof of the hind-brain, between the 
 brain and the auditory involution, and abutting against the wall 
 of the latter. 
 
 Rudiments of the spinal nerves are also seen at intervals 
 as projections from the dorsal angles of the spinal cord (Plate 
 36, fig. 29, sp.1t.}. They extend only for a short distance 
 outwards, gradually tapering off to a point, and situated 
 
 1 " Beitrii^e zur Enlwick. <1. Knociuinfische,'' Zcit.f. li'iss. Zool. Vol. xxiil. 1873, 
 taf. m. tig. ix. 2. 
 
750 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 between the epiblast and the dorsal angles of the mesoblastic 
 somites. 
 
 The process of formation of the cranial nerves and dorsal 
 roots of the spinal nerves is, it will be seen, essentially the same 
 as that already known in the case of Elasmobranchii, Aves, &c. 
 The nerVes arise as outgrowths of a special crest of cells, the 
 neural crest of Marshall, which is placed along the dorsal angle 
 of the cord. The peculiar position of the dorsal roots of the 
 spinal nerves is also very similar to what has been met with in 
 the early stages of these structures by Marshall in Birds 1 , and 
 by one of us in Elasmobranchs 2 . 
 
 In the parietal region a cavity has now appeared in part of 
 the trunk between the splanchnic and somatic layers of the 
 mesoblast (Plate 36, fig. 29, b.c,\ the somatic layer (so.) consist- 
 ing of a single row of columnar cells on the dorsal side, while 
 the remainder of each somite is formed of the splanchnic layer 
 (sp>). In many of the sections the somatic layer is separated by 
 a considerable interval from the epiblast. 
 
 We have been able to some extent to follow the develop- 
 ment of the segmental duct. The imperfect preservation of our 
 specimens has, as in other instances, rendered the study of the 
 point somewhat difficult, but we believe that the figure represent- 
 ing the development of the duct some way behind its front end 
 (Plate 36, fig. 29) is an accurate representation of what may be 
 seen in a good many of our sections. 
 
 It appears from these sections that the duct (Plate 36, fig. 29, 
 .jg-.) is developed as a hollow ridge-like outgrowth of the somatic 
 layer of mesoblast, directed towards the epiblast, in which it 
 causes a slight bulging. The cavity of the ridge freely com- 
 municates with the body- cavity. The anterior part of this ridge 
 appears to be formed first. Very soon, in fact, in an older 
 embryo belonging to this stage, the greater part of the groove 
 becomes segmented off as a duct lying between the epiblast and 
 somatic mesoblast (Plate 36, fig. 28, sg.), while the front end still 
 remains, as we believe, in communication with the body-cavity 
 by an anterior pore. 
 
 1 Journal of A 'tint and Pliysiol. Vol. xi. p. 491, plates xx. and xxi. 
 
 2 "Elasmobranch Fishes/' p. 156, plates 10 and 13. [This edition, p. 378, 
 pi. 11, 14-] 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 751 
 
 This mode of development corresponds in every particular 
 with that observed in Teleostei by Rosenberg and Oellacher. 
 
 The structure of the notochord (nc.) at this stage is very 
 similar to that observed by one of us in Elasmobranchii 1 . The 
 cord is formed of transversely arranged flattened cells, the outer 
 parts of which are vacuolated, while the inner parts are granular, 
 and contain the nuclei. This structure gives rise to the appear- 
 ance in transverse sections of an axial darker area and a periphe- 
 ral lighter portion. 
 
 The hypoblast retains for the most part its earlier constitution, 
 but underneath the notochord, in the trunk, it is somewhat thick- 
 ened, and the cells at the two sides spread in to some extent 
 under the thickened portion (Plate 36, fig. 29, s.nc.). This thick- 
 ening, as is shewn in transverse sections at the stage when the 
 segmental duct becomes separated from the somatic mesoblast 
 (Plate 36, fig. 28, s.nc.), is the commencement of the subnoto- 
 chordal rod. 
 
 The tail end of the embryo still retains its earlier characters. 
 Seventh day after impregnation. Our series of specimens of 
 this stage is very imperfect, and we are only able to call attention 
 to the development of a certain number of organs. 
 
 Our sections clearly establish the fact that the optic vesicles 
 are now hollow processes of the fore-brain. Their outer ends 
 are dilated, and are in contact with the external skin. The 
 formation of the optic cup has not, however, commenced. The 
 nervous layer of the skin adjoining the outer wall of the optic 
 cup is very slightly thickened, constituting the earliest rudiment 
 of the lens. 
 
 In one of our embryos of this day the developing auditory 
 vesicle still has the form of a pit, but in the other it is a closed 
 vesicle, already constricted off from the nervous layer of the 
 epidermis. 
 
 With reference to the development of the excretory duct we 
 cannot add much to what we have already stated in "describing 
 the last stage. 
 
 The duct is considerably dilated anteriorly (Plate 36, fig. 31, 
 .sg-.); but our sections throw no light on the nature of the ab- 
 dominal pore. The posterior part of the duct has still the form 
 
 1 " Elasmobranch Fishes," p. 136, plate u, fig. 10. [This edition, p. 354, pi. 12.] 
 
752 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 of a hollow ridge united with somatic mesoblast (Plate 36, fig. 
 32, s&). 
 
 During this stage, the embryo becomes to a small extent 
 folded off from the yolk-sack both in front and behind, and in the 
 course of this process the anterior and posterior extremities of 
 the alimentary tract become definitely established. 
 
 We have not got as clear a view of the process of formation 
 of these two sections of the alimentary tract as we could desire, 
 but our observations appear to shew that the process is in many 
 respects similar to that which takes place in the formation of 
 the anterior part of the alimentary tract in Elasmobranchii 1 . 
 One of us has shewn that in Elasmobranchs the ventral wall of 
 the throat is formed not by a process of folding in of the hypo- 
 blastic sheet as in Birds, but by a growth of the ventral face of 
 the hypoblastic sheet on each side of and at some little distance 
 from the middle line. Each growth is directed inwards, and 
 the two eventually meet and unite, thus forming a complete 
 ventral wall for the gut. Exactly the same process would seem 
 to take place in Lcpidosteus, and after the lumen of the gut is in 
 this way established, a process of mesoblast on each side also 
 makes its appearance, forming a mesoblastic investment on the 
 ventral side of the alimentary tract. Some time after the ali- 
 mentary tract has been thus formed, the epiblast becomes folded 
 in, in exactly the same manner as in the Chick, the embryo 
 becoming thereby partially constricted off from the yolk (Plate 
 36, figs. 33, 34). 
 
 The form of the lumen of the alimentary tract differs some- 
 what in front and behind. In front, the hypoblastic sheet 
 remains perfectly flat during the formation of the throat, and thus 
 the lumen of the latter has merely, the form of a slit. The lumen 
 of the posterior end of the alimentary tract is, however, narrower 
 and deeper (Plate 36, figs. 33, 34, /.). Both in front and behind, 
 the lateral parts of the hypoblastic sheet become separated from 
 the true alimentary tract as soon as the lumen of the latter is 
 established. 
 
 It is quite possible that at the extreme posterior end of the 
 embryo a modification of the above process may take place, for 
 
 1 F. M. Balfour, "Monograph on the Development of Elasmobranch Fishes," 
 p. 87, plate 9, fig. 2. [This edition, p. 303, pi. 10.] 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 753 
 
 in this region the hypoblast appears to us to have the form of 
 a solid cord. 
 
 We could detect no true neurenteric canal, although a more 
 or less complete fusion of the germinal layers at the tail end of 
 the embryo may still be traced. 
 
 During this stage the protoplasm of the notochordal cells, 
 which in the last stage formed a kind of axial rod in the centre 
 of the notochord, begins to spread outwards toward the sheath 
 of the notochord. 
 
 Eighth day after impregnation. The external form of the em- 
 bryo (Plate 34, fig. 9) shews a great advance upon the stage last 
 figured. Both head and body are much more compressed later- 
 ally and raised from the yolk, and the head end is folded off for 
 some distance. The optic vesicles are much less prominent 
 externally. A commencing opercular fold is distinctly seen. 
 Our figure of this stage is not, however, so satisfactory as we 
 could wish. 
 
 A thickening of the nervous layer of the external epiblast 
 which will form the lens (Plate 36, fig. 35, /.) is more marked 
 than in the last stage, and presses against the slightly concave 
 exterior wall of the optic vesicle (op.). The latter has now 
 a large cavity, and its stalk is considerably narrowed. 
 
 The auditory vesicles (Plate 36, fig. 36, an.} are closed, ap- 
 pearing as hollow sacks one on each side of the brain, and are no 
 longer attached to the epiblast. 
 
 The anterior opening of the segmental duct can be plainly 
 seen close behind the head. The lumen of the duct is consider- 
 ably larger. 
 
 The two vertebral portions of the mesoblast are now sepa- 
 rated by a considerable space from the epiblast on one side and 
 from the notochord on the other, and the cells composing them 
 have become considerably elongated from side to side (Plate 36, 
 
 fig- 37. "") 
 
 In some sections the aorta can be seen (Plate 36, fig. 37, ao.) 
 lying close under the sub- notochordal rod, between it and the 
 hypoblast, and on either side of it a slightly larger cardinal vein 
 (cd. v.}. 
 
 The protoplasm of the notochord has now again retreated 
 towards the centre, shewing a clear space all round. This is 
 
754 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 most marked in the region of the trunk (Plate 36, fig. 37). The 
 sub-notochordal rod (s. nc.} lies close under it. 
 
 A completely closed fore-gut, lined by thickened hypoblast, 
 extends about as far back as the auditory sacks (Plate 36, figs. 35 
 and 36, a!.). In the trunk the hypoblast, which will form the 
 walls of the alimentary tract, is separated from the notochord 
 by a considerable interval. 
 
 Ninth day after impregnation: External characters. Very 
 considerable changes have taken place in the external characters 
 of the embryo. It is about 8 millims. in length, and has assumed 
 a completely piscine form. The tail especially has grown in 
 length, and is greatly flattened from side to side : it is wholly 
 detached from the yolk, and bends round towards the head, 
 usually with its left side in contact with the yolk. It is pro- 
 vided with well-developed dorsal and ventral fin-folds, which 
 meet each other round the end of the tail, the tail fin so formed 
 being nearly symmetrical. The head is not nearly so much 
 folded off from the yolk as the tail. At its front end is placed 
 a disc with numerous papillae, of which we shall say more here- 
 after. This disc is somewhat bifid, and is marked in the centre 
 by a deep depression. 
 
 Dorsal to it, on the top of the head, are two widely separated 
 nasal pits. On the surface of the yolk, in front of the head, is to 
 be seen the heart, just as in Sturgeon embryos. Immediately 
 below the suctorial disc is a slit-like space, forming the mouth. 
 It is bounded below by the two mandibular arches, which meet 
 ventrally in the median line. A shallow but well-marked de- 
 pression on each side of the head indicates the posterior boundary 
 of the mandibular arch. Behind this is placed the very con- 
 spicuous hyoid arch with its rudimentary opercular flap ; and in 
 the depression, partly covered over by the latter, may be seen a 
 ridge, the external indication of the first branchial arch. 
 
 Eleventh day after impregnation : External characters. The 
 embryo (Plate 34, fig. 10) is now about 10 millims. in length, and 
 in several features exhibits an advance upon the embryo of the 
 previous stage. 
 
 The tail fin is now obviously not quite symmetrical, and 
 the dorsal fin-fold is continued for nearly the whole length of the 
 trunk. The suctorial disc (Plate 34, fig. 1 1, s.d.} is much more 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 755 
 
 prominent, and the papillae (about 30 in number) covering it are 
 more conspicuous from the surface. It is not obviously com- 
 posed of two symmetrical halves. The opercular flap is larger, 
 and the branchial arches behind it (two of which may be made 
 out without dissection) are more prominent. 
 
 The anterior pair of limbs is now visible in the form of two 
 longitudinal folds projecting in a vertical direction from the 
 surface of the yolk-sack at the sides of the body. 
 
 The stages subsequent to hatching have been investigated 
 with reference to the external features and to the habits by 
 Agassiz, and we shall enrich our own account by copious quota- 
 tions from his memoir. 
 
 He states that the first batch were hatched on the eighth 1 
 day after being laid. " The young Fish possessed a gigantic 
 yolk-bag, and the posterior part of the body presented nothing 
 specially different from the general appearance of a Teleostean 
 embryo, with the exception of the great size of the chorda. The 
 anterior part, however, was most remarkable ; and at first, on 
 seeing the head of this young Lepidosteus, with its huge mouth- 
 cavity extending nearly to the gill-opening, and surmounted by 
 a hoof-shaped depression edged with a row of protuberances 
 acting as suckers, I could not help comparing this remarkable 
 structure, so utterly unlike anything in Fishes or Ganoids, to the 
 Cyclostomes, with which it has a striking analogy. This organ 
 is also used by Lepidosteus as a sucker, and the moment the 
 young Fish is hatched he attaches himself to the sides of the 
 disc, and there remains hanging immovable ; so firmly attached, 
 indeed, that it requires considerable commotion in the water to 
 make him loose his hold. Aerating the water by pouring it from 
 a height did not always produce sufficient disturbance to loosen 
 the young Fishes. The eye, in this stage, is rather less advanced 
 than in corresponding stages in bony Fishes ; the brain is also 
 comparatively smaller, the otolith ellipsoidal, placed obliquely in 
 the rear above the gill-opening. . . . Usually the gill-cover is 
 pressed closely against the sides of the body, but in breathing an 
 opening is seen through which water is constantly passing, a 
 
 1 This statement of Agassi/ tides not correspond with the dates on the specimens 
 >ent to us a fact no doubt due In the hatching not taking place at the same time for 
 all the larva.-. 
 
756 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 strong current being made by the rapid movement of the pectorals, 
 against the base of which the extremity of the gill-cover is closely 
 pressed. The large yolk-bag is opaque, of a bluish-gray colour. 
 The body of the young Lepidosteus is quite colourless and trans- 
 parent. The embryonic fin is narrow, the dorsal part commencing 
 above the posterior end of the yolk-bag ; the tail is slightly 
 rounded, the anal opening nearer the extremity of the tail than 
 the bag. . The intestine is narrow, and the embryonic fin extend- 
 ing from the vent to the yolk-bag is quite narrow. In a some- 
 what more advanced stage, hatched a few hours earlier, the 
 upper edge of the yolk-bag is covered with black pigment cells, 
 and minute black pigment cells appear on the surface of the 
 alimentary canal. There are no traces of embryonic fin-rays 
 either in this stage or the one preceding ; the structure of the 
 embryonic fin is as in bony Fishes previous to the appearance 
 of these embryonic fin-rays finely granular. Seen in profile, 
 the yolk-bag is ovoid ; as seen from above, it is flattened, rect- 
 angular in front, with rounded corners, tapering to a rounded 
 point towards the posterior extremity, with re-entering sides." 
 
 We have figured an embryo of 1 1 millims. in length, shortly 
 after hatching (Plate 34, fig. 12), the most important characters 
 of which are as follows : The yolk-sack, which has now become 
 much reduced, forms an appendage attached to the ventral 
 surface of the body, and has a very elongated form as compared 
 with its shape just before hatching. The mouth, as also noticed 
 by Agassiz, has a very open form. It is (Plate 34, fig. 13, ?;z.) 
 more or less rhomboidal, and is bounded behind by the mandi- 
 bular arch (;;z.) and laterally by the superior maxillary processes 
 (s. mx). In front of the mouth is placed the suctorial 'disc (s. d.}, the 
 central papillae of which are arranged in groups. The opercular 
 fold (Ji. op.} is very large, covering the arches behind. A well- 
 marked groove is present between the mandibular and opercular 
 arches, but so far as we can make out it is not a remnant of the 
 hyomandibular cleft. 
 
 The pectoral fins (Plate 34, fig. 12, /<:./) are very prominent 
 longitudinal ridges, which, owing to their being placed on the 
 surface of the yolk-sack, project in a nearly vertical direction : a 
 feature which is also found in many Teleostean embryos with 
 large yolk-sacks. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 757 
 
 No traces of the pelvic fins have yet become developed, _ 
 
 The positions of the permanent dorsal, anal, and caudal fins, 
 as pointed out by Agassiz, are now indicated by a deposit of 
 pigment in the embryonic fin. 
 
 In an embryo on the sixth day after hatching, of about 15 
 millims. in length, of which we have also given a figure (Plate 
 34, fig. 14), the following fresh features deserve special notice. 
 
 In the region of the head there is a considerable elongation 
 of the pre-oral part, forming a short snout, at the end of which 
 is placed the suctorial disc. At the sides of the snout are placed 
 the nasal pits, which have become somewhat elongated anteriorly. 
 
 The mouth has lost its open rhomboidal shape, and has 
 become greatly narrowed in an antero-posterior direction, so 
 that its opening is reduced to a slit. The mandibles and max- 
 illary processes are nearly parallel, though both of them are 
 very much shorter than in the adult. The operculum is now a 
 very large flap, and has extended so far backwards as to cover 
 the insertion of the pectoral fin. The two opercular folds nearly 
 meet ventrally. 
 
 The yolk-sack is still more reduced in size, one important 
 consequence of which is that the pectoral fins (pc.f.} appear to 
 spring out more or less horizontally from the sides of the body, 
 and at the same time their primitive line of attachment to the 
 body becomes transformed from a longitudinal to a more or less 
 transverse one. 
 
 The first traces of the pelvic fins are now visible as slight 
 longitudinal projections near the hinder end of the yolk-sack 
 
 The pigmentation marking the regions of the permanent fins 
 has become more pronounced, and it is to be specially noted 
 that the ventral part of the caudal fin (the permanent caudal) is 
 considerably more prominent than the dorsal fin opposite to it. 
 
 The next changes, as Agassiz points out, " are mainly in the 
 lengthening of the snout ; the increase in length both of the 
 lower and upper jaw ; the concentration of the sucker of the 
 sucking disc ; and the adoption of the general colouring of 
 somewhat older Fish. The lobe of the pectoral has become 
 specially prominent, and the outline of the fins is now indicated 
 by a fine milky granulation. Seen from above, the gill-cover is 
 
758 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 seen to leave a large circular opening leading to the gill-arches, 
 into which a current of water is constantly passing, by the lateral 
 expansion and contraction of the gill-cover; the outer extremity 
 of the gill-cover covers the base of the pectorals. In a some- 
 what older stage the snout has become more elongated, the 
 sucker more concentrated, and the disproportionate size of the 
 terminal sucking-disc is reduced ; the head, when seen from 
 above, becoming slightly elongated and pointed." 
 
 In a larva of about 18 days old and 21 millims. in length, of 
 which we have not given a figure, the snout has grown greatly 
 in length, carrying with it the nasal organs, the openings of 
 which now appear to be divided into two parts. The suctorial 
 disc is still a prominent structure at the end of the snout. The 
 lower jaw has elongated correspondingly with the upper, so that 
 the gape is very considerable, though still very much less than 
 in the adult. 
 
 The opercular flaps overlap ventrally, the left being super- 
 ficial. They still cover the bases of the pectoral fins. The 
 latter are described by Agassiz as being " kept in constant rapid 
 motion, so that the fleshy edge is invisible, and the vibration 
 seems almost involuntary, producing a constant current round 
 the opening leading into the cavity of the gills." 
 The pelvic fins are somewhat more prominent. 
 The yolk-sack, as pointed out by Agassiz, has now dis- 
 appeared as an external appendage. 
 
 After the stage last described the young Fish rapidly ap- 
 proaches the adult form. To shew the changes effected we 
 have figured the head of a larva of about a. month old and 
 23 millims. in length (Plate 34, fig. 15). The suctorial disc, 
 though much reduced, is still prominent at the end of the snout. 
 Eventually, as shewn by Agassiz, it forms the fleshy globular 
 termination of the upper jaw. 
 
 The most notable feature in which the larva now differs in 
 its external form from the adult is in the presence of an ex- 
 ternally heterocercal tail, caused by the persistence of the primi- 
 tive caudal fin as an elongated filament projecting beyond the 
 permanent caudal (Plate 41, fig. 68). 
 
 Delicate dermal fin-rays are now conspicuous in the peri- 
 pheral parts of all the permanent fins. These rays closely 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 759 
 
 resemble the horny fin-rays in the fins of embryo Elasrno- 
 branchs in their development and structure. They appear 
 gradually to enlarge to form the permanent rays, and we have 
 followed out some of the stages of their growth, which is in 
 many respects interesting. Our observations are not, however, 
 complete enough to publish, and we can only say here that their 
 early development and structure proves their homology with 
 the horny fibres or rays in fins of Elasmobranchii. The skin is 
 still, however, entirely naked, and without a trace of its future 
 armour of enamelled scales. 
 
 The tail of a much older larva, 1 1 centims. in length, in 
 which the scales have begun to be formed, is shewn in Plate 34, 
 fig. 1 6. 
 
 We complete this section of our memoir by quoting the 
 following passages from Agassiz as to the habits of the young 
 fish at the stages last described : 
 
 " In the stages intervening between plate iii, fig. 19, and 
 plate iii, fig. 30, the young Lepidosteus frequently swim about, 
 and become readily separated from their point of attachment. 
 In the stage of plate iii, fig. 30, they remain often perfectly quiet 
 close to the surface of the water; but, when disturbed, move 
 very rapidly about through the water. . . . The young 
 already have also the peculiar habit of the adult of coming to 
 the surface to swallow air. When they go through the process 
 under water of discharging air again they open their jaws wide, 
 and spread their gill-covers, and swallow as if they were choking, 
 making violent efforts, until a minute bubble of air has become 
 liberated, when they remain quiet again. The resemblance to a 
 Sturgeon in the general appearance of this stage of the young 
 Lcpidostcus is quite marked." 
 
 BRAIN. 
 I. Anatomy. 
 
 The brain of Lepidostcus has been figured by Busch (whose 
 figure has been copied by Miklucho-Maclay, and apparently by 
 Huxley), by Owen and by Wilder (No. 15). The figure of the 
 latter author, representing a longitudinal section through the 
 
760 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 brain, is the most satisfactory, the other figures being in many 
 respects inaccurate; but even Wilder's figure and description, 
 though taken from the fresh object, appear to us in some 
 respects inadequate. He offers, moreover, fresh interpretations 
 of certain parts of the brain which we shall discuss in the sequel. 
 
 We have examined two brains which, though extremely soft, 
 were, nevertheless, sufficiently well preserved to enable us to 
 study the external form. We have, moreover, made a complete 
 series of transverse sections through one of the brains, and our 
 sections, though utterly valueless from a histological point of view, 
 have thrown some light on the topographical anatomy of the 
 brain. 
 
 Plate 38, figs. 47 A, P, and C, represent three views of the 
 brain, viz.: from the side, from above, and from below. We will 
 follow in our description the usual division of the brain into fore- 
 brain, mid-brain, and hind-brain. 
 
 The fore-brain consists of an anterior portion forming the 
 cerebrum, and a posterior portion constituting the thalamen- 
 cephalon. 
 
 The cerebrum at first sight appears to be composed of (a) 
 a pair of posterior and somewhat dorsal lobes, forming what have 
 usually been regarded as the true cerebral hemispheres, but 
 called by Wilder the prothalami, and (b} a pair of anterior and 
 ventral lobes, usually regarded as the olfactory lobes, from which 
 the olfactory nerves spring. Mainly from a comparison with 
 our embryonic brains described in the sequel, we are inclined to 
 think that the usual interpretations are not wholly correct, but 
 that the true olfactory lobes are to be sought for in small enlarge- 
 ments (Plate 38. figs. 47 A, B, and C, off.) at the front end of the 
 brain 1 from which the olfactory nerves spring. The cerebrum 
 proper would then consist of a pair of anterior and ventral lobes 
 (ce.}, and of a pair of posterior lobes (ce'.\ both pairs uniting to 
 form a basal portion behind. 
 
 The two pairs of lobes probably correspond with the two 
 parts of the cerebrum of the Frog, the anterior of which, like 
 that of Lepidosteus, was held to be the olfactory lobe, till Gotte's 
 researches shewed that this view was not tenable. 
 
 1 The homoiogies of the olfactory lobes throughout the group of Fishes require 
 further investigation. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 761 
 
 The anterior lobes of the cerebrum have a conical fornv taper- 
 ing anteriorly, and are completely separated from each other. 
 The posterior lobes, as is best shewn in side views, have a 
 semicircular form. Viewed from above they appear as rounded 
 prominences, and their dorsal surface is marked by two con- 
 spicuous furrows (Plate 38, fig. 47 B, ce'.}, which have been noticed 
 by Wilder, and are similar to those present in many Teleostei. 
 Their front ends overhang the base of the anterior cerebral 
 lobes. The basal portion of the cerebrum is an undivided lobe, 
 the anterior wall of which forms the lamina terminalis. 
 
 What we have above described as the posterior cerebral 
 lobes have been described by Wilder as constituting the everted 
 dorsal border of the basal portion of the cerebrum. 
 
 The portion of the cerebro-spinal canal within the cerebrum 
 presents certain primitive characters, which are in some respects 
 dissimilar to those of higher types, and have led Wilder to 
 hold the posterior cerebral lobes, together with what we have 
 called the basal portion of the cerebrum, to be structures 
 peculiar to Fishes, for which he has proposed the name " pro- 
 thalami." 
 
 In the basal portion of the cerebrum there is an unpaired 
 slit-shaped ventricle, the outer walls of which are very thick. 
 It is provided with a floor formed ot nervous matter, in part of 
 which, judging from Wilder's description, a well-marked com- 
 missure is placed. We have found in the larva a large com- 
 missure in this situation (Plate 37, figs. 44 and 45, a.c.) ; and 
 it may be regarded as the homologue of the anterior commissure 
 of higher types. This part of the ventricle is stated by Wilder 
 to be without a roof. This appears to us highly improbable. We 
 could not, however, determine the nature of the roof from our 
 badly preserved specimens, but if present, there is no doubt that 
 it is extremely thin, as indeed it is in the larva (Plate 37, fig. 
 46 B). In a dorsal direction the unpaired ventricle extends so 
 as to separate the two posterior cerebral lobes. Anteriorly the 
 ventricle is prolonged into two horns, which penetrate for a 
 short distance, as the lateral ventricles, into the base of the 
 anterior cerebral lobes. The front part of each anterior cerebral 
 lobe, as well as of the whole of the posterior lobes, appears solid 
 in our sections ; but Wilder describes the anterior horns of the 
 B. 49 
 
762 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 ventricle as being prolonged for the whole length of the anterior 
 lobes. 
 
 In the embryos of all Vertebrates the cerebrum is not at 
 first divided into two lobes, so that the fact of the posterior part 
 of the cerebrum in Lepidosteus and probably other Ganoids 
 remaining permanently in the undivided condition does not 
 appear to us a sufficient ground for giving to the lobes of this 
 part of the cerebrum the special name of prothalami, as pro- 
 posed by Wilder, or for regarding them as a section of the 
 brain peculiar to Fishes. 
 
 The thalamencephalon (///.) contains the usual parts, but is 
 is some respects peculiar. Its lateral walls, forming the optic 
 thalami, are thick, and are not sharply separated in front from 
 the basal part of the cerebrum ; between them is placed the 
 third ventricle. The thalami are of considerable extent, though 
 partially covered by the optic lobes and the posterior lobes of 
 the cerebrum. They are not, however, relatively so large as 
 in other Ganoid forms, more especially the Chondrostei and 
 Polyptertis. 
 
 On the roof of the thalamencephalon is placed a large thin- 
 walled vesicle (Plate 38, figs. 47 A and B, v.th.), which undoubtedly 
 forms the most characteristic structure connected with this part 
 of the brain. Owing to the wretched state of preservation of 
 the specimens, we have found it impossible to determine the 
 exact relations of this body to the remainder of the thalamen- 
 cephalon; but it appears to be attached to the roof of the 
 thalamencephalon by a narrow stalk only. It extends forwards 
 so as to overlap part of the cerebrum in front, and is closely 
 invested by a highly vascular layer of the pia mater. 
 
 No mention is made by Wilder of this body ; nor is it repre- 
 sented in his figures or in those of the other anatomists who 
 have given drawings of the brain of Lepidosteus. It might at 
 first be interpreted as a highly-developed pineal gland, but a 
 comparison with the brain of the larva (vide p. 764) shews that 
 this is not the case, but that the body in question is represented 
 in the larva by a special outgrowth of the roof of the thalamen- 
 cephalon. The vesicle of the roof of the thalamencephalon is 
 therefore to be regarded as a peculiar development of the tela 
 choroidea of the third ventricle. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 763 
 
 How far this vesicle has a homologue in the brains of -other 
 Ganoids is not certain, since negative evidence on this subject is 
 all but valueless. It is possible that a vesicular sack covering 
 over the third ventricle of the Sturgeon described by Stannius 1 , 
 and stated by him to be wholly formed of the membranes of the 
 brain, is really the homologue of our vesicle. 
 
 Wiedersheim 2 has recently described in Protopterus a body 
 which is undoubtedly homologous with our vesicle, which he 
 describes in the following way : 
 
 " Dorsalwarts ist das Zwischenhirn durch ein tiefes, von 
 Hirnschlitz eingenommenes Thai von Vorderhirn abgesetzt ; 
 dasselbe ist jedoch durch eine hautige, mit der Pia mater zusam- 
 menhangende Kuppel oder Kapsel uberbruckt." 
 
 This " Kuppel " has precisely the same relations and a very 
 similar appearance to our vesicle. The true pineal gland is 
 placed behind it. It appears to us possible that the body found 
 by Huxley 3 in Ceratodus, which he holds to be the pineal gland, 
 is in reality this vesicle. It is moreover possible that what has 
 usually been regarded as the pineal gland in Petromyzon may 
 in reality be the homologue of the vesicle we have found in 
 Lepidosteus. 
 
 We have no observations on the pineal gland of the adult, 
 but must refer the reader for the structure and relations of this 
 body to the embryological section. 
 
 The infundibulum (Plate 38, fig. 47 A, in.} is very elongated. 
 Immediately in front of it is placed the optic chiasma (Plate 38, 
 figs. 47 A and C, op.c/i.} from which the optic fibres can be traced 
 passing along the sides of the optic thalami and to the optic 
 lobes, very much as in Muller's figure of the brain of Po- 
 lypterus. 
 
 On the sides of the infundibulum are placed two promi- 
 nent bodies, the lobi inferiores (/.#/.), each of which contains a 
 cavity continuous with the prolongation of the third ventricle 
 
 1 " Ueb. d. Gehirn des Stors," Muller's Archii\ 1843, and Lehrbuch d. z-ergl. Anat. 
 d. Wirbdthiere. Cattie, Archives tie Biologic, Vol. in. 1882, has recently described 
 in Acipcnscr sturio a vesicle on the roof of the thalamencephalon, whose cavity is 
 continuous with the third ventricle. This vesicle is clearly homologous with that in 
 Lepidostetis. (June 28, 1882.) 
 
 2 R. Wiedersheim, Morphol. SfuJitn, 1880, p. 71. 
 
 a " On Certitihtm h'orstirij' kc., /'roc. tool. Soc. ^76. 
 
 492 
 
764 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 into the infundibulum. The apex of the infundibulum is enlarged, 
 and to it is attached a pituitary body (pt.}. 
 
 The mid-brain is of considerable size, and consists of a basal 
 portion connecting the optic thalami with the medulla, and a 
 pair of large optic lobes (<?/./.). The iter a tertio ad quartum 
 ventriculum, which forms the ventricle of this part of the brain, 
 is prolonged into each optic lobe, and the floor of each prolon- 
 gation is taken up by a dome-shaped projection, the homologue 
 of the torus semicircularis of Teleostei. 
 
 The hind-brain consists of the usual parts, the medulla 
 oblongata and the cerebellum. The medulla presents no peculiar 
 features. The sides of the fourth ventricle are thickened and 
 everted, and marked with peculiar folds (Plate 38, figs. 47 A 
 and B, m.o.). 
 
 The cerebellum is much larger than in the majority of 
 Ganoids, and resembles in all essential features the cerebellum 
 of Teleostei. In side views it has a somewhat S-shaped form, 
 from the presence of a peculiar lateral sulcus (Plate 38, fig. 47 A, 
 cb.}. As shewn by Wilder, its wall actually has in longitudinal 
 section this form of curvature, owing to its anterior part pro- 
 jecting forwards into the cavity of the iter 1 . This forward pro- 
 jection is not, however, so conspicuous as in most Teleostei. 
 The cerebellum contains a large unpaired prolongation of the 
 fourth ventricle. 
 
 II. Development. 
 
 The early development of the brain has already been de- 
 scribed ; and, although we do not propose to give any detailed 
 account of the later stages of its growth, we have thought it 
 worth while calling attention to certain developmental features 
 which may probably be regarded as to some extent characteristic 
 of the Ganoids. With this view we have figured (Plate 37, figs. 
 44, 45) longitudinal sections of the brain at two stages, viz.: 
 of larvae of 15 and 26 millims., and transverse sections (Plate 37, 
 figs. 46 A G) of the brain of a larva at about the latter stage 
 (25 millims.). 
 
 1 In Wilder'* figure the walls of the cerebellum are represented as much too thin. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 765 
 
 The original embryonic fore-brain is divided in both embryos 
 into a cerebrum (ce.) in front and a thalamencephalon (th.} behind. 
 In the younger embryo the cerebrum is a single lobe, as it is 
 in the brains of all Vertebrate embryos ; but in the older larva 
 it is anteriorly (Plate 37, fig. 46 A) completely divided into 
 two hemispheres. The roof of the undivided posterior part of 
 the cerebrum is extremely thin (Plate 37, fig. 46 B). Near the 
 posterior border of the base of the cerebrum there is a great 
 development of nervous fibres, which may probably be regarded 
 as in part equivalent to the anterior commissure (Plate 37, figs. 
 44, 45 a.c.). 
 
 Even in the oldest of the two brains the olfactory lobes are 
 very slightly developed, constituting, however, small lateral and 
 ventral prominences of the front end of the hemispheres. From 
 each of them there springs a long olfactory nerve, extending for 
 the whole, length of the rostrum to the olfactory sack. 
 
 The thalamencephalon presents a very curious structure, and 
 is relatively a more important part of the brain than in the 
 embryo of any other form which we know of. Its roof, instead 
 of being, as usual, compressed antero-posteriorly 1 , so as to be 
 almost concealed between the cerebral hemispheres and the optic 
 lobes (mid-brain), projects on the surface for a length quite equal 
 to that of the cerebral hemispheres (Plate 37, figs. 44 and 45, ///.). 
 In the median line the roof of the thalamencephalon is thin 
 and folded ; at its posterior border is placed the opening of 
 the small pineal gland. This body is a papilliform process of 
 the nervous matter of the roof of this part of the brain, and 
 instead of being directed forwards, as in most Vertebrate types, 
 tends somewhat backwards, and rests on the mid-brain behind 
 (Plate 37, figs. 44, 45, and 46 C and D, /.). The roof of the 
 thalamencephalon immediately in front of the pineal gland forms 
 a sort of vesicle, the sides of which extend laterally as a pair 
 of lobes, shewn in transverse sections in Plate 37, figs. 46 C and 
 D, as tli.l. This vesicle becomes, we cannot doubt, the vesicle 
 on the roof of the thalamencephalon which we have described in 
 the adult brain. Immediately in front of the pineal gland the 
 roof of the thalamencephalon contains a transverse commissure 
 
 1 Vide F. M. Balfour, Comparative Embryology, Vol. n. figs. 248 and 250. 
 
766 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 (Plate 37, fig. 46 C, z.}, which is the homologue of a similarly- 
 situated commissure present in the Elasmobranch brain 1 , while 
 behind the pineal gland is placed the posterior commissure. The 
 sides of the thalamencephalon are greatly thickened, forming 
 the optic thalami (Plate 37, figs. 46 C and D, op.th^, which are 
 continuous in front with the thickened outer walls of the hemi- 
 spheres. Below, the thalamencephalon is produced into a very 
 elongated infundibulum (Plate 37, figs. 44, 45, 46 E, in.}, the 
 apex of which is trilobed as in Elasmobranchii and Teleostei. 
 The sides of the infundibulum exhibit two lobes, the lobi infe- 
 riores (Plate 37, fig. 46 E, l.in.}, which are continued posteriorly 
 into the crura cerebri. 
 
 The pituitary body 2 (Plate 37, figs. 44, 45, 46 E, pt} is small, 
 not divided into lobes, and provided with a very minute lumen. 
 
 In front of the infundibulum is the optic chiasma (Plate 37, 
 fig. 46 D, op. c/i.}, which is developed very early. It is A as stated 
 by Miiller, a true chiasma. 
 
 The mid-brain (Plate 37, figs. 44 and 45, m. b.} is large, and 
 consists in both stages of (i) a thickened floor forming the crura 
 cerebri, the central canal of which constitutes the iter a tertio ad 
 quartum ventriculum ; and (2} the optic lobes (Plate 37, figs. 46 
 E, F, G, op. /.) above, each of which is provided with a cavity 
 continuous with the median iter. The optic lobes are separated 
 dorsally and in front by a well-marked median longitudinal 
 groove. Posteriorly they largely overlap the cerebellum. In the 
 anterior part of the optic lobes, at the point where the iter joins 
 the third ventricle, there may be seen slight projections of the 
 floor into the lumen of the optic lobes (Plate 37, fig. 46 E). 
 These masses probably become in the adult the more conspicuous 
 
 1 Vide F. M. Balfour, Comparative Embryology, Vol. n. pp. 355 6 [the original 
 edition], where it is suggested that this commissure is the homologue of the grey 
 commissure of higher types. 
 
 2 We have not been able to work out the early development of the pituitary body 
 as satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an 
 invagination of the oral epithelium to form it ; in Plate 37, figs. 41 and 42, it is repre- 
 sented in transverse section in two consecutive sections. Anteriorly it is still con- 
 nected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible 
 that an earlier stage of it is shewn in Plate 36, fig. 35. Were it not for the evidence 
 in other types of its being derived from the epiblast we should be inclined to regard it 
 as hypoblastic in origin. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 767 
 
 prominences of the floor of the ventricles of the optic_Jo_bes, 
 which we regard as homologous with the tori semicirculares of 
 the brain of the Teleostei. 
 
 The hind-brain is formed of the usual divisions, viz. : cere- 
 bellum and medulla oblongata (Plate 37, figs. 44 and 45, cb.,md^). 
 The former constitutes a bilobed projection of the roof of the 
 hind-brain. Only a small portion of it is during these stages left 
 uncovered by the optic lobes, but the major part extends forwards 
 for a considerable distance under the optic lobes, as shewn in 
 the transverse sections (Plate 37, figs. 46 F and G, cb.) ; and 
 its two lobes, each with a prolongation of its cavity, are con- 
 tinued forwards beyond the opening of the iter into the fourth 
 ventricle. 
 
 It is probable that the anterior horns of the cerebellum are 
 equivalent to the prolongations of the cerebellum into the central 
 cavity of the optic lobes of Teleostei, which are continuous with 
 the so-called fornix of Gottsche. 
 
 III. Comparison of the larval and adult brain of Lepidosteus, 
 togetJier with some observations on tJie systematic value of the 
 characters of the Ganoid brain. 
 
 The brain of the older of the two larvae, which we have 
 described, sufficiently resembles in most of its features that of 
 the adult to render material assistance in the interpretation of 
 certain of the parts of the latter. It will be remembered that in 
 the adult brain the parts usually held to be olfactory lobes were 
 described as the anterior cerebral lobes. The grounds for this 
 will be apparent by a comparison of the cerebrum of the larva 
 and adult. In the larva the cerebrum is formed of (i) an unpaired 
 basal portion with a thin roof, and (2) of a pair of anterior lobes, 
 with small olfactory bulbs at their free extremities. 
 
 The basal portion in the larva clearly corresponds in the 
 adult with the basal portion, together with the two posterior 
 cerebral lobes, which are merely special outgrowths of the dorsal 
 edge of the primitive basal portion. The pair of anterior lobes 
 have exactly the same relations in the larva as in the adult, 
 except that in the former the ventricles are prolonged for their 
 
768 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 whole length instead of being confined to their proximal portions. 
 If, therefore, our identifications of the larval parts of the brain 
 are correct, there can hardly be a question as to our identifications 
 of the parts in the adult. As concerns these identifications, the 
 comparison of the brain of our two larvae appears conclusive in 
 favour of regarding the anterior lobes as parts of the cerebrum, 
 as distinguished from the olfactory lobes, in that they are clearly 
 derived from the undivided anterior portion of the cerebrum of 
 the younger larva. 
 
 The comparison of the larval brain with that of the adult 
 again appears to us to leave no doubt that the vesicle attached 
 to the roof of the thalamencephalon in the adult is the same 
 structure as the bilobed outgrowth of this roof in the larva ; and 
 since there is in addition a well-developed pineal gland in the 
 larva with the usual relations, there can be no ground for identify- 
 ing the vesicle in the adult with the pineal gland. 
 
 Muller, in his often quoted memoir (No. 13), states that the 
 brains of Ganoids are peculiar and distinct from those both of 
 Teleostei and Elasmobranchii ; but in addition to pointing out 
 that the optic nerves form a chiasma he does not particularly 
 mention the features, to which he alludes in general terms. More 
 recently Wilder (No. 15) has returned to this subject; and 
 though, as we have already had occasion to point out, we cannot 
 accept all his identifications of the parts of the Ganoid brain, yet 
 he has called attention to certain characteristic features of the 
 cerebrum which have an undoubted systematic value. 
 
 The distinctive characters of the Ganoid brain are, in our 
 opinion, (i) the great elongation of the region of the thalamen- 
 cephalon ; and (2) the unpaired condition of the posterior part 
 of the cerebrum, and the presence of so thin a roof to the 
 ventricle of this part as to cause it to appear open above. 
 
 The immense length of the region of the thalamencephalon 
 is a feature in the Ganoid brain which must at once strike any 
 one who examines figures of the brains of Chondrostei, Polypterus, 
 or Amia. It is less striking in the adult Lepidosteus, though here 
 also we have shewn that the thalamencephalon is really very 
 greatly developed ; but in the larva of Lepidosteus this feature is 
 still better marked, so that the brain of the larva may be described 
 as being more characteristically Ganoid than that of the adult. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 769 
 
 The presence of a largely developed thalamencephajpn at 
 once distinguishes a Ganoid brain from that of a Teleostean 
 Fish, in which the optic thalami are very much reduced ; but 
 Lepidosteus shews its Teleostean affinities by a commencing 
 reduction of this part of the brain. 
 
 The large size of the thalamencephalon is also characteristic of 
 the Ganoid brain in comparison with the brain of the Dipnoi ; 
 but is not however so very much more marked in the Ganoids 
 than it is in some Elasmobranchii. 
 
 On the whole, we may consider the retention of a large 
 thalamencephalon as a primitive character. 
 
 The second feature which we have given as characteristic 
 of the Ganoid brain is essentially that which has been insisted 
 upon by Wilder, though somewhat differently expressed by 
 him. 
 
 The simplest condition of the cerebrum is that found in the 
 larva of Lepidosteus, where there is an anterior pair of lobes, and 
 an undivided posterior portion with a simple prolongation of the 
 third ventricle, and a very thin roof. The dorsal edges of the 
 posterior portion, adjoining the thin roof, usually become some- 
 what everted (cf. Wilder), and in Lepidosteus these edges have in 
 the adult a very great development, and form (vide Plate 38, fig. 
 47 A C, /.) two prominent lobes, which we have spoken of as 
 the posterior cerebral lobes. 
 
 These characters of the cerebrum are perhaps even more 
 distinctive than those of the thalamencephalon. 
 
 In Teleostei the cerebrum appears to be completely divided 
 into two hemispheres, which are, however, all but solid, the lateral 
 ventricles being only prolonged into their bases. In Dipnoi 
 again there is either (Protoptcrus, Wiedersheim 1 ) a completely 
 separated pair of oval hemispheres, not unlike those of the lower 
 Amphibia, or the oval hemispheres are not completely separated 
 from each other (Ccratodus, Huxley 2 , Lepidosiren, Hyrtl 3 ) ; in 
 either case the hemispheres are traversed for the whole length by 
 lateral ventricles which are either completely or nearly completely 
 separated from each other. 
 
 l. Stiidicn, in. Jena, 1880. 
 2 "On Ceratodns Forsleri," Proc. Zool. Soc. 1876. 
 ;| Ltpidosireit paradexa. Prag. 1845. 
 
770 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 In Elasmobranchii the cerebrum is an unpaired though 
 bilobed body, but traversed by two completely separated lateral 
 ventricles, and without a trace of the peculiar membranous roof 
 found in Ganoids. 
 
 Not less interesting than the distinguishing characters of the 
 Ganoid brain are those cerebral characters which indicate affinities 
 between Lepidosteus and other groups. The most striking of 
 these are, as might have been anticipated, in the direction of the 
 Teleostei. 
 
 Although the foremost division of the brain is very dissimilar 
 in the two groups, yet the hind-brain in many Ganoids and the 
 mid-brain also in Lepidosteus approaches closely to the Teleostean 
 type. The most essential feature of the cerebellum in Teleostei 
 is its prolongation forwards into the ventricles of the optic 
 vesicles as the valvula cerebelli. We have already seen that 
 there is a homologous part of the cerebellum in Lepidosteus ; 
 Stannius also describes this part in the Sturgeon, but no such 
 part is represented in M tiller's figure of the brain of Polypterus, 
 or described by him in the text. 
 
 The cerebellum is in most Ganoids relatively smaller, and 
 this is even the case with Amia; but the cerebellum of Lepidosteus 
 is hardly less bulky than that of most Teleostei. 
 
 The presence of tori semicirculares on the floor of the mid- 
 brain of Lepidosteus again undoubtedly indicates its affinities with 
 the Teleostei, and such processes are stated by Stannius to be 
 absent in the Sturgeon, and have not, so far as we are aware, 
 been described in other Ganoids. Lastly we may point to the 
 presence of well-developed lobi inferiores in the brain of Lepi- 
 dosteus as an undoubted Teleostean character. 
 
 On the whole, the brain of Lepidosteus, though preserving its 
 true Ganoid characters, approaches more closely to the brain 
 of the Teleostei than that of any other Ganoid, including even 
 A mia. 
 
 It is not easy to point elsewhere to such marked resemblances 
 of the Ganoid brain, as to the brain of the Teleostei. 
 
 The division of the cerebrum into anterior and posterior 
 lobes, which is found in Lepidosteus, probably reappears again, 
 as already indicated, in the higher Amphibia. The presence of 
 the peculiar vesicle attached to the roof of the thalamencephalon 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 771 
 
 has its parallel in the brain of Protopterus, and as pointing in 
 the same direction a general similarity in the appearance of the 
 brain of Polypterus to that of the Dipnoi may be mentioned. 
 
 There appears to us to be in no points a close resemblance 
 between the brain of Ganoids and that of Elasmobranchii. 
 
 SENSE ORGANS. 
 
 Olfactory organ. 
 
 Development. The nasal sacks first arise during the late em- 
 bryonic period in the form of a pair of thickened patches of the 
 nervous layer of the epiblast on the dorsal surface of the front 
 end of the head (Plate 37, fig. 39, ol.). The patches very soon 
 become partially invaginated ; and a small cavity is developed 
 between them and the epidermic layer of the epiblast (Plate 37, 
 figs. 42 and 43, ol.}. Subsequently, the roof of this space, formed 
 by the epidermic layer of the epiblast, is either broken through 
 or absorbed ; and thus open pits, lined entirely by tlie nervous 
 layer of tlic epidermis, are formed. 
 
 We are not acquainted with any description of an exactly 
 similar mode of origin of the olfactory pits, though the process 
 is almost identical with that of the other sense organs. 
 
 We have not worked out in detail the mode of formation of 
 the double openings of the olfactory pits, but there can be but 
 little doubt that it is caused by the division of the single open- 
 ing into two. 
 
 The olfactory nerve is formed very early (Plate 37, fig. 39, I), 
 and, as Marshall has found in Aves and Elasmobranchii, it 
 arises at a stage prior to the first differentiation of an olfactory 
 bulb as a special lobe of the brain. 
 
 The Eye. 
 
 Anatomy. We have not made a careful histological examin- 
 ation of the eye of Lcpidostcns, which in our specimens was not 
 sufficiently well preserved for such a purpose ; but we have 
 
77 2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 found a vascular membrane enveloping the vitreous humour on 
 its retinal aspect, which, so far as we know, is unlike anything 
 which has so far been met with in the eye of any other adult 
 Vertebrate. 
 
 The membrane itself is placed immediately outside the hya- 
 loid membrane, i.e. on the side of the hyaloid membrane bound- 
 ing the vitreous humour. It is easily removed from the retina, 
 to which it is only adherent at the entrance of the optic nerve. 
 In both the eyes we examined it also adhered, at one point, to 
 the capsule of the lens, but we could not make out whether this 
 adhesion was natural, or artificially produced by the coagulation 
 of a thin layer of albuminous matter. In one instance, at any 
 rate, the adhesion appeared firmer than could easily be produced 
 artificially. 
 
 The arrangement of the vessels in the membrane is shewn 
 diagrammatically in Plate 38, fig. 49, while the characteristic 
 form of the capillary plexus is represented in Plate 38, fig. 50. 
 
 The arterial supply appears to be derived from a vessel per- 
 forating the retina close to the optic nerve, and obviously homo- 
 logous with the artery of the processus falciformis and pecten 
 of Teleostei and Birds, and with the arteria centralis retinae of 
 Mammals. From this vessel branches diverge and pursue a 
 course towards the periphery. They give off numerous branches, 
 the blood from which enters a capillary plexus (Plate 38, figs. 
 49 and 50) and is collected again by veins, which pass outwards 
 and finally bend over and fall into (Plate 38, fig. 49) a circular 
 vein (cr. v.) placed at the outer edge of the retina along the 
 insertion of the iris (z>). The terminal branches of some of the 
 main arteries appear also to fall directly into this vein. 
 
 The membrane supporting the vessels just described is com- 
 posed of a transparent matrix, in which numerous cells are 
 embedded (Plate 38, fig. 50). 
 
 Development. In the account of the first stages of develop- 
 ment of Lepidosteus, the mode of formation of the optic cup, the 
 lens, Sec., have been described (vide Plates 35 and 36, figs. 23, 
 26, 35). With reference to the later stages in the development 
 of the eye, the only subject with which we propose to deal is the 
 growth of the mesoblastic processes which enter the cavity of 
 the vitreous humour through the choroid slit. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 773 
 
 
 
 Lepidosteus is very remarkable for the great number of meso- 
 blast cells which thus enter the cavity of the vitreous humour, 
 and for the fact that these cells are at first unaccompanied by any 
 vascular structures (Plate 37, fig. 43, vJt). The mesoblast cells 
 are scattered through the vitreous humour, and there can be no 
 doubt that during early larval life, at a period however when 
 the larva is certainly able to see, every histologist would con- 
 sider the vitreous humour to be a tissue formed of scattered 
 cells, with a large amount of intercellular substance ; and the 
 fact that it is so appears to us to demonstrate that Kessler's 
 view of the vitreous humour being a mere transudation is not 
 tenable. 
 
 In the larva five or six days after hatching, and about 
 15 millims. in length, the choroid slit is open for its whole 
 length. The edges of the slit near the lens are folded, so as to 
 form a ridge projecting into the cavity of the vitreous humour, 
 while nearer the insertion of the optic nerve they cease to ex- 
 hibit any such structure. The mesoblast, though it projects 
 between the lips of the ridge near the lens, only extends through 
 the choroid slit into the cavity of the vitreous humour in the 
 neighbourhood of the optic nerve. Here it forms a lamina with 
 a thickened edge, from which scattered cells in the cavity of the 
 vitreous humour seem to radiate. 
 
 At a slightly later stage than that just described, blood- 
 vessels become developed within the cavity of the vitreous 
 humour, and form the vascular membrane already described in 
 the adult, placed close to the layer of nerve-fibres of the retina, 
 but separated from this layer by the hyaloid membrane (Plate 
 38, fig. 48, v.sh). The artery bringing the blood to the above 
 vascular membrane is bound up in the same sheath as the optic 
 nerve, and passes through the choroid slit very close to the optic 
 nerve. Its entrance into the cavity of the vitreous humour is 
 shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in 
 Plate 37, fig. 46, C and D (vs.). 
 
 The above sheath has, so far as we know, its nearest analogue 
 in the eye of Alytes, where, however, it is only found in the 
 larva. 
 
 The reader who will take the trouble to refer to the account 
 of the imperfectly-developed processus falciformis of the Elas- 
 
774 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 mobranch eye in the treatise On Comparative Embryology, by 
 one of us 1 , will not fail to recognize that the folds of the retina 
 at the sides of the choroid slit, and the mesoblastic process 
 passing through this slit, are strikingly similar in Lepidosteus 
 and Elasmobranchii ; and that, if we are justified in holding 
 them to be an imperfectly-developed processus falciformis in the 
 one case, we are equally so in the other. 
 
 Johannes Mliller mentions the absence of a processus falci- 
 formis as one of the features distinguishing Ganoids and Te- 
 leostei. So far as the systematic separation of the two groups 
 is concerned, he is probably perfectly justified in this course ; 
 but it is interesting to notice that both in Ganoids and Elasmo- 
 branchii we have traces of a structure which undergoes a very 
 special development in the Teleostei, and that the processus 
 falciformis of Teleostei is therefore to be regarded, not as an 
 organ peculiar to them, but as the peculiar modification within 
 the group of a primitive Vertebrate organ. 
 
 SUCTORIAL Disc. 
 
 One of the most remarkable organs of the larval Lepidosteus 
 is the suctorial disc, placed at the front end of the head, to 
 which we have made numerous allusions in the first section of 
 this memoir. 
 
 The external features of the disc have been fully dealt with 
 by Agassiz, and he also explained its function by observations 
 on the habits of the larva. We have already quoted (p. 755) 
 a passage from Agassiz' memoir shewing how the young Fishes 
 use the disc to attach themselves firmly to any convenient 
 object. The discs appear in fact to be highly efficient organs of 
 attachment, in that the young Fish can remain suspended by 
 them to the sides of the jar, even after the water has been 
 lowered below the level at which they are attached. 
 
 The disc is formed two or three days before hatching, and 
 from Agassiz' statements, it appears to come into use imme- 
 diately the young Fish is liberated from the egg membranes. 
 
 We have examined the histological structure of the disc at 
 various ages of its growth, and may refer the reader to Plate 34, 
 
 1 Vol. ii. p. 414 [the original edition]. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 775 
 
 figs. 1 1 and 13, and Plate 37, figs. 40 and 44. The result-of-our 
 examination has been to shew that the disc is provided with 
 a series of papillae often exhibiting a bilateral arrangement. 
 The papillae are mainly constituted of highly modified cells of 
 the mucous layer of the epidermis. These cells have the form 
 of elongated columns, the nucleus being placed at the base, and 
 the main mass of the cells being filled with a protoplasmic reti- 
 culum. They may probably be regarded as modified mucous 
 cells. In the mesoblast adjoining the suctorial disc there are 
 numerous sinus-like vascular channels. 
 
 It does not appear probable that the disc has a true sucking 
 action. It is unprovided with muscular elements, and there 
 appears to be no mechanism by which it could act as a sucking 
 organ. We must suppose, therefore, that its adhesive power 
 depends upon the capacity of the cells composing its papillae to 
 pour out a sticky secretion. 
 
 MUSCULAR SYSTEM. 
 
 There is a peculiarity in the muscular system of Lcpidostens, 
 which so far as we know has not been previously noticed. It is 
 that the lateral muscles of each side are not divided, either in 
 the region of the trunk or of the tail, into a dorso-lateral and 
 ventro-lateral division. 
 
 This peculiarity is equally characteristic of the .older larvae 
 as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73, 
 and Plate 42, figs. 74 76. In the Cyclostomata the lateral 
 muscles are not divided into dorsal and ventral sections ; but 
 except in this group such a division has been hitherto considered 
 as invariable amongst Fishes. 
 
 This character must, without doubt, be held to be the indica- 
 tion of a very primitive arrangement of the muscular system. 
 In the embryos of all Fishes with the usual type of the lateral 
 muscles, the undivided condition of the muscles precedes the 
 divided condition ; and in primitive forms such as the Cyclosto- 
 mata and Amphioxus the embryonic condition is retained, as it 
 is in Lepidostcus. 
 
776 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 SKELETON. 
 PART I. Vertebral column and ribs of the adult. 
 
 A typical vertebra from the trunk of Lepidosteus has the 
 following characters (Plate 42, figs. 80 and 81). 
 
 The centrum is slightly narrower in the middle than at its 
 two extremities. It articulates with adjacent vertebrae by a 
 convex face in front and a concave face behind, being thus, 
 according to Owen's nomenclature, opisthoccelous. It presents 
 on its under surface a well-marked longitudinal ridge, which in 
 many vertebrae is only united at its two extremities with the 
 main body of the vertebra. 
 
 From the lateral borders of the centrum there project, at a 
 point slightly nearer the front than the hind end, a pair of pro- 
 minent haemal processes (h.a.}, to the ends of which are articu- 
 lated the ribs. These processes have a nearly horizontal direc- 
 tion in the greater part of the trunk, though bent downwards in 
 the tail. 
 
 The neural arches (n.a.) have a somewhat complicated form. 
 They are mainly composed of two vertical plates, the breadth 
 of the basal parts of which is nearly as great as the length of 
 the vertebrae, so that comparatively narrow spaces are left be- 
 tween the neural arches of successive vertebrae for the passage 
 of the spinal nerves. Some little way from its dorsal extremity 
 each neural arch sends a horizontal process inwards, which meets 
 its fellow and so forms a roof for the spinal canal. These pro- 
 cesses appear to be confined to the posterior parts of the ver- 
 tebrae, so that at the front ends of the vertebrae, and in the 
 spaces between them, the neural canal is without an osseous 
 roof. Above the level of this osseous roof there is a narrow 
 passage, bounded laterally by the dorsal extremities of the 
 neural plates. This passage is mainly filled up by a series of 
 cartilaginous elements (Plate 42, figs. 80 and 81, i.e.) (probably 
 fibre-cartilage), which rest upon the roof of the neural canal. 
 Each element is situated intervertebrally, its anterior end being 
 wedged in between the two dorsal processes of the neural arch 
 of the vertebra in front, and its posterior end extending for some 
 
. STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 777 
 
 distance over the vertebra behind. The successive elements arc 
 connected by fibrous tissue, and are continuous dorsally with 
 a fibrous band, known as the ligamentum longitudinale superius 
 (Plate 42, figs. 80 and 81, /./.), characteristic of Fishes generally, 
 and running continuously for the whole length of the vertebral 
 column. Each of the cartilaginous elements is, as will be after- 
 wards shewn, developed as two independent pieces of cartilage, 
 and might be compared with the dorsal element which usually 
 forms the keystone of the neural arch in Elasmobranchs, were 
 not the latter vertebral instead of intervertebral in position. 
 More or less similar elements are described by Gotte in the 
 neural arches of many Teleostei, which also, however, appear to 
 be vertebral ly placed, and he has compared them and the corre- 
 sponding elements in the Sturgeon with the Elasmobranch 
 cartilages forming the keystone of the neural arch. Gotte does 
 not, however, appear to have distinguished between the carti- 
 laginous elements, and the osseous elements forming the roof of 
 the spinal canal, which are true membrane bones ; it is probable 
 that the two are not so clearly separated in other types as in 
 Lcpidosteus. 
 
 The posterior ends of the neural plates of the neural arches 
 are continued into the dorsal processes directed obliquely up- 
 wards and backwards, which have been somewhat unfortunately 
 described by Stannius as rib-like projections of the neural arch. 
 The dorsal processes of the two sides do not meet, but between 
 them is placed a median free spinous element, also directed 
 obliquely upwards and backwards, which forms a kind of roof 
 for the groove in which the cartilaginous elements and the liga- 
 mentum longitudinale are placed. 
 
 The vertebrae are wholly formed of a very cellular osseous 
 tissue, in which a distinction between the bases of the neural 
 and haemal processes and the remainder of the vertebra is not 
 recognizable. The bodies of the vertebrae are, moreover, directly 
 continuous with the neural and haemal arches. 
 
 The ribs in the region of the trunk arc articulated to the 
 ends of the long haemal processes. They envelop the body- 
 cavity, their proximal parts being placed immediately outside 
 the peritoneal membrane, along the bases of the intermuscular 
 septa. Their distal ends do not, however, remain close to the 
 B. 50 
 
7/8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. , 
 
 peritoneal membrane, but pass outzvards along the intermuscular 
 septa till their free ends come into very close proximity with the 
 skin. This peculiarity, which holds good in the adult for all the 
 free ribs, is shewn in one of the anterior ribs of an advanced 
 larva in Plate 41, fig. 72 (rb.}. We are not aware that this has 
 been previously noticed, but it appears to us to be a point not 
 without interest in all questions which concern the homology of 
 rib-like structures occupying different positions in relation to the 
 muscles. Its bearings are fully dealt with in the section of this 
 paper devoted to the consideration of the homologies of the ribs 
 in Fishes. 
 
 As regards the behaviour of the ribs in the transitional region 
 between the trunk and the tail, we cannot do better than trans- 
 late the description given by Gegenbaur of this region (No. 6, 
 p. 411): "Up to the 34th vertebra the ribs borne by the late- 
 rally and posteriorly directed processes present nothing remark- 
 able, though they have gradually become shorter. The ribs of 
 the 35th vertebra exhibit a slight curvature outwards of their 
 free ends, a peculiarity still more marked in the 36th. The last 
 named pair of ribs converge somewhat in their descent back- 
 wards so that both ribs decidedly approach before bending out- 
 wards. The 37th vertebra is no longer provided with freely 
 terminating ribs, but on the contrary, the same pair of processes 
 which in front was provided with ribs, bears a short forked 
 process as the haemal arch. The two, up to this point separated 
 ribs, have here formed a hcemal arch by the fusion of their lower 
 ends, which arch is movable just like the ribs, and, like them, 
 is attached to the vertebral column" 
 
 In the region of the tail-fin the haemal arches supporting the 
 caudal fin-rays are very much enlarged. 
 
 PART II. Development of the vertebral column and ribs. 
 
 The first development and early histological changes of the 
 notochord have already been given, and we may take up the 
 history of the vertebral column at a period when the notochord 
 forms a large circular rod, whose cells are already highly vacuo- 
 latcd, while the septa between the vacuoles form a delicate 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 779 
 
 wide-meshed reticulum. Surrounding the notochord is the 
 usual cuticular sheath, which is still thin. 
 
 The first indications of the future vertebral column are to be 
 found in the formation of a distinct mesoblastic investment of 
 the notochord. On the dorsal aspect of the notochord, the 
 mesoblast forms two ridges, one on each side, which are pro- 
 longed upwards so as to meet above the neural canal, for which 
 they form a kind of sheath. On the ventral side of the noto- 
 chord there are also two ridges, which are, however, except on 
 the tail, much less prominent than the dorsal ridges. 
 
 The changes which next ensue are practically identical with 
 those which take place in Teleostei. Around the cuticular 
 sheath of the notochord there is formed an elastic membrane 
 the membrana elastica externa. At the same time the basal 
 parts of the dorsal, or as we may perhaps more conveniently call 
 them, the neural ridges of the notochord become enlarged at 
 each intermuscular septum, and the tissue of these enlargements 
 soon becomes converted into cartilage, thus forming a series of 
 independent paired neural processes riding on the membrana 
 elastica externa surrounding the notochord, and extending about 
 two-thirds of the way up the sides of the medullary cord. They 
 are shewn in transverse section in Plate 41, fig. 67 (n.a.), and in 
 a side view in fig. 68 (n.a.}. 
 
 Simultaneously with the neural arches, the haemal arches 
 also become established, and arise by the formation of similar 
 enlargements of the ventral or haemal ridges. In the trunk they 
 are very small, but in the region of the tail their condition is 
 very different. At the front end of the anal fin the paired 
 haemal arches suddenly enlarge and extend ventralwards (Plate 
 41, fig. 67, /i.a.}. 
 
 Each succeeding pair of arches becomes larger than the one 
 in front, and the two elements of each arch first nearly meet 
 below the caudal vein (Plate 41, fig. 67) and finally actually do 
 so, forming in this way a completely closed haemal canal. At 
 the point where they first meet the permanent caudal fin com- 
 mences, and here (Plate 41, fig. 68) we find that not only do the 
 haemal arches meet and coalesce below the caudal vein, but they 
 are actually produced into long spines supporting the fin-rays of 
 the caudal fin, which thus differs from the other fins in being 
 
 50 2 
 
780 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 supported by parts of the true vertebral column and not by 
 independently formed elements of the skeleton. 
 
 Each of the large caudal haemal arches, including the spine, 
 forms a continous whole, and arises at an earlier period of larval 
 life than any other part of the vertebral column. We noticed 
 the first indications of the neural arches in the larva of about a 
 week old, while they are converted into fully formed cartilage in 
 the larva of three weeks. 
 
 The neural and haemal arches, resting on the membrana 
 elastica externa, do not at this early stage in the least constrict 
 the notochord. They grow gradually more definite, till the larva 
 is five or six weeks old and about 26 millims. in length, but 
 otherwise for a long time undergo no important changes. Dur- 
 ing the same period, however, the true sheath of the notochord 
 greatly increases in thickness, and the membrana elastica ex- 
 terna becomes more definite. So far it would be impossible to 
 distinguish the development of the vertebral column of Lepidos- 
 teus from that of a Teleostean Fish. 
 
 Of the stages immediately following we have unfortunately 
 had no examples, but we have been fortunate enough to obtain 
 some young specimens of Lepidosteus 1 , which have enabled us to 
 work out with tolerable completeness the remainder of the de- 
 velopmental history of the vertebral column. In the next oldest 
 larva, of about 5 '5 centims., the changes which have taken place 
 are already sufficient to differentiate the vertebral column of 
 Lepidosteus from that of a Teleostean, and to shew how certain 
 of the characteristic features of the adult take their origin. 
 
 In the notochord the most important and striking change 
 consists in the appearance of a series of very well marked verte- 
 bral constrictions opposite the insertions of the neural and hcemal 
 arches. The first constrictions of the notochord are thus, as in 
 other Fishes, vertebral; and although, owing to the growth of 
 the -inter vertebral cartilage, the vertebral constrictions are subse- 
 quently replaced by intervertebral constrictions, yet at the same 
 time the primitive occurrence of vertebral constrictions demon- 
 strates that the vertebral column of Lepidosteus is a modification 
 of a type of vertebral column with biconcave vertebrae. 
 
 1 These specimens were given to us by Professor W. K. Parker, who received 
 them from Professor Hurt G. Wilder. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 781 
 
 The structure of the gelatinous body of the notochorct has 
 undergone no important change. The sheath, however, exhibits 
 certain features which deserve careful description. In the first 
 place the attention of the observer is at once struck by the fact 
 that, in the vertebral regions, the sheath is much thicker ('014 
 millim.) than in the intervertebral ('005 millim.), and a careful 
 examination of the sheath in longitudinal sections shews that 
 the thickening is due to the special differentiation of a superficial 
 part (PJate 41, fig. 69, sh.} of the sheath in each vertebral region. 
 This part is somewhat granular as compared to the remainder, 
 especially in longitudinal sections. It forms a cylinder (the wall 
 of which is about - oi millim. thick) in each vertebral region, 
 immediately within the membrana elastica externa. Between 
 it and the gelatinous tissue of the notochord within there is a 
 very thin unmodified portion of the sheath, which is continuous 
 with the thinner intervertebral parts of the sheath. This part of 
 the sheath is faintly, but at the same time distinctly, concentri- 
 cally striated a probable indication of concentric fibres. The 
 inner unmodified layer of the sheath has the appearance in 
 transverse sections through the vertebral regions of an inner 
 membrane, and may perhaps be Kolliker's "membrana elastica 
 interna." 
 
 We are not aware that any similar modification of the sheath 
 has been described in other forms. 
 
 The whole sheath is still invested by a very distinct mem- 
 brana elastica externa (m.e/). 
 
 The changes which have taken place in the parts which form 
 the permanent vertebrae will be best understood from Plate 41, 
 figs. 69 71. From the transverse section (fig. 70) it will be 
 seen that there are still neural and haemal arches resting upon 
 the membrana elastica externa ; but longitudinal sections (fig. 69) 
 shew that laterally these arches join a cartilaginous tube, embrac- 
 ing the intervertebral regions of the notochord, and continuous 
 from one vertebra to the next. 
 
 It will be convenient to treat separately the neural arches, 
 the haemal arches with their appendages, and the intervertebral 
 cartilaginous rings. 
 
 The neural arches, except in the fact of embracing a relatively 
 smaller part of the neural tube than in the earlier stage, do not 
 
782 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 at first sight appear to have undergone any changes. Viewed 
 from the side, however, in dissected specimens, they are seen to 
 be prolonged upwards so as to unite above with bars of cartilage 
 directed obliquely backwards. An explanation of this appear- 
 ance is easily found in the sections. The cartilaginous neural 
 arches are invested by a delicate layer of homogeneous bone, 
 developed in the perichondrium, and this bone is prolonged 
 beyond the cartilage and joins a similar osseous investment of 
 the dorsal bars above mentioned. The whole of these parts 
 may, it appears to us, be certainly reckoned as parts of the 
 neural arches, so that at this stage each neural arch consists of: 
 (i) a pair of basal portions resting on the notochord consisting 
 of cartilage invested by bone, (2) of a pair of dorsal cartilaginous 
 bars invested in bone (n.a '.), and (3) of osseous bars connecting 
 (i) and (2). 
 
 Though, in the absence of the immediately preceding stages, 
 it is not perfectly certain that the dorsal pieces of cartilage are 
 developed independently of the ventral, there appears to us every 
 probability that this is so ; and thus the cartilage of each neural 
 arch is developed discontinuously, while the permanent bony 
 neural arch, which commences as a deposit of bone partly in the 
 perichondrium and partly in the intervening membrane, forms a 
 continuous structure. 
 
 Analogous occurrences have been described by Gotte in 
 Teleostei. 
 
 The .dorsal portion of each neural arch becomes what we 
 have called the dorsal process of the adult arch. 
 
 Between the dorsal processes of the two sides there is placed 
 a median rod of cartilage (Plate 41, fig. 70, i. s.), which in its 
 development is wholly independent of the true neural arches, 
 and which constitutes the median spinous element of the adult. 
 In tracing these backwards it becomes obvious that they are 
 homologous with the interspinous elements supporting the dorsal 
 fin, in that they are replaced by these interspinous elements in 
 the region of the dorsal fin, and that the interspinous bones 
 occupy the same position as the median spinous processes. 
 This homology was first pointed out by Gotte in the case of the 
 Teleostei. 
 
 Immediately beneath this rod is placed the longitudinal 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 783 
 
 ligament (Plate 41, fig. 70, /./.), but there is as yet no trace of a 
 junction between the neural arches of the two sides in the space 
 between the longitudinal ligament and the spinal cord. 
 
 The basal parts of the neural arches of the two sides are 
 united dorsally by a thin cartilaginous layer resting on the 
 sheath of the notochord, but they are not united ventrally with 
 the haemal arches. 
 
 The haemal processes in the trunk are much more prominent 
 than in the preceding stage, and their bases are united ventrally 
 by a tolerably thick layer of cartilage. In the trunk they are 
 continuous with the so-called ribs of the adult (Plate 41, fig. 70) ; 
 but in order to study the nature of these ribs it is necessary to 
 trace the modifications undergone by the haemal arches in pass- 
 ing from the tail to the trunk. 
 
 It will be remembered that at an earlier stage the haemal 
 arches in the region of the tail-fin were fully formed, and that 
 through the anterior part of the caudal region the haemal pro- 
 cesses were far advanced in development, and just in front of 
 the caudal fin had actually met below the caudal vein. 
 
 The mode of development of the haemal arches in the tail as 
 uiijointed cartilaginous bars investing the caudal arteries and 
 veins is so similar to that of the caudal haemal arches of 
 Elasmobranchii, that it appears to us impossible to doubt their 
 identity in the two groups 1 . 
 
 The changes which have taken place by this stage with 
 reference to the haemal arches of the tail are not very con- 
 siderable. 
 
 In the case of a few more vertebrae the haemal processes 
 
 1 Gegenbaur (No. 6) takes a different view on this subject, as is clear from the 
 following passage in this memoir (pp. 369 370): "Each vertebra of Lepidostens 
 thus consists of a section of the notochord, and of the cartilaginous tissue surrounding 
 its sheath, which gives origin to the upper arches for the whole length of the vertebral 
 column, and in the caudal region to that of the lower arches also. The latter do not 
 however complete the enclosure of a lower canal, but this is effected by special independent 
 elements^ which are to be interpreted as homologues of the ribs." (The italics are 
 ours.) While we fully accept the homology between the ribs and the lower elements 
 of the haemal arches of the tail, the view expressed in the italicised section, to the 
 effect that the lower parts of the caudal arches are not true haemal arches but are 
 independently formed elements, is entirely opposed to our observations, and has we 
 believe only arisen from the fact that Gegenbaur had not the young larvae to work 
 with by which alone this question could be settled. 
 
784 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 have united into an arch, and the spinous processes of the arches 
 in the region of the caudal fin have grown considerably in 
 length. A more important change is perhaps the commence- 
 ment of a segmentation of the distal parts of the haemal arches 
 from the proximal. This process has not, however, as yet re- 
 sulted in a complete separation of the two, such as we find in 
 the adult. 
 
 If the haemal processes are traced forwards (Plate 42, figs. 
 75 and 76) from the anterior segment where they meet ventrally, 
 it will be found that each haemal process consists of a basal 
 portion, adjoining the notochord, and a peripheral portion. 
 These two parts are completely continuous, but the line of a 
 future separation is indicated by the structure of the cartilage, 
 though not shewn in our figures. As the true body-cavity of 
 the trunk replaces the obliterated body-cavity of the caudal 
 region, no break of continuity will be found in the structure of 
 the haemal processes (Plates 41 and 42, figs. 73 and 74), but 
 while the basal portions graw somewhat larger, the peripheral 
 portions gradually elongate and take the form of delicate rods 
 of cartilage extending ventralwards, on each side of the body- 
 cavity, immediately outside the peritoneal membrane, and along 
 the lines of insertion of the intermuscular septa. These rods 
 obviously become the ribs of the adult. 
 
 As one travels forwards the ribs become continually longer 
 and more important, and though they are at this stage united 
 with the haemal processes in every part of the trunk, yet they 
 are much more completely separated from these processes in 
 front than behind (Plate 41, fig. 72). 
 
 In front (Plate 41, fig. 72), each rib (rb.), after continuing its 
 ventral course for some distance, immediately outside the peri- 
 toneal membrane, turns outwards, and passes along one of the 
 intermuscular septa till it reaches the epidermis. This feature 
 in the position of the ribs is, as has been already pointed out in 
 the anatomical part of this section, characteristic of all the ribs 
 of the adult. 
 
 It is unfortunate that we have had no specimens shewing the 
 ribs at an earlier stage of development ; but it appears hardly 
 open to doubt that the ribs are originally continuous ivith the 
 hcstnal processes, and that the indications of a separation between 
 
STRUCTURE AND DEVELOPMENT OF LEPTDOSTEUS. 785 
 
 those two parts at this stage are not due to a secondary fusion, 
 but to a commencing segmentation. 
 
 It further appears, as Mtiller, Gegenbaur and others have 
 stated, that the ribs and haemal processes of the tail are serially 
 homologous structures ; but that the view maintained by Gotte 
 in his very valuable memoirs on the Vertebrate skeleton is also 
 correct to the effect that the Jicemal arches of tlie tail are Jwmo- 
 logous throughout the series of FisJies. 
 
 To this subject we shall return again at the end of the 
 section. 
 
 Before leaving the haemal arches it may be mentioned that 
 behind the region of the ventral caudal fin the two haemal pro- 
 cesses merge into one, and form an unpaired knob resting 
 on the ventral side of the notochord, and not perforated by 
 a canal. 
 
 There are now present well -developed intervertebral rings of 
 cartilage, each of which eventually becomes divided into two 
 parts, and converted into the adjacent faces of the contiguous 
 vertebrae. These rings are united with the neural and haemal 
 arches of the vertebrae in front and behind. 
 
 Each ring, as shewn by the transverse section (Plate 41, fig. 
 71), is not uniformly thick, but exhibits four projections, two 
 dorsal and two ventral. These four projections are continuous 
 with the bases of the neural and haemal arches of the adjacent 
 vertebrae, and afford presumptive evidence of the derivation of 
 the intervertebral rings from the neural and haemal arches; in 
 that had they so originated, it would be natural to anticipate 
 the presence of four thickenings indicating the four points from 
 which the cartilage had spread, while if the rings had originated 
 independently, it would not be easy to give any explanation of 
 the presence of such thickenings. Gegenbaur (No. 6), from the 
 investigation of a much older larva than that we are now describ- 
 ing, also arrived at the conclusion that the intervertebral carti- 
 lages were derived from the neural and haemal arches ; but as 
 doubts have been thrown upon this conclusion by Gotte, and 
 as it obviously required further confirmation, we have considered 
 it important to attempt to settle this point. From the description 
 given above, it is clear that we have not. however, been able 
 absolutely to trace the origin of this cartilage, but at the same 
 
786 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 time we think that we have adduced weighty evidence in corrobo- 
 ration of Gegenbaur's view. 
 
 As shewn in longitudinal section (Plate 41, fig. 69, iv.r.}, the 
 intervertebral rings are thicker in the middle than at the two 
 ends. In this thickened middle part the division of the cartilage 
 into two parts to form the ends of two contiguous vertebrae is 
 subsequently effected. The curved line which this segmentation 
 will follow is, however, already marked out, and from surface 
 views it might be supposed that this division had actually 
 occurred. 
 
 The histological structure of the intervertebral cartilage is 
 very distinct from that of the cartilage of the bases of the 
 arches, the nuclei being much more closely packed. In parts, 
 indeed, the intervertebral cartilage has almost the character of 
 fibre-cartilage. On each side of the line of division separating 
 two vertebrae it is invested by a superficial osseous deposit. 
 
 The next oldest larva we have had was 1 1 centims. in length. 
 The filamentous dorsal lobe of the caudal fin still projected far 
 beyond the permanent caudal fin (Plate 34, fig. 16). 
 
 The vertebral column was considerably less advanced in deve- 
 lopment than that dissected by Gegenbaur, though it shews a 
 great advance on the previous stage. Its features are illustrated 
 by two transverse sections, one through the median plane of a 
 vertebral region (Plate 42, fig. 78) and the other through that of 
 an intervertebral region (Plate 42, fig. 79), and by a horizontal 
 section (Plate 42, fig. 77). 
 
 In the last stage the notochord was only constricted verte- 
 brally. Now, however, by the great growth of intervertebral 
 cartilage there have appeared (Plate 42, fig. 77) very well- 
 marked intervertebral constrictions, by the completion of which the 
 vertebras of Lepidosteus acquire their unique character amongst 
 Fishes. 
 
 These constrictions still, however, coexist with the earlier, 
 though at this stage relatively less conspicuous, vertebral con- 
 strictions. 
 
 The gelatinous body of the notochord retains its earlier 
 condition. The sheath has, however, undergone some changes. 
 In the vertebral regions there is present in any section of the 
 sheath (i) externally, the membrana elastica externa (m.el.) ; 
 
STRUCTURE ANJ) DEVELOPMENT OF LEPIDOSTEUS. 787 
 
 i 
 
 then (2) the external layer of the sheath (s/i.)>, which is, however, 
 
 less thick than before, and exhibits a very faint form of radial 
 striation ; and (3) internally, a fairly thick and concentrically 
 striated layer. The whole thickness is, on an average, O'i8 
 millim. 
 
 In the intervertebral regions the membrana elastica externa 
 is still present in most parts, but has become absorbed at the 
 posterior border of each vertebra, as shewn in longitudinal section 
 in Plate 42, fig. 77. It is considerably puckered transversely. 
 The sheath of the notochord within the membrana elastica 
 externa is formed of a concentrically striated layer, continuous 
 with the innermost layer of the sheath in the vertebral regions. 
 It is puckered longitudinally. Thus, curiously enough, the 
 membrana elastica externa and the sheath of the notochord 
 in the intervertebral regions are folded in different directions, 
 the folds of the one being only visible in transverse sections 
 (Plate 42, fig. 79), and those of the other in longitudinal sections 
 (Plate 42, fig. 77). 
 
 The osseous and cartilaginous structures investing the noto- 
 chord may conveniently be dealt with in the same order as 
 before, viz. : the neural arches, the haemal arches, and the 
 intervertebral cartilages. 
 
 The cartilaginous portions of the neural arches are still 
 unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped 
 masses resting on the sheath of the notochord. They are in- 
 vested by a thick layer of bone prolonged upwards to meet 
 the dorsal processes (n.a'.}, which are still formed of cartilage 
 invested by bone. 
 
 It will be remembered that in the last stage there was no 
 key-stone closing in the neural arch above. This deficiency is 
 now however supplied, and consists of (i) two bars of cartilage 
 repeated for each vertebra, but intervertebral ly placed, which are 
 directly differentiated from the ligamentum longitudinale supe- 
 rius, into which they merge above ; and (2) two osseous plates 
 placed on the outer sides of these cartilages, which are continuous 
 with the lateral osseous bars of the neural arch. The former 
 of these elements gives rise to the cartilaginous elements above 
 the osseous bridge of the neural arch in the adult. The two 
 osseous plates supporting these cartilages clearly form what we 
 
788 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 have called in our description of the adult the osseous roof of 
 the spinal canal. 
 
 A comparison of the neural arch at this stage with the arch 
 in the adult, and in the stage last described, shews that the 
 greater part of the neural arch of the adult is formed of mem- 
 brane-bone, there being preformed in cartilage only a small basal 
 part, a dorsal process, and paired key-stones below the ligamen- 
 tum longitudinale superius. 
 
 The haemal arches (Plate 42, fig. 78) are still largely carti- 
 laginous, and rest upon the sheath of the notochord. They are 
 invested by a thick layer of bone. The bony layer investing 
 the neural and haemal arches is prolonged to form a continuous 
 investment round the vertebral portions of the notochord (Plate 
 42, fig. 78). This investment is at the sides prolonged outwards 
 into irregular processes (Plate 42, fig. 78), which form the com- 
 mencement of the outer part of the thick but cellular osseous 
 cylinder forming the middle part of the vertebral body. 
 
 The intervertebral cartilages are much larger than in the 
 earlier stage (Plate 42, figs. 77 and 79), and it is by their growth 
 that the intervertebral constrictions of the notochord are pro- 
 duced. They have ceased to be continuous with the cartilage 
 of the arches, the intervening portion of the vertebral body 
 between the two being only formed of bone. They are not yet 
 divided into two masses to form the contiguous ends of adjacent 
 vertebrae. 
 
 Externally, the part of each cartilage which will form the 
 hinder end of a vertebral body is covered by a tube of bone, 
 having the form of a truncated funnel, shewn in longitudinal 
 section in Plate 42, fig. 77, and in transverse section in Plate 42, 
 
 ng- 79- 
 
 At each end, the intervertebral cartilages are becoming 
 penetrated and replaced by beautiful branched processes from 
 the homogeneous bone which was first of all formed in the peri- 
 chondrium (Plate 42, fig. 77). 
 
 This constitutes the latest stage which we have had. 
 
 Gegenbaur (No. 6) has described the vertebral column in 
 a somewhat older larva -of 18 centims. 
 
 The chief points in which the vertebral column of this larva 
 differed from ours are: (i) the disappearance of all trace of the 
 
STRUCTURE AND DEVELOPMENT OF I.EPIDOSTEUS. 789 
 
 primitive vertebral constriction of the notochord ; (2) the-ne-arly 
 completed constriction of the notochord in the intervertebral 
 regions ; (3) the complete ossification of the vertebral portions 
 of the bodies of the vertebrae, the terminal so-called intervertebral 
 portions alone remaining cartilaginous ; (4) the complete ossifi- 
 cation of the basal portions of the haemal and neural processes 
 included within the bodies of the vertebrae, so that in the case 
 of the neural arch all trace of the fact that the greater part 
 was originally not formed in cartilage had become lost. The 
 cartilage of the dorsal spinous processes was, however, still 
 persistent. 
 
 The only points which remain obscure in the later history 
 of the vertebral column are the history of the notochord and of 
 its sheath. We do not know how far these are either simply 
 absorbed or partially or wholly ossified. 
 
 Gotte in his memoir on the formation of the vertebral bodies 
 of the Teleostei attempts to prove (i) that the so-called mem- 
 brana elastica externa of the Teleostei is not a homogeneous 
 elastica, but is formed of cells, and (2) that in the vertebral regions 
 ossification first occurs in it. 
 
 In Lepidostcus we have met with no indication that the mem- 
 brana elastica externa is composed of cells ; though it is fair to 
 Gotte to state that we have not examined such isolated portions 
 of it as he states are necessary in order to make out its structure. 
 But further than this we have satisfied ourselves that during 
 the earlier stage of ossification this membrane is not ossified, 
 and indeed in part becomes absorbed in proximity to the inter- 
 vertebral cartilages ; and Gcgenbaur met with no ossification of 
 this membrane in the later stage described by him. 
 
 Summary of the development of the vertebral column and ribs. 
 
 A mesoblastic investment is early formed round the noto- 
 chord, which is produced into two dorsal and two ventral ridges, 
 the former uniting above the neural canal. Around the cuticular 
 sheath of the notochord an elastic membrane, the membrana 
 elastica externa, is next developed. The neural ridges become 
 enlarged at each inter-muscular septum, and these enlargements 
 
790 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 soon become converted into cartilage, thus forming a series of 
 neural processes riding on the membrana elastica externa, and 
 extending about two-thirds of the way up the sides of the neural 
 canal. The haemal processes arise simultaneously with, and in 
 the same manner as, the neural. They are small in the trunk, 
 but at the front end of the anal fin they suddenly enlarge and 
 extend ventralwards. Each succeeding pair of haemal arches 
 becomes larger than the one in front, each arch finally meeting its 
 fellow below the caudal vein, thus forming a completely closed 
 haemal canal. These arches are moreover produced into long 
 spines supporting the fin-rays of the caudal fin, which thus 
 differs from the other unpaired fins in being supported by parts 
 of the vertebral column, and not by separately formed skeletal 
 elements. 
 
 In the next stage which we have had the opportunity of study- 
 ing (larva of 5^ centims.), a series of very well-marked vertebral 
 constrictions are to be seen in the notochord. The sheath is now 
 much thicker in the vertebral than in the intervertebral regions : 
 this is due to a special differentiation of a superficial part of 
 the sheath, which appears more granular than the remainder. 
 This granular part of the sheath thus forms a cylinder in each 
 vertebral region. Between it and the gelatinous tissue of the 
 notochord there remains a thin unmodified portion of the sheath, 
 which is continuous with the intervertebral parts of the sheath. 
 The neural and haemal arches are seen to be continuous with a 
 cartilaginous tube embracing the intervertebral regions of the 
 notochord, and continuous from one vertebra to the next. A 
 delicate layer of bone, developed in the perichondrium, invests 
 the cartilaginous neural arches, and this bone grows upwards 
 so as to unite above with the osseous investment of separately 
 developed bars of cartilage, which are directed obliquely back- 
 wards. These bars, or dorsal processes, may be reckoned as 
 parts of the neural arches. Between the dorsal processes of the 
 two sides is placed a median rod of cartilage, which is developed 
 separately from the true neural arches, and which constitutes 
 the median spinous element of the adult. Immediately below 
 this rod is placed the ligamentum longitudinale superius. There 
 is now a commencement of separation between the dorsal and 
 ventral parts of the haemal arches, not only in the tail, but also 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 791 
 
 in the trunk, where they pass ventralwards on each side-oil the 
 body-cavity, immediately outside the peritoneal membrane, along 
 the lines of insertion of the intermuscular septa. These are 
 obviously the ribs of the adult, and there is no break of con- 
 tinuity of structure between the haemal processes of the tail and 
 the ribs. In the anterior part of the trunk the ribs pass out- 
 wards along the intermuscular septa till they reach the epidermis. 
 Thus the ribs are originally continuous with the haemal pro- 
 cesses. Behind the region of the ventral caudal fin the two 
 haemal processes merge into one, which is not perforated by 
 a canal. 
 
 Each of the intervertebral rings of cartilage becomes eventually 
 divided into two parts, and converted into the adjacent faces of 
 contiguous vertebrae, the curved line where this will be effected 
 being plainly marked out. These rings are united with the 
 neural and haemal arches of the vertebrae next in front and 
 behind. As these rings are formed originally by the spreading 
 of the cartilage from the primitive neural and haemal processes, 
 the intervertebral cartilages are clearly derived from the neural 
 and haemal arches. The intervertebral cartilages are thicker in 
 the middle than at their two ends. 
 
 In our latest stage (u centims.), the vertebral constrictions 
 of the notochord are rendered much less conspicuous by the 
 growth of the intervertebral cartilages giving rise to marked 
 intervertebral constrictions. In the intervertebral regions the 
 membrana elastica externa has become aborted at the posterior 
 border of each vertebra, and the remaining part is considerably 
 puckered transversely. The inner sheath of the notochord is 
 puckered longitudinally in the intervertebral regions. The 
 granular external layer of the sheath in the vertebral regions is 
 less thick than in the last stage, and exhibits faint radial 
 striations. 
 
 Two closely approximated cartilaginous elements now form 
 a key-stone to the neural arch above : these are directly differen- 
 tiated from the ligamentum longitudinale superius, into which 
 they merge above. An osseous plate is formed on the outer side 
 of each of these cartilages. These plates are continuous with 
 the lateral osseous bars of the neural arches, and also give rise 
 to the osseous roof of the spinal canal of the adult. 
 
792 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Thus the greater part of the neural arches is formed of mem- 
 brane bone. The haemal arches are invested by a thick layer of 
 bone, and there is also a continuous osseous investment round 
 the vertebral portions of the notochord. The intervertebral 
 cartilages become penetrated by branched processes of bone. 
 
 Comparison of the vertebral column of Lepidosteus with that of 
 
 other forms. 
 
 The peculiar form of the articulatory faces of the vertebrae of 
 Lepidosteus caused L. Agassiz (No. 2) to compare them with the 
 vertebrae of Reptiles, and subsequent anatomists have suggested 
 that they more nearly resemble the vertebras of some Urodelous 
 Amphibia than those of any other form. 
 
 If, however, Gotte's account of the formation of the am- 
 phibian vertebrae is correct, there are serious objections to a 
 comparison between the vertebrae of Lepidosteus and Amphibia 
 on developmental grounds. The essential point of similarity 
 supposed to exist between them consists in the fact that in both 
 there is a great development of intervertebral cartilage which 
 constricts the notochord intervertebrally, and forms the articular 
 faces of contiguous vertebrae. 
 
 In Lepidosteus this cartilage is, as we have seen, derived from 
 the bases of the arches ; but in Amphibia it is held by Gotte to 
 be formed by a special thickening of a cellular sheath round the 
 notochord which is probably homologous with the cartilaginous 
 sheath of the notochord of Elasmobranchii, and therefore with 
 part of the notochordal sheath placed within the membrana 
 elastica externa. 
 
 If the above statements with reference to the origin of the 
 intervertebral cartilage in the two types are true, it is clear that 
 no homology can exist between structures so differently de- 
 veloped. Provisionally, therefore, we must look elsewhere 
 than in Lepidosteus for the origin of the amphibian type of 
 vertebrae. 
 
 The researches which we have recorded demonstrate, how- 
 ever, in a very conclusive manner that the vertebras of Lepi- 
 dosteus have very close affinities with those of Teleostei. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 793 
 
 In support of this statement we may point: (i) To the 
 structure of the sheath of the notochord ; (2) to the formation of 
 the greater part of the bodies of the vertebrae from ossification 
 in membrane around the notochord ; (3) to the early biconcave 
 form of the vertebras, only masked at a later period by the de- 
 velopment of intcrvertebral cartilages ; (4) to the character of 
 the neural arches. 
 
 This latter feature will be made very clear if the reader will 
 compare our figures of the sections of later vertebrae (Plate 42, 
 fig. 78) with Gotte's 1 figure of the section of the vertebra of a 
 Pike (Plate 7, fig. i). In Gotte's figure there are shewn similar 
 intercalated pieces of cartilage to those which we have found, 
 and similar cartilaginous dorsal processes of the vertebrae. Thus 
 we are justified in holding that whether or no the opisthoccelous 
 form of the vertebrae of Lepidosteus is a commencement of a 
 type of vertebrae inherited by the higher forms, yet in any case 
 the vertebrae are essentially built on the type which has become 
 inherited by the Teleostei from the bony Ganoids. 
 
 PART III. The ribs of Fishes. 
 
 The nature and homologies of the ribs of Fishes have long 
 been a matter of controversy ; but the subject has recently been 
 brought forward in the important memoirs of Gotte' 2 on the 
 Vertebrate skeleton. The alternatives usually adopted are, 
 roughly speaking, these : Either the haemal arches of the tail 
 are homologous throughout the piscine series, while the ribs 
 of Ganoids and Teleostei are not homologous with those of 
 Elasmobranchii ; or the ribs are homologous in all the piscine 
 groups, and the haemal arches in the tail are differently formed 
 in the different types. Gotte has brought forward a great body 
 of evidence in favour of the first view; while Gegenbaur 3 may 
 
 1 "Beitrage zur vergl. Morphol. d. Skeletsystems d. Wirbelthiere." Archiv /. 
 Mikr. Anat. Vol. XVI. 1879. 
 
 - " Beitrage z. vergl. Morph. d. Skeletsystems d. Wirbelthiere. II. Die Wir- 
 belsaule u. ihre Anhange." Archiv f. Mikr. Anat., Vol. xv., 1878, and Vol. xvi., 
 1879. 
 
 3 " Ueb. d. Entwick. d. Wirbelsiiule d. Lepidosteus, mil. vergl. Anat. Bemer- 
 kungen. " Jcnaisc/ic Zeilschrift, Bd. in., 1863. 
 
 B. 5 I 
 
794 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 be regarded as more especially the champion of the second 
 view. 
 
 One of us held in a recent publication 1 that the question was 
 not yet settled, though the view that the ribs are homologous 
 throughout the series was provisionally accepted. 
 
 It is admitted by both Gegenbaur and Gotte that in Lepido- 
 stens the ribs, in the transition from the trunk to the tail, bend 
 inwards, and finally unite in the region of the tail to form the 
 ventral parts of the haemal arches, and our researches have 
 abundantly confirmed this conclusion. 
 
 Are the haemal arches, the ventral parts of which are thus 
 formed by the coalescence of the ribs, homologous with the 
 haemal arches in Elasmobranchii ? The researches recorded in 
 the preceding pages appear to us to demonstrate in a conclusive 
 manner that they are so. 
 
 The development of the haemal arches in the tail in these two 
 groups is practically identical ; they are formed in both as simple 
 elongations of the primitive haemal processes, which meet below 
 the caudal vein. In the adult there is an apparent difference 
 between them, arising from the fact that in Lepidosteus the 
 peripheral parts of the haemal processes are only articulated with 
 the basal portions, and not, as in Elasmobranchii, continuous 
 with them. This difference does not, however, exist in the early 
 larva, since in the larval Lepidosteus the haemal arches of the tail 
 are unsegmented cartilaginous arches, as they permanently are 
 in Elasmobranchii. If, however, the homology between the 
 haemal arches of the two types should still be doubted, the fact 
 that in both types the haemal arches are similarly modified to 
 support the fin-rays of the ventral lobe of the caudal fin, while in 
 neither type are they modified to support the anal fin, may 
 be pointed out as a very strong argument in confirmation of 
 their homology. 
 
 The demonstration of the homology of the haemal arches of 
 the tail in Lepidosteus and Elasmobranchii might at first sight be 
 taken as a conclusive argument in favour of Gotte's view, that 
 the ribs of Elasmobranchii are not homologous with those of 
 Ganoidei. This view is mainly supported by two facts : 
 
 1 Comparative Embryology, Vol. II., pp. 462, 463 [the original edition]. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 795 
 
 (1) In the first place, the ribs in Elasmobranchii do not at 
 first sight appear to be serially homologous with the ventral 
 parts of the haemal arches of the tail, but would rather seem to 
 be lateral offshoots of the haemal processes, while the haemal 
 arches of the tail appear to be completed by the coalescence of 
 independent ventral prolongations of the haemal processes. 
 
 (2) In the second place, the position of the ribs is different 
 in the two groups. In Elasmobranchii they are situated between 
 the dorso-lateral and ventro-lateral muscles (woodcut, fig. I, rb.}, 
 
 FIG. i. 
 
 II, 
 
 Diagrammatic section through the trunk of an advanced embryo of Scyllium, to shew 
 the position of the ribs. 
 
 ao., aorta; c. sh., cartilaginous notochordal sheath; cv., cardinal vein; hp., hremal 
 process; k., kidney; l.s., ligamentum longitudinale superius ; m.el 9 membrana 
 elastica externa ; na., neural arch; no., notochord ; //., lateral line; rb., rib; 
 sp.c., spinal cord. 
 
 while in Lepidosteus and other Ganoids they immediately girth 
 the body-cavity. 
 
 There is much, therefore, to be said in favour of Gottc's view. 
 At the same time, there is another possible interpretation of the 
 facts which would admit the homology of the ribs as well as of 
 the haemal arches throughout the Pisces. 
 
 512 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Let us suppose, to start with, that the primitive arrangement 
 of the parts is more or less nearly that found in Lepidosteus, 
 where we have well-developed ribs in the region of the trunk, 
 girthing the body-cavity, and uniting in the caudal region to 
 form the ventral parts of the haemal arches. It is easy to con- 
 ceive that the ribs in the trunk might somewhat alter their 
 position by passing into the muscles, along the inter-muscular 
 septa, till they come to lie between the dorso-lateral and ventro- 
 lateral muscles, as in Elasmobranchii. Lepidosteus itself affords 
 a proof that such a change in the position of the ribs is not 
 impossible, in that it differs from other Ganoids and from Teleostei 
 in the fact that the free ends of the ribs leave the neighbourhood 
 of the body-cavity and penetrate into the muscles. 
 
 If it be granted that the mere difference in position between 
 the ribs of Ganoids and Elasmobranchii is not of itself sufficient 
 to disprove their homology, let us attempt to picture what would 
 take place at the junction of the trunk and tail in a type in 
 which the ribs had undergone the above change in position. On 
 nearing the tail it may be supposed that the ribs would gradually 
 become shorter, and at the same time alter their position, till 
 finally they shaded off into ordinary haemal processes. If, how- 
 ever, the haemal canal became prolonged forwards by the forma- 
 tion of some additional complete or nearly complete haemal 
 arches, an alteration in the relation of the parts would necessarily 
 take place. Owing to the position of the ribs, these structures 
 could hardly assist in the new formation of the anterior part of 
 the haemal canal, but the continuation forwards of the canal 
 would be effected by prolongations of the haemal processes 
 supporting the ribs. The new arches so formed would naturally 
 be held to be homologous with the haemal arches of the tail, 
 though really not so, while the true nature of the ribs would 
 also be liable to be misinterpreted, in that the ribs would appear 
 to be lateral outgrowths of the haemal processes of a wholly 
 different nature to the ventral parts of the haemal arches of the 
 tail. 
 
 In some Elasmobranchii, as shewn in the accompanying 
 woodcut (fig. 2), in the transitional vertebrae between the trunk 
 and the tail, the ribs are supported by lateral outgrowths of the 
 haemal processes, while the wholly independent prolongations of 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 797 
 
 the haemal processes appear to be about to give rise tcr the 
 haemal arches of the tail. 
 
 This peculiar state of things led Gotte, and subsequently one 
 of us, to deny for Elasmobranchs all homology between the ribs 
 and any part of the haemal arches of the tail ; but in view of the 
 explanation just suggested, this denial was perhaps too hasty. 
 
 FIG. 2. 
 
 r.p - 
 
 . .V. COLXL*. 
 
 Transverse segtion through the ventral part of the notochord, and adjoining structures 
 of an advanced Scyllium embryo at the root of the tail. 
 
 Vb., cartilaginous sheath of the notochord ; ha., haemal process ; ;-./., process to 
 which the rib is articulated ; m.el., membrana elastica externa ; ck., notochord ; 
 ao., aorta; V.cau., caudal vein. 
 
 We are the more inclined to take this view because the re- 
 searches of Gotte appear to shew that an occurrence, in many 
 respects analogous, has taken place in some Teleostei. 
 
 In Teleostei, Johannes Muller, and following him Gegenbaur, 
 do not admit that the haemal arches of the tail are in any part 
 formed by the ribs. Gegenbaur (Elements of Comp. Anat., trans- 
 lation, p. 431) says, "In the Teleostei, the costiferous transverse 
 processes" (what we have called the haemal processes) "gradually 
 
798 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 converge in the caudal region, and form inferior arches, which 
 are not homologous with those of Selachii and Ganoidei, although 
 they also form spinous processes." 
 
 The opposite view, that the haemal arches of the tail in Tele- 
 ostei contain parts serially homologous with the basal parts of 
 the haemal processes as well as with the ribs, has been also 
 maintained by many anatomists, e.g., Meckel, Aug. Muller, &c., 
 and has recently found a powerful ally in Gotte. 
 
 In many cases, the relations of the parts appear to be funda- 
 mentally those found in Lepidosteus and Amia, and Gotte has 
 shewn by his careful embryological investigations on Esox and 
 Anguilla, that in these two forms there is practically conclusive 
 evidence that the ribs as well as the haemal costiferous pro- 
 cesses of Gegenbaur, which support them, enter into the forma- 
 tion of the haemal arches of the tail. 
 
 In a great number of Teleostei, e.g., the Salmon and most 
 Cyprinoids, &c., the haemal arches in the region of transition 
 from the trunk to the tail have a structure which at first sight 
 appears to support Johannes Muller's and Gegenbaur's view. 
 The hsemal processes grow larger and meet each other ventrally; 
 while the ribs articulated to them gradually grow smaller and 
 disappear. 
 
 The Salmon is typical in this respect, and has been carefully 
 studied by Gotte, who attempts to shew (with, in our opinion, 
 complete success) that the anterior haemal arches are really not 
 entirely homologous with the true haemal arches behind, but 
 that in the latter, the closure of the arch below is effected by the 
 haemal spine, which is serially homologous with a pair of coal- 
 esced ribs, while in the anterior haemal arches, i.e., those of the 
 trunk, the closure of the arch is effected by a bridge of bone 
 uniting the haemal processes. 
 
 The arrangement of the parts just described, as well as the 
 view of Gotte with reference to them, will be best understood 
 from the accompanying woodcut (fig. 3), copied from Gotte's 
 memoir. 
 
 Gotte sums up his own results on this point in the following 
 words (p. 138): "It follows from this, that the half rings, forming 
 the haemal canal in the hindermost trunk vertebrae of the Sal- 
 mon, are not (with the exception of the last) completely homo- 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 799 
 
 logous with those of the tail, but are formed by a connecting 
 piece between the basal stumps (haemal processes), which origi- 
 nates as a paired median process of these stumps." 
 
 The incomplete homology between the anterior haemal arches 
 and the true caudal haemal arches which follow them is exactly 
 what we suggest may be the case in Elasmobranchii, and if it be 
 admitted in the one case, we see no reason why it should not 
 also be admitted in the other. 
 
 If this admission is made, the only ground for not regarding 
 the ribs of Elasmobranchii as homologous with those of Ganoids 
 
 FIG. 3. 
 
 Semi -diagrammatic transverse sections through the first caudal vertebra (A), the last 
 trunk vertebra (B), and the two trunk vertebra- in front (C and D), of a Salmon 
 embryo of 2-3 centims. (From Gbtte.) 
 
 ttb., hremal arch; ul>'., haemal process; ui>"., rib; c., notochord ; a., aorta; v., vein; 
 h., connecting pieces between haemal processes ; u., kidney ; </., intestine ; 
 sp'., hnemal spine ; m'. , muscles. 
 
 is their different position, and we have already attempted to 
 prove that this is not a fundamental point. 
 
 The results of our researches appear to us, then, to leave two 
 alternatives as to the ribs of Fishes. One of these, which may 
 be called Gotte's view, may be thus stated: The haemal arches 
 
80O STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 are homologous throughout the Pisces : in Teleostei, Ganoidei, 
 and Dipnoi 1 , the ribs, placed on the inner face of the body-wall, 
 are serially homologous with the ventral parts of the haemal 
 arches of the tail; in Elasmobranchii, on the other hand, the ribs 
 are neither serially homologous with the haemal arches of the 
 tail nor homologous with the ribs of Teleostei and Ganoidei, but 
 are outgrowths of the haemal processes into the space between 
 the dorso-lateral and ventro-lateral muscles, which may perhaps 
 have their homologues in Teleostei and Ganoids in certain 
 accessory processes of the vertebrae. 
 
 The other view, which we are inclined to adopt, and the 
 arguments for which have been stated in the preceding pages, is 
 as follows : The Teleostei, Ganoidei, Dipnoi, and Elasmobran- 
 chii are provided with homologous haemal arches, which are 
 formed by the coalescence below the caudal vein of simple pro- 
 longations of the primitive haemal processes of the embryo. The 
 canal enclosed by the haemal arches can be demonstrated em- 
 bryologically to be the aborted body-cavity. 
 
 In the region of the trunk the haemal processes and their 
 prolongations behave somewhat differently in the different types. 
 
 In Ganoids and Dipnoi, in which the most primitive arrange- 
 ment is probably retained, the ribs are attached to the haemal 
 processes,and are placed immediately without the peritoneal mem- 
 brane at the insertions of the intermuscular septa. These ribs are 
 in many instances (Lepidosteus, Acipenser}, and very probably in 
 all, developed continuously with the haemal processes, and be- 
 come subsequently segmented from them. They are serially 
 homologous with the ventral parts of the haemal arches of the 
 tail, which, like them, are in many instances (Ceratodus, Lepidos- 
 teus, Polypterus, and to some extent in Amia] segmented off 
 from the basal parts of the haemal arches. 
 
 In Teleostei the ribs have the same position and relations as 
 those in Ganoids and Dipnoi, but their serial homology with the 
 ventral parts of the hsemal processes of the tail, is often (e.g., the 
 S Imon) obscured by some of the anterior haemal arches in the 
 I "erior part of the trunk being completed, not by the ribs, but 
 
 1 We find the serial homology of the ribs and ventral parts of the hsemal arches to 
 be very clear in Ceratodtis. Wiedersheim states that it is not clear in Protopterus, 
 although he holds that the facts are in favour of this view. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8oi 
 
 by independent outgrowths of the basal parts of the haemal pro- 
 cesses. 
 
 In Elasmobranchii a still further divergence from the primi- 
 tive arrangement is present. The ribs appear to have passed 
 outwards along the intermuscular septa into the muscles, and are 
 placed between the dorso-lateral and ventro-lateral muscles (a 
 change of position of the ribs of the same nature, but affecting 
 only their ends, is observable in Lepidosteus). This change of 
 position, combined probably with the secondary formation of a 
 certain number of anterior haemal arches similar to those in the 
 Salmon, renders their serial homology with the ventral parts of 
 the haemal processes of the tail far less clear than in other types, 
 and further proof is required before such homology can be con- 
 sidered as definitely established. 
 
 This is not the place to enter into the obscure question as to 
 how far the ribs of the Amphibia and Amniota are homologous 
 with those of Fishes, It is to be remarked, however, that the 
 ribs of the Urodela (i) occupy the same position in relation to 
 the muscles as the Elasmobranch ribs, (2) that they are con- 
 nected with the neural arches, and (3) that they coexist in the 
 tail with the haemal arches, and seem, therefore, to be as differ- 
 ent as possible from the ribs of the Dipnoi. 
 
 PART IV. The skeleton of the ventral lobe of the tail fin, and its 
 bearing on tlie nature of the tail fin of tlie various types of Pisces. 
 
 In the embryos or larvae of all the Elasmobranchii, Ganoidei, 
 and Teleostei which have up to this time been studied, the un- 
 paired fins arise as median longitudinal folds of the integument 
 on the dorsal and ventral sides of the body, which meet at the 
 apex of the tail. The tail at first is symmetrical, having a form 
 which has been called diphycercal or protocercal. At a later 
 stage, usually, though not always, parts of these fins atrophy, 
 while other parts undergo a special development and constit! e 
 the permanent unpaired fins. 
 
 Since the majority of existing as well as extinct Fishes are 
 provided with discontinuous fins, those forms, such as the Eel 
 (Angtiilla), in which the fins are continuous, have probably re- 
 
802 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 verted to an embryonic condition : an evolutional process which 
 is of more frequent occurrence than has usually been admitted. 
 
 In the caudal region there is almost always developed in the 
 larvae of the above groups a special ventral lobe of the em- 
 bryonic fin a short distance from the end of the tail. In Elasmo- 
 branchii and Chondrostean Ganoids the portion of the em- 
 bryonic tail behind this lobe persists through life, and a special 
 type of caudal fin, which is usually called heterocercal, is thus 
 produced. This type of caudal fin appears to have been the 
 most usual in the earlier geological periods. 
 
 Simultaneously with the formation of the ventral lobe of the 
 heterocercal caudal fin, the notochord with the vertebral tissues 
 surrounding it, becomes bent somewhat dorsalwards, and thus 
 the primitive caudal fin forms a dorsally directed lobe of the 
 heterocercal tail. We shall call this part the dorsal lobe of the 
 tail-fin, and the secondarily formed lobe the ventral lobe. 
 
 Lepidosteus and Amia (Wilder, No. 15) amongst the bony 
 Ganoids, and, as has recently been shewn by A. Agassiz 1 , most 
 Teleostei acquire at an early stage of their development hetero- 
 cercal caudal fins, like those of Elasmobranchii and the Chondro- 
 stean Ganoids ; but in the course of their further growth the 
 dorsal lobe partly atrophies, and partly disappears as such, 
 owing to the great prominence acquired by the ventral lobe. A 
 portion of the dorsally flexed notochord and of the cartilage or 
 bone replacing or investing it remains, however, as an indication 
 of the original dorsal lobe, though it does not project backwards 
 beyond the level of the end of the ventral lobe, which in these 
 types forms the terminal caudal fin. 
 
 The true significance of the dorsally flexed portion of the 
 vertebral axis was first clearly stated by Huxley 2 , but as 
 A. Agassiz has fairly pointed out in the paper already quoted, 
 this fact does not in any way militate against the view put 
 forward by L. Agassiz that there is a complete parallelism be- 
 tween the embryonic development of the tail in these Fishes 
 and the palseontological development of this organ. We think 
 
 1 " On the Young Stages of some Osseous Fishes. I. The Development of the 
 Tail," Proc. of the American Academy of Arts and Sciences^ Vol. XIIL, 1877. 
 
 2 "Observations on the Development of some Parts of the Skeleton of Fishes," 
 Quart. Journ. of Micr. Science, Vol. VII., 1859. 
 
STRUCTURE AND DEVELOPMENT OF LEl'IDOSTKUS. 803 
 
 that it is moreover convenient to retain the term homocercal for 
 those types of caudal fin in which the dorsal lobe has atrophied 
 so far as not to project beyond the ventral lobe. 
 
 We have stated these now well-known facts to enable the 
 reader to follow us in dealing with the comparison between the 
 skeleton supporting the fin-rays of the ventral lobe of the caudal 
 fin, and that supporting the fin-rays of the remaining unpaired 
 fins. 
 
 It has been shewn that in Lepidosteus the unpaired fins fall 
 into two categories, according to the nature of the skeletal parts 
 supporting them. The fin-rays of the true ventral lobe of the 
 caudal fin are supported by the spinous processes of certain of 
 the haemal arches. The remaining unpaired fins, including the 
 anal fin, are supported by the so-called interspinous bones, 
 which are developed independently of the vertebral column arid 
 its arches. 
 
 The question which first presents itself is, how far does this 
 distinction hold good for other Fishes ? This question, though 
 interesting, does not appear to have been greatly discussed by 
 anatomists. Not unfrequently the skeletal supports of the 
 ventral lobe of the caudal fin are assumed to be the same as 
 those of the other fins. 
 
 Davidofif 1 , for instance, in speaking of the unpaired fins of 
 Elasmobranch embryos, says (p. 514): "The cartilaginous rays 
 of the dorsal fins agreed not only in number with the spinous 
 processes (as indeed is also found in the caudal fin of the full- 
 grown Dog-fish)," &c. 
 
 Thacker 2 , again, in his memoir on the Median and Paired 
 Fins, states at p. 284 : " We shall here consider the skeleton of 
 the dorsal and anal fins alone. That of the caudal fin has 
 undergone peculiar modifications by the union of fin-rays with 
 luemal spines." 
 
 Mivart 3 goes into the question more fully. He points out 
 (p. 471) that there is an essential difference between the dorsal 
 and ventral parts of the caudal fin in Elasmobranchs, in that in 
 
 1 " Beitrage z. vergl. Anat. d. hinteren Gliedmassen d. Fische," Morph. Jahrbuch, 
 Vol. v., 1879. 
 
 * J^rans. of the Connecticut Accui., Vol. III., 1877. 
 
 :l St George Mivart, "Fins of Elasmobranchs," Zool. Trans., Vol. X. 
 
804 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 the former the radials are more numerous than the vertebrae and 
 unconformable to them, while in the latter they are equal in 
 number to the vertebrae and continuous with them. " This," he 
 goes on to say, "seems to point to a difference in nature be- 
 tween the dorsal and ventral portions of the caudal fin, in at 
 least most Elasmobranchs." He further points out that Polyodon 
 resembles Elasmobranchs. As to Teleostei, he does not express 
 himself decidedly except in the case of Murcena, to which we 
 shall return. 
 
 Mivart expresses himself as very doubtful as to the nature of 
 the supports of the caudal fin, and thinks "that the caudal fin of 
 different kinds of Fishes may have arisen in different ways in 
 different cases." 
 
 An examination of the ventral part of the caudal fin in various 
 Ganoids, Teleostei, and Elasmobranchii appears to us to shew 
 that there can be but little doubt that, in the majority of the 
 members of these groups at any rate, and we believe in all, the 
 same distinction between the ventral lobe of the caudal fin and 
 the remaining unpaired fins is found as in Lepidosteus. 
 
 In the case of most Elasmobranchii, a simple inspection of 
 the caudal fin suffices to prove this, and the anatomical features 
 involved in this fact have usually been recognized ; though, in the 
 absence of embryological evidence, the legitimate conclusion has 
 not always been drawn from them. 
 
 The difference between the ventral lobe of the caudal fin and 
 the other fins in the mode in which the fin-rays are supported is 
 as obvious in Chondrostean Ganoids as it is in Elasmobranchii ; 
 it would appear also to hold good for Amia. Polypterus we have 
 had no opportunity of examining, but if, as there is no reason to 
 doubt, the figure of its skeleton given by Agassiz (Poissons 
 Fossiles) is correct, there can be no question that the ventral lobe 
 of the caudal fin is supported by the haemal arches, and not 
 by interspinous bones. In Calamoicthys, the tail of which we 
 have had an opportunity of dissecting through the kindness 
 of Professor Parker, the fin -rays of the ventral lobe of the 
 true caudal fin are undoubtedly supported by true haemal 
 arches. 
 
 There is no unanimity of opinion as to the nature of the 
 elements supporting the fin-rays of the caudal fin of Teleostei. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 805 
 
 Huxley 1 in his paper on the development of the caudal fin of 
 the Stickleback, holds that these elements are of the nature of 
 interhaemal bones. He says (p. 39) : " The last of these rings lay 
 just where the notochord began to bend up. It was slightly 
 longer than the bony ring which preceded it, and instead of 
 having its posterior margin parallel with the anterior, it sloped 
 from above downwards and backwards. Two short osseous 
 plates, attached to the anterior part of the inferior surface of the 
 penultimate ring, or rudimentary vertebral centrum, passed down- 
 wards and a little backwards, and abutted against a slender 
 elongated mass of cartilage. Similar cartilaginous bodies occupy 
 the same relation to corresponding plates of bone in the anterior 
 vertebrae in the region of the anal fin ; and it is here seen, that 
 while the bony plates coalesce and form the inferior arches of 
 the caudal vertebrae, the cartilaginous elements at their ex- 
 tremities become the interhsemal bones. The cartilage connected 
 with the inferior arch of the penultimate centrum is therefore an 
 " interhaemal " cartilage. The anterior part of the inferior surface 
 of the terminal ossification likewise has its osseous inferior arch, 
 but the direction of this is nearly vertical, and though it is con- 
 nected below with an element which corresponds in position 
 with the interhaemal cartilage, this cartilage is five or six times 
 as large, and constitutes a broad vertical plate, longer than it is 
 deep, and having its longest axis inclined downwards and back- 
 wards. . . . 
 
 " Immediately behind and above this anterior hypural apo- 
 physis (as it may be termed) is another very much smaller vertical 
 cartilaginous plate, which may be called the posterior hypural 
 apophysis." 
 
 We have seen that Mivart expresses himself doubtful on the 
 subject. Gegcnbaur 2 appears to regard them as haemal arches. 
 
 The latter view appears to us without doubt the correct one. 
 An examination of the tail of normal Teleostei shews that the 
 fin-rays of that part of the caudal fin which is derived from the 
 ventral lobe of the larva are supported by elements serially 
 homologous with the haemal arches, but in no way homologous 
 
 1 "Observations on the Development of some parts of the Skeleton of Fishes," 
 Quart. Journ. Micr. Science, Vol. VII., 1859. 
 
 - Elements of Comparative Anatomy. (Translation), p. 43 r. 
 
806 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 with the interspinous bones of the anal fin. The elements in 
 question formed of cartilage in the larva, become ossified in the 
 adult, and are known as the- hypural bones. They may appear 
 in the form of a series of separate haemal arches, corresponding 
 in number with the primitive somites of this region, which 
 usually, however, atrophy in the adult, or more often are from 
 the first imperfectly segmented, and have in the adult the form 
 of two or three or even of a single broad bony plate. The 
 transitional forms between this state of things and that, for 
 instance, in Lepidosteus are so numerous, that there can be no 
 doubt that even the most peculiar forms of the hypural bones of 
 Teleostei are simply modified haemal arches. 
 
 This view of the hypural bones is, moreover, supported by 
 embryological evidence, since Aug. Miiller 1 (p. 205) describes 
 their development in a manner which, if his statements are to be 
 trusted, leaves no doubt on this point. 
 
 There are a considerable number of Fishes which are not 
 provided with an obvious caudal fin as distinct from the remain- 
 ing unpaired fins, i.e. Chimaera, Eels, and various Eel-like forms 
 amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold 
 that these Fishes ought to be classed together in relation to the 
 structure of the caudal portion of their vertebral column, as he 
 says on p. 431 of his Comparative A natomy (English Translation): 
 " In the Chimaerae, Dipnoi, and many Teleostei, the caudal 
 portion of the vertebral column ends by gradually diminishing in 
 size, but in most Fishes, &c." 
 
 For our purpose it will, however, be advisable to treat them 
 separately. 
 
 The tail of Chimaera appears to us to be simply a peculiar 
 modification of the typical Elasmobranch heterocercal tail, in 
 which the true ventral lobe of the caudal fin may be recognized 
 in the fin-fold immediately in front of the filamentous portion of 
 the tail. In the allied genus Callorhynchus this feature is more 
 distinct. The filamentous portion of the tail of Chimaera con- 
 stitutes, according to the nomenclature adopted above, the true 
 dorsal lobe, and may be partially paralleled in the filamentous 
 dorsal lobe of the tail of the larval Lepidosteus (Plate 34, fig. 16). 
 
 1 " Beobachtungen zur vergl. Anat. d. Wirbelsaule," Miiller's Archiv, 1853. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 807 
 
 The tail of the eel-like Teleostei is again undoubtedly a 
 modification of the normal form of tail characteristic of the 
 Teleostei, in which, however, the caudal fin has become very 
 much reduced and merged into the prolongations of the anal and 
 dorsal fins. 
 
 This can be very clearly seen in Siluroid forms with an Eel- 
 like tail, such as Cnidoglanis. Although the dorsal and ventral 
 fins appear to be continuous round the end of the tail, and 
 there is superficially no distinct caudal fin, yet an examination 
 of the skeleton of Cnidoglanis shews that the end of the vertebral 
 column is modified in the usual Teleostean fashion, and that the 
 haemal arches of the modified portion of the vertebral column 
 support a small number of fin-rays ; the adjoining ventral fin- 
 rays being supported by independent osseous fin-supports (inter- 
 spinous bones). 
 
 In the case of the Eel (Anguilla anguilla} Huxley (loc. cit.} 
 long ago pointed out that the terminal portion of the vertebral 
 column was modified in an analogous fashion to that of other 
 Teleostei, and we have found that the modified haemal arches of 
 this part support a few fin-rays, though a still smaller number 
 than in Cnidoglanis. The fin-rays so supported clearly consti- 
 tute an aborted ventral lobe of the caudal fin. 
 
 Under these circumstances we think that the following state- 
 ment by Mivart (Zool. Trans. Vol. X., p. 471) is somewhat mis- 
 leading : 
 
 "As to the condition of this part (i.e. the ventral lobe of the 
 tail-fin) in Teleosteans generally, I will not venture as yet to 
 say anything generally, except that it is plain that in snch forms 
 as Murcsna, the dorsal and ventral parts of the caudal fin are 
 similar in nature and homotypal with ordinary dorsal and anal 
 fins\" 
 
 The italicized portion of this sentence is only true in respect 
 to that part of the fringe of fin surrounding the end of the body, 
 which is not only homotypal with, but actually part of, the 
 dorsal and anal fins. 
 
 Having settled, then, that the tails of Chimaera and of Eel- 
 like Teleostei arc simply special modifications of the typical 
 form of tail of the group of Fishes to which they respectively 
 
 1 The italics arc ours. 
 
808 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 belong, we come to the consideration of the Dipnoi, in which the 
 tail-fin presents problems of more interest and greater difficulty 
 than those we have so far had to deal with. 
 
 The undoubtedly very ancient and primitive character of the 
 Dipnoi has led to the view, implicitly if not definitely stated in 
 most text-books, that their tail-fin retains the character of the 
 piscine tail prior to the formation of the ventral caudal lobe, a 
 stage which is repeated embryologically in the pre-heterocercal 
 condition of the tail in ordinary Fishes. 
 
 Through the want of embryological data, and in the absence 
 of really careful histological examination of the tail of any of 
 the Dipnoi, we are not willing to speak with very great confi- 
 dence as to its nature ; we are nevertheless of the opinion that 
 the facts we can bring forward on this head are sufficient to 
 shew that the tail of the existing Dipnoi is largely aborted, so 
 that it is more or less comparable with that of the Eel. 
 
 We have had opportunities of examining the structure of the 
 tail of Ceratodus and Protopterus in dissected specimens in the 
 Cambridge Museum. The vertebral axis runs to the ends of 
 the tail without shewing any signs of becoming dorsally flexed. 
 At some distance from the end of the tail the fin-rays are sup- 
 ported by what are apparently segmented spinous prolongations 
 of the neural and haemal arches. The dorsal elements are 
 placed above the longitudinal dorsal cord, and occupy therefore 
 the same position as the independent elements of the neural 
 arches of Lepidosteiis. They are therefore to be regarded as 
 homologous with the dorsal fin-supports or interspinous bones 
 of other types. The corresponding ventral elements are there- 
 fore also to be regarded as interspinous bones. 
 
 In view of the fact that the fin-supports, whenever their 
 development has been observed, are found to be formed inde- 
 pendently of the neural and haemal arches, we may fairly assume 
 that this is also true for what we have identified as the inter- 
 spinous elements in the Dipnoi. 
 
 The interspinous elements become gradually shorter as the 
 end of the tail is approached, and it is very difficult from a 
 simple examination of dissected specimens to make out how far 
 any of the posterior fin-rays are supported by the haemal arches 
 only. To this question we shall return, but we may remark 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 809 
 
 that, although there is a prolongation backwards of the-verte- 
 bral axis beyond the last interspinous elements, composed it 
 would seem of the coalesced neural and haemal arches but 
 without the notochord, yet by far the majority of the fin-rays 
 which constitute the apparent caudal fin are supported by inter- 
 spinous elements. 
 
 The grounds on which we hold that the tail of the Dipnoi is 
 to be regarded as a degenerate rather than primitive type of tail 
 are the following : 
 
 (1) If it be granted that a diphycercal or protocercal form 
 of tail must have preceded a heterocercal form, it is also clear 
 that the ventral fin-rays of such a tail must have been supported, 
 as in Polypterus and Calamoicthys, by haemal arches, and not by 
 interspinous elements ; otherwise, a special ventral lobe, giving 
 a heterocercal character to the tail, and provided with fin-rays 
 supported only by haemal arches, could never have become 
 evolved from the protocercal tail-fin. Since the ventral fin-rays 
 of the tail of the Dipnoi are supported by interspinous elements 
 and not by haemal arches, this tail-fin cannot claim to have the 
 character of t/iat primitive type of diphycercal or protocercal 
 tail from which the heterocercal tail must be supposed to have 
 been evolved. 
 
 (2) Since the nearest allies of the Dipnoi are to be found in 
 Polypterus and the Crossopterygidae of Huxley, and since in 
 these forms (as evinced by the structure of the tail-fin of Polyp- 
 terus, and the transitional type between a heterocercal and 
 diphycercal form of fin observable in fossil Crossopterygidae) the 
 ventral fin-rays of the caudal fin were clearly supported by 
 haemal arches and not by interspinous elements, it is rendered 
 highly probable that the absence of fin-rays so supported in the 
 Dipnoi is a result of degeneration of the posterior part of the 
 tail. 
 
 [We use this argument without offering any opinion as to 
 whether the diphycercal character of the tail of many Crossop- 
 terygidae is primary or secondary.] 
 
 (3) The argument just used is supported by the degenerate 
 and variable state of the end of the vertebral axis in the Dipnoi 
 a condition most easily explained by assuming that the terminal 
 part of the tail has become aborted. 
 
 B. 52 
 
8 10 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 (4) We believe that in Ceratodus we have been able to trace 
 a small number of the ventral fin-rays supported by haemal 
 arches only, but these rays are so short as not to extend so 
 far back as some of the rays attached to the interspinous elements 
 in front. These rays may probably be interpreted, like the more 
 or less corresponding rays in the tail of the Eel, as the last 
 remnant of a true caudal fin. 
 
 The above considerations appear to us to shew with very 
 considerable probability that the true caudal fin of the Dipnoi 
 has become all but aborted like that of various Teleostei ; and 
 that the apparent caudal fin is formed by the anal and dorsal fins 
 meeting round the end of the stump of the tail. 
 
 From the adult forms of Dipnoi we are, however, of opinion 
 that no conclusion can be drawn as to whether their ancestors 
 were provided with a diphycercal or a heterocercal form of 
 caudal fin. 
 
 The general conclusions with reference to the tail-fin at which 
 we have arrived are the following : 
 
 (1) The ventral lobe of the tail-fin of Pisces differs from the 
 other unpaired fins in the fact that its fin- rays are directly 
 supported by spinous processes of certain of the haemal arches 
 instead of independently developed interspinous bones. 
 
 (2) The presence or absence of fin-rays in the tail-fin 
 supported by haemal arches may be used in deciding whether 
 apparently diphycercal tail-fins are aborted or primitive. 
 
 EXCRETORY AND GENERATIVE ORGANS. 
 
 I. Anatomy, 
 
 The excretory organs of Lepidosteus have been described by 
 Muller (No. 13) and Hyrtl (No. 11). These anatomists have 
 given a fairly adequate account of the generative ducts in the 
 female, and Hyrtl has also described the male generative ducts 
 and the kidney and its duct, but his description is contradicted 
 by our observations in some of the most fundamental points. 
 
 In the female example of ioo'5 centims. which we dissected, 
 the kidney forms a paired gland, consisting of a narrow strip of 
 glandular matter placed on each side of the vertebral column, on 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8ll 
 
 the dorsal aspect of the body-cavity. It is covered ^>n- its 
 ventral aspect by the oviduct and by its own duct, but is sepa- 
 rated from both of these by a layer of the tough peritoneal 
 membrane, through which the collecting tubes pass. It extends 
 forwards from the anus for about three-fifths of the length of 
 the body-cavity, and in our example had a total length of about 
 28 centims. (Plate 39, fig. 60, k}. Anteriorly the two kidneys 
 are separated by a short interval in the median line, but poste- 
 riorly they come into contact, and are so intimately united as 
 almost to constitute a single gland. 
 
 A superficial examination might lead to the supposition that 
 the kidney extended forwards for the whole length of the body- 
 cavity up to the region of the branchial arches, and Hyrtl appears 
 to have fallen into this error ; but what appears to be its anterior 
 continuation is really a form of lymphatic tissue, something like 
 that of the spleen, filled with numerous cells. This matter 
 (Plate 39, fig. 60, ly.) continues from the kidney forwards with- 
 out any break, and has a colour so similar to that of the kidney 
 as to be hardly distinguishable from it with the naked eye. The 
 true anterior end of the kidney is placed about 3 centims. in 
 front on the left side, and on the same level on the right side 
 as the wide anterior end of the generative duct (Plate 39, fig. 
 60, od.}. It is not obviously divided into segments, and is richly 
 supplied with malpighian bodies. 
 
 It is clear from the above description that there is no trace of 
 head-kidney or pronephros visible in the adult. To this subject 
 we shall, however, again return. 
 
 As will appear from the embryological section, the ducts 
 of the kidneys are probably simply the archinephric ducts, but 
 to avoid the use of terms involving a theory, we propose in the 
 anatomical part of our work to call them kidney ducts. They 
 are thin-walled widish tubes coextensive with the kidneys. If 
 cut open there may be seen on their inner aspect the numerous 
 openings of the collecting tubes of the kidneys. They are 
 placed ventrally to and on the outer border of the kidneys 
 (Plate 39, fig. 60, s.g.\ Posteriorly they gradually enlarge, and 
 approaching each other in the median line, coalesce, forming 
 an unpaired vesicle or bladder (bl.} about 6 centims. long in 
 our example opening by a median pore on a more or less 
 
 52 2 
 
8l2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 prominent papilla (u.g.} behind the anus. The dilated portions 
 of the two ducts are called by Hyrtl the horns of the bladder. 
 
 The sides of the bladder and its so-called horns are pro- 
 vided with lateral pockets into which the collecting tubes of the 
 kidney open. These pockets, which we have found in two 
 female examples, are much larger in the horns of the bladder 
 than in the bladder itself. Similar pockets, but larger than 
 those we have found, have been described by Hyrtl in the male, 
 but are stated by him to be absent in the female. It is clear 
 from our examples that this is by no means always the case. 
 
 Hyrtl states that the wide kidney ducts, of which his de- 
 scription differs in no material point from our own, suddenly 
 narrow in front, and, perforating the peritoneal lining, are con- 
 tinued forwards to supply the anterior part of the kidney. We 
 have already shewn that the anterior part of the kidney has no 
 existence, and the kidney ducts supplying it are, according to 
 our investigations, equally imaginary. 
 
 It was first shewn by Miiller, whose observations on this point 
 have been confirmed by Hyrtl, &c., that the ovaries of Lepidosteus 
 are continuous with their ducts, forming in this respect an 
 exception to other Ganoids. 
 
 In our example of Lepidosteus the ovaries (Plate 39, fig. 60, ov.) 
 were about 18 centims. in length. They have the form of simple 
 sacks, filled with ova, and attached about their middle to their 
 generative duct, and continued both backwards and forwards 
 from their attachment into a blind process. 
 
 With reference to these sacks Muller has pointed out and 
 the importance of this observation will become apparent when 
 we deal with the development that the ova are formed in the 
 thickness of the inner wall of the sack. We hope to shew that 
 the inner wall of the sack is alone equivalent to the genital ridge 
 of, for instance, the ovary of Scyllium. The outer aspect of 
 this wall i.e., that turned towards the interior of the sack is 
 equivalent to the outer aspect of the Elasmobranch genital ridge, 
 on which alone the ova are developed 1 . The sack into which 
 the ova fall is, as we shall shew in the embryological section, a 
 special section of the body-cavity shut off from the remainder, 
 
 1 Treatise on Comparative Embryology, Vol. i., p. 43 [the original edition]. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 813 
 
 and the dehiscence of the ova into this cavity is equivalent to 
 their discharge into the body-cavity in other forms. 
 
 The oviduct (Plate 39, fig. 60, od.} is a thin-walled duct of 
 about 21 centims. in length in the example we are describing, 
 continuous in front with the ovarian sack, and gradually tapering 
 behind, till it ends (od'.} by opening into the dilated terminal 
 section of the kidney duct on the inner side, a short distance 
 before the latter unites with its fellow. It is throughout closely 
 attached to the ureter and placed on its inner, and to some 
 extent on its ventral, aspect. The hindermost part of the oviduct 
 which runs beside the enlarged portion of the kidney duct 
 that portion called by Hyrtl the horn of the urinary bladder is 
 so completely enveloped by the wall of the horn of the urinary 
 bladder as to appear like a projection into the lumen of the 
 latter structure, and the somewhat peculiar appearance which 
 it presents in Hyrtl's figure is due to this fact. In our examples 
 the oviduct was provided with a simple opening into the kidney 
 duct, on a slight papilla ; the peculiar dilatations and processes 
 of the terminal parts of the oviduct, which have been described 
 by Hyrtl, not being present. 
 
 The results we have arrived at with reference to the male 
 organs are very different indeed from those of our predecessor, 
 in that we find the testicular products to be carried off by a series 
 of vasa cffercntia, which traverse tlte mesorchium, and are con- 
 tinuous with tlie nriniferous tubuli ; so tJiat tJie semen passes 
 througJi tlic uriniferous tubuli into the kidney duct and so to the 
 exterior. We have moreover been unable to find in tJu male a duct 
 homologous with the oviduct of the female. 
 
 This mode of transportation outwards of the semen has not 
 hitherto been known to occur in Ganoids, though found in all 
 Elasmobranchii, Amphibia, and Amniota. It is not, however, 
 impossible that it exists in other Ganoids, but has hitherto been 
 overlooked. 
 
 Our male example of Lepidostcns was about 60 centims. in 
 length, and was no doubt mature. It was smaller than any 
 of our female examples, but this according to Garman (vide, 
 p. 361) is usual. The testes (Plate 39, fig. 58 A. t.} occupied 
 a similar position to the ovaries, and were about 21 centims. 
 long. They were, as is frequently the case with piscine testes, 
 
8 14 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 divided into a series of lobes (10 12), and were suspended by 
 a delicate mesentery (mesorchium) from the dorsal wall of the 
 abdomen on each side of the dorsal aorta. Hyrtl (No. 11) 
 states that air or quicksilver injected between the limbs of the 
 mesentery, passed into a vas deferens homologous with the 
 oviduct which joins the ureter. We have been unable to find 
 such a vas deferens ; but we have found in the mesorchium a 
 number of tubes of a yellow colour, the colour being due to 
 a granular substance quite unlike coagulated blood, but which 
 appeared to us from microscopic examination to be the remains 
 of spermatozoa 1 . These tubes to the number of 40 50 con- 
 stitute, we believe, the vasa efferentia. Along the line of suspen- 
 sion of the testis on its inner border these tubes unite to form 
 an elaborate network of tubes placed on the inner face of the 
 testis an arrangement very similar to that often found in Elas- 
 mobranchii (vide F. M. Balfour, Monograph on the Development of 
 Elasmobranch Fislies, plate 20, figs. 4 and 8). 
 
 We have figured this network on the posterior lobe of the 
 testis (fig. 58 B), and have represented a- section through it 
 (fig. 59 A, n.v.e.), and through one of the vasa efferentia (v.e.) 
 in the mesorchium. Such a section conclusively demonstrates 
 the real nature of these passages : they are filled with sperm 
 like that in the body of the testis, and are, as may be seen 
 from the section figured, continuous with the seminal tubes of the 
 testis itself. 
 
 At the attached base of the mesorchium the vasa efferentia 
 unite into a longitudinal canal, placed on the inner side of the 
 kidney duct (Plate 39, fig. 58 A, I.e., also shewn in section in 
 Plate 39, fig. 59 B, I.e.). From this canal tubules pass off which 
 are continuous with the tubuli uriniferi, as may be seen from 
 fig. 59 B, but the exact course of these tubuli through the kidney 
 could not be made out in the preparations we were able to 
 make of the badly conserved kidney. Hyrtl describes the 
 arrangement of the vascular trunks in the mesorchium in the 
 following way (No. 11, p. 6): "The mesorchium contains vas- 
 cular trunks, viz., veins, which through their numerous anasto- 
 
 1 The females we examined, which were no doubt procured at the same time as 
 the male, had their oviducts filled with ova : and it is therefore not surprising that 
 the vasa efferentia should be naturally injected with sperm. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 815 
 
 moses form a plexus at the hilus of the testis, whose efferent 
 trunks, 13 in number, again unite into a plexus on the vertebral 
 column, which is continuous with the cardinal veins." The 
 arrangement (though not the number) of Hyrtl's vessels is 
 very similar to that of our vasa efferentia, and we cannot help 
 thinking that a confusion of the two may have taken place ; 
 which, in badly conserved specimens, not injected with semen, 
 would be very easy. 
 
 We have, as already stated, been unable to find in our dis- 
 sections any trace of a duct homologous with the oviduct of 
 the female, and our sections through the kidney and its ducts 
 equally fail to bring to light such a duct. The kidney ducts are 
 about 19 centims. in length, measured from the genital aperture 
 to their front end. These ducts are generally similar to those 
 in the female ; they unite about 2 centims. from the genital 
 pore to form an unpaired vesicle. Their posterior parts are 
 considerably enlarged, forming what Hyrtl calls the horns of 
 the urinary bladder. In these enlarged portions, and in the 
 wall of the unpaired urinary bladder, numerous transverse 
 partitions are present, as correctly described by Hyrtl, which are 
 similar to those in the female, but more numerous. They give 
 rise to a series of pits, at the blind ends of which are placed the 
 openings of the kidney tubules. The kidney duct without doubt 
 serves as vas deferens, and we have found in it masses of yellowish 
 colour similar to the substance in the vasa efferentia identified 
 by us as remains of spermatozoa. 
 
 II. Development. 
 
 In the general account of the development we have already 
 called attention to the earliest stages of the excretory system. 
 
 We may remind the reader that the first part of the system 
 to be formed is the segmental or archinephric duct (Plate 36, 
 figs. 28 and 29, sg^]. This duct arises, as in Teleostei and 
 Amphibia, by the constriction of a hollow ridge of the somatic 
 mesoblast into a canal, which is placed in contiguity with the 
 epiblast, along the line of junction between the mesoblastic 
 somites and the lateral plates of mesoblast. Anteriorly the duct 
 
8l6 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 does not become shut off from the body-cavity, and also bends 
 inwards towards the middle line. The inflected part of the duct 
 is the first rudiment of the pronephros, and very soon becomes 
 considerably dilated relatively to the posterior part of the duct. 
 
 The posterior part of each segmental duct acquires an opening 
 into the cloacal section of the alimentary tract. Apart from 
 this change, the whole of the ducts, except their pronephric 
 sections, remain for a long time unaltered, and the next changes 
 we have to speak of concern the definite establishment of the 
 pronephros. 
 
 The dilated incurved portion of each segmental duct soon 
 becomes convoluted, and by the time the embryo is about 10 
 millims. in length, but before the period of hatching, an important 
 change is effected in the relations of their peritoneal openings 1 . 
 
 Instead of leading into the body-cavity, they open into an 
 isolated chamber on each side (Plate 38, fig. $i,pr.c.}, which we 
 will call the pronephric chamber. The pronephric chamber is not, 
 however, so far as we can judge, completely isolated from the 
 body-cavity. We have not, it is true, detected with certainty at 
 this stage a communication between the two ; but in later stages, 
 in larvae of from 1 1 to 26 millims., we have found a richly ciliated 
 passage leading from the body-cavity into the pronephros on 
 each side (Plate 38, fig. 52, p.f.p^. We have not succeeded in 
 determining with absolute certainty the exact relations between 
 this passage and the tube of the pronephros, but we are inclined 
 to believe that it opens directly into the pronephric chamber just 
 spoken of. 
 
 As we hope to shew, this chamber soon becomes largely 
 filled by a vascular glomerulus. On the accomplishment of 
 these changes, the pronephros is essentially provided with all 
 the parts typically present in a segment of the mesonephros 
 (woodcut, fig. 4). There is a peritoneal tube (/) 2 , opening into 
 a vesicle (v) ; from near the neck of the peritoneal tube there 
 
 1 The change is probably effected somewhat earlier than would appear from our 
 description, but our specimens were not sufficiently well preserved to enable us to 
 speak definitely as to the exact period. 
 
 2 We feel fairly confident that there is only one pronephric opening on each side, 
 though we have no single series of sections sufficiently complete to demonstrate this 
 fact with absolute certainty. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 817 
 
 comes off a convoluted tube (pr.n.}, forming the main mass of 
 the pronephros, and ending in the segmental duct (sd.\ 
 
 FIG. 
 
 Diagrammatic views of the pronephros of Lepidostetis. 
 
 A, pronephros supposed to be isolated and seen from the side ; B, section through 
 the vesicle of the pronephros and the ciliated peritoneal funnel leading into it ; 
 pr.n., coiled tube of pronephros; s</., segmental or archinephric duct ; /, peri- 
 toneal funnel ; v., vesicle of pronephros ; &v., blood vessel of glomerulus ; 
 /., glomerulus. 
 
 The different parts do not, however, appear to have the same 
 morphological significance as those in the mesonephros. 
 
 Judging from the analogy of Teleostei, the embryonic structure 
 of whose pronephros is strikingly similar to that of Lepidosteus, 
 the two pronephric chambers into which the segmental ducts 
 open are constricted off sections of the body-cavity. 
 
8l8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 With the formation of the convoluted duct opening into the 
 isolated section of the body-cavity we may speak of a definite 
 pronephros as having become established. The pronephros is 
 placed, as can be made out in later stages, on the level of the 
 opening of the air-bladder into the throat. 
 
 The pronephros increases in size, so far as could be determined, 
 by the further convolution of the duct of which it is mainly 
 formed ; and the next change of importance which we have 
 noticed is the formation of a vascular projection into the pro- 
 nephric chamber, forming the glomerulus already spoken of 
 (vide woodcut, fig. 4, "/.), which is similar to that of the pronephros 
 of Teleostei. We first detected these glomeruli in an embryo of 
 about 15 millims., some days after hatching (Plate 38, fig. $2.,gl\ 
 but it is quite possible that they may be formed considerably 
 earlier. 
 
 In the same embryo in which the glomeruli were found we 
 also detected for the first time a mesonepkros consisting of a 
 series of isolated segmental or nephridial tubes, placed posteriorly 
 to the pronephros along the dorsal wall of the abdomen. 
 
 These were so far advanced at this stage that we are not in a 
 position to give any account of their mode of origin. They are, 
 however, formed independently of the segmental ducts, and in 
 the establishment of the junction between the two structures, 
 there is no outgrowth from the segmental duct to meet the 
 segmental tubes. We could not at this stage find peritoneal 
 funnels of the segmental tubes, though we have met with them 
 at a later stage (Plate 38, fig. 53, //.), and our failure to find 
 them at this stage is not to be regarded as conclusive against 
 their existence. 
 
 A very considerable space exists between the pronephros 
 and the foremost segmental tube of the mesonephros. The 
 anterior mesonephric tubes are, moreover, formed earlier than 
 the posterior. 
 
 In the course of further development, the mesonephric tubules 
 increase in size, so that there ceases to be an interval between 
 them, the mesonephros thus becoming a continuous gland. In 
 an embryo of 26 millims. there was no indication of the forma- 
 tion of segmental tubes to fill up the space between the pronephros 
 and mesonephros. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 The two segmental ducts have united behind into an unpaired 
 structure in an embryo of 1 1 millims. This structure is no doubt 
 the future unpaired urinogenital chamber (Plate 39, figs. 58 A, 
 and 60, bl.}. Somewhat later, the hypoblastic cloaca becomes 
 split into two sections, the hinder one receiving the coalesced 
 segmental ducts, and the anterior remaining continuous with the 
 alimentary tract. The opening of the hinder one forms the 
 urinogenital opening, and that of the anterior the anus. 
 
 In an older larva of about 5 '5 centims. the pronephros did 
 not exhibit any marked signs of atrophy, though the duct between 
 it and the mesonephros was somewhat reduced and surrounded 
 by the trabecular tissue spoken of in connection with the adult. 
 In the region between the pronephros and the front end of the 
 fully developed part of the mesonephros very rudimentary tubules 
 had become established. 
 
 The latest stage of the excretory system which we have studied 
 is in a young Fish of about 1 1 centims. in length. The special 
 interest of this stage depends upon the fact that the ovary is 
 already developed, and not only so, but the formation of the 
 oviducts has commenced, and their condition at this stage throws 
 considerable light on the obscure problem of their nature in the 
 Ganoids. 
 
 Unfortunately, the head of the young Fish had been removed 
 before it was put into our hands, so that it was impossible for us 
 to determine whether the pronephros was still present ; but as we 
 shall subsequently shew, the section of the segmental duct, 
 originally present between the pronephros and the front end of 
 the permanent kidney or mesonephros, has in any case dis- 
 appeared. 
 
 In addition to an examination of the excretory organs in 
 situ, which shewed little except the presence of the generative 
 ridges, we made a complete series of sections through the excre- 
 tory organs for their whole length (Plate 39, figs. 54 57). 
 
 Posteriorly these sections shewed nothing worthy of note, 
 the excretory organs and their ducts differing in no important 
 particular from these organs as we have described them in the 
 adult, except in the fact that the segmental ducts are not joined 
 by the oviducts. 
 
 Some little way in front of the point where the two segmental 
 
82O STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 ducts coalesce to form the urinary bladder, the genital ridge 
 comes into view. For its whole extent, except near its anterior 
 part (of which more hereafter) this ridge projects freely into the 
 body-cavity, and in this respect the young Fish differs entirely 
 from the adult. As shewn in Plate 39, figs. 56 and 57 (g.r.), it 
 is attached to the abdominal wall on the ventral side of, and near 
 the inner border of each kidney. The genital ridge itself has a 
 structure very similar to that which is characteristic of young 
 Elasmobranchii, and it may be presumed of young Fishes 
 generally. The free edge of the ridge is swollen, and this part 
 constitutes the true generative region of the ridge, while its dorsal 
 portion forms the supporting mesentery. The ridge itself is 
 formed of a central stroma and a germinal epithelium covering 
 it. The epithelium is thin on the whole of the inner aspect of 
 the ridge, but, just as in Elasmobranchii, it becomes greatly 
 thickened for a band-like strip on the outer aspect. Here, the 
 epithelium is several layers deep, and contains numerous primitive 
 germinal cells (p.o.}. 
 
 Though the generative organs were not sufficiently advanced 
 for us to decide the point with certainty, the structure of the 
 organ is in favour of the view that this specimen was a female, 
 and, as will be shewn directly, there can on other grounds be no 
 doubt that this is so. The large size of the primitive germinal 
 cells (primitive ova) reminded us of these bodies in Elasmo- 
 branchii. 
 
 In the region between the insertion of the genital ridge (or 
 ovary, as we may more conveniently call it) and the segmental 
 duct we detected the openings of a series of peritoneal funnels of 
 the excretory tubes (Plate 39 , fig. 57, /./), which clearly there- 
 fore persist till the young Fish has reached a very considerable 
 size. 
 
 As we have already said, the ovary projects freely into the 
 body-cavity for the greater part of its length. Anteriorly, how- 
 ever, we found that a lamina extended from the free ventral 
 edge of the ovary to the dorsal wall of the body-cavity, to which 
 it was attached on the level of the outer side of the segmental 
 duct. A somewhat triangular channel was thus constituted, the 
 inner wall of which was formed by the ovary, the outer by the 
 lamina just spoken of, and the roof by the strip of the peritoneum 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 821 
 
 of the abdominal wall covering that part of the ventral mirface 
 of the kidney in which the openings of the peritoneal funnels of 
 the excretory tubes are placed. The structure of this canal 
 will be at once understood by the section of it shewn in Plate 39, 
 
 fig- 55- 
 
 There can be no doubt that this canal is the commencing 
 ovarian sack. On tracing it backwards we found that the lamina 
 forming its outer wall arises as a fold growing upwards from the 
 free edge of the genital ridge meeting a downward growth of the 
 peritoneal membrane from the dorsal wall of the abdomen ; and 
 in Plate 39, fig. 56, these two laminae may be seen before they 
 have met. Anteriorly the canal becomes gradually smaller and 
 "smaller in correlation with the reduced size of the ovarian ridge, 
 and ends blindly nearly on a level with the front end of the 
 excretory organs. 
 
 It should be noted that, owing to the mode of formation of 
 the ovarian sack, the outer side of the ovary with the band of 
 thickened germinal epithelium is turned towards the lumen of 
 the sack; and thus the fact of the ova being formed on the 
 inner wall of the genital sack in the adult is explained, and the 
 comparison which we instituted in our description of the adult 
 between the inner wall of the genital sack and the free genital 
 ridge of Elasmobranchs receives its justification. 
 
 It is further to be noticed that, from the mode of formation 
 of the ovarian sack, the openings of the peritoneal funnels of the 
 excretory organs ought to open into its lumen; and if these 
 openings persist in the adult, they will no doubt be found in 
 this situation. 
 
 Before entering on further theoretical considerations with 
 reference to the oviduct, it will be convenient to complete our 
 description of the excretory organs at this stage. 
 
 When we dissected the excretory organs out, and removed 
 them from the body of the young Fish, we were under the im- 
 pression that they extended for the whole length of the body- 
 cavity. Great was our astonishment to find that slightly in 
 front of the end of the ovary both excretory organs and seg- 
 mental ducts grew rapidly smaller and finally vanished, and that 
 what we had taken to be the front part of the kidney was 
 nothing else but a linear streak of tissue formed of cells with 
 
822 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 peculiar granular contents supported in a trabecular work 
 (Plate 39, fig. 54). This discovery first led us to investigate 
 histologically what we, in common with previous observers, had 
 supposed to be the anterior end of the kidneys in the adult, and 
 to shew that they were nothing else but trabecular tissue with 
 cells like that of lymphatic glands. The interruption of the 
 segmental duct at the commencement of this tissue demonstrates 
 
 o 
 
 that if any rudiment of the pronephros still persists, it is quite 
 functionless, in that it is not provided with a duct. 
 
 Ill . Theoretical considerations. 
 
 There are three points in our observations on the urino- 
 genital system which appear to call for special remark. The 
 first of these concerns the structure and fate of the pronephros, 
 the second the nature of the oviduct, and the third the presence 
 of vasa efferentia in the male. 
 
 Although the history we have been able to give of the prone- 
 phros is not complete, we have nevertheless shewn that in most 
 points it is essentially similar to the pronephros of Teleostei. 
 In an early stage we find the pronephros provided with a peri- 
 toneal funnel opening into the body-cavity. At a later stage we 
 find that there is connected with the pronephros on each side, a 
 cavity the pronephric cavity into which a glomerulus projects. 
 This cavity is in communication on the one hand with the lumen 
 of the coiled tube which forms the main mass of the pronephros, 
 and on the other hand with the body-cavity by means of a 
 richly ciliated canal (woodcut, fig. 4, p. 817). 
 
 In Teleostei the pronephros has precisely the same charac- 
 ters, except that the cavity in which the glomerulus is placed is 
 without a peritoneal canal. 
 
 The questions which naturally arise in connection with the 
 pronephros are: (i) what is the origin of the above cavity with 
 its glomerulus ; and (2) what is the meaning of the ciliated canal 
 connecting this cavity with the peritoneal cavity ? 
 
 We have not from our researches been able to answer the 
 first of these questions. In Teleostei, however, the origin of this 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 823 
 
 cavity has been studied by Rosenberg 1 and Gotte 2 . According 
 to the account of the latter, which we have not ourselves con- 
 firmed but which has usually been accepted, the front end of the 
 scgmental duct, instead of becoming folded off from the body- 
 cavity, becomes included in a kind of diverticulum of the body- 
 cavity, which only communicates with the remainder of the 
 body-cavity by a narrow opening. On the inner wall of this 
 diverticulum a projection is formed which becomes a glomerulus. 
 At this stage in the development of the pronephros we have 
 essentially the same parts as in the fully formed pronephros of 
 Lepidosteus, the only difference being that the passage con- 
 necting the diverticulum containing the glomerulus with the 
 remainder of the body-cavity is short in Teleostei, and in Lepi- 
 dosteus forms a longish ciliated canal. In Teleostei the opening 
 into the body-cavity becomes soon closed. If the above com- 
 parison is justified, and if the development of these parts in 
 Lepidosteus takes place as it is described as doing in Tele- 
 ostei, there can, we think, be no doubt that the ciliated canal 
 of Lepidosteus, which connects the pronephric cavity with 
 the body-cavity, is a persisting communication between this 
 cavity and the body-cavity; and that Lepidosteus presents 
 in this respect a more primitive type of pronephros than 
 Teleostei. 
 
 It may be noted that in Lepidosteus the whole pronephros 
 has exactly the character of a single segmental tube of the 
 mesonephros. The pronephric cavity with its glomerulus is 
 identical in structure with a malpighian body. The ciliated 
 canal is similar in its relations to the peritoneal canal of such a 
 segmental tube, and the coiled portion of the pronephros re- 
 sembles the secreting part of the ordinary segmental tube. This 
 comparison is no doubt an indication that the pronephros is 
 physiologically very similar to the mesonephros, and so far 
 justifies Sedgwick's 3 comparison between the two, but it does 
 not appear to us to justify the morphological conclusions at 
 
 1 Rosenberg, Untersuch, ueb. d. Entwick. d. Teleoslierniere, Dorpat, 1867. 
 
 2 Gotte, Ent-wick. d. Unke, p. 826. 
 
 3 Sedgwick, " Early Development of the Wolffian Duct and anterior Wolffian 
 Tubules in the Chick; with some Remarks on the Vertebrate Excretory System," 
 Quart. Journ. of Micros. Science, Vol. xxi., 1881. 
 
824 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 which he has arrived, or to necessitate any modification in the 
 views on this subject expressed by one of us 1 . 
 
 The genital ducts of Ganoids and Teleostei have for some 
 time been a source of great difficulty to morphologists ; and any 
 contributions with- reference to the ontogeny of these structures 
 are of interest. 
 
 The essential point which we have made out is that the ante- 
 rior part of the oviduct of Lepidosteus arises by a fold of the 
 peritoneum attaching itself to the free edge of the genital ridge. 
 We have not, unfortunately, had specimens old enough to decide 
 how the posterior part of the oviduct is formed ; and although 
 in the absence of such stages it would be rash in the extreme to 
 speak with confidence as to the nature of this part of the duct, it 
 may be well to consider the possibilities of the case in relation 
 to other Ganoids and Teleostei. 
 
 The simplest supposition would be that the posterior part of 
 the genital duct had the same origin as the anterior, i.e., that it 
 was formed for its whole length by the concrescence of a peri- 
 toneal fold with the genital ridge, and that the duct so formed 
 opened into the segmental duct. 
 
 The other possible supposition is that a true Miillerian duct 
 i.e., a product of the splitting of the segmental duct is sub- 
 sequently developed, and that the open end of this duct coalesces 
 with the duct which has already begun to be formed in our 
 oldest larva. 
 
 In attempting to estimate the relative probability of these 
 two views, one important element is the relation of the oviducts 
 of Lepidosteus to those of other Ganoids. 
 
 In all other Ganoids (vide Hyrtl, No. 1 1) there are stated to 
 be genital ducts in both sexes which are provided at their ante- 
 rior extremities with a funnel-shaped mouth open to the abdo- 
 minal cavity. At first sight, therefore, it might be supposed 
 that they had no morphological relationship with the oviducts 
 of Lepidosteus, but, apart from the presence of a funnel-shaped 
 mouth, the oviducts of Lepidosteus are very similar to those of 
 Chondrostean Ganoids, being thin-walled tubes opening on a 
 projecting papilla into the dilated kidney ducts (ho'rns of the 
 
 1 F. M. Balfour, Comparative Embryology, Vol. n., pp. 600603 [the original 
 edition]. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 825 
 
 urinary bladder, Hyrtl). These relations seem to prove beyond 
 a doubt that the oviduct of Lepidostens is for its major part 
 homologous with the genital ducts of other Ganoids. 
 
 The relationship of the genital ducts to the kidney ducts in 
 Amia and Polypterus is somewhat different from that in the 
 Chondrostei and Lcpidosteus. In Amia the ureters are so small 
 that they may be described rather as joining the coalesced 
 genital ducts than vice versa, although the apparent coalesced 
 portion of the genital ducts is shewn to be really part of the 
 kidney ducts by receiving the secretion of a number of meso- 
 nephric tubuli. In Polypterus the two ureters are stated to 
 unite, and open by a common orifice into a sinus formed by the 
 junction of the two genital ducts, which has not been described 
 as receiving directly the secretion of any part of the meso- 
 nephros. 
 
 It has been usual to assume that the genital ducts of Ganoids 
 are true Miillerian ducts in the sense above defined, on the 
 ground that they are provided with a peritoneal opening and 
 that they are united behind with the kidney ducts. In the 
 absence of ontological evidence this identification is necessarily 
 provisional. On the assumption that it is correct we should 
 have to accept the second of the two alternatives above sug- 
 gested as to the development of the posterior parts of the oviduct 
 in Lepidostens. 
 
 There appear to us, however, to be sufficiently serious objec- 
 tions to this view to render it necessary for us to suspend our 
 judgment with reference to this point. In the first place, if the 
 view that the genital ducts are Mullerian ducts is correct, the 
 true genital ducts of Lepidosteus must necessarily be developed 
 at a later period than the secondary attachment between their 
 open mouths and the genital folds, which would, to say the least 
 of it, be a remarkable inversion of the natural order of develop- 
 ment. Secondly, the condition of our oldest larva shews that 
 the Mullerian duct, if developed later, is only split off from quite 
 the posterior part of the segmental duct ; yet in all types in 
 which the development of the Mullerian duct has been followed, 
 its anterior extremity, with the abdominal opening, is split off 
 from either the foremost or nearly the foremost part of the seg- 
 mental duct. 
 
 B- 53 
 
826 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Judging from the structure of the adult genital ducts of other 
 Ganoids they must also be developed only from the posterior 
 part of the segmental duct, and this peculiarity so struck one of 
 us that in a previous paper 1 the suggestion was put forward that 
 the true Ganoid genital ducts were perhaps not Mullerian ducts, 
 but enlarged segmental tubes with persisting abdominal funnels 
 belonging to the mesonephros. 
 
 If the possibility of the oviduct of Lepidostcus not being 
 a Mullerian duct is admitted, a similar doubt must also exist as 
 to the genital ducts of other Ganoids, and we must be prepared 
 to shew that there is a reasonable ground for scepticism on this 
 point. We would in this connexion point out that the second 
 of the two arguments urged against the view that the genital 
 duct of Lepidosteus is not a Mullerian duct applies with equal 
 force to the case of all other Ganoids. 
 
 The short funnel-shaped genital duct of the Chondrostei is 
 also very unlike undoubted Mullerian ducts, and could moreover 
 easily be conceived as originating by a fold of the peritoneum, 
 a slight extension of which would give rise to a genital duct-like 
 that of Lepidosteus. 
 
 The main difficulty of the view that the genital ducts of 
 Ganoids are not Mullerian ducts lies in the fact that they open 
 into the segmental duct. While it is easy to understand the 
 genesis of a duct from a folding of the peritoneum, and also easy 
 to understand how such a duct might lead to the exterior by 
 coalescing, for instance, with an abdominal pore, it is not easy 
 to see how such a duct could acquire a communication with the 
 segmental duct. 
 
 We do not under these circumstances wish to speak dog- 
 matically, either in favour of or against the view that the genital 
 ducts of Ganoids are Mullerian ducts. Their ontogeny would 
 be conclusive on this matter, and we trust that some of the 
 anatomists who have the opportunity of studying the develop- 
 ment of the Sturgeon will soon let us know the facts of the case. 
 If there are persisting funnels of the mesonephric segmental 
 tubes in adult Sturgeons, some of them ought to be situated 
 within the genital ducts, if the latter are not Mullerian ducts ; 
 
 1 F. M. Balfour, "On the Origin and History of the Urinogenital Organs of 
 Vertebrates," Journ. of Anat. and P/iys., Vol. X., 1876 [This edition, No. VII]. 
 
STRUCTURK AND DEVELOPMENT OF LEPIDOSTEUS. 827 
 
 and naturalists who have the opportunity ought also to look out 
 for such openings. 
 
 The mode of origin of the anterior part of the genital duct 
 of Lepidostcus appears to us to tell strongly in favour of the 
 view, already regarded as probable by one of us 1 , that the 
 Teleostean genital ducts are derived from those of Ganoids ; 
 and if, as appears to us indubitable, the most primitive type of 
 Ganoid genital ducts is found in the Chondrostei, it is interesting 
 to notice that the remaining Ganoids present in various ways 
 approximations to the arrangement typically found in Teleostei. 
 Lepidosteus obviously approaches Teleostei in the fact of the 
 ovarian ridge forming part of the wall of the oviduct, but differs 
 from the Teleostei in the fact of the oviduct opening into the 
 kidney ducts, instead of each pair of ducts having an independ- 
 ent opening in the cloaca, and in the fact that the male genital 
 products are not carried to the exterior by a duct homologous 
 with the oviduct. Aviia is closer to the Teleostei in the arrange- 
 ment of the posterior part of the genital ducts, in that the two 
 genital ducts coalesce posteriorly ; while Polypterus approaches 
 still nearer to the Teleostei in the fact that the two genital ducts 
 and the two kidney ducts unite with each other before they 
 join ; and in order to convert this arrangement into that charac- 
 teristic of the Teleostei we have only to conceive the coalesced 
 ducts of the kidneys acquiring an independent opening into the 
 cloaca behind the genital opening. 
 
 The male genital ducts. The discovery of the vasa efferentia 
 in Lepidosteus, carrying off the semen from the testis, and trans- 
 porting it to the mesonephros, and thence through the mesone- 
 phric tubes to the segmental duct, must be regarded as the most 
 important of our results on the excretory system. 
 
 It proves in the first place that the transportation outwards 
 of the genital products of both sexes by homologous ducts, 
 which has been hitherto held to be universal in Ganoids, and 
 which, in the absence of evidence to the contrary, must still 
 be assumed to be true for all Ganoids except Lepidosteus, is 
 a secondary arrangement. This conclusion follows from the 
 fact that in Elasmobranchs, &c., which are not descendants of 
 
 1 F. M. Balfour, Comparative Embryology, Vol. II., p. 605 [the original edition]. 
 
 532 
 
828 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 the Ganoids, the same arrangement of seminal ducts is found 
 as in Lepidosteus, and it must therefore have been inherited from 
 an ancestor common to the two groups. 
 
 If, therefore, the current statements about the generative 
 ducts of Ganoids are true, the males must have lost their vasa 
 efferentia, and the function of vas deferens must have been taken 
 by the homologue of the oviduct, presumably present in the 
 male. The Teleostei must, moreover, have sprung from Ganoidei 
 in which the vasa efferentia had become aborted. 
 
 Considerable phylogenetic difficulties as to the relationships 
 of Ganoidei and Elasmobranchii are removed by the discovery 
 that Ganoids were originally provided with a system of vasa 
 efferentia like that of Elasmobranchii. 
 
 THE ALIMENTARY CANAL AND ITS APPENDAGES. 
 
 I. Anatomy. 
 
 Agassiz (No. 2) gives a short description with a figure of the 
 viscera of Lepidosteus as a whole. Van der Hceven has also 
 given a figure of them in his memoir on the air-bladder of this 
 form (No. 8), and Johannes Miiller first detected the spiral valve 
 and gave a short account of it in his memoir (No. 13). Stan- 
 nius, again, makes several references to the viscera of Lepi- 
 dosteus in his anatomy of the Vertebrata, and throws some doubt 
 on Miiller's determination of the spiral valve. 
 
 The following description refers to a female Lepidosteus of 
 100-5 centims. (Plate 40, fig. 66). 
 
 With reference to the mouth and pharynx, we have nothing 
 special to remark. Immediately behind the pharynx there 
 comes an elongated tube, which is not divisible into stomach 
 and cesophagus, and may be called the stomach (st\ It is about 
 44-6 centims. long, and gradually narrows from the middle to- 
 wards the hinder or pyloric extremity. It runs straight back- 
 wards for the greater part of its length, the last 3*8 centims., 
 however, taking a sudden bend forwards. For about half its 
 length the walls are thin, and the mucous membrane is smooth ; 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 829 
 
 in the posterior half the walls are thick, and the mucous~mem- 
 brane is raised into numerous longitudinal ridges. The peculiar 
 glandular structure of the epithelium of this part in the embryo 
 is shewn in Plate 40, fig. 62 (st.}. Its opening into the duo- 
 denum is provided with a very distinct pyloric valve (/J.). 
 This valve projects into a kind of chamber, freely communi- 
 cating with the duodenum, and containing four large pits (c'}, 
 into each of which a group of pyloric caeca opens. These caeca 
 form a fairly compact gland (c.] about 6 - 5 centims. long, which 
 overlaps the stomach anteriorly, and the duodenum posteriorly. 
 
 Close to the pyloric valve, on its right side, is a small papilla, 
 on the apex of which the bile duct opens (b.d'}. 
 
 A small, apparently glandular, mass closely connected with 
 the bile duct, in the position in which we have seen the pancreas 
 in the larva (Plate 40, figs. 62 and 63, /.), is almost certainly a 
 rudimentary pancreas, like that of many Teleostei ; but its 
 preservation was too bad for histological examination. We be- 
 lieve that the pancreas of Lepidosteus has hitherto been over- 
 looked. 
 
 The small intestine passes straight backwards for about 
 8 centims., and then presents three compact coils. From the 
 end of these a section, about 5 centims. long, the walls of which 
 are much thicker, runs forwards. The intestine then again turns 
 backwards, making one spiral coil. This spiral part passes 
 directly, without any sharp line of demarcation, into a short and 
 straight tube, which tapers slightly from before backwards, and 
 ends at the anus. The mucous membrane of the intestine for 
 about the first 3^5 centims. is smooth, and the muscular walls 
 thin : the rest of the small intestine has thick walls, and the 
 mucous membrane is reticulated. 
 
 A short spiral valve (sp. v.}, with a very rudimentaiy epithelial 
 fold, making nearly two turns, begins in about the posterior half 
 of the spiral coil of the intestine, extending backwards for 
 slightly less than half the straight terminal portion of the in- 
 testine, and ending 4 centims. in front of the anus. Its total 
 length in one example was about 4*5 centims. 
 
 The termination of the spiral valve is marked by a slight 
 constriction, and we may call the straight portion of the in- 
 testine behind it the rectum (re.}. 
 
830 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 The posterior part of the intestine, from the beginning of the 
 spiral valve to the anus, is connected with the ventral wall of the 
 abdomen by a mesentery. 
 
 The air-bladder (a.b.} is 45 centims. long, and opens into the 
 alimentary canal by a slit-like aperture (a.fr.) on the median 
 dorsal line, immediately behind the epipharyngeal teeth. Each 
 lip of this aperture is largely formed by a muscular cushion, 
 thickest at its posterior end, and extending about 6 millims. 
 behind the aperture itself. A narrow passage is bounded by 
 these muscular walls, which opens dorsally into the air-bladder. 
 
 The air-bladder is provided with two short anterior cornua, 
 and tapers to a point behind : it shews no indication of any 
 separation into two parts. A strong band of connective tissue 
 runs along the inner aspect of its whole dorsal region, from 
 which there are given off on each side at intervals of about 
 12 millims. anteriorly, gradually increasing to 18 millims. pos- 
 teriorly bands of muscle, which pass outwards towards its side 
 walls, and then spread out into the numerous reticulations with 
 which the air-bladder is lined throughout. By the contraction 
 of these muscles the cavity of the air-bladder can doubtless be 
 very much diminished. 
 
 The main muscular bands circumscribe a series of more or 
 less complete chambers, which were about twenty- seven in 
 number on each side in our example. The chambers are con- 
 fined to the sides, so that there is a continuous cavity running 
 through the central part of the organ. The whole organ has the 
 characteristic structure of a simple lung. 
 
 The liver (lr.} consists of a single elongated lobe, about 32 
 centims. long, tapering anteriorly and posteriorly, the anterior 
 half being on the average twice as thick as the posterior half. 
 The gall-bladder (g.b.} lies at its posterior end, and is of con- 
 siderable size, tapering gradually so as to pass insensibly into 
 the bile duct The hepatic duct (hp.d.) opens into the gall- 
 bladder at its anterior end. 
 
 The spleen (s.) is a large, compact, double gland, one lobe 
 lying in the turn of the intestine immediately above the spiral 
 valve, and the other on the opposite side of the intestine, so that 
 the intestine is nearly embraced between the two lobes. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 831 
 
 II. Development. 
 
 We have already described in detail the first formation of 
 the alimentary tract so far as we have been able to work it out, 
 and we need only say here that the anterior and posterior ends 
 of the canal become first formed, and that these two parts 
 gradually elongate, so as to approach each other ; the growth of 
 the posterior part is, however, the most rapid. The junction of 
 the two parts takes place a very short distance behind the 
 opening of the bile duct into the intestine. 
 
 For some time after the two parts of the alimentary tract 
 have nearly met, the ventral wall of the canal at this point is 
 not closed ; so that there is left a passage between the alimentary 
 canal and the yolk-sack, which forms a vitelline duct. 
 
 After the yolk-sack has ceased to be visible as an external 
 appendage it still persists within the abdominal cavity. It has, 
 however, by this stage ceased to communicate with the gut, so 
 that the eventual absorption of the yolk is no doubt entirely 
 effected by the vitelline vessels. At these later stages of de- 
 velopment we have noticed that numerous yolk nuclei, like 
 those met with in Teleostei and Elasmobranchii 1 , are still to be 
 found in the yolk. 
 
 It will be convenient to treat the history of sections of the 
 alimentary tract in front of and behind the vitelline duct 
 separately. The former gives rise to the pharyngeal region, the 
 oesophagus, the stomach, and the duodenum. 
 
 The pharyngeal region, immediately after it has become 
 established, gives rise to a series of paired pouches. These may 
 be called the branchial pouches, and are placed between the 
 successive branchial arches. The first or hyomandibular pouch, 
 placed between the mandibular and hyoid arches, has rather 
 the character of a double layer of hypoblast than of a true 
 pouch, though in parts a slight space is developed between its 
 two walls. It is shewn in section in Plate 37, fig. 43 (//.;.), from 
 an embryo of about 10 millims., shortly before hatching. It 
 
 1 For a history of similar nuclei, vide Coif>. Emlyol., Vol. II., chapters III. 
 and iv. 
 
832 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 does not appear to undergo any further development, and, so far 
 as we can make out, disappears shortly after the embryo is 
 hatched, without acquiring an opening to the exterior. 
 
 It is important to notice that this cleft, which in the cartila- 
 ginous Ganoids and Polypterus remains permanently open as the 
 spiracle, is rudimentary even in the embryo of Lepidosteus. 
 
 The second pouch is the hyobranchial pouch : its outer end 
 meets the epiblast before the larva is hatched, and a perforation 
 is effected at the junction of the two layers, converting the pouch 
 into a visceral cleft. 
 
 Behind the hyobranchial pouch there are four branchial 
 pouches, which become perforated and converted into branchial 
 clefts shortly after hatching. 
 
 The region of the cesophagus following the pharynx is not 
 separated from the stomach, unless a glandular posterior region 
 (vide description of adult) be regarded as the stomach, a non- 
 glandular anterior region forming the cesophagus. The lumen 
 of this part appears to be all but obliterated in the stages im- 
 mediately before hatching, giving rise for a short period to a 
 solid cesophagus like that of Elasmobranchii and Teleostei 1 . 
 
 From the anterior part of the region immediately behind the 
 pharynx the air-bladder arises as a dorsal unpaired diverticulum. 
 From the very first it has an elongated slit-like mouth (Plate 40, 
 fig. 64, a.b'.}, and is placed in the mesenteric attachment of the 
 part of the throat from which it springs. 
 
 We have first noticed it in the stages immediately after 
 hatching. At first very short and narrow, it grows in succeeding 
 stages longer and .wider, making its way backwards in the 
 mesentery of the alimentary tract (Plate 40, fig. 65, a.b.). In 
 the larva of a month and a half old (26 millims.) it has still a 
 perfectly simple form, and is without traces of its adult lung-like 
 structure ; but in the larva of 1 1 centims. it has the typical adult 
 structure. 
 
 The stomach is at first quite straight, but shortly after the 
 larva is hatched its posterior end becomes bent ventralwards and 
 forwards, so that the flexure of its posterior end (present in the 
 adult) is very early established. The stomach is continuous be- 
 
 1 Vide Comf, Embryol., Vol. 11., pp. 5063 [the original edition]. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 833 
 
 hind with the duodenum, the commencement of which is indicated 
 by the opening of the bile duct. 
 
 The liver is the first-formed alimentary gland, and is already 
 a compact body before the larva is hatched. We have nothing 
 to say with reference to its development, except that it exhibits 
 the same simple structure in the embryo that it does in the 
 adult. 
 
 A more interesting glandular body is the pancreas. It has 
 already been stated that in the adult we have recognized a small 
 body which we believe to be the pancreas, but that we were 
 unable to study its histological characters. 
 
 In the embryo there is a well-developed pancreas which 
 arises in the same position and the same manner as in those 
 Vertebrata in which the pancreas is an important gland in the 
 adult. 
 
 We have first noticed the pancreas in a stage shortly after 
 hatching (Plate 40, fig. 6i,/.). It then has the form of a funnel- 
 shaped diverticulum of the dorsal wall of the duodenum, imme- 
 diately behind the level of the opening of the bile duct. From 
 the apex of this funnel numerous small glandular tubuli soon 
 sprout out. 
 
 The similarity in the development of the pancreas in Lepi- 
 dosteus to that of the same gland in Elasmobranchii is very 
 striking 1 . 
 
 The pancreas at a later stage is placed immediately behind 
 the end of the liver in a loop formed by the pyloric section of the 
 stomach (Plate 40, fig. 62,/.). During larval life it constitutes a 
 considerable gland, the anterior end of which partly envelopes 
 the bile duct (Plate 40, fig. 63,^.). 
 
 Considering the undoubted affinities between Lepidosteus and 
 the Teleostei, the facts just recorded with reference to the 
 pancreas appear to us to demonstrate that the small size and 
 occasional absence (?) of this gland in Teleostei is a result of the 
 degeneration of this gland ; and it seems probable that the 
 pancreas will be found in the larvae of most Teleostei. These 
 conclusions render intelligible, moreover, the great development 
 of the pancreas in the Elasmobranchii. 
 
 1 Vide F. M. Balfour, "Monograph on Development of Elasmobranch Fishes," 
 p. 226 [This edition, No. X., p. 454]. 
 
834 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 We have first noticed the pyloric caeca arising as outgrowths 
 of the duodenum in larvae of about three weeks old, and they 
 become rapidly longer and more prominent (Plate 40, fig. 62, c.}. 
 
 The portion of the intestine behind the vitelline duct is, as in 
 all the Vertebrata, at first straight. In Elasmobranchs the lumen 
 of the part of the intestine in which a spiral valve is present in 
 the adult, very early acquires a more or less semilunar form by 
 the appearance of a fold which winds in a long spiral. In Lepi- 
 dostens there is a fold similar in every respect (Plate 38, fig. 53, 
 sp.v.\ forming an open spiral round the intestine. This fold is 
 the first indication of the spiral valve, but it is relatively very 
 much later in its appearance than in Elasmobranchs, not being 
 formed till about three weeks after hatching. It is, moreover, in 
 correlation with the small extent of the spiral valve of the adult, 
 confined to a much smaller portion of the intestine than in 
 Elasmobranchii, although owing to the relative straightness of 
 the anterior part of the intestine it is proportionately longer in 
 the embryo than in the adult. 
 
 The similarity of the embryonic spiral valve of Lepidosteus to 
 that of Elasmobranchii shews that Stannius' hesitation in accept- 
 ing Miiller's discovery of the spiral valve in Lepidosteus is not 
 justified. 
 
 J. Muller (Ban u. Entwick. d. Myxinoideii) holds that the so- 
 called bursa entiana of Elasmobranchii (i.e., the chamber placed 
 between the part of the intestine with the spiral valve and the 
 end of the pylorus) is the homologue of the more elongated 
 portion of the small intestine which occupies a similar position 
 in the Sturgeon. This portion of the small intestine is no doubt 
 homologous with the still more elongated and coiled portion of 
 the small intestine in Lepidosteus placed between the chamber 
 into which the pyloric caeca, &c., open and the region of the 
 spiral valve. The fact that the vitelline duct in the embryo 
 Lepidosteus is placed close to the pyloric end of the stomach, and 
 that the greater portion of the small intestine is derived from 
 part of the alimentary canal behind this, shews that Muller is 
 mistaken in attempting to homologise the bursa entiana of 
 Elasmobranchii, which is placed in front of the vitelline duct, 
 with the coiled part of the small intestine of the above forms. 
 The latter is cither derived from an elongation of the very short 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 835 
 
 portion of the intestine between the vitelline duct and the" primi- 
 tive spiral valve, or more probably by the conversion of the 
 anterior part of the intestine, originally provided with a spiral 
 valve into a coiled small intestine not so provided. 
 
 We have already called attention to the peculiar mesentery 
 present in the adult attaching the posterior straight part of the 
 intestine to the ventral wall of the body. This mesentery, which 
 together with the dorsal mesentery divides the hinder section of 
 the body-cavity into two lateral compartments is, we believe, a 
 persisting portion of the ventral mesentery which, as pointed out 
 by one of us 1 , is primitively present for the whole length of the 
 body-cavity. The persistence of such a large section of it as 
 that found in the adult Lepidostens is, so far as we know, quite 
 exceptional. This mesentery is shewn in section in the embryo 
 in Plate 38, fig. 53 (?>.;///.). The small vessel in it appears to be 
 the remnant of the subintestinal vein. 
 
 THE GILL ON THE HYOID ARCH. 
 
 It is well known that Lepidostens is provided with a gill on 
 the hyoid arch, divided on each side into two parts. An excellent 
 figure of this gill is given by Muller (No. 13, plate 5, fig. 6), who 
 holds from a consideration of the vascular supply that the two 
 parts of this gill represent respectively the hyoid gill and the 
 mandibular gill (called by Muller pseudobranch). Miiller's views 
 on this subject have not usually been accepted, but it is the 
 fashion to regard the whole of the gill as the hyoid gill divided 
 into two parts. It appeared to us not improbable that embryo- 
 logy might throw some light on the history of this gill, and 
 accordingly we kept a look out in our embryos for traces of gills 
 on the hyoid and mandibular arches. The results we have arrived 
 at are purely negative, but are not the less surprising for this 
 fact. The hyomandibular cleft as shewn above, is never fully 
 developed, and early undergoes a complete atrophy a fact which 
 is, on the whole, against Miiller's view ; but what astonished us 
 most in connection with the gill in question is that we have been 
 
 1 Comparative Embryology, Vol. II. p. 514 [the original edition]. 
 
836 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 unable to find any trace of it even in the oldest larva whose head 
 we have had (26 millims.), and .at a period when the gills on the 
 hinder arches have reached their full development. 
 
 We imagined the gill in question to be the remnant of a gill 
 fully formed in extinct Ganoid types, and therefore expected to 
 find it better developed in the larva than in the adult. That the 
 contrary is the fact appears to us fairly certain, although we can- 
 not at present offer any explanation of it. 
 
 SYSTEMATIC POSITION OF LEPIDOSTEUS. 
 
 A. Agassiz concludes his memoir on the development of 
 Lepidosteus by pointing out that in spite of certain affinities in 
 other directions this form is " not so far removed from the bony 
 Fishes as has been supposed." Our own observations go far to 
 confirm Agassiz' opinion. 
 
 Apart from the complete segmentation, the general develop- 
 ment of Lepidosteus is strikingly Teleostean. In addition to the 
 general Teleostean features of the embryo and larva, which can 
 only be appreciated by those who have had an opportunity of 
 practically working at the subject, we may point to the following 
 developmental features 1 as indicative of Teleostean affinities : 
 
 (1) The formation of the nervous system as a solid keel of 
 the epiblast. 
 
 (2) The division of the epiblast into a nervous and epidermic 
 stratum. 
 
 (3) The mode of development of the gut (vide pp. 752 754). 
 
 (4) The mode of development of the pronephros ; though, 
 as shewn on p. 822, the pronephros of Lepidosteus has primitive 
 characters not retained by Teleostei. 
 
 (5) The early stages in the development of the vertebral 
 column (vide p. 779). 
 
 In addition to these, so to speak, purely embryonic characters 
 
 there are not a few important adult characters : 
 
 (i) The continuity of the oviducts with the genital glands. 
 
 1 The features enumerated above are not in all cases confined to Lepidosteus and 
 Teleostei, but are always eminently characteristic of the latter. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 837 
 
 (2) The small size of the pancreas, and the presence of 
 numerous so-called pancreatic caeca. 
 
 (3) The somewhat coiled small intestine. 
 
 (4) Certain characters of the brain, e.g.. the large size of 
 the cerebellum ; the presence of the so-called lobi inferiores 
 on the infundibulum ; and of tori semicirculares in the mid- 
 brain. 
 
 In spite of the undoubtedly important list of features to which 
 we have just called attention, a list containing not less important 
 characters, both embryological and adult, separating Lepidosteus 
 from the Teleostei, can be drawn up : 
 
 (1) The character of the truncus arteriosus. 
 
 (2) The fact of the genital ducts joining the ureters. 
 
 (3) The presence of vasa efferentia in the male carrying the 
 semen from the testes to the kidney, and through the tubules of 
 the latter into the kidney duct. 
 
 (4) The presence of a well-developed opercular gill. 
 
 (5) The presence of a spiral valve; though this character 
 may possibly break down with the extension of our knowledge. 
 
 (6) The typical Ganoid characters of the thalamencephalon 
 and the cerebral hemispheres (vide pp. 769 and 770). 
 
 (7) The chiasma of the optic nerves. 
 
 (8) The absence of a pecten, and presence of a vascular mem- 
 brane between the vitreous humour and the retina. 
 
 (9) The opisthoccelous form of the vertebrae. 
 
 (10) The articulation of the ventral parts of the haemal arches 
 of the tail with processes of the vertebral column. 
 
 (11) The absence of a division of the muscles into dorso- 
 lateral and ventro-lateral divisions. 
 
 (12) The complete segmentation of the ovum. 
 
 The list just given appears to us sufficient to demonstrate 
 that Lepidosteus cannot be classed with the Teleostei ; and we 
 hold that Muller's view is correct, according to which Lepidosteus 
 is a true Ganoid. 
 
 The existence of the Ganoids as a distinct group has, how- 
 ever, recently been challenged by so distinguished an Ichthyolo- 
 gist as Giinther, and it may therefore be well to consider how 
 far the group as defined by Mliller is a natural one for living 
 
838 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 forms 1 , and how far recent researches enable us to improve upon 
 Muller's definitions. In his classical memoir (No. 13) the charac- 
 ters of the Ganoids are thus shortly stated : 
 
 " These Fishes are either provided with plate-like angular or 
 rounded cement-covered scales, or they bear osseous plates, or 
 are quite naked. The fins are often, but not always, beset with 
 a double or single row of spinous plates or splints. The caudal 
 fin occasionally embraces in its upper lobe the end of the ver- 
 tebral column, which may be prolonged to the end of the upper 
 lobe. Their double nasal openings resemble those of Teleostei. 
 The gills are free, and lie in a branchial cavity under an oper- 
 culum, like those of Teleostei. Many of them have an accessory 
 organ of respiration, in the form of an opercular gill, which is 
 distinct from the pseudobranch, and can be present together 
 with the latter ; many also have spiracles like Elasmobranchii. 
 They have many valves in the stem of the aorta like the latter, 
 also a muscular coat in the stem of the aorta. Their ova are 
 transported from the abdominal cavity by oviducts. Their optic 
 nerves do not cross each other. The intestine is often provided 
 with a spiral valve, like Elasmobranchii. They have a swim- 
 ming-bladder with a duct, like many Teleostei. Their pelvic 
 fins are abdominal. 
 
 " If we include in a definition only those characters which 
 are invariable, the Ganoids may be shortly defined as being 
 those Fish with numerous valves to the stem of the aorta, which 
 is also provided with a muscular coat ; with free gills and an 
 operculum, and with abdominal pelvic fins." 
 
 To these distinctive characters, he adds in an appendix to 
 his paper, the presence of the spiral valve, and the absence of a 
 processus falciformis and a choroid gland. 
 
 To the distinctive set of characters given by Miiller we may 
 probably add the following : 
 
 (1) Oviducts and urinary ducts always unite, and open by a 
 common urinogenital aperture behind the anus. 
 
 (2) Skull hyostylic. 
 
 1 We do not profess to be able to discuss this question for extinct forms of Fish, 
 though of course it is a necessary consequence of the theory of descent that the various 
 groups should merge into each other as we go back in geological time. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 839 
 
 (3) Segmentation complete in the types so far investigated, 
 though perhaps Amia may be found to resemble the Teleostei in 
 this particular. 
 
 (4) A pronephros of the Teleostean type present in the larva. 
 
 (5) Thalamencephalon very large and well developed. 
 
 (6) The ventricle in the posterior part of the cerebrum is not 
 divided behind into lateral halves, the roof of the undivided part 
 being extremely thin. 
 
 (7) Abdominal pores always present. 
 
 The great number of characters just given are amply sufficient 
 to differentiate the Ganoids as a group ; but, curiously enough, 
 the only characters amongst the whole series which have been 
 given, which can be regarded as peculiar to the Ganoids, are (i) 
 the characters of the brain, and (2) the fact of the oviducts and 
 kidney ducts uniting together and opening by a common pore to 
 the exterior. 
 
 This absence of characters peculiar to the Ganoids is an indi- 
 cation of how widely separated in organization are the different 
 members of this great group. 
 
 At the same time, the only group with which existing Ganoids 
 have close affinities is the Teleostei. The points they have in 
 common with the Elasmobranchii are merely such as are due to 
 the fact that both retain numerous primitive Vertebrate charac- 
 ters 1 , and the gulf which really separates them is very wide. 
 
 There is again no indication of any close affinity between the 
 Dipnoi and, at any rate, existing Ganoids. 
 
 Like the Ganoids, the Dipnoi are no doubt remnants of a 
 very primitive stock ; but in the conversion of the air-bladder 
 into a true lung, the highly specialized character of their limbs 2 , 
 their peculiar autostylic skulls, the fact of their ventral nasal 
 openings leading directly into the mouth, their multisegmented 
 bars (interspinous bars), directly prolonged from the neural and 
 haemal arches and supporting the fin-rays of the unpaired dorsal 
 and ventral fins, and their well-developed cerebral hemispheres, 
 
 1 As instances of this we may cite (i) the spiral valve; (2) the frequent presence 
 of a spiracle; (3) the frequent presence of a communication between the pericardium 
 and the body-cavity ; (4) the heterocercal tail. 
 
 2 Vide F. M. Balfour, " On the Development of the Skeleton of the Paired Fins 
 of Elasmobranchs," Proc. Zool. Soc., 1881 [This edition, No. XX.]. 
 
840 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 very unlike those of Ganoids and approaching the Amphibian 
 type, they form a very well-defined group, and one very dis- 
 tinctly separated from the Ganoids. 
 
 No doubt the Chondrostean Ganoids are nearly as far re- 
 moved from the Teleostei as from the Dipnoi, but the links 
 uniting these Ganoids with the Teleostei have been so fully pre- 
 served in the existing fauna of the globe, that the two groups 
 almost run into each other. If, in fact, we were anxious to make 
 any radical change in the ordinary classification of Fishes, it 
 would be by uniting the Teleostei and Ganoids, or rather con- 
 stituting the Teleostei into one of the sub-groups of the Ganoids, 
 equivalent to the Chondrostei. We do not recommend such an 
 arrangement, which in view of the great preponderance of the 
 Teleostei amongst living Fishes would be highly inconvenient, 
 but the step from Amia to the Teleostei is certainly not so great 
 as that from the Chondrostei to Amia, and is undoubtedly less 
 than that from the Selachii to the Holocephali. 
 
 LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF 
 LEPIDOSTEUS. 
 
 1. Agassiz, A. "The Development of Lepidosteus." Part I., Proc. 
 Amer. A cad. Arts and Sciences, Vol. xiv. 1879. 
 
 2. Agassiz, L. Recherches s. I. Poissons Fossiles. Neuchatel. 1833 
 45. 
 
 3. Boas, J. E. " Ueber Herz u. Arterienbogen bei Ceradotus u. Protop- 
 terus," Morphol. Jahrbuch, Vol. VI. 1880. 
 
 4. Davidoff, M. von. " Beitrage z. vergleich. Anat. d. hinteren Glied- 
 massen d. Fische," Morphol. Jahrbuch, Vol. vi. 1880. 
 
 5. Gegenbaur, C. Uutersuch. z. vergleich. Anat. d. Wirbelthiere, 
 Heft II., Schnltergiirtel d. Wirbelthiere. Brustflosse der Fische. Leipzig, 
 1865. 
 
 6. Gegenbaur, C. " Zur Entwick. d. Wirbelsaule d. Lepidosteus, &c." 
 Jenaische Zeitschrift, Vol. ill. 1867. 
 
 7. Hertwig, O. "Ueber d. Hautskelet d. Fische (Lepidosteus u. 
 Polypterus)? Morphol. Jahrbuch, Vol. v. 1879. 
 
 8. Hceven, Van der. " Ueber d. zellige Schwimmblase d. Lepidostens? 
 M tiller's Archiv, 1841. 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 84! 
 
 9. Hyrtl, J. " Ueber d. Schwimmblase von Lepidosteus osseus" *>'//-. 
 d. Wiener Akad. Vol. vill. 1852. 
 
 10. Hyrtl, J. "Ueber d. Pori abdominales, d. Kiemen-Arterien, u. d. 
 Glandula thyroidea d. Ganoiden," Sitz. d. Wiener Akad. Vol. vm. 1852. 
 
 u. H y r 1 1, J . Ueber d. Zussammenhang d. Geschlechts u. Harnwerkzeuge 
 bei d. Ganoiden, Wien, 1855. 
 
 12. Kolliker, A. Ueber d. Ende d. Wirhelsaule b. Ganoiden, Leipzig, 
 1860. 
 
 13. Miille r, J. "Ueber d. Ban u. d. Grenzen d. Ganoiden," Berlin 
 Akad, 1844. 
 
 14. Schneider, H. "Ueber d. Augenmuskelnerven d. Ganoiden," 
 Jenaische Zeitschrift, Vol. XV. 1881. 
 
 15. Wilder, Burt G. "Notes on the North American Ganoi Is, Amia, 
 Lepidosteus, Acipenser, and Polyodon? Proc. Amer. Assoc.for the Advance- 
 ment of Science, 1875. 
 
 LIST OK REFERENCE LETTERS. 
 
 a. Anus, a b. Air-bladder, a b'. Aperture of air-bladder into throat, ac. An- 
 terior commissure, af. Anal fin. al. Alimentary canal, ao. Aorta, ar. Artery. 
 au. Auditory pit. b. Brain, be. Body-cavity, bd. Bile duct. bit. Aperture of 
 bile duct into duodenum, bl. Coalesced portion of segmental ducts, forming urino- 
 genital bladder. bra. Branchial arches, br c. Branchial clefts, c. Pyloric casca. 
 c'. Apertures of caeca into duodenum. cb. Cerebellum. c>fv. Cardinal vein. 
 ce. Cerebrum : in figs. 47 A and B, anterior lobe of cerebrum, ce'. Posterior lobe of 
 cerebrum, cf. Caudal fin. en. Centrum, ch. Choroidal fissure, crv. Circular 
 vein of vascular membrane of eye. csh. Cuticular sheath of notochord. cv. Caudal 
 vein. d. Duodenum, d c. Dorsal cartilage of neural arch. df. Dermal fin-rays. 
 dl. Dorsal lobe of caudal fin. dlf. Dorsal fin. e. Eye. ed. Epidermis, ep. Epi- 
 blast. fb. Fore-brain, fe. Pyriform bodies surrounding the zona radiata of the 
 ovum, probably the remains of epithelial cells, gb. Gall-bladder, gd. Genital duct. 
 gl. Glomerulus. gr. Genital ridge. h. Heart, ha. Haemal arch. hb. Hind- 
 brain, he. Head-cavity, hpd. Hepatic duct, h m. Hyomandibular cleft, hop. 
 Operculum. hy. Hypoblast ; in fig. 10, hyoid arch. hyl. Hyaloid membrane, ic. 
 Intercalated cartilaginous elements of the neural arches, in. Infundibulum. ir. Iris. 
 is. Interspinous cartilage or bones, iv. Sub-intestinal vein. ivr. Intervertebral 
 ring of cartilage, k. Kidney. /. Lens. le. Longitudinal canal, formed by union of 
 the vasa efierentia. I in. Lobi inferiores. //. Ligamentiun longitudinale supcrius. 
 Ir. Liver. It. Lateral line. ly. Lymphatic body in front of kidney, m. Mouth. 
 mb. Mid-brain. me. Medullary cord. met. Membrana elastica externa. mes. 
 Mesorchium. /. Mandible, md. and mo. Medulla oblongata. nis. Mesoblast. 
 n a. Neural arch. na'. Dorsal element of neural arch. nc. Notochord. nve. Net- 
 work formed by vasa efferentia on inner face of testis. od. Oviduct, oif. Aperture 
 of oviduct into bladder, ol. Nasal pit or aperture, olf. Olfactory lobe. op. Optic 
 vesicle, opch. Optic chiasma. of> I. Optic lobes, oplh. Optic thalami. or ef>. 
 
 B. 54 
 
842 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Oral epithelium, ov. Ovary. />. Pancreas, pc. Pericardium, pcf. Pectoral fin. 
 p ck. Pigmented layer of choroid. pf. Peritoneal funnel of segmental tube of meso- 
 nephros. pfp~ Peritoneal funnel leading into pronephric chamber, p g. Pectoral 
 girdle. /// Pelvic fin. pn. Pineal gland, po. Primitive germinal cells, pr. 
 Mesoblastic somite, prc. Pronephric chamber, prn. Pronephros. prri. Opening 
 of pronephros into pronephric chamber. //. Pituitary body. py. Pyloric valve. 
 / z. Parietal zone of blastoderm, r. Rostrum. rb. Rib. re. Rectum, s. Spleen. 
 s c. Seminal vessels passing from the longitudinal canal into the kidney, sd. Suc- 
 torial disc. sg. Segmental or archinephric duct. j t. Segmental tubules, sh. 
 Granular outer portion of the sheath of the notochord in the vertebral regions, s mx. 
 Superior maxillary process. s nc. Sub-notochordal rod. so. Somatic mesoblast. 
 sp. Splanchnic mesoblast. sp n. Spinal nerve, sp v. Spiral valve, st. Stomach. 
 s t. Seminal tubes of the testis. sup. Suctorial papillae, t. Testis. th. Thalamen- 
 cephalon. thl. Lobes of the roof of the thalamencephalon. tr. Trabeculse. tig. 
 Urinogenital aperture, v. Ventricle, v e. Vasa efferentia. v h. Vitreous humour. 
 v 1. Ventral lobe of the caudal fin. v mt. Ventral mesentery, v n. Vein. vs. Blood- 
 vessel, v sh. Vascular sheath between the hyaloid membrane and the vitreous 
 humour, v th. Vesicle of the thalamencephalon. x. Groove in epiblast, probably 
 formed in process of hardening, y. Yolk. z. Commissure in front of pineal gland. 
 z r. Outer striated portion of investing membrane (zona radiata) of ovum. zr*. Inner 
 non-striated portion of investing membrane of ovum. I. Olfactory nerve. II. Optic 
 nerve. III. Oculomotor nerve. V. Trigeminal nerve. VIII. Facial and auditory 
 
 EXPLANATION OF PLATES 3442. 
 PLATE 34. 
 
 Figs. 14. Different stages in the segmentation of the ovum. 
 Fig. i. Ovum with a single vertical furrow, from above. 
 Fig. 2. Ovum with two vertical furrows, from above. 
 Fig. 3. Side view of an ovum with a completely formed blastodermic disc. 
 Fig. 4. The same ovum as fig. 3, from below, shewing four vertical furrows 
 nearly meeting at the vegetative pole. 
 
 Fig 5 - 5 ro- External views of embryos up to time of hatching. 
 
 Fig. 5. Embryo, 3-5 millims. long, third day after impregnation. 
 Fig. 6. Embryo on the fifth day after impregnation. 
 Fig. 7. Posterior part of same embryo as fig. 6, shewing tail swelling. 
 Fig. 8. Embryo on the sixth day after impregnation. 
 Fig. 9. Embryo on the seventh day after impregnation. 
 Fig. 10. Embryo on the eleventh day after impregnation (shortly before 
 hatching). 
 
 Fig. ir. Head of embryo about the same age as fig. 10, ventral aspect. 
 
 Fig. 12. Side view of a larva about u millims. in length, shortly after hatching. 
 
 Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect. 
 
EXPLANATION OF PLATES 35, 36. 843 
 
 Fig. 14. Side view of a larva about 15 millims. long, five days after hatching. 
 Fig. 15. Head of a larva 23 millims. in length. 
 Fig. 16. Tail of a larva ir centims. in length. 
 
 Fig. 17. Transverse section through the egg-membranes of a just-laid ovum. 
 We are indebted to Professor W. K. Parker for figs. 12, 14 and 15. 
 
 PLATE 35. 
 
 Figs. 18 22. Transverse sections of embryo on the third day after impregnation. 
 
 Fig. 18. Through head, shewing the medullary keel. 
 
 Fig. 19. Through anterior part of trunk. 
 
 Fig. 20. Through same region as fig. 19, shewing a groove (x) in the 
 epiblast, probably artificially formed in the process of harden- 
 ing. 
 
 Fig. 21. Through anterior part of tail region, shewing partial fusion of 
 layers. 
 
 Fig. 22. Through posterior part of tail region, shewing more complete 
 fusion of layers than fig. 21. 
 
 Figs. 23 25. Transverse sections of an embryo on the fifth day after impregna- 
 tion. 
 
 Fig. 23. Through fore-brain and optic vesicles. 
 
 Fig. 24. Through hind-brain and auditory pits. 
 
 Fig. 25. Through anterior part of trunk. 
 
 Figs. 26 27. Tranverse sections of the head of an embryo on the sixth day after 
 impregnation. 
 
 Fig. 26. Through fore-brain and optic vesicles. 
 Fig. 27. Through hind-brain and auditoiy pits. 
 
 PLATE 36. 
 
 Figs. 28 29. Transverse sections of the trunk of an embryo on the sixth day 
 after impregnation. 
 
 Fig. 28. Through anterior part of trunk (from a slightly older embryo than 
 
 the other sections of this stage). 
 Fig. 29. Slightly posterior to fig. 28, shewing formation of segmental duct 
 
 as a fold of the somatic mesoblast. 
 
 Fig. 30. Longitudinal horizontal section of embryo on the sixth day after impreg- 
 nation, passing through the mesoblastic somites, notochord, and medullary canal. 
 
 Figs- 3 1 34- Transverse sections through an embryo on the seventh day after 
 impregnation. 
 
 Fig. 31. Through anterior part of trunk. 
 
 Fig. 32. Through the trunk somewhat behind fig. 31. 
 
 Fig. 33. Through tail region. 
 
 F 'g- 34- Further back than fig. 33, shewing constriction of tail from the 
 yolk. 
 
 542 
 
844 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Figs. 35 37. Transverse sections through an embryo on the eighth day after 
 impregnation. 
 
 Fig- 35- Through fore-brain and optic vesicles. 
 
 Fig. 36. Through hind-brain, shewing closed auditory pits, &c. 
 
 Fig. 37- Through anterior part of trunk. 
 
 Fig. 38. Section through tail of an embryo on the ninth day after impregnation. 
 
 PLATE 37. 
 
 Fig. 39. Section through the olfactory involution and part of fore-brain of a 
 larva on the ninth day after impregnation, shewing olfactory nerve. 
 
 Fig. 40. Section through the anterior part of the head of the same larva, shewing 
 pituitary involution. 
 
 Figs. 41 43. Transverse sections through an embryo on the eleventh day after 
 impregnation. 
 
 Fig. 41. Through fore-part of head, shewing the pituitary body still con- 
 nected with the oral epithelium. 
 Fig. 42. Slightly further back than fig. 41, shewing the pituitary body 
 
 constricted off from the oral epithelium. 
 
 Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution, eye, 
 and hyomandibular cleft. 
 
 Fig. 44. Longitudinal section of the head of an embryo of 15 millims. in length, 
 a few days after hatching, shewing the structure of the brain. 
 
 Fig. 45. Longitudinal section of the head of an embryo, about five weeks after 
 hatching, 26 millims. in length, shewing the structure of the brain. In the front part 
 of the brain the section passes slightly to one side of the median line. 
 
 Figs. 46 A to 46 G. Transverse sections through the brain of an embryo 25 
 millims. in length, about a month after hatching. 
 
 Fig. 46 A. Through anterior lobes of cerebrum. 
 
 Fig. 46 B. Through posterior lobes of cerebrum. 
 
 Fig. 46 C. Through thalamencephalon. 
 
 Fig. 46 D. Through optic thalami and optic chiasma. 
 
 Fig. 46 E. Through optic lobes and infundibulum. 
 
 Fig. 46 F. Through optic lobes and cerebellum. 
 
 Fig. 46 G. Through optic lobes and cerebellum, slightly in front of fig. 46 F. 
 
 PLATE 38. 
 
 Figs. 47 A, B, C. Figures of adult brain. 
 Fig. 47 A. From the side. 
 Fig. 47 B. From above. 
 Fig. 47 C. From below. 
 
 Fig. 48. Longitudinal vertical section through the eye of an embryo, about a 
 week after hatching, shewing the vascular membrane surrounding the vitreous 
 humour. 
 
EXPLANATION OF PLATES 38, 39. 845 
 
 Fig. 49. Diagram shewing the arrangement of the vessels in the vascular mem- 
 brane of the vitreous humour of adult eye. 
 
 Fig. 50. Capillaries of the same vascular membrane. 
 
 Fig. 51. Transverse section through anterior part of trunk of an embryo on the 
 ninth day after impregnation, shewing the pronephros and pronephric chamber. 
 
 Fig. 52. Transverse section through the region of the stomach of an embryo 
 15 millims. in length, shortly after hatching, to shew the glomerulus and peritoneal 
 funnel of pronephros. 
 
 Fig- 53- Transverse section through posterior part of the body of an embryo, 
 about a month after hatching, shewing the structure of the mesonephros, the spiral 
 valve, &c. 
 
 PLATE 39. 
 
 Figs. 54, 55, 56, and 57 are a series of transverse sections through the genital 
 ridge and mesonephros of one side from a larva of rr centims. 
 
 Fig. 54. Section of the lymphatic organ which lies in front of the mesone- 
 phros. 
 
 Fig- 55- Section near the anterior end of the mesonephros, where the 
 genital sack is completely formed. 
 
 Fig. 56. Section somewhat further back, shewing the mode of formation of 
 the genital sack. 
 
 Fig- 57. Section posterior to the above, the formation of the genital sack not 
 having commenced, and the genital ridge with primitive ger- 
 minal cells projecting freely into the body-cavity. 
 
 Fig. 58 A. View of the testis, mesorchium, and duct of the kidney of the left side 
 of an adult male example of Lepidosteus, 60 centims. in length, shewing the vasa 
 efferentia and the longitudinal canal at the base of the mesorchium. The kidney 
 ducts have been cut open posteriorly to shew the structure of the interior. 
 
 Fig. 58 B. Inner aspect of the posterior lobe of the testis from the same example, 
 to shew the vasa efferentia forming a network on the face of the testis. 
 
 Figs. 59 A and B. Two sections shewing the structure and relations of the 
 efferent ducts of the testis in the same example. 
 
 Fig. 59 A. Section through the inner aspect of a portion of the testis and 
 mesorchium, to shew the network of the vasa efferentia (n ve) 
 becoming continuous with the seminal tubes (st). The granu- 
 lar matter nearly filling the vasa efferentia and the seminal 
 tubes represent the spermatozoa. 
 
 Fig. 59 B. Section through part of the kidney and its duct and the longitu- 
 dinal canal (Ic) at the base of the mesorchium. Canals (s c) 
 are seen passing off from the latter, which enter the kidney and 
 join the uriniferous tubuli. Some of the latter (as well as the 
 seminal tubes) are seen to be filled with granular matter, 
 which we believe to be the remains of spermatozoa. 
 
846 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
 
 Fig. 60. Diagram of the urinogenital organs of the left side of an adult female 
 example of Lepidosteus 100 centims. in length. This figure shews the oviduct (od) 
 continuous with the investment of the ovary, opening at od' into the dilated part of 
 the kidney duct (segmental duct). It also shews the segmental duct and the junction 
 of the latter with its fellow of the right side to form the so-called bladder, this part 
 being represented as cut open. The kidney (k) and lymphatic organ (ly) in front of it 
 are also shewn. 
 
 PLATE 40. 
 
 Fig. 6 1. Transverse section through the developing pancreas (p) of a larva n 
 millims. in length. 
 
 Fig. 62. Longitudinal section through portions of the stomach, liver, and duode- 
 num of an embryo about a month after hatching, to shew the relations of the pancreas 
 (/) to the surrounding parts. 
 
 Fig- 63. External view of portions of the liver, stomach, duodenum. &c., of a 
 young Fish, n centims. in length, to shew the pancreas (/). 
 
 Fig. 64. Transverse section through the anterior part of the trunk of an embryo, 
 about a month after hatching, shewing the connection of the air-bladder with the 
 throat (a b'). 
 
 Fig. 65. Transverse section through the same embryo as fig. 64 further back, 
 shewing the posterior part of the air-bladder (a b). 
 
 Fig. 66. Viscera of an adult female, 100 centims. in length, shewing the alimen- 
 tary canal with its appended glands in natural position, and the air-bladder with its 
 aperture into the throat (a b'). The proximal part of the duodenum and the terminal 
 part of the intestine are represented as cut open, the former to shew the pyloric valve 
 and the apertures of the pyloric caeca and bile duct, and the latter to shew the spiral 
 valve. 
 
 This figure was drawn for us by Professor A. C. Haddon. 
 
 PLATE 41. 
 
 Fig. 67. Transverse section through the tail of an advanced larva, shewing the 
 neural and haemal processes, the independently developed interneural and interhsemal 
 elements (i s), and the commencing dermal fin-rays (df). 
 
 Fig. 68. Side view of the tail of a larva, 21 millims. in length, dissected so as to 
 shew the structure of the skeleton. 
 
 Fig. 69. Longitudinal horizontal section through the vertebral column of a larva, 
 5-5 centims. in length, on the level of the haemal arches, shewing the intervertebral 
 rings of cartilage continuous with the arches, the vertebral constriction of the noto- 
 chord, &c. 
 
 Figs. 70 and 71. Transverse sections through the vertebral column of a larva of 
 5 '5 centims. The red represents bone, and the blue cartilage. 
 
 Fig. 70. Through the vertebral region, shewing the neural and haemal 
 
 arches, the notochordal sheath, &c. 
 
 Fig. 71. Through the intervertebral region, shewing the intervertebral car- 
 tilage. 
 
EXPLANATION OF PLATES 41, 42. 847 
 
 Figs. 72 and 73. Transverse sections through the trunk of a larva of 5*5 centims. 
 to shew the structure of the ribs and haemal arches. 
 
 Fig. 72. Through the anterior part of the trunk. 
 Fig- 73- Through the posterior part of the trunk. 
 
 PLATE 42. 
 
 Figs. 74 76. Transverse sections through the trunk of the same larva as figs. 72 
 and 73. 
 
 Fig. 74. Through the posterior part of the trunk (rather further back than 
 
 % 73)- 
 
 Fig- 75- Through the anterior part of the tail. 
 Fig. 76. Rather further back than fig. 75. 
 
 Fig- 77- Longitudinal horizontal section through the vertebral column of a larva 
 of 1 1 centims., passing through the level of the haemal arches, and shewing the inter- 
 vertebral constriction of the notochord, the ossification of the cartilage, &c. 
 
 Fig. 78. Transverse section through a vertebral region of the vertebral column of 
 a larva 1 1 centims. in length. 
 
 Fig. 79. Transverse section through an intervertebral region of the same larva as 
 fig. 78. 
 
 Fig. 80. Side view of two trunk vertebrae of an adult Lepidosteus. 
 Fig. 8 r. Front view of a trunk vertebra of adult. 
 
 In figures 80 and 81 the red does not represent bone as in the other figures, but 
 simply the ligamentum longitudinale superius. 
 
XXIII. ON THE NATURE OF THE ORGAN IN ADULT TE- 
 
 LEOSTEANS AND GANOIDS, WHICH IS USUALLY RE- 
 GARDED AS THE HEAD-KIDNEY OR PRONEPHROs 1 . 
 
 WHILE working at the anatomy of Lepidosteus I was led to 
 doubt the accuracy of the accepted accounts of the anterior part 
 of the kidneys in this* and in allied species of Fishes. In order 
 to test my doubts I first examined the structure of the kidneys 
 in the Sturgeon (Acipenser), of which I fortunately had a well- 
 preserved specimen. 
 
 The bodies usually described as the kidneys consist of two 
 elongated bands, attached to the dorsal wall of the abdomen, 
 and extending for the greater part of the length of the abdo- 
 minal cavity. In front each of these bands first becomes con- 
 siderably narrowed, and then expands and terminates in a great 
 dilatation, which is usually called the head-kidney. Along the 
 outer border of the hinder part of each kidney is placed a wide 
 ureter, which ends suddenly in the narrow part of the body, 
 some little way behind the head-kidney. To the naked eye 
 there is no distinction in structure between the part of the so- 
 called kidney in front of the ureter and that in the region of the 
 ureter. Any section through the kidney in the region of the 
 ureter suffices to shew that in this part the kidney is really 
 formed of uriniferous tubuli with numerous Malpighian bodies. 
 Just in front, however, of the point where the ureter ends the 
 true kidney substance rapidly thins out, and its place is taken 
 by a peculiar tissue formed of a trabecular work filled with cells, 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. XXII., 1882. 
 
 2 I am about to publish, in conjunction with Mr Parker, a full account of the 
 anatomy and development of Lepidosteus [No. XXII. of this edition], and shall 
 therefore in this paper make no further allusion to it. 
 
I II. AD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 849 
 
 which I shall in future call lymphatic tissue. Thus tJiejwhole 
 of that part of the apparent kidney in front of the ureter, including 
 the whole of the so-called head-kidney, is simply a great mass of 
 lympJiatic tissue, and does not contain a single urinifcrous tubule 
 or Malpigldan body. 
 
 The difference in structure between the anterior and posterior 
 parts of the so-called kidney, although not alluded to in most 
 modern works on the kidneys, appears to have been known to 
 Stannius, at least I so interpret a note of his in the second edi- 
 tion of his Comparative Anatomy, p. 263, where he describes the 
 kidney of the Sturgeon as being composed of two separate parts, 
 viz. a spongy vascular substance (no doubt the so-called head- 
 kidney) and a true secretory substance. 
 
 After arriving at the above results with reference . to the 
 Sturgeon I proceeded to the examination of the structure of the 
 so-called head-kidney in Teleostei. 
 
 I have as yet only examined four forms, viz. the Pike (Esox 
 Indus), the Smelt (Osmerus eperlanus], the Eel (Anguilla an- 
 guilld], and the Angler (LopJiius piscatorius). 
 
 The external features of the apparent kidney of the Pike 
 have been accurately described by Hyrtl 1 . He says: "The 
 kidneys extend from the second trunk vertebra to the end of 
 the abdominal cavity. Their anterior extremities, which have 
 the form of transversely placed coffee beans, are united together, 
 and lie on the anterior end of the swimming bladder. The con- 
 tinuation of the kidney backwards forms two small bands, sepa- 
 rated from each other by the whole breadth of the vertebral 
 column. They gradually, however, increase in breadth, so that 
 about the middle of the vertebral column they unite together 
 and form a single symmetrical, keel-shaped body," &c. 
 
 The Pike I examined was a large specimen of about 58 
 centimetres in length, and with an apparent kidney of about 25^ 
 centimetres. The relations of lymphatic tissue and kidney 
 tissue were much as in the Sturgeon. The whole of the ante- 
 rior swelling, forming the so-called head-kidney, together with 
 a considerable portion of the part immediately behind, forming 
 not far short of half the whole length of the apparent kidney, 
 
 1 "Das UropoctUche System der Knochenfische," .v/':. <l. /r/V//. Aka<l., isuo. 
 
850 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 
 
 was entirely formed of lymphatic tissue. The posterior part of 
 the kidney was composed of true kidney substance, but even at 
 1 6 centimetres from the front end of the kidney the lymphatic 
 tissue formed a large portion of the whole. 
 
 A rudiment of the duct of the kidney extended forwards for 
 a short way into the lymphatic substance beyond the front part 
 of the functional kidney. 
 
 In the Smelt (Osmerus eperlanus] the kidney had the typical 
 Teleostean form, consisting of two linear bands stretching for 
 the whole length of the body-cavity, and expanding into a great 
 swelling in front on the level of the ductus Cuvieri, forming the 
 so-called head-kidney. The histological examination of these 
 bodies shewed generally the same features as in the case of the 
 Sturgeon and Pike. The posterior part was formed of the 
 usual uriniferous tubuli and Malpighian bodies. The anterior 
 swollen part of these bodies, and the part immediately follow- 
 ing, were almost wholly formed of a highly vascular lymphatic 
 tissue ; but in a varying amount in different examples portions 
 of uriniferous tubules were present, mainly, however, in the 
 region behind the anterior swelling. In some cases I could find 
 no tubules in the lymphatic tissue, and in all cases the number 
 of them beyond the region of the well-developed part of the 
 kidney was so slight, that there can be little doubt that they are 
 functionless remnants of the anterior part of the larval kidney. 
 Their continuation into the anterior swelling, when present, con- 
 sisted of a single tube only. 
 
 In the Eel (Anguilla anguilla), which, however, I have not 
 examined with the same care as the Smelt, the true excretory 
 part of the kidney appears to be confined to the posterior por- 
 tion, and to the portion immediately in front of the anus, the 
 whole of the anterior part of each apparent kidney, which is 
 not swollen in front, being composed of lymphatic tissue. 
 
 Lophius piscatorius is one of the forms which, according to 
 Hyrtl 1 , is provided with a head -kidney only, i.e. with that part 
 of the kidney which corresponds with the anterior swelling of 
 the kidney of other types. For this reason I was particularly 
 anxious to investigate the structure of its kidneys. 
 
 1 "Das Uropoetische System der Knochenfische," Sitz. d. Wien. Akad., 1850. 
 
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 851 
 
 Each of these bodies forms a compact oval mass, with the 
 ureter springing from its hinder extremity, situated in a forward 
 position in the body-cavity. Sections through the kidneys 
 shewed that they were throughout penetrated by uriniferous 
 tubules, but owing to the bad state of preservation of my speci- 
 mens I could not come to a decision as to the presence of 
 Malpighian bodies. The uriniferous tubules were embedded in 
 lymphatic tissue, similar to that which forms the anterior part of 
 the apparent kidneys in other Teleostean types. 
 
 With reference to the structure of the Teleostean kidneys, 
 the account given by Stannius is decidedly more correct than 
 that of most subsequent writers. In the note already quoted he 
 gives it as his opinion that there is a division of the kidney into 
 the same two parts as in the Sturgeon, viz. into a spongy 
 vascular part and a true secreting part ; and on a subsequent 
 page he points out the absence or poverty of the uriniferous 
 tubules in the anterior part of the kidney in many of our native 
 Fishes. 
 
 Prior to the discovery that the larvae of Teleosteans and 
 Ganoids were provided with two very distinct excretory organs, 
 viz. a pronephros or head-kidney, and a mesonephros or Wolf- 
 fian body, which are usually separated from each other by a 
 more or less considerable interval, it was a matter of no very 
 great importance to know whether the anterior part of the so- 
 called kidney was a true excretory organ. In the present state 
 of our knowledge the question is, however, one of considerable 
 interest. 
 
 In the Cyclostomata and Amphibia the pronephros is a 
 purely larval organ, which either disappears or ceases to be 
 functionally active in the adult state. 
 
 Rosenberg, to whom the earliest satisfactory investigations 
 on the development of the Teleostean pronephros are due, stated 
 that he had traced in the Pike (Esox Indus} the larval organ into 
 the adult part of the kidney, called by Hyrtl the pronephros ; 
 and subsequent investigators have usually assumed that the so- 
 called head-kidney of adult Teleosteans and Ganoids is the 
 persisting larval pronephros. 
 
 We have already seen that Rosenberg was entirely mistaken 
 on this point, in that the so-called head-kidney of the adult is 
 
852 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 
 
 not part of the true kidney. From my own studies on young 
 Fishes I do not believe that the oldest larvae investigated by 
 Rosenberg were sufficiently advanced to settle the point in 
 question ; and, moreover, as Rosenberg had no reason for doubt- 
 ing that the so-called head-kidney of the adult was part of the 
 excretory organ, he does not appear to have studied the histo- 
 logical structure of the organ which he identified with the em- 
 bryonic pronephros in his oldest larva. 
 
 The facts to which I have called attention in this paper 
 demonstrate that in the Sturgeon the larval pronephros un- 
 doubtedly undergoes atrophy before the adult stage is reached. 
 The same is true for Lepidosteus, and may probably be stated 
 for Ganoids generally. 
 
 My observations on Teleostei are clearly not sufficiently ex- 
 tensive to prove that the larval pronephros never persists in this 
 group. They appear to me, however, to shew that in the normal 
 types of Teleostei the organ usually held to be the pronephros 
 is actually nothing of the kind. 
 
 A different interpretation might no doubt be placed upon 
 my observations on Lophius piscatorius, but the position of the 
 kidney in this species appears to me to be far from affording a 
 conclusive proof that it is homologous with the anterior swelling 
 of the kidney of more normal Teleostei. 
 
 When, moreover, we consider that Lophius, and the other 
 forms mentioned by Hyrtl as being provided with a head-kidney 
 only, are all of them peculiarly modified and specialized types 
 of Teleostei, it appears to me far more natural to hold that their 
 kidney is merely the ordinary Teleostean kidney, which, like 
 many of their other organs, has become shifted in position, than 
 to maintain that the ordinary excretory organ present in other 
 Teleostei has been lost, and that a larval organ has been retained, 
 which undergoes atrophy in less specialized Teleostei. 
 
 As the question at present stands, it appears to me that the 
 probabilities are in favour of there being no functionally active 
 remains of the pronephros in adult Teleostei, and that in any 
 case the burden of proof rests with those who maintain that 
 such remnants are to be found. 
 
 The general result of my investigations is thus to render it 
 probable that the pronephros, though found in the larva or em- 
 
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 853 
 
 bryos of almost all the IchtJiyopsida, except the 'Elasmobmnehii, is 
 always a ptirely larval organ, which never constitutes an active 
 part of the excretory system in the adult state. 
 
 This conclusion appears to me to add probability to the view 
 of Gegenbaur that the pronephros is the primitive excretory 
 gland of the Chordata ; and that the mesonephros or Wolffian 
 body, by which it is replaced in existing Ichthyopsida, is phylo- 
 genetically a more recent organ. 
 
 In the preceding pages I have had frequent occasion to 
 allude to the lymphatic tissue which has been usually mistaken 
 for part of the excretory organ. This tissue is formed of tra- 
 becular work, like that of lymphatic glands, in the meshes of 
 which an immense number of cells are placed, which may fairly 
 be compared with the similarly placed cells of lymphatic glands. 
 In the Sturgeon a considerable number of cells are found with 
 peculiar granular nuclei, which are not found in the Teleostei. 
 In both groups, but especially in the Teleostei, the tissue is 
 highly vascular, and is penetrated throughout by a regular 
 plexus of very large capillaries, which appear to have distinct 
 walls, and which pour their blood into the posterior cardinal 
 vein as it passes through the organ. The relation of this tissue 
 to the lymphatic system I have not made out. 
 
 The function of the tissue is far from clear. Its great 
 abundance, highly vascular character, and presence before the 
 atrophy of the pronephros, appear to me to shew that it cannot 
 be merely the non-absorbed remnant of the latter organ. From 
 its size and vascularity it probably has an important function ; 
 and from its structure this must either be the formation of lymph 
 corpuscles or of blood corpuscles. 
 
 In structure it most resembles a lymphatic gland, though, till 
 it has been shewn to have some relation to the lymphatic system, 
 this can go for very little. 
 
 On the whole, I am provisionally inclined to regard it as a 
 form of lymphatic gland, these bodies being not otherwise repre- 
 sented in fishes. 
 
XXIV. A RENEWED STUDY OF THE GERMINAL LAYERS OF 
 THE CHICK. BY F. M. BALFOUR AND F. DEiGHTON 1 . 
 
 (With Plates 43, 44, 45-) 
 
 THE formation of the germinal layers in the chick has been 
 so often and so fully dealt with in recent years, that we consider 
 some explanation to be required of the reasons which have in- 
 duced us to add to the long list of memoirs on this subject. 
 Our reasons are twofold. In the first place the principal results 
 we have to record have already been briefly put forward in a 
 Treatise on Comparative Embryology by one of us ; and it seemed 
 desirable that the data on which the conclusions there stated 
 rest should be recorded with greater detail than was possible in 
 such a treatise. In the second place, our observations differ 
 from those of most other investigators, in that they were pri- 
 marily made with the object of testing a theory as to the nature 
 of the primitive streak. As such they form a contribution to 
 comparative embryology ; since our object has been to in- 
 vestigate how far the phenomena of the formation of the germinal 
 layers in the chick admit of being compared with those of lower 
 and less modified vertebrate types. 
 
 We do not propose to weary the reader by giving a new 
 version of the often told history of the views of various writers 
 on the germinal layers in the chick, but our references to other 
 investigators will be in the main confined to a comparison of 
 our results with those of two embryologists, who have published 
 their memoirs since our observations were made. One of them 
 is L. Gerlach, who published a short memoir 2 in April last, and 
 
 1 From the Quarterly Journal of Microscopical Science, Vol. xxn. N. S. 1882. 
 - " Ueb. d. entodermale Entstehungs\veise d. Chorda dorsalis," Biol. Centralblait, 
 Vol. I. Nos. i and 2. 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 855 
 
 the other is C. Koller, who has published his memoir 1 still- more 
 recently. Both of them cover part of the ground of our in- 
 vestigations, and their results are in many, though not in all 
 points, in harmony with our own. Both of them, moreover, lay 
 stress on certain features in the development which have escaped 
 our attention. We desired to worjc over these points again, but 
 various circumstances have prevented our doing so, and we have 
 accordingly thought it best to publish our observations as they 
 stand, in spite of their incompleteness, merely indicating where 
 the most important gaps occur. 
 
 Our observations commence at a stage a few hours after 
 hatching, but before the appearance of the primitive streak. 
 
 The area pellucida is at this stage nearly spherical. In it 
 there is a large oval opaque patch, which is continued to the 
 hinder border of the area. This opaque patch has received the 
 name of the embryonic shield a somewhat inappropriate name, 
 since the structure in question has no very definite connection 
 with the formation of the embryo. 
 
 Koller describes, at this stage, in addition to the so-called 
 embryonic shield, a sickle-shaped opaque appearance at the 
 hinder border of the area pellucida. 
 
 We have not made any fresh investigations for the purpose 
 of testing Roller's statements on this subject. 
 
 Embryologists are in the main agreed as to the structure of 
 the blastoderm at this stage. There is (PI. 43, Ser. A, I and 2) 
 the epiblast above, forming a continuous layer, extending over 
 the whole of the area opaca and area pellucida. In the former 
 its cells are arranged as a single row, and are cubical or slightly 
 flattened. In the latter the cells are more columnar, and form, 
 in the centre especially, more or less clearly, a double row ; 
 many of them, however, extend through the whole- thickness of 
 the layer. 
 
 We have obtained evidence at this stage which tends to shew 
 that at its outer border the epiblast grows not merely by the 
 division of its own cells, but also by the addition of cells derived 
 from the yolk below. The epiblast has been observed to extend 
 itself over the yolk by a similar process in many invertebrate forms. 
 
 1 " Untersuch. lib. d. Blatterbildung im Hiihnerkeim," Arfhiv f. niikr. Anat. 
 Vol. xx. i88r. 
 
856 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 Below the epiblast there is placed, in the peripheral part of 
 the area opaca, simply white yolk ; while in a ring immediately 
 outside and concentric with the area pellucida, there is a closely- 
 packed layer of cells, known as the germinal wall. The con- 
 stituent cells of this wall are in part relatively small, of a 
 spherical shape, with a distinct nucleus, and a granular and not 
 very abundant protoplasm ; and in part large and spherical, 
 filled up with highly refracting yolk particles of variable size, 
 which usually render the nucleus (which is probably present) 
 invisible (A, I and 2). This mass of cell rests, on its outer side, 
 on a layer of white yolk. 
 
 The sickle-shaped structure, visible in surface veins, is stated 
 by Roller to be due to a special thickening of the germinal wall. 
 We have not found this to be a very distinctly marked structure 
 in our sections. 
 
 In the region of the area pellucida there is placed below the 
 epiblast a more or less irregular layer of cells. This layer is 
 continuous, peripherally, with the germinal wall ; and is com- 
 posed of cells, which are distinguished both by their flattened 
 or oval shape and more granular protoplasm from the epiblast- 
 cells above, to which, moreover, they are by no means closely 
 attached. Amongst these cells a few larger cells are usually 
 present, similar to those we have already described as forming 
 an important constituent of the germinal wall. 
 
 We have figured two sections of a blastoderm of this age 
 (Ser. A, i and 2) mainly to shew the arrangement of these cells. 
 A large portion of them, considerably more flattened than the 
 remainder, form a continuous membrane over the whole of the 
 area pellucida, except usually for a small area in front, where 
 the membrane is more or less interrupted. This layer is the 
 hypoblast (fry.). The remaining cells are interposed between 
 this layer and the epiblast. In front of the embryonic shield 
 there are either comparatively few or none of these cells present 
 (Ser. A, i), but in the region of the embryonic shield they are 
 very numerous (Ser. A, 2), and are, without doubt, the main 
 cause of the opacity of this part of the area pellucida. These 
 cells may be regarded as not yet completely differentiated seg- 
 mentation spheres. 
 
 In many blastoderms, not easily distinguishable in surface 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 857 
 
 views from those which have the characters just described, the 
 hypoblastic sheet is often much less completely differentiated, 
 and we have met with other blastoderms, again, in which the 
 hypoblastic sheet was completely established, except at the 
 hinder part of the embryonic shield ; where, in place of it and 
 of the cells between it and the epiblast, there was only to be 
 found a thickish layer of rounded cells, continuous behind with 
 the germinal wall. 
 
 In the next stage, of which we have examined surface views 
 and sections, there is already a well-formed primitive streak. 
 
 The area pellucida is still nearly spherical, the embryonic 
 shield has either disappeared or become much less obvious, but 
 there is present a dark linear streak, extending from the pos- 
 terior border of the area pellucida towards the centre, its total 
 length being about one third, or even less, of the diameter of 
 the area. This streak is the primitive streak. It enlarges con- 
 siderably behind, where it joins the germinal wall. By Roller 
 and Gerlach it is described as joining the sickle-shaped struc- 
 ture already spoken of. We have in some instances found the 
 posterior end of the primitive streak extending laterally in the 
 form of two wings (PI. 45, fig. L). These extensions are, no 
 doubt, the sickle ; but the figures given by Koller appear to us 
 somewhat diagrammatic. One or two of the figures of early 
 primitive streaks in the sparrow, given by Kupffer and Benecke 1 , 
 correspond more closely with what we have found, except that 
 in these figures the primitive streak does not reach the end of 
 the area pellucida, which it certainly usually does at this early 
 stage in the chick. 
 
 Sections through the area pellucida (PI. 43, Sen B and c) 
 give the following results as to the structure of its constituent 
 parts. 
 
 The epiblast cells have undergone division to a considerable 
 extent, and in the middle part, especially, are decidedly more 
 columnar than at an earlier stage, and distinctly divided into two 
 rows, the nuclei of which form two more or less distinct layers. 
 
 In the region in front of the primitive streak the cells of the 
 lower part of the blastoderm have arranged themselves as a 
 
 1 " Photogramme d. Ontogenie d. Vogcl." Nova Acts. K. Leop. Carol, Dciits- 
 chen A had. d. Naturj\>r. Bd. X. 41, 1879. 
 
 B. 55 
 
858 RENEWED STUDY OF -GERMINAL LAYERS OF THE CHICK. 
 
 definite layer, the cells of which are not so flat as is the case 
 with the hypoblast cells of the posterior part of the blastoderm, 
 and in the older specimens of this stage they are very decidedly 
 more columnar than in the younger specimens. 
 
 The primitive streak is however the most interesting structure 
 in the area pellucida at this stage. 
 
 The feature which most obviously strikes the observer in 
 transverse sections through it is the fact, proved by Kolliker, that 
 it is mainly due to a proliferation of the epiblast cells along an 
 axial streak, which, roughly speaking, corresponds with the dark 
 line visible in surface views In the youngest specimens and at 
 the front end of the primitive streak, the proliferated cells do not 
 extend laterally beyond the region of their origin, but in the 
 older specimens they have a considerable lateral extension. 
 
 The hypoblast can, in most instances, be traced as a distinct 
 layer underneath the primitive streak, although it is usually less 
 easy to follow it in that region than elsewhere, and in some 
 cases it can hardly be distinctly separated from the superjacent 
 cells. 
 
 The cells, undoubtedly formed by a proliferation of the epi- 
 blast, form a compact mass extending downwards towards the 
 hypoblast ; but between this mass and the hypoblast there are 
 almost always present along the whole length of the primitive 
 streak a number of cells, more or less loosely arranged, and 
 decidedly more granular than the proliferated cells. Amongst 
 these loosely arranged cells there are to be found a certain 
 number of large spherical cells filled with yolk granules. Some- 
 times these cells are entirely confined to the region of the primi- 
 tive streak, at other times they are continuous laterally with cells 
 irregularly scattered between the hypoblast and epiblast (Ser.C,2), 
 which are clearly the remnants of the undifferentiated cells of 
 the embryonic shield. The junction between these cells and 
 the cells of the primitive streak derived from the epiblast is 
 often obscure, the two sets of cells becoming partially inter- 
 mingled. The facility with which the cells we have just spoken 
 of can be recognized varies moreover greatly in different in- 
 stances. In some cases they are very obvious (Sen c), while in 
 other cases they can only be distinguished by a careful ex- 
 amination of good sections. 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 859 
 
 The cells of the primitive streak between the epibTast and 
 the hypoblast are without doubt mesoblastic, and constitute the 
 first portion of the mesoblast which is established. The section 
 of these cells attached to the epiblast, in our opinion, clearly 
 originates from the epiblast ; while the looser cells adjoining 
 the hypoblast must, it appears to us, be admitted to have their 
 origin in the indifferent cells of the embryonic shield, placed 
 between the epiblast and the hypoblast, and also very probably 
 in a distinct proliferation from the hypoblast below the primitive 
 streak. 
 
 Posteriorly the breadth of the streak of epiblast which buds 
 off the cells of the primitive streak widens considerably, and in 
 the case of the blastoderm with the earliest primitive streaks 
 extends into the region of the area opaca. The widening of the 
 primitive streak behind is shewn in Ser. B, 3 ; Ser. C, 2 ; and Ser. 
 E, 4. Where very marked it gives rise to the sickle-shaped 
 appearance upon which so much stress has been laid by Roller 
 and Gerlach. In the case of one of the youngest of our blasto- 
 derms of this stage in which we found in surface views (PI. 45, 
 fig. L) a very well-marked sickle-shaped appearance at the hind 
 end of the primitive streak, the appearance was caused, as is 
 clearly brought out by our sections, by a thickening of the hypo- 
 blast of the germinal wall. 
 
 There is a short gap in our observations between the stage 
 with a young primitive streak and the first described stage in 
 which no such structure is present. This gap has been filled up 
 both by Gerlach and Koller. 
 
 Gerlach states that during this period a small portion of the 
 epiblast, within the region of the area opaca, but close to the 
 posterior border of the area pellucida, becomes thickened by a 
 proliferation of its cells. This portion gradually grows out- 
 wards laterally, forming in this way a sickle-shaped structure. 
 From the middle of this sickle a process next grows forward 
 into the area pellucida. This process is the primitive streak, 
 and it is formed, like the sickle, of proliferating epiblast cells. 
 
 Koller 1 described the sickle and the growth forwards from it 
 of the primitive streak in surface views somewhat before Gerlach; 
 
 1 " Beitr. z. Kenntniss d. Hiihnerkeims im Beginne d. Bebriitung," Sffs. d. k. 
 Akad. Wiss. \\. Abth. 1879. 
 
 552 
 
860 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 and in his later memoir has entered with considerable detail 
 into the part played by the various layers in the formation of 
 this structure. 
 
 He believes, as already mentioned, that the sickle-shaped 
 structure, which appears according to him at an earlier stage 
 than is admitted by Gerlach, is in the first instance due to a 
 thickening of the hypoblast. At a later stage he finds that the 
 epiblast in the centre of the sickle becomes thickened, and that 
 a groove makes its appearance in this thickening which he calls 
 the "Sichel-rinne." This groove is identical with that first 
 described by Kupffer and Benecke 1 in the sparrow and fowl. 
 We have never, however, found very clear indications of it in 
 our sections. 
 
 In the next stage, Koller states that, in the region immedi- 
 ately in front of the "Sichel-rinne," a prominence appears which 
 he calls the Sichelknopf, and from this a process grows forwards 
 which constitutes the primitive streak. This structure is in main 
 derived from a proliferation of epiblast cells, but Koller admits 
 that some of the cells just above the hypoblast in the region of 
 the Sichelknopf are probably derived from the hypoblast. Since 
 these cells form part of the mesoblast it is obvious that Keller's 
 views on the origin of the mesoblast of the primitive streak 
 closely approach those which we have put forward. 
 
 The primitive streak starting, as we have seen, at the hinder 
 border of the area pellucida, soon elongates till it eventually 
 occupies at least two-thirds of the length of the area. As Koller 
 \loc. cit.} has stated, this can only be supposed to happen in one 
 of two ways, viz. either by a progression forward of the region 
 of epiblast budding off mesoblast, or by an interstitial growth of 
 the area of budding epiblast. Koller adopts the second of these 
 alternatives, but we cannot follow him in doing so. The simplest 
 method of testing the point is by measuring the distance between 
 the front end of the primitive streak and the front border of the 
 area pellucida at different stages of growth of the primitive 
 streak. If this distance diminishes with the elongation of the 
 primitive streak then clearly the second of the two alternatives 
 is out of the question. 
 
 1 Die erstt Entvriek. an Eier d. Reptilim. Konigsbcrg, 1878. 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 86l 
 
 We have made measurements to test this point, and~find that 
 the diminution of the space between the front end of the primi- 
 tive streak and the anterior border of the area pellucida is very 
 marked up to the period in which the medullary plate first be- 
 comes established. We can further point in support of our view 
 to the fact that the extent of the growth lateralwards of the 
 mesoblast from the sides of the primitive streak is always less in 
 front than behind; which would seem to indicate that the front 
 part of the streak is the part formed latest. Our view as to the 
 elongation of the primitive streak appears to be that adopted by 
 Gerlach. 
 
 Our next stage includes roughly the period commencing 
 slightly before the first formation of a groove along the primi- 
 tive streak, known as the primitive groove, and terminating 
 immediately before the first trace of the notochord makes its 
 appearance. After the close of the last stage the primitive 
 streak gradually elongates, till it occupies fully two-thirds of 
 the diameter of the area pellucida. The latter structure also 
 soon changes its form from a circular to an oval, and finally 
 becomes pyriform with the narrow end behind, while the primi- 
 tive streak occupying two-thirds of its long axis becomes in most 
 instances marked by a light linear band along the centre, which 
 constitutes the primitive groove. 
 
 In surface views the primitive streak often appears to stop 
 short of the hinder border of the area pellucida. 
 
 During the period in which the external changes, which we 
 have thus briefly described, take place in the area pellucida, 
 great modifications are effected in the characters of the germinal 
 layers. The most important of these concern the region in front 
 of the primitive streak; but they will be better understood if we 
 commence our description with the changes in the primitive 
 streak itself. 
 
 In the older embryos belonging to our last stage we pointed 
 out that the mesoblast of the primitive streak was commencing 
 to extend outwards from the median line in the form of two 
 lateral sheets. This growth of the mesoblast is continued 
 rapidly during the present stage, so that during the latter part 
 of it any section through the primitive streak has approximately 
 the characters of Ser. I, 5. 
 
862 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 The mesoblast is attached in the median line to the epiblast. 
 Laterally it extends outwards to the edge of the area pellu- 
 cida, and in older embryos may even form a thickening beyond 
 the edge (fig. G). Beneath the denser part of the mesoblast, and 
 attached to the epiblast, a portion composed of stellate cells 
 may in the majority of instances be recognized, especially in the 
 front part of the primitive streak. We believe these stellate 
 cells to be in the main directly derived from the more granular 
 cells of the previous stage. The hypoblast forms a sheet of 
 flattened cells, which can be distinctly traced for the whole 
 breadth of the area pellucida, though closely attached to the 
 mesoblast above. 
 
 In sections we find that the primitive streak extends back 
 to the border of the area pellucida, and even for some distance 
 beyond. The attachment to the epiblast is wider behind; but 
 the thickness of the mesoblast is not usually greater in the 
 median line than it is laterally, and for this reason probably the 
 posterior part of the streak fails to shew up in surface views. 
 The thinning out of the median portion of the mesoblast of the 
 primitive streak is shewn in a longitudinal section of a duck's 
 blastoderm of this stage (fig. D). The same figure also shews 
 that the hypoblastic sheet becomes somewhat thicker behind, 
 and more independent of the parts above. 
 
 A careful study of the peripheral part of the area pellucida, 
 in the region of the primitive streak, in older embryos of this 
 stage, shews that the hypoblast is here thickened, and that its 
 upper part, i.e. that adjoining the mesoblast, is often formed 
 of stellate cells, many of which give the impression of being 
 in the act of passing into the mesoblast above. At a later 
 stage the mesoblast of the vascular area undoubtedly receives 
 accessions of cells from the yolk below; so that we see no 
 grounds for mistrusting the appearances just spoken of, or for 
 doubting that they are to be interpreted in the sense suggested. 
 
 We have already stated that during the greater part of the 
 present stage a groove, known as the primitive groove, is to be 
 found along the dorsal median line of the primitive streak. 
 
 The extent to which this groove is developed appears to be 
 subject to very great variation. On the average it is, perhaps, 
 slightly deeper than it is represented in Ser. I, 5. In some cases 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 863 
 
 it is very much deeper. One of the latter is represented in 
 fig. G. It has here the appearance of a narrow slit, and sec- 
 tions of it give the impression of the mesoblast originating 
 from the lips of a fold; in fact, the whole structure appears 
 like a linear blastopore, from the sides of which the mesoblast 
 is growing out ; and this as we conceive actually to be the true 
 interpretation of the structure. Other cases occur in which the 
 primitive groove is wholly deficient, or at the utmost repre- 
 sented by a shallow depression along the median axial line of a 
 short posterior part of the primitive streak. 
 
 We may now pass to the consideration of the part of the 
 area pellucida in front of the primitive streak. 
 
 We called attention to a change in the character of the hypo- 
 blast cells of this region as taking place at the end of the last 
 stage. During the very early part of this stage the change in 
 the character of these cells becomes very pronounced. 
 
 What we consider to be our earliest stage in this change we 
 have only so far met with in the duck, and we have figured a 
 longitudinal and median section to shew it (PI. 43, fig. D). The 
 hypoblast (hy) has become a thick layer of somewhat cubical 
 cells several rows deep. These cells, especially in front, are 
 characterized by their numerous yolk spherules, and give the 
 impression that part of the area pellucida has been, so to speak, 
 reclaimed from the area opaca. Posteriorly, at the front end of 
 the primitive streak, the thick layer of hypoblast, instead of being 
 continuous with the flattened hypoblast under the primitive streak, 
 falls, in the axial line, into the mesoblast of the primitive streak 
 (PI. 43, fig. D). 
 
 In a slightly later stage, of which we have specimens both of 
 the duck and chick, but have only figured selected sections of a 
 chick series, still further changes have been effected in the con- 
 stitution of the hypoblast (PI. 44, Ser. H, I and 2). 
 
 Near the front border of the area pellucida (i) it has the 
 general characters of the hypoblast of the duck's blastoderm just 
 described. Slightly further back the cells of the hypoblast have 
 become differentiated into stellate cells several rows deep, which 
 can hardly be resolved in t/ie axial line into hypoblast and meso- 
 blast, though one can fancy that in places, especially laterally, 
 they are partially differentiated into two layers. The axial 
 
864 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 sheet of stellate cells is continuous laterally with cubical hypo- 
 blast cells. 
 
 As the primitive streak is approached an axial prolongation 
 forwards of the rounded and closely-packed mesoblastic ele- 
 ments of the primitive streak is next met with ; and at the front 
 end of the primitive streak, where this prolongation unites with 
 the epiblast, it also becomes continuous with the stellate cells 
 just spoken of. In fact, close to the end of the primitive streak 
 it becomes difficult to say which mesoblast cells are directly 
 derived from the primitive layer of hypoblast in front of the 
 primitive streak, and which from the forward growth of the 
 mesoblast of the primitive streak. There is, in fact, as in the 
 earlier stage, a fusion of the layers at this point. 
 
 Sections of a slightly older chick blastoderm are represented 
 in PI. 45, Ser. I, i, 2, 3, 4 and 5. 
 
 Nearly the whole of the hypoblast in front of the primitive 
 streak has now undergone a differentiation into stellate cells. 
 In the second section the products of the differentiation of this 
 layer form a distinct mesoblast and hypoblast laterally, while in 
 the median line they can hardly be divided into two distinct 
 layers. 
 
 In a section slightly further back the same is true, except 
 that we have here, in the axial line above the stellate cells, 
 rounded elements derived from a forward prolongation of the 
 cells of the primitive streak. In the next section figured, pass- 
 ing through the front end of the primitive streak, the axial cells 
 have become continuous with the axial mesoblast of the primi- 
 tive streak, while below there is an independent sheet of flattened 
 hypoblast cells. 
 
 The general result of our observations on the part of the 
 blastoderm in front of the primitive streak during this stage is 
 to shew that the primitive hypoblast of this region undergoes 
 considerable changes, including a multiplication of its cells; and 
 that these changes result in its becoming differentiated on each 
 side of the middle line, with more or less distinctness, into (i) a 
 hypoblastic sheet below, formed of a single row of flattened cells, 
 and (2) a mesoblast plate above formed of stellate cells, while in 
 the middle line there is a strip of stellate cells in which there is 
 no distinct differentiation into two lavers. 
 
RENEWED STUDY OF GERMINAL LAYERS .OF THE CHICK. 865 
 
 Since the region in which these changes take place is-that in 
 which the medullary plate becomes subsequently formed, the 
 lateral parts of the mesoblast plate are clearly the permanent 
 lateral plates of the trunk, from which the mesoblastic somites, 
 &c., become subsequently formed ; so that the main part of the 
 mesoblast of the trunk is not directly derived from the primitive 
 streak. 
 
 Before leaving this stage we would call attention to the pre- 
 sence, in one of our blastoderms of this stage, of a deep pit at 
 the junction of the primitive streak with the region in front of it 
 (PI. 44, Ser. F, I and 2). Such a pit is unusual, but we think 
 it may be regarded as an exceptionally early commencement 
 of that most variable structure in the chick, the neurenteric 
 canal. 
 
 The next and last stage we have to deal with is that during 
 which the first trace of the notochord and of the medullary plate 
 make their appearance. 
 
 In surface views this stage is marked by the appearance of a 
 faint dark line, extending forwards, from the front end of the 
 primitive streak, to a fold, which has in the mean time made its 
 appearance near the front end of the area pellucida, and consti- 
 tutes the head fold. 
 
 PI. 45, Ser. K, represents a series of sections through a blas- 
 toderm of this stage, which have been selected to illustrate the 
 mode of formation of the notochord. 
 
 In a section immediately behind the head fold the median 
 part of the epiblast is thicker than the lateral parts, forming the 
 first indication of a medullary plate (Ser. K, i). Below the 
 median line of the epiblast is a small cord of cells, not divided 
 into two layers, but continuous laterally, both with the hypo- 
 blast and mesoblast, which are still more distinctly separated 
 than in the previous stage. 
 
 A section or so further back (Ser. K, 2) the axial cord, which 
 we need scarcely say is the rudiment of the notochord, is thicker, 
 and causes a slight projection in the epiblast above. It is, as 
 before, continuous laterally, both with the mesoblast and with 
 the hypoblast. The medullary plate is more distinct, and a 
 shallow but unmistakable medullary groove has made its ap- 
 pearance. 
 
866 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 As we approach the front end of the primitive streak the 
 notochord becomes (Ser. K, 3) very much more prominent, 
 though retaining the same relation to the germinal layers as in 
 front. 
 
 In the section immediately behind (Ser. K, 4) the convex 
 upper surface of the notochord has become continuous with the 
 epiblast for a very small region. The section, in fact, traverses 
 the front end of the primitive streak. 
 
 In the next section the attachment between the epiblast and 
 the cells below becomes considerably wider. It will be noticed 
 that this part of the primitive streak is placed on the floor of the 
 wide medullary groove, and there forms a prominence known as 
 the anterior swelling of the primitive streak. 
 
 It will further be noticed that in the two sections passing 
 through the primitive streak, the hypoblast, instead of simply 
 becoming continuous with the axial thickening of the cells, as in 
 front, forms a more or less imperfect layer underneath it. This 
 layer becomes in the sections following still more definite, and 
 forms part of the continuous layer of hypoblast present in the 
 region of the primitive streak. 
 
 A comparison of this stage with the previous one shews very 
 clearly that the notochord is formed out of the median plate of 
 cells of the earlier stage, which was not .divided into mesoblast 
 and -hypoblast, together with the short column of cells which 
 grew forwards from the primitive streak. 
 
 The notochord, from its mode of origin, is necessarily con- 
 tinuous behind with the axial cells of the primitive streak. 
 
 The sections immediately behind the last we have represented 
 shew a rudiment of the neurenteric canal of the same form as 
 that first figured by Gasser, viz. a pit perforating the epiblast 
 with a great mass of rounded cells projecting upwards through it. 
 
 The observations just recorded practically deal with two 
 much disputed points in the ontogeny of birds, viz. the origin of 
 the mesoblast and the origin of the notochord. 
 
 With reference to the first of these our results are briefly as 
 follows : 
 
 The first part of the mesoblast to be formed is that which 
 arises in connection with the primitive streak. This part is in 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 867 
 
 the main formed by a proliferation from an axial strip of the 
 epiblast along the line of the primitive streak, but in part also 
 from a simultaneous differentiation of hypoblast cells also along 
 the axial line of the primitive streak. The two parts of the 
 mesoblast so formed become subsequently indistinguishable. 
 The second part of the mesoblast to be formed is that which 
 gives rise to the lateral plates of mesoblast of the head and 
 trunk of the embryo. This part appears as two plates one on 
 each side of the middle line which arise by direct differentia- 
 tion from the hypoblast in front of the primitive streak. They 
 are continuous behind with the lateral wings of mesoblast 
 which grow out from the primitive streak, and on their inner 
 side are also at first continuous with the cells which form the 
 notochord. 
 
 In addition to the parts of mesoblast, formed as just de- 
 scribed, the mesoblast of the vascular area is in a large measure 
 developed by a direct formation of cells round the nuclei of the 
 germinal wall. 
 
 The mesoblast formed in connection with the primitive 
 streak gives rise in part to the mesoblast of the allantois, and 
 ventral part of the tail of the embryo (?), and in part to the 
 vascular structures found in the area pellucida. 
 
 With reference to the formation of the mesoblast of the 
 primitive streak, our conclusions are practically in harmony 
 with those of Roller ; except that Koller is inclined to mini- 
 mise the share taken by the hypoblast in the formation of the 
 mesoblast of the primitive streak. 
 
 Gerlach, with reference to the formation of this part of the 
 mesoblast, adopts the now generally accepted view of Kolliker, 
 according to which the whole of the mesoblast of the primitive 
 streak is derived from the epiblast. 
 
 As to the derivation of the lateral plates of mesoblast of the 
 trunk from the hypoblast of the anterior part of the primitive 
 streak, our general result is in complete harmony with Gerlach's 
 results, although in our accounts of the details of the process we 
 differ in some not unimportant particulars. 
 
 As to the origin of the notochord, our main result is that 
 this structure is formed as an actual thickening of the primitive 
 hypoblast of the anterior part of the area pellucida. We find 
 
868 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 that it unites posteriorly with a forward growth of the axial 
 tissue of the primitive streak, while it is laterally continuous, at 
 first, both with the mesoblast of the lateral plates and with the 
 hypoblast. At a later period its connection with the mesoblast 
 is severed, while the hypoblast becomes differentiated as a con- 
 tinuous layer below it. 
 
 As to the hypoblastic origin of the notochord, we are again 
 in complete accord with Gerlach ; but we differ from him in 
 admitting that the notochord is continuous posteriorly with the 
 axial tissue of the primitive streak, and also at first continuous 
 with the lateral plates of mesoblast. 
 
 The account we have given of the formation of the mesoblast 
 may appear to the reader somewhat fantastic, and on that ac- 
 count not very credible. We believe, however, that if the view 
 which has been elsewhere urged by one of us, that the primitive 
 streak is the homologue of the blastopore of the lower ver- 
 tebrates is accepted, the features we have described receive an 
 adequate explanation. 
 
 The growth outwards of part of the mesoblast from the axial 
 line of the primitive streak is a repetition of the well-known 
 growth from the lips of the blastopore. It might have been 
 anticipated that all the layers would fuse along the line of the 
 primitive streak, and that the hypoblast as well as part of the 
 mesoblast would grow out from it. There is, however, clearly a 
 precocious formation of the hypoblast ; but the formation of the 
 mesoblast of the primitive streak, partly from the epiblast and 
 partly from the hypoblast, is satisfactorily explained by re- 
 garding the whole structure as the blastopore. The two parts 
 of the mesoblast subsequently become indistinguishable, and 
 their difference in origin is, on the above view, to be regarded as 
 simply due to a difference of position, and not as having a deeper 
 significance. 
 
 The differentiation 'of the lateral plates of mesoblast of the 
 trunk directly from the hypoblast is again a fundamental feature 
 of vertebrate embryology, occurring in all types from Am- 
 phioxus upwards, the meaning of which has been fully dealt 
 with in the Treatise on Comparative Embryology by one of us. 
 Lastly, the formation of the notochord from the hypoblast is 
 the typical vertebrate mode of formation of this organ, while 
 
EXPLANATION OF PLATES. 869 
 
 the fusion of the layers at the front end of the primitive 
 streak is the universal fusion of the layers at the dorsal lip 
 of the blastopore, which is so well known in the lower verte- 
 brate types. 
 
 EXPLANATION OF PLATES 4345. 
 N. B. The series of sections are in all cases numbered from before backwards. 
 
 LIST OF REFERENCE LETTERS. 
 
 a. p. Area pellucida. ep. Epiblast. ch. Notochord. gr. Germinal wall. hy. 
 Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s. 
 Primitive streak, yk. Yolk of germinal wall. 
 
 PLATE 43. 
 
 SERIES A, i and 2. Sections through the blastoderm before the appearance of 
 primitive streak. 
 
 i. Section through anterior part of area pellucida in front of embryonic 
 shield. The hypoblast here forms an imperfect layer. The figure represents about 
 half the section. 2. Section through same blastoderm, in the region of the embry- 
 onic shield. Between the epiblast and hypoblast are a number of undifferentiated 
 cells. The figure represents considerably more than half the section. 
 
 SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primi- 
 tive streak. 
 
 i. Section through the anterior part of the area pellucida in front of the 
 primitive streak, i. Section through about the middle of the primitive streak. 
 3. Section through the posterior part of the primitive streak. 
 
 SERIES C, i and 2. Sections through a blastoderm with a young primitive streak, 
 i. Section through the front end of the primitive streak. 2. Section through 
 the primitive streak, somewhat behind r. Both figures shew very clearly the differ- 
 ence in character between the cells of the epiblastic mesoblast of the primitive streak, 
 and the more granular cells of the mesoblast derived from the hypoblast. 
 
 FIG. D. Longitudinal section through the axial line of the primitive streak, and 
 the part of the blastoderm in front of it, of an embryo duck with a well-developed 
 primitive streak. 
 
 PLATE 44. 
 
 SERIES E, 1,2, 3 and 4. Sections through blastoderm with a primitive streak, 
 towards the end of the first stage. 
 
 i. Section through the anterior part of the area pellucida. 2. Section a little 
 way behind i shewing a forward growth of mesoblast from the primitive streak. 3. 
 Section through primitive streak. 4. Section through posterior part of primitive 
 streak, shewing the great widening of primitive streak behind. 
 
87 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 
 
 SERIES F, i and 2. Sections, through a blastoderm with primitive groove. 
 
 i. Section shewing a deep pit in front of primitive streak, probably an early 
 indication of the neurenteric canal. 2. Section immediately following i. 
 
 FIG. G. Section through blastoderm with well-developed primitive streak, shew- 
 ing an exceptionally deep slit-like primitive groove. 
 
 SERIES H, i and i. Sections through a blastoderm with a fully-developed primi- 
 tive streak. 
 
 i. Section through the anterior part of area pellucida, shewing the cubical 
 granular hypoblast cells in this region. 2. Section slightly behind i, shewing the 
 primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved 
 in the middle line into hypoblast and mesoblast. 
 
 PLATE 45. 
 
 SERIES I, i, 2, 3, 4 and 5. Sections through blastoderm somewhat older than 
 Series H. 
 
 i. Section through area pellucida well in front of primitive streak. 2. Section 
 through area pellucida just in front of primitive streak. 3. Section through the front 
 end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4. 
 
 SERIES K, 1,2, 3, 4 and 5. Sections through a blastoderm in which the first 
 traces of notochord and medullary groove have made their appearance. Rather more 
 than half the section is represented in each figure, but the right half is represented in 
 i and 3, and the left in 2 and 4. 
 
 i. Section through notochord immediately behind the head-fold. 2. Section 
 shewing medullary groove a little behind i. 3. Section just in front of the primitive 
 streak. 4 and 5. Sections through the front end of the primitive streak. 
 
 FIG. L. Surface view of blastoderm with a very young primitive streak. 
 
XXV. THE ANATOMY AND DEVELOPMENT OF PERIPATUS 
 
 CAPENSIS 1 . 
 (With Plates 4653.) 
 
 INTRODUCTION. 
 
 THE late Professor Balfour was engaged just before his death 
 in investigating the structure and embryology of Peripatus 
 capensis, with the view of publishing a complete monograph 
 of the genus. He left numerous drawings intended to serve 
 as illustrations to the monograph, together with a series of 
 notes and descriptions of a large part of the anatomy of 
 Peripatus capensis. Of this manuscript some portions were 
 ready for publication, others were more or less imperfect ; while 
 of the figures many were without references, and others were 
 provided with only a few words of explanation. 
 
 It was obviously necessary that Professor Balfour's work 
 embodying as it did much important discovery should be pub- 
 lished without delay; and the task of preparing his material 
 for the press was confided to us. We have printed all his 
 notes and descriptions without alteration 2 . Explanations which 
 appeared to be necessary, and additions to the text in cases in 
 which he had prepared figures without writing descriptions, to- 
 gether with full descriptions of all the plates, have been added 
 by us, and are distinguished by enclosure in square brackets*. 
 
 We have to thank Miss Balfour, Professor Balfour's sister, 
 for the important service which she has rendered by preparing 
 
 1 From the Quarterly Journal of Microscopical Science, April, 1883. 
 
 2 Excepting in an unimportant matter of change of nomenclature used with regard 
 to the buccal cavity. 
 
 3 The account of the external characters, generative organs, and development, has 
 been written by the editors. 
 
872 ANATOMY AND DEVELOPMENT 
 
 a large part of the beautiful drawings with which the mono- 
 graph is illustrated. Many of these had been executed by her 
 under Professor Balfour's personal supervision ; and the know- 
 ledge of his work which she then acquired has been of the 
 greatest assistance to us in preparing the MSS. and drawings for 
 publication. 
 
 Since his death she has spared no pains in studying the 
 structure of Peripatus, so as to enable us to bring out the 
 first part of the monograph in as complete a state as possible. 
 It is due to her skill that the first really serviceable and accurate 
 representation of the legs of any species of Peripatus available 
 for scientific purposes are issued with the present memoir 1 . 
 
 We have purposely refrained from introducing comments on 
 the general bearing of the new and important results set forth in 
 this memoir, and have confined ourselves to what was strictly 
 necessary for the presentation of Mr Balfour's discoveries in a 
 form in which they could be fully comprehended. 
 
 Mr Balfour had at his disposal numerous specimens of 
 Peripatus novcz zealaudicz, collected for him by Professor Jeffrey 
 Parker, of Christchurch, New Zealand ; also specimens from 
 the Cape of Good Hope collected by Mr Lloyd Morgan, 
 and brought to England by Mr Roland Trimen in 1881 ; and 
 others given to him by Mr Wood Mason, together with all the 
 material collected by Mr Moseley during the "Challenger" 
 voyage. 
 
 A preliminary account of the discoveries as to the em- 
 bryology of Peripatus has already been communicated to the 
 Royal Society 2 . It is intended that the present memoir shall 
 be followed by others, comprising a complete account of all the 
 species of the genus Peripatus. 
 
 H. M. MOSELEY. 
 A. SEDGWICK. 
 
 1 The drawings on PI. 47, figs. 9 and 10 on PI. 48, and the drawings of the 
 embryos (except fig. 37), have been made by Miss Balfour since Professor Balfour's 
 death. 
 
 4 Proc. Royal Soc. 1883. 
 
OF PERIPATUS CAPENSIS. 873 
 
 PART I. 
 DESCRIPTION OF THE SPECIES. 
 
 Peripatus capensis (fig. i). 
 
 [The body is elongated, and slightly flattened dorso-ventrally. 
 The dorsal surface is arched, and darkly pigmented ; while the 
 ventral surface is nearly flat, and of a lighter colour. 
 
 The mouth is placed at the anterior end of the body, on the 
 ventral surface. 
 
 The anus is posterior and terminal. 
 
 The generative opening is single and median, and placed 
 in both sexes on the ventral surface, immediately in front of 
 the anus. 
 
 There are a pair of ringed antennae projecting from the an- 
 terior end of the head, and a pair of simple eyes, placed on the 
 dorsal surface at the roots of the antennae. 
 
 The appendages of the body behind the antennae are dis- 
 posed in twenty pairs. 
 
 1. The single pair of jaws placed within the buccal cavity 
 in front of the true mouth opening, and consisting each of a 
 papilla, armed at its termination with two cutting blades. 
 
 2. The oral papillae placed on each side of the mouth. At 
 their apices the ducts of the slime glands open. 
 
 3. The seventeen pairs of ambulatory appendages, each pro- 
 vided with a pair of chitinous claws at its extremity. 
 
 4. The anal papillae placed on each side of the generative 
 opening. 
 
 Colour. The following statements on this head are derived 
 from observations of spirit specimens. The colour varies in 
 different individuals. It always consists of a groundwork of 
 green and bluish grey, with a greater or less admixture of 
 brown. The chief variations in the appearance of the animal, 
 so far as colour is concerned, depend on the shade of the green. 
 In some it is dark, as in the specimen figured (fig. I) ; in others 
 it is of a lighter shade. 
 
 There is present in most specimens a fairly broad light band 
 on each side of the body, immediately dorsal to the attachment 
 B. 56 
 
8/4 ANATOMY AND DEVELOPMENT 
 
 of the legs. This band is more prominent in the lighter coloured 
 varieties than in the dark, and is especially conspicuous in large 
 individuals. It is due to a diminution in the green pigment, and 
 an increase in the brown. 
 
 There is a dark line running down the middle of the dorsal 
 surface, in the middle of which is a fine whitish line. 
 
 The ventral surface is almost entirely free from the green 
 pigment, but possesses a certain amount of light brown. This 
 brown pigment is more conspicuous and of a darker shade on 
 the spinous pads of the foot. 
 
 In parts of the body where the pigment is scarce, it is seen 
 to be confined to the papillae. This is especially evident round 
 the mouth, where the sparse green pigment is entirely confined 
 to the papillae. 
 
 In some specimens a number of white papillae, or perhaps 
 light brown, are scattered over the dorsal surface ; and some- 
 times there is a scattering of green papillae all over the ventral 
 surface. These two peculiarities are more especially noticeable 
 in small specimens. 
 
 Ridges and Papilla of the Skin. The skin is thrown into 
 a number of transverse ridges, along which the primary wart- 
 like papillae are placed. 
 
 The papillae, which are found everywhere, are specially de- 
 veloped on the dorsal surface, less so on the ventral. The 
 papillae round the lips differ from the remaining papillae of the 
 ventral surface in containing a green pigment. Each papilla 
 bears at its extremity a well-marked spine. 
 
 The ridges of the skin are not continued across the dorsal 
 middle line, being interrupted by the whitish line already 
 mentioned. Those which lie in the same transverse line as 
 the legs are not continued on to the latter, but stop at the 
 junction of the latter with the body. All the others pass round 
 to the ventral surface and are continued across the middle line ; 
 they do not, however, become continuous with the ridges of the 
 other side, but passing between them gradually thin off and 
 vanish. 
 
 The ridges on the legs are directed transversely to their 
 long axes, i.e. are at right angles to the ridges of the rest of the 
 body. 
 
OF PERIPATUS CAPENSIS. 8/5 
 
 The antennae are ringed and taper slightly till near- their 
 termination, where they present a slight enlargement in spirit 
 specimens, which in its turn tapers to its termination. 
 
 The rings consist essentially of a number of coalesced primary 
 papillae, and are, therefore, beset by a number of spines like 
 those of the primary papillae (described below). They are more 
 deeply pigmented than the rest of the antenna. 
 
 The free end of the antenna is covered by a cap of tissue like 
 that of the rings. It is followed by four or more rings placed 
 close together on the terminal enlargement. There appears to 
 be about thirty rings on the antennae of all adults of this species. 
 But they are difficult to count, and a number of small rings 
 occur between them, which are not included in the thirty. 
 
 The antennae are prolongations of the dorso-lateral parts of 
 the anterior end of the body. 
 
 The eyes are paired and are situated at the roots of the 
 antennae on the dorso-lateral parts of the head. Each is placed 
 on the side of a protuberance which is continued as the an- 
 tenna, and presents the appearance of a small circular crystal- 
 line ball inserted on the skin in this region. 
 
 The rings of papillae on that part of the head from which 
 the antennae arise lose their transverse arrangement. They 
 are arranged concentrically to the antennal rings, and have a 
 straight course forwards between the antennae. 
 
 The oral papillae are placed at the side of the head. They 
 are attached ventro-laterally on each side of the lips. The 
 duct of the slirne gland opens through their free end. They 
 possess two main rings of projecting tissue, which are especially 
 pigmented on the dorsal side ; and their extremities are covered 
 by papillae irregularly arranged. 
 
 The buccal cavity, jaws, and lips are described below. 
 
 The Ambulatory Appendages. The claw-bearing legs are 
 usually seventeen in number ; but in two cases of small females 
 we have observed that the anal papillae bear claws, and pre- 
 sent all the essential features of the ambulatory appendages. 
 In one small female specimen there were twenty pairs of claw- 
 bearing appendages, the last being like the claw-bearing anal 
 papillae last mentioned, and the generative opening being placed 
 between them. 
 
 56-2 
 
876 ANATOMY AND DEVELOPMENT 
 
 The ambulatory appendages, with the exception of the fourth 
 and fifth pairs in both sexes, and the last pair (seventeenth) in 
 the male, all resemble each other fairly closely. A typical ap- 
 pendage (figs. 2 and 3) will first be described, and the small 
 variations found in the appendages just mentioned will then 
 be pointed out. Each consists of two main divisions, a larger 
 proximal portion, the leg, and a narrow distal claw-bearing 
 portion, the foot. 
 
 The leg has the form of a truncated cone, the broad end of 
 which is attached to the ventro-lateral body-wall, of which it 
 appears to be, and is, a prolongation. It is marked by a number 
 of rings of primary papillae, placed transversely to the long axis 
 of the leg, the dorsal of which contain a green and the ventral a 
 brown pigment. These rings of papillae, at the attachment of 
 the leg, gradually change their direction and merge into the 
 body rings. At the narrow end of the cone there are three 
 ventrally placed pads, in which the brown pigment is dark, and 
 which are covered by a number of spines precisely resembling 
 the spines of the primary papillae. These spinous pads are con- 
 timaed dorsally, each into a ring of papillae. 
 
 "The papillae of the ventral row next the proximal of these 
 spinous pads are intermediate in character between the primary 
 papillae and the spinous pads. Each of these papillae is larger 
 than a normal papilla, and bears several spines (fig. 2). This 
 character of the papilla of this row is even more marked in 
 some of the anterior legs than in the one figured ; it seems 
 probable that the pads have been formed by the coalescence of 
 several rows of papillae on the ventral surface of the legs. On 
 the outer and inner sides of these pads the spines are absent, 
 and secondary papillae only are present. 
 
 In the centre of the basal part of the ventral surface of the 
 foot there are present a group of larger papillae, which are of a 
 slightly paler colour than the others. They are arranged so as 
 to form a groove, directed transversely to the long axis of the 
 body, and separated at its internal extremity by a median papilla 
 from a deep pit which is placed at the point of junction of the 
 body and leg. The whole structure has the appearance, when 
 viewed with the naked eye, of a transverse slit placed at the base 
 of the leg. The segmental organs open by the deep pit placed 
 
OF FERIPATUS CAPENSIS. 877 
 
 at the internal end of this structure. The exact arrangement of 
 the papillae round the outer part of the slit does not appear to be 
 constant. 
 
 The foot is attached to the distal end of the leg. It is 
 slightly narrower at its attached extremity than at its free end, 
 which bears the two claws. The integument of the foot is 
 covered with seco'ndary papillae, but spines and primary pa- 
 pilla? are absent, except at the points now to be described. 
 
 On each side of the middle ventral line of the proximal end 
 of the foot is placed an elliptical elevation of the integument 
 covered with spines. Attached to the proximal and lateral end 
 of this is a primary papilla. At the distal end of the ventral 
 side of the foot on each side of the middle line is a group of in- 
 conspicuous pale elevations, bearing spines. 
 
 On the front side of the distal end of the foot, close to the 
 socket in which the claws are placed, are two primary papillae, 
 one dorsal and the other ventral. 
 
 On the posterior side of the foot the dorsal of these only 
 is present. The claws are sickle-shaped, and placed on papillae 
 on the terminal portion of the foot. The part of the foot on 
 which they are placed is especially retractile, and is generally 
 found more or less telescoped into the proximal part (as in the 
 figure). 
 
 The fourth and fifth pairs of legs exactly resemble the others, 
 except in the fact that the proximal pad is broken up into 
 three, a small central and two larger lateral. The enlarged 
 segmental organs of these legs open on the small central di- 
 vision. 
 
 The last (17) leg of the male (PI. 47, fig. 4) is character- 
 ized by possessing a \vell-marked white papilla on the ventral 
 surface. This papilla, which presents a slit- like opening at its 
 apex, is placed on the second row of papillae counting from the 
 innermost pad, and slightly posterior to the axial line of the leg. 
 
 The anal papillae, or as they should be called, generative 
 papillae, are placed one on each side of the generative aperture. 
 They are most marked in small and least so in large specimens. 
 That they are rudimentary ambulatory appendages is shewn by 
 the fact that they are sometimes provided with claws, and resem- 
 ble closely the anterior appendages.] 
 
8/8 ANATOMY AND DEVELOPMENT 
 
 PART II. 
 ALIMENTARY CANAL. 
 
 The alimentary canal of Peripatns capensis forms, in the 
 extended condition of the animal, a nearly straight tube, slightly 
 longer than the body, the general characters of which are shewn 
 in figs. 6 and 7. 
 
 For the purposes of description, it may conveniently be di- 
 vided into five regions, viz. (i) the buccal cavity with the tongue, 
 jaws, and salivary glands, (2) pharynx, (3) the oesophagus, (4) 
 the stomach, (5) the rectum. 
 
 The Buccal Cavity. The buccal cavity has the form of a 
 fairly deep pit, of a longitudinal oval form, placed on the ventral 
 surface of the head, and surrounded by a tumid lip. 
 
 [The buccal cavity has been shewn by Moseley to be formed 
 in the embryo by the fusion of a series of processes surrounding 
 the true mouth-opening, and enclosing in their fusion the jaws.] 
 
 The lip is covered by a soft skin, in which are numerous 
 organs of touch, similar to those in other parts of the skin having 
 their projecting portions enclosed in delicate spines formed by 
 the cuticle. The skin of the lips differs, however, from the re- 
 mainder of the skin, in the absence of tubercles, and in the great 
 reduction of the thickness of the dermis. It is raised into a 
 series of papilliform ridges, whose general form is shewn in fig. 5 ; 
 of these there is one unpaired and median behind, and a pair, 
 differing somewhat in character from the remainder, in front, and 
 there are, in addition, seven on each side. 
 
 The structures within the buccal cavity are shewn as they 
 appear in surface views in figs. 5 and 7, but their real nature is 
 best seen in sections, and is illustrated by PI. 49, figs, n and 12, 
 representing the oral cavity in transverse section, and by PL 49, 
 figs. 17 and 1 8, representing it in horizontal longitudinal sections. 
 In the median line of the buccal cavity in front is placed a thick 
 muscular protuberance, which may perhaps conveniently be 
 called the tongue, though attached to the dorsal instead of 
 the ventral wall of the mouth. It has the form of an elongated 
 
OF PERIPATUS CAPENSIS. 879 
 
 ridge, which ends rather abruptly behind, becoming continuous 
 with the dorsal wall of the pharynx. Its projecting edge is 
 armed by a series of small teeth, which are thickenings of the 
 chitinous covering, prolonged from the surface of the body over 
 the buccal cavity. Where the ridge becomes flatter behind, the 
 row of teeth divides into two, with a shallow groove between 
 them (PI. 48, fig. 7). 
 
 The surface of the tongue is covered by the oral epithelium, 
 in parts of which are organs of special sense, similar to those in 
 the skin; but its interior is wholly formed of powerful muscles. 
 The muscles form two groups, intermingled amongst each other. 
 There are a series of fibres inserted in the free edge of the 
 tongue, which diverge, more or less obliquely, towards the skin 
 at the front of the head anteriorly, and towards the pharynx 
 behind. The latter set of fibres are directly continuous with 
 the radial fibres of the pharynx. The muscular fibres just 
 described are clearly adapted to give a sawing motion to the 
 tongue, whose movements may thus, to a certain extent, be com- 
 pared to those of the odontophor of a mollusc. 
 
 In addition to the above set of muscles, there are also trans- 
 verse muscles, forming laminae between the fibres just described. 
 They pass from side to side across the tongue, and their action 
 is clearly to narrow it, and so cause it to project outwards from 
 the buccal cavity. 
 
 On each side of the tongue are placed the jaws, which are, 
 no doubt, a pair of appendages, modified in the characteristic 
 arthropodan manner, to subserve mastication. Their structure 
 has never been satisfactorily described, and is very complicated. 
 They are essentially short papillae, moved by an elaborate 
 and powerful system of muscles, and armed at their free ex- 
 tremities by a pair of cutting blades or claws. The latter struc- 
 tures are, in all essential points, similar to the claws borne by 
 the feet, and, like these, are formed as thickenings of the cuticle. 
 They have therefore essentially the characters of the claws and 
 jaws of the Arthropoda, and are wholly dissimilar to the setae of 
 Chaetopoda. The claws are sickle-shaped and, as shewn in PI. 
 47, fig. 5, have their convex edge directed nearly straight for- 
 wards, and their concave or cutting edge pointed backwards. 
 Their form differs somewhat in the different species, and, as will 
 
880 ANATOMY AND DEVELOPMENT 
 
 be shewn in the systematic part of this memoir 1 , forms a good 
 specific character. In Peripatns capensis (PI. 48, fig. 10) the 
 cutting surface of the outer blade is smooth and without teeth, 
 while that of the inner blade (fig. 9), which is the larger of the 
 two, is provided with five or six small teeth, in- addition to the 
 main point. A more important difference between the two blades 
 than that in the character of the cutting edge just spoken of, is 
 to be found in their relation to the muscles which move them. 
 The anterior parts of both blades are placed on two epithelial 
 ridges, which are moved by muscles common to both blades (PI. 
 49, fig. 1 1). Posteriorly, however, the behaviour of the two blades 
 is very different. The epithelial ridge bearing "the outer blade 
 is continued back for a short distance behind the blade, but 
 the cuticle covering it becomes very thin, and it forms a 
 simple epithelial ridge placed parallel to the inner blade. The 
 cuticle covering the epithelial ridge of the inner blade is, on the 
 contrary, prolonged behind the blade itself as a thick rod, which, 
 penetrating backwards along a deep pocket of the buccal epithe- 
 lium, behind the main part of the buccal cavity for the whole 
 length of the pharynx, forms a very powerful lever, on which 
 a great part of the muscles connected with the jaws find their 
 insertion. The relations of the epithelial pocket bearing this 
 lever are somewhat peculiar. 
 
 The part of the epithelial ridge bearing the proximal part of 
 this lever is bounded on both its outer and inner aspect by a deep 
 groove. The wall of the outer groove is formed by the epithe- 
 lial ridge of the outer blade, and that of the inner by a special 
 epithelial ridge at the side of the tongue. Close to the hinder 
 border of the buccal cavity (as shewn in PI. 49, fig. 12, on the 
 right hand side), the outer walls of these two grooves meet over 
 the lever, so as completely to enclose it in an epithelial tube, 
 and almost immediately behind this point the epithelial tube is 
 detached from the oral epithelium, and appears in section as 
 a tube with a chitinous rod in its interior, lying freely in the 
 body-cavity (shewn in PI. 49, figs. 13 16 /?). This apparent 
 tube is the section of the deep pit already spoken of. It may 
 
 1 Some material for this memoir was left by Prof. Balfour, which will be published 
 separately- 
 
OF PEKIPATUS CAPENSIS. 88 1 
 
 be traced back even beyond the end of the pharynx, and serves 
 along its whole length for the attachment of muscles. 
 
 The greater part of the buccal cavity is filled with the tongue 
 and jaws just described. It opens dorsally'and behind by the 
 mouth into the pharynx, there being no sharp line of demarca- 
 tion between the buccal cavity and the pharynx. Behind the 
 opening into the pharynx there is a continuation of the buccal 
 cavity shewn in transverse section in fig. 13, and in longitudinal 
 and horizontal section in fig. 17, into which there opens the 
 common junction of the two salivary glands. This diverticulum 
 is wide at first and opens by a somewhat constricted mouth into 
 the pharynx above (PL 49, fig. 13, also shewn in longitudinal 
 and horizontal section in fig. 17). Behind it narrows, passing 
 insensibly into what may most conveniently be regarded as a 
 common duct for the two salivary glands (PL 49, fig. 17). 
 
 The Salivary Glands. These two bodies were originally 
 described by Grube, by whom their nature was not made out, 
 and subsequently by Moseley, who regarded them as fat bodies. 
 They are placed in the lateral compartments of the body-cavity 
 immediately dorsal to the ventral nerve cords, and extend for 
 a very variable distance, sometimes not more than half the 
 length of the body, and in other instances extending for nearly 
 its whole length. Their average length is perhaps about two- 
 thirds that of the body. Their middle portion is thickest, and 
 they thin off very much behind and to a slight extent in front. 
 Immediately behind the mouth and in front of the first pair of 
 legs, they bend inwards and downwards, and fall (fig. 7) one on 
 each side into the hind end of the narrow section of the oral 
 diverticulum just spoken of as the common duct for the two 
 salivary glands. The glandular part of these organs is that 
 extending back from the point where they bend inwards. This 
 part (fig. 1 6) is formed of very elongated cells supported by 
 a delicate membrana propria. The section of this part is some- 
 what triangular, and the cells are so long as to leave a compara- 
 tively small lumen. The nuclei of the cells are placed close to 
 the supporting membrane, and the remainder of the cells are 
 filled with very closely packed secretory globules, which have 
 a high index of refraction. It was the presence of these globules 
 which probably led Moseley to regard the salivary glands as fat 
 
882 ANATOMY AND DEVELOPMENT 
 
 bodies. The part of each gland which bends inwards must be 
 regarded as the duct. 
 
 The cells lining the ducts are considerably less columnar 
 than those of the gland proper. Their nuclei (fig. 14) are 
 situated at the free extremities instead of at the base of the cells, 
 and they are without secretory globules. The cells lining the 
 ducts of the salivary glands pass, without any sharp line of 
 demarcation, into those of the oral epithelium, which are flatter 
 and have their nuclei placed in the middle. 
 
 The Pharynx. The Pharynx is a highly muscular tube (fig. 
 7) with a triangular lumen (figs. 14, 15), which extends from 
 the mouth to about half way between the first and second pair 
 of legs. It is lined by a flattish epithelium bounded by a cuticle 
 continuous with that of the mouth. On the dorsal side is a 
 ridge projecting into the lumen of the pharynx. This ridge 
 may be traced forwards (PI. 49, figs. 1 1 14) into the tongue, 
 and the two grooves at the side of this ridge, forming the two 
 upper angles of the triangular lumen, may be followed into those 
 at the sides of the tongue. The muscles of the pharynx are 
 very highly developed, consisting of an intrinsic and an extrinsic 
 set. The former consists, as is best seen in longitudinal sections, 
 of (PI. 51, fig. 23) radial fibres, arranged in somewhat wedge- 
 shaped laminae, between which are rings of circular fibres. The 
 latter are thicker externally than internally, and so also appear 
 wedge-shaped in longitudinal sections. Very characteristic of 
 the pharynx are the two sympathetic nerves placed close to the 
 two dorsal angles of the triangular lumen (fig. 14, sy). 
 
 The pharynx of Peripatus is interesting in that it is unlike, 
 so far as I know, the pharynx of any true Arthropod, in all of 
 which the region corresponding with the pharynx of Peripatus 
 is provided with relatively very thin walls. 
 
 The pharynx of Peripatus has, on the other hand, a very 
 close and obvious resemblance to that of many of the Chaeto- 
 poda, a resemblance which is greatly increased by the character- 
 istic course of the sympathetic nerves. 
 
 The form of the lumen, as already pointed out by Grube, 
 resembles that of the Nematoda. 
 
 T/ie (Esophagus. Behind the pharynx there follows a narrow 
 oesophagus (fig. 7, o e] shewn in section in fig. 16. It has some- 
 
OF PERIPATUS CAPENSIS. 883 
 
 what folded and fairly thick walls, and lies freely in the central 
 division of the body-cavity without any mesenteric support. Its 
 walls are formed of five layers, viz. from without inwards. 
 
 (1) A peritoneal investment. 
 
 (2) A layer of longitudinal fibres. 
 
 (3) A layer of circular fibres, amongst which are numerous 
 nuclei. 
 
 (4) A connective-tissue layer supporting (5) a layer of fairly 
 columnar hyaline epithelium, bounded on its inner aspect by 
 a cuticle continued from that of the pharynx. In front it passes 
 insensibly into the pharynx, and beyond the region where the 
 dorsal walls of the pharynx have clearly commenced, the ventral 
 walls still retain the characters of the cesophageal walls. The 
 oesophagus is vertically oval in front, but more nearly circular 
 behind. Characteristic of the oesophagus is the junction of the 
 two sympathetic nerves on its dorsal wall (fig. 16). These 
 nerves cannot be traced far beyond their point of junction. 
 
 Tlie Stomach. The next section of the alimentary tract is 
 the stomach or rnesenteron (fig. 6). It is by far the largest 
 part of the alimentary tract, commencing at about the second 
 pair of legs and extending nearly to the hind end of the body. 
 It tapers both in front and behind, and is narrowest in the 
 middle, and is marked off sharply both from the oesophagus in 
 front and the rectum behind, and is distinguished from both of 
 these by its somewhat pinker hue. In the retracted condition 
 of the animal it is, as pointed out by Moseley, folded in a single 
 short dorsal loop, at about the junction of its first with its second 
 third, and also, according to my observations, at its junction 
 with the rectum ; but in the extended condition it is nearly 
 straight, though usually the posterior fold at the junction of th 
 rectum is not completely removed. Its walls are always marked 
 by plications which, as both Moseley and Grube have stated, do 
 not in any way correspond with the segmentation of the body. 
 In its interior I have frequently found the chitinous remains of 
 the skins of insects, so that we are not justified in considering 
 that the diet is purely vegetable. It lies free, and is, like the 
 remainder of the, alimentary tract, without a mesentery. The 
 structure of the walls of the stomach has not hitherto been very 
 satisfactorily described. 
 
884 ANATOMY AND DEVELOPMENT 
 
 The connective tissue and muscular coats are extremely 
 thin. There is present everywhere a peritoneal covering, and 
 in front a fairly well-marked though very thin layer of muscles 
 formed of an external circular and an internal longitudinal 
 layer. In the middle and posterior parts, however, I was un- 
 able to recognize these two layers in section ; although in surface 
 view Grube found an inner layer of circular fibres and an outer 
 layer formed of bands of longitudinal fibres, which he regards as 
 muscular. 
 
 The layer supporting the epithelium is reduced to a base- 
 ment membrane. The epithelial part of the wall of the stomach 
 is by far the thickest (fig. 20), and is mainly composed of enor- 
 mously elongated, fibre-like cells, which in the middle part of 
 the stomach, where they are longest, are nearly half a millimetre 
 in length, and only about -006 mm. in breadth. Their nuclei, as 
 seen in fig. 20, are very elongated, and are placed about a quar- 
 ter of the length from the base. 
 
 The cells are mainly filled with an immense number of 
 highly refracting spherules, probably secretory globules, but 
 held by Grube, from the fact of their dissolving in ether, to be 
 fat. The epithelial cells are raised into numerous blunt pro- 
 cesses projecting into the lumen of the stomach. 
 
 In addition to the cells just described there are present in 
 the anterior part of the stomach a fair sprinkling of mucous 
 cells. There are also everywhere present around the bases of 
 the columnar cells short cells with spherical nuclei, which are 
 somewhat irregularly scattered in the middle and posterior parts 
 of the stomach, but form in the front part a definite layer. I 
 have not been able to isolate these cells, and can give no ac- 
 count of their function. 
 
 The rectum extends from the end of the stomach to the 
 anus. The region of junction between the stomach and the 
 rectum is somewhat folded. The usual arrangement of the 
 parts is shewn in fig. 6, where the hind end of the stomach is 
 seen to be bent upon itself in a U-shaped fashion, and the 
 rectum extending forwards under this bent portion and joinino- 
 the front end of the dorsal limb of the U. The structure of 
 the walls of the rectum is entirely different to that of the 
 stomach, and the transition between the two is perfectly sudden. 
 
OF PERIPATUS CAPENSIS. 885 
 
 Within the peritoneal investment comes a well-developed mus- 
 cular layer with a somewhat unusual arrangement of its layers, 
 there being an external circular layer and an internal layer 
 formed of isolated longitudinal bands. The epithelium is fairly 
 columnar, formed of granular cells with large nuclei, and is lined 
 by a prolongation of the external cuticle. It is raised into 
 numerous longitudinal folds, which are visible from the surface, 
 and give a very characteristic appearance to this part of the 
 alimentary tract. The muscular layers do not penetrate into 
 the epithelial folds, which are supported by a connective tissue 
 layer. 
 
 NERVOUS SYSTEM. 
 
 The central nervous system consists of a pair of supra-ceso- 
 phageal ganglia united in the middle line, and of a pair of 
 widely divaricated ventral cords, continuous in front with the 
 supra-cesophageal ganglia. 
 
 It will be convenient in the first instance to deal with the 
 general anatomy of the nervous system and then with the 
 histology. 
 
 Ventral Cords. The ventral cords at first sight appear to be 
 without ganglionic thickenings, but on more careful examina- 
 tion they are found to be enlarged at each pair of legs (PI. 48, 
 fig. 8). These enlargements may be regarded as imperfect 
 ganglia. There are, therefore, seventeen such pairs of ganglia 
 corresponding to the seventeen pairs of legs. There is in addi- 
 tion a ganglionic enlargement at the commencement of the 
 cesophageal commissures, where the nerves to the oral papillae 
 are given off (PI. 51, fig. 22 or. g.), and the region of junction 
 between the cesophageal commissures with the supra-cesophageal 
 ganglia, where another pair of nerves are given off to the jaws 
 (PI. 51, fig. 227 ri), may be regarded as the anterior ganglion of 
 the ventral cords. There are, therefore, according to the above 
 reckoning, nineteen pairs of ganglia connected with the ventral 
 cords. 
 
 The ventral cords are placed each in the lateral compart- 
 ments of the body-cavity, immediately within the longitudinal 
 layer of muscles. 
 
886 ANATOMY AND DEVELOPMENT 
 
 They are connected with each other, rather like the pedal 
 nerves of Chiton and the lower Prosobranchiata, by a number 
 of commissures. These commissures exhibit a fairly regular 
 arrangement from the region included between the first and the 
 last pair of true feet. There are nine or ten of them between 
 each pair of feet (PI. 52, fig. 26). They pass along the ventral 
 wall of the body, perforating the ventral mass of longitudinal 
 muscles. On their way they give off nerves which innervate 
 the skin. 
 
 In Peripatus nova zealandia, and probably also in P. capen- 
 sis, two of these nerves, coming off from each pair of ganglia, 
 are distinguished from the remainder by the fact that they are 
 provided with numerous nerve-cells, instead of being composed 
 of nerve-fibres only, like the remaining commissures (PI. 52, fig. 
 26 g cd). In correlation with the nerves given off from them to 
 the skin the commissures are smaller in the middle than at the 
 two ends. 
 
 Posteriorly the two nerve-cords nearly meet immediately in 
 front of the generative aperture, and between this aperture and 
 the last pair of feet there are about six commissures passing 
 between them (PI. 48, fig. 8). Behind the generative aperture 
 the two cords bend upwards, and, as is shewn in fig. 8, fall into 
 each other dorsally to the rectum. The section of the two cords 
 placed dorsally to the rectum is solely formed of nerve-fibres; 
 the nerve-cells, present elsewhere, being here absent. 
 
 In front of the ganglion of the first foot the commissures 
 have a more dorsal situation than in the remainder of the body. 
 The median longitudinal ventral muscle here gradually thins 
 out and comes to an end, while the commissures pass imme- 
 diately below the wall of the pharynx (PI. 49, figs. 14, 15). The 
 ventral cords themselves at first approach very close to each 
 other in this region, separating again, however, to envelope be- 
 tween them the pharynx (PI. 51, fig. 22). 
 
 There are eleven commissures in front of the first pair of legs 
 (PI. 51, fig. 22). The three foremost of these are very close 
 together, the middle one arising in a more ventral position than 
 the other two, and joining in the median ventral line a peculiar 
 mass of cells placed in contact with the oral epithelium (fig. 14). 
 It is probably an organ of special sense. 
 
OF PERIPATUS CAPENSIS. 887 
 
 The ventral cords give off" a series of nerves from their outer 
 borders, which present throughout the trunk a fairly regular 
 arrangement. From each ganglion two large nerves (figs. 8, 22, 
 26) are given off, which, diverging somewhat from each other, 
 pass into the feet, and, giving off branches on their way, may be 
 traced for a considerable distance within the feet along their 
 anterior and posterior borders. 
 
 In front of each of the pair of pedal nerves a fairly large 
 nerve may be seen passing outwards towards the side of the 
 body (fig. 22). In addition to this nerve there are a number of 
 smaller nerves passing off from the main trunk, which do not 
 appear to be quite constant in number, but which are usually 
 about seven or eight. Similar nerves to those behind are given 
 off from the region in front of the first pair of legs, while at the 
 point where the two ventral cords pass into the cesophageal 
 commissures two large nerves (fig. 22), similar to the pairs of 
 pedal nerves, take their origin. These nerves may be traced 
 forwards into the oral papillae, and are therefore to be regarded 
 as the nerves of these appendages. On the ventral side of the 
 cords, where they approach most closely, between the oral 
 papillae and the first pair of legs, a number of small nerves are 
 given off to the skin, whose distribution appears to be to the 
 same region of the skin as that of the branches from the 
 commissures behind the first pair of legs. 
 
 From the cesophageal commissures, close to their junction 
 with the supra-cesophageal ganglia, a nerve arises on each side 
 which passes to the jaws, and a little in front of this, apparently 
 from the supra-cesophageal ganglion itself, a second nerve to the 
 jaws also takes its origin (PI. 51, fig. 22 j ri). These two nerves 
 I take to be homologous with a pair of pedal nerves. 
 
 Between the nerves to the jaws and those to the oral papillae 
 a number of small nerves take their origin. Three of these on 
 each side pass in a dorsal direction and one or two in a ventral 
 one. 
 
 TJie Supra-cesopJiagcal Ganglia. The supra-cesophageal gan- 
 glia (figs. 8 and 22) are large, somewhat oval masses, broader in 
 front than behind, completely fused in the middle, but free at 
 their extremities. Each of them is prolonged anteriorly into an 
 antennary nerve, and is continuous behind with one of the 
 
888 ANATOMY AND DEVELOPMENT 
 
 cesophageal commissures. On the ventral surface of each, rather 
 behind the level of the eye, is placed a very peculiar pro- 
 tuberance (fig. 22 d], of which I shall say more in dealing with 
 the histology of the nervous system. 
 
 A number of nerves arise from the supra-cesophageal ganglia, 
 mainly from their dorsal surface. 
 
 In front are the immense antennary nerves extending along 
 the whole length of each antenna, and giving off numerous 
 lateral twigs to the sense organs. Near the origin of the an- 
 tennary nerves, and rather on the dorsal surface, there spring 
 a few small twigs, which pass to the skin, and are presumably 
 sensory. The largest of them is shewn in PL 50, fig. 19 A. 
 About one-third of the way back the two large optic nerves take 
 their origin, also arising laterally, but rather from the dorsal 
 surface (PL 50, fig. 19 D and E). Each of them joins a large 
 ganglionic mass placed immediately behind the retina. Nearly 
 on a level with the optic nerves and slightly nearer the middle 
 dorsal line a pair of small nerves (fig. 19 D) spring from the 
 brain and pass upwards, while nearly in the same line with the 
 optic nerves and a little behind them a larger pair of nerves take 
 their origin. 
 
 Behind all these nerves there arises from the line of suture 
 between the two supra-cesophageal ganglia a large median nerve 
 which appears to supply the integument of the dorsal part of 
 the head (PL 48, fig. 8 ; PL 49, figs. 1 1 14 dn}. 
 
 Sympathetic System. In addition to the nerves just de- 
 scribed there are two very important nerves which arise near 
 the median ventral line, close to the hind end of the supra- 
 cesophageal ganglia. The origin of these two nerves is shewn 
 in the surface view (fig. 22 sy, and in section in fig. 11). They 
 at first tend somewhat forwards and pass into the muscles near 
 the epithelium lining the groove on each side of the tongue. 
 Here they suddenly bend backwards again and follow the 
 grooves into the pharynx. 
 
 The two grooves are continuous with the two dorsal angles 
 of the pharynx ; and embedded in the muscles of the pharynx, 
 in juxtaposition with the epithelium, these two nerves may 
 easily be traced in sections. They pass backwards the whole 
 length of the pharynx till the latter joins the oesophagus. 
 
OF PERIPATUS CAPEXSIS. 889 
 
 Here they at once approach and shortly meet in the~rrredian 
 dorsal line (fig. 16). They can only be traced for a very short 
 distance beyond their meeting point. These nerves are, without 
 doubt, the homologues of the sympathetic system of Chaetopods, 
 occupying as they do the exact position which Semper has 
 shewn to be characteristic of the sympathetic nerves in that 
 group, and arising from an almost identical part of the brain 1 . 
 
 Histology of the Nervous System. 
 
 Ventral Cords. The histology of the ventral cords and 
 oesophageal commissures is very simple and uniform. They 
 consist of a cord almost wholly formed of nerve-fibres, placed 
 dorsally, and a ventral layer of ganglion cells (figs. 16 and 20). 
 
 The fibrous portion of the cord has the usual structure, being 
 formed mainly of longitudinal fibres, each probably being a 
 bundle of fibres of various sizes, enveloped in a sponge-work 
 of connective tissue. The larger bundles of fibres are placed 
 near the inner borders of the cords. In this part of the cord 
 there are placed a very small number of ganglion cells. 
 
 The layer of ganglion cells is somewhat crescent-shaped in 
 section, and, as shewn in figs. 16 and 20, envelopes the whole 
 ventral aspect of the fibrous parts of the cord, and even creeps 
 up slightly on to the dorsal side. It is thicker on the inner 
 than on the outer side, and increases considerably in bulk at 
 each ganglionic enlargement. The cells of which it is com- 
 posed are for the most part of a nearly uniform size, but at the 
 border of the fibrous matter a fair sprinkling of larger cells is 
 found. 
 
 The tracheal vessels supplying the nervous system are placed 
 amongst the larger cells, at the boundary between the ganglionic 
 and fibrous regions of the cords. 
 
 With reference to the peripheral nerve-stems there is not 
 much to be said. They have for the most part a similar struc- 
 ture to the fibrous parts of the main cord, but are provided with 
 a somewhat larger number of cells. 
 
 1 Vide Spengel, " Oligognatluis Ronelliac." Naph-s Mittht-ihiti^-tt, IJd. in. pi. iv. 
 
 fur. 5:. 
 
 B. 57 
 
890 ANATOMY AND DEVELOPMENT 
 
 Sheath of the Ventral Cords. The ventral cords are en- 
 veloped by a double sheath, the two layers of which are often in 
 contact, while in other cases they may be somewhat widely 
 separated from each other. The inner layer is extremely thin 
 and always very closely envelopes the nerve-cords. The outer 
 layer is thick and fibrous, and contains a fair sprinkling of 
 nuclei. 
 
 Supra-cesophageal Ganglia. In the present state of our know- 
 ledge a very detailed description of the histology of the supra- 
 oesophageal ganglia would be quite superfluous, and I shall 
 confine myself to a description of the more obvious features in 
 the arrangement of the ganglionic and fibrous portions (PI. 50, 
 fig. 19 A G). 
 
 The ganglion cells are in the first place confined, for the 
 most part, to the surface. Along the under side of each gan- 
 glion there is a very thick layer of cells, continuous behind, 
 with the layer of ganglion cells which is placed on the under 
 surface of the cesophageal commissures. These cells have, 
 moreover, an arrangement very similar to that in the ventral 
 cords, so that a section through the supra-cesophageal ganglia 
 has an obvious resemblance to what would be the appearance 
 of a section through the united ventral cords. On the outer 
 borders of the ganglia the cells extend upwards, but they end 
 on about the level of the optic nerve (fig. 19 D). Immediately 
 dorsal to this point the fibrous matter of the brain is exposed 
 freely on the surface (fig. 19 A, B, &c., a}. I shall call the region 
 of fibrous matter so exposed the dorso-lateral horn of white 
 matter. 
 
 Where the two ganglia separate in front the ganglion cells 
 spread up the inner side, and arch over so as to cover part of 
 the dorsal side. Thus, in the anterior part, where the two 
 ganglia are separate, there is a complete covering of ganglionic 
 substance, except for a narrow strip, where the dorso-lateral 
 lobe of white matter is exposed on the surface (fig. 19 A). From 
 the point where the two ganglia meet in front the nerve-cells 
 extend backwards as a median strip on the dorsal surface (fig. 
 19 D and E). This strip, becoming gradually smaller behind, 
 reaches nearly, though not quite, the posterior limit of the junc- 
 tion of the ganglia. Behind it there is, however, a region where 
 
OF PERIPATUS CAPENSIS. 89! 
 
 the whole dorsal surface of the ganglia is without any~covering 
 of nerve-cells. 
 
 This tongue of ganglion cells sends in, slightly behind the 
 level of the eyes, a transverse vertical prolongation inwards into 
 the white matter of the brain, which is shewn in the series of 
 transverse sections in fig. 19 E, and also in the vertical longi- 
 tudinal section (PI. 51, fig. 21), and in horizontal section in 
 PI. 51, fig. 22. 
 
 On the ventral aspect of each lobe of the brain there is pre- 
 sent a very peculiar, bluntly conical protuberance of ganglion 
 cells (PI. 51, fig. 22), which was first detected by Grube (No. 10), 
 and described by him as "a white thick body of a regular 
 tetrahedral form, and exhibiting an oval dark spot in the middle 
 of two of the faces." He further states that it is united by a 
 delicate nerve to the supra-cesophageal ganglion, and regards it 
 as an organ of hearing. 
 
 In Pcripatus capensis the organ in question can hardly be 
 described as tetrahedral. It is rather of a flattened oval form, 
 and consists, as shewn in sections (PI. 50, fig. 19 C and D, d\ 
 mainly of ganglion cells. In its interior is a cayity with a distinct 
 bounding membrane : the cells of which it is composed vary 
 somewhat in size, being smallest near the point of attachment. 
 At its free end is placed a highly refractive, somewhat oval 
 body, probably forming what Grube describes as a dark spot, 
 half embedded in its substance, and kept in place by the sheath 
 of nervous matter surrounding it. This body appears to have 
 fallen out in my sections. The whole structure is attached to 
 the under surface of the brain by a very short stalk formed of a 
 bundle of cells and nervous fibres. 
 
 It is difficult to offer any interpretation of the nature of this 
 body. It is removed considerably from the surface of the 
 animal, and is not, therefore, so far as I can see, adapted to serve 
 as an organ of hearing. 
 
 The distribution of the white or fibrous matter of the ganglia 
 is not very easy to describe. 
 
 There is a central lobe of white matter (fig. 19 E), which 
 is continuous from ganglion to ganglion, where the two are 
 united. It is smaller behind than in front. On its ventral side 
 it exhibits fairly well-marked transverse commissural fibres, con- 
 
 572 
 
892 ANATOMY AND DEVELOPMENT 
 
 necting the two halves of the ganglion. Laterally and some- 
 what ventrally it is prolonged into a horn (fig. 19 D, E, b], which 
 I propose calling the ventro-lateral horn. In front it is placed 
 in a distinct protuberance of the brain, which is placed ventrally 
 to and nearly in the same vertical plane as the optic nerve. 
 This protuberance is best shewn in the view of the brain from 
 below given in PI. 51, fig. 22. This part of the horn is charac- 
 terized by the presence of large vertically-directed bundles of 
 nerve-fibres, shewn in transverse section in fig. 190. Posteriorly 
 the diameter of this horn is larger than in front (fig. 19, E, F, G), 
 but does not give rise to a protuberance on the surface of the 
 brain owing to the smaller development of the median lobe 
 behind. 
 
 The median lobe of the brain is also prolonged into a dorso- 
 lateral lobe (fig. 19, a), which, as already mentioned, is freely 
 exposed on the surface. On its ventral border there springs the 
 optic nerve, and several pairs of sensory nerves already de- 
 scribed (fig. 19 D, E), while from its dorsal border a pair of 
 sensory nerves also spring, nearly in the same vertical plane as 
 the optic nerves. 
 
 Posteriorly where the dorsal surface of the brain is not 
 covered in with ganglion cells the dorso-lateral horn and median 
 lobe of the brain become indistinguishable. 
 
 In the front part of the brain the median lobe of white matter 
 extends dorsalwards to the dorsal strip of ganglion cells, but 
 behind the region of the transverse prolongation of these cells, 
 into the white matter already described (p. 890), there is a more 
 or less distinctly defined lobe of white matter on the dorsal 
 surface, which I propose calling the postero-dorsal lobe of white 
 matter. It is shewn in the transverse sections (fig. 19 F and 
 G, c). It gradually thins away and disappears behind. It is 
 mainly characterized by the presence on the ventral border of 
 definite transverse commissural fibres. 
 
OF PERIPATUS CAPENSIS. 893 
 
 THE SKIN. 
 
 The skin is formed of three layers. 
 
 1. The cuticle. 
 
 2. The epidermis or hypodermis. 
 
 3. The dermis. 
 
 The cuticle is a layer of about O'CO2 mm. in thickness. Its 
 surface is not, however, smooth, but is everywhere, with the 
 exception of the perioral region, raised into minute secondary 
 papillae, the base of which varies somewhat in diameter, but is 
 usually not far from O'O2 mm. On the ventral surface of the 
 body these papillae are for the most part somewhat blunt, but 
 on the dorsal surface they are more or less sharply pointed. In 
 most instances they bear at their free extremity a somewhat 
 prominent spine. The whole surface of each of the secondary 
 papillas just described, is in its turn covered by numerous 
 minute spinous tubercles. In the perioral region, where the 
 cuticle is smooth, it is obviously formed of two layers which 
 easily separate from each other, and there is I believe a similar 
 division elsewhere, though it is not so easy to see. It is to be 
 .presumed that the cuticle is regularly shed. 
 
 The epidermis, placed immediately within the cuticle, is 
 composed of a single row of cells, which vary, however, a good 
 deal in size in different regions of the body. The cells excrete 
 the cuticle, and, as shewn in fig. 32, they stand in a very re- 
 markable relation to the secondary papillae of the cuticle just 
 described. Each epidermis cell is in fact placed within one of 
 these secondary papillae, so that the cuticle of each secondary 
 papilla is the product of a single epidermis cell. This relation 
 is easily seen in section, while it may also be beautifully shewn 
 by taking a part of the skin which is not too much pigmented, 
 and, after staining it, examining from the surface. 
 
 In fig. 32 a region of the epidermis is figured, in which the 
 cells are exceptionally columnar. The cwticle has, moreover, 
 in the process of cutting the section, been somewhat raised and 
 carried away from the subjacent cells. The cells of the epi- 
 dermis are provided with large oval nuclei, which contain a well- 
 
894 ANATOMY AND DEVELOPMENT 
 
 developed reticulum, giving with low powers a very granular 
 appearance to the nuclei. The protoplasm of the cells is also 
 somewhat granular, and the granules are frequently so disposed 
 as to produce a very well-marked appearance of striation on 
 the inner end of the cells. The pigment which gives the charac- 
 teristic colour to the skin is deposited in the protoplasm of the 
 outer ends of the cells in the form of small granules. An at- 
 tempt is made to shew this in fig. 32. 
 
 At the apex of most, if not all, the primary wart-like papillae 
 there are present oval aggregations, or masses of epidermis 
 cells, each such mass being enclosed in a thickish capsule (fig. 
 31). The cells of these masses appear to form the wall of a 
 cavity which leads into the hollow interior of a long spine. 
 These spines when carefully examined with high objectives 
 present a rather peculiar structure. The base of the spine is 
 enveloped by the normal cuticle, but the spine itself, which 
 terminates in a very fine point, appears, as shewn in fig. 31, to 
 be continuous with the inner layer of the cuticle. In the 
 perioral region the outer layer of the cuticle, as well as the 
 inner, appear to be continued to the end of the spines. Within 
 the base of the spine there is visible a finely striated substance 
 which may often be traced into the cavity enclosed by the cells, 
 and appears to be continuous with the cells. Attached to the 
 inner ends of most of the capsules of these organs a delicate 
 fibrillated cord may be observed, and although I have not in any 
 instance succeeded in tracing this cord into one of the nerve- 
 stems, yet in the antennae, where the nerve-stems are of an 
 enormous size, I have satisfied myself that the minute nerves 
 leaving the main nerve-stems and passing out towards the skin 
 are histologically not to be distinguished from these fibrillated 
 cords. I have therefore but little hesitation in regarding these 
 cords as nerves. 
 
 In certain regions of the body the oval aggregations of cells 
 are extremely numerous ; more especially is this the case in the 
 antennae, lips, and oral papillae. On the ventral surface of the 
 peripheral rings of Jhe thicker sections of the feet they are 
 also very thick set (fig. 20 P). They here form a kind of pad, 
 and have a more elongated form than in other regions. In the 
 antennae they are thickly set side by side on the rings of skin 
 
OF PERIPATUS CAPENSIS. 895 
 
 which give such an Arthropod appearance to these organs in 
 Peripatus. 
 
 The arrangement of the cells in the bodies just described led 
 me at first to look upon them as glands, but a further inves- 
 tigation induced me to regard them as a form of tactile organ. 
 The arguments for this view are both of a positive and a nega- 
 tive kind. 
 
 The positive arguments are the following : 
 
 (1) The organs are supplied with large nerves, which is dis- 
 tinctly in favour of their being sense organs rather than glands. 
 
 (2) The peculiar striae at the base of the spines appear to me 
 like the imperfectly preserved remains of sense hairs. 
 
 (3) The distribution of the.se organs favours the view that 
 they are tactile organs. They are most numerous on the an- 
 tennas, where such organs would naturally be present, especially 
 in a case like that of Peripatus, where the nerve passing to 
 the antennae is simply gigantic. On the other hand, the an- 
 tennae would not be a natural place to look for an enormous 
 development of dermal glands. 
 
 The lips, oral papillae, and under surface of the legs, where 
 these bodies are also very numerous, are situations where tactile 
 organs would be of great use. 
 
 Under the head of negative arguments must be classed those 
 which tell against these organs being glandular. The most im- 
 portant of these is the fact that they have no obvious orifice. 
 Their cavities open no doubt into the spines, but the spines 
 terminate in such extremely fine points that the existence of an 
 orifice at their apex is hardly credible. 
 
 Another argument, from the distribution of these organs over 
 the body is practically the converse of that already used. The 
 distribution being as unfavourable to the view that they are 
 glands, as it is favourable to that of their being sense organs. 
 
 THE TRACHEAL SYSTEM. 
 
 The apertures of the tracheal system are placed in the de- 
 pressions between the papillae or ridges of the skin. Each of 
 them leads into a tube, which I shall call the tracheal pit (fig. 
 30), the walls of which are formed of epithelial cells bounded 
 
896 ANATOMY AND DEVELOPMENT 
 
 towards the lumen of the pit by a very delicate cuticular mem- 
 brane continuous with the cuticle covering the surface of the 
 body. The pits vary somewhat in depth; the pit figured was 
 about O'OQ mm. It perforates the dermis and terminates in the 
 subjacent muscular layer. The investigation of the inner end of 
 the pit gave me some little trouble. 
 
 Transverse sections (fig. 30) through the trunk containing a 
 tracheal opening shew that the walls of the pit expanded inter- 
 nally in a mushroom-like fashion, the narrow part being, how- 
 ever, often excentric in relation to the centre of the expanded 
 part. 
 
 Although it was clear that the tracheae started from the ex- 
 panded region of the walls of the pit, I could not find that the 
 lumen of the pit dilated into a large vesicle in this part, and 
 further investigation proved that the tracheae actually started 
 from the slightly swollen inner extremity of the narrow part of 
 the pit, the expanded walls of the pit forming an umbrella-like 
 covering for the diverging bundles of tracheae. 
 
 I have, in fig. 30, attempted to make clear this relation be- 
 tween the expanded walls of the tracheal pits and the tracheae. 
 In longitudinal sections of the trunk the tracheal pits do not 
 exhibit the lateral expansion which I have just described, which 
 proves that the divergence of the bundles of tracheae only takes 
 place laterally and not in an antero-posterior direction. Cells 
 similar in general character to those of the walls of the tracheal 
 pits are placed between the branches of tracheae, and somewhat 
 similar cells, though generally with more elongated nuclei, ac- 
 company the bundles of tracheae as far as they can be followed 
 in my sections. The structure of these parts in the adult would, 
 in fact, lead one to suppose that the tracheae had originated at 
 the expense of the cells of pits of the epidermis, and that the 
 cells accompanying the bundles of tracheae were the remains of 
 cords of cells which sprouted out from the blind ends of the 
 epidermis pits and gave rise in the first instance to the tracheae. 
 
 The tracheae themselves are extremely minute, unbranched 
 (so far as I could follow them) tubes. Each opening by a sepa- 
 rate aperture into the base of the tracheal pit, and measuring 
 about 0-002 mm. in diameter. They exhibit a faint transverse 
 striation, which I take to be the indication of a spiral fibre. 
 
OF PERIPATUS CAPENSIS. 897 
 
 [Moseley (Phil. Trans., 1874, PL 73, fig. i) states that the 
 tracheae branch, but only exceptionally.] 
 
 Situation of the tracJieal apertures. Moseley states (No. 13) 
 that the tracheae arise from the skin all over the surface of the 
 body, but are especially developed in certain regions. He finds 
 "a row of minute oval openings on the ventral surface of the 
 body," the openings being "situate with tolerable regularity in 
 the centres of the interspaces between the pairs of members, but 
 additional ones occurring at irregular intervals. Other similar 
 openings occur in depressions on the inner side of the conical 
 foot protuberance." It is difficult in preserved specimens to 
 make out the exact distributions of the tracheal apertures, but I 
 have been able to make out certain points about them. 
 
 There is a double row of apertures on each side of the 
 median dorsal line, forming two sub-dorsal rows of apertures. 
 The apertures are considerably more numerous than the legs. 
 There is also a double row of openings, again more numerous 
 than the legs, on each side of the median ventral line between 
 the insertions of the legs. Moseley speaks of a median row in 
 this position. I think this must be a mistake. 
 
 Posteriorly the two inner rows approach very close to each 
 other in the median ventral line, but I have never seen them 
 in my section opening quite in the middle line. Both the dorsal 
 and ventral rows are very irregular. 
 
 I have not found openings on the ventral or dorsal side of 
 the feet but there are openings at the anterior and posterior 
 aspects of the feet. There are, moreover, a considerable num- 
 ber of openings around the base of the feet. 
 
 The dorsal rows of tracheal apertures are continued into 
 the head and give rise in this situation to enormous bundles of 
 tracheae. 
 
 In front of the mouth there is a very large median ventral 
 tracheal pit, which gives off tracheae to the ventral part of the 
 nervous system, and still more in front a large number of such 
 pits close together. The tracheae to the central nervous system 
 in many instances enter the nervous system bound up in the 
 same sheath as the nerves. 
 
898 ANATOMY AND DEVELOPMENT 
 
 THE MUSCULAR SYSTEM. 
 
 The general muscular system consists of (i) the general 
 wall of the body; (2) the muscles connected with the mouth, 
 pharynx, and jaws; (3) the muscles of the feet; (4) the muscles 
 of fhe alimentary tract. 
 
 The muscular wall of the body is formed of (i) an external 
 layer of circular fibres; (2) an internal layer of longitudinal 
 muscles; (3) a layer of transverse fibres. 
 
 The layer which I have spoken of as formed of circular fibres 
 is formed of two strata of fibres which girth the body somewhat 
 obliquely (PI. 51, fig. 25). In the outer stratum the rings are 
 arranged so that their ventral parts are behind, while the ventral 
 parts of the rings of the inner stratum are most forward. Both 
 in the median dorsal and ventral lines the layer of circular fibres 
 become somewhat thinner, and where the legs are attached the 
 regularity of both strata is somewhat interfered with, and they 
 become continuous with a set of fibres inserted in the wall of the 
 foot. 
 
 The longitudinal muscles are arranged as five bands (vide 
 fig. 1 6), viz. two dorsal, two lateral, and three ventral. The 
 three ventral may be spoken of as the latero-ventral and medio- 
 ventral bands. 
 
 The transverse fibres consist of (i) a continuous sheet on 
 each side inserted dorsally in the cutis, along a line opposite 
 the space between the dorsal bands of longitudinal fibres, and 
 ventrally between the ventro-median and ventro-lateral bands. 
 Each sheet at its insertion slightly breaks up into separate 
 bands. They divide the body-cavity into three regions a 
 median, containing the alimentary tract, slime glands, &c., and 
 two lateral, which are less well developed, and contain the nerv- 
 ous system, salivary glands, segmental organs, &c. 
 
 (2) Inserted a little dorsal to the transverse band just de- 
 scribed is a second band which immediately crosses the first, 
 and then passes on the outer side of the nervous cord and 
 salivary gland, where such is present, and is inserted ventrally 
 in the space between the ventro-lateral and lateral longitudinal 
 band. 
 
OF PERIPATUS CAPENSIS. 899 
 
 Where the feet are given off the second transverse band be- 
 comes continuous with the main retractor muscular fibres in the 
 foot, which are inserted both on to the dorsal side and ventral 
 side. 
 
 Muscular system of the feet. This consists of the retractors 
 of the feet connected with the outer transverse muscle and the 
 circular layer of muscles. In addition to these muscles there are 
 intrinsic transverse muscles which cross the cavity of the feet in 
 various directions (PI. 51, fig. 20). There is no special circular 
 layer of fibres. 
 
 Histology of the muscle. The main muscles of the body are 
 unstriated and divided into fibres, each invested by a delicate 
 membrane. Between the membrane and muscle are scattered 
 nuclei, which are never found inside the muscle fibres. The 
 muscles attached to the jaws form an exception in that they are 
 distinctly transversely striated. 
 
 THE BODY-CAVITY AND VASCULAR SYSTEM. 
 
 The body-cavity, as already indicated, is formed of three 
 compartments one central and two lateral. The former is by 
 far the largest, and contains the alimentary tract, the generative 
 organs, and the mucous glands. It is lined by a delicate endo- 
 thelial layer, and is not divided into compartments nor traversed 
 by muscular fibres. 
 
 The lateral divisions are much smaller than the central, and 
 are shut off from it by the inner transverse band of muscles. 
 They are almost entirely filled with the nerve-cord and salivary 
 gland in front and with the nerve-cord alone behind, and their 
 lumen is broken up by muscular bands. They further contain 
 the segmental organs which open into them. They are pro- 
 longed into the feet, as is the embryonic body-cavity of most 
 Arthropoda. 
 
 The vascular system is usually stated to consist of a dorsal 
 heart. I find between the dorsal bands of longitudinal fibres 
 a vessel in a space shut off from the body-cavity by a con- 
 tinuation of the endothelial lining of the latter (fig. 16). The 
 vessel has definite walls and an endothelial lining, but I could 
 not make out whether the walls were muscular. The ventral 
 
ANATOMY AND DEVELOPMENT 
 
 part of it is surrounded by a peculiar cellular tissue, probably, as 
 suggested by Moseley, equivalent to the fat bodies of insects. 
 It is continued from close to the hind end of the body to the 
 head, and is at its maximum behind. In addition to this vessel 
 there is present a very delicate ventral vessel, by no means easy 
 to see, situated between the cutis and the outer layer of circular 
 muscles. 
 
 SEGMENTAL ORGANS. 
 
 A series of glandular organs are found in Peripatus which 
 have their external openings situated on the ventral surface of a 
 certain number of the legs, and which, to the best of my belief, 
 end internally by opening into the lateral compartments of the 
 body-cavity. These organs are probably of an excretory nature, 
 and I consider them homologous with the nephridia or seg- 
 mental organs of the Chaetopoda. 
 
 In Peripatus capensis they are present in all the legs. In all 
 of them (except the first three) the following parts may be 
 recognized : 
 
 (1) A vesicular portion opening to the exterior by a narrow 
 passage. 
 
 (2) A coiled portion, which is again subdivided into several 
 sections. 
 
 (3) A terminal section ending by a somewhat enlarged open- 
 ing into the lateral compartment of the body-cavity. 
 
 The last twelve pairs of these organs are all constructed in a 
 very similar manner, while the two pairs situated in the fourth 
 and fifth pairs of legs are considerably larger than those behind, 
 and are in some respects very differently constituted. 
 
 It will be convenient to commence with one of the hinder 
 nephridia. Such a nephridium from the ninth pair of legs is 
 represented in fig. 28. The external opening is placed at the 
 outer end of a transverse groove placed at the base of one of the 
 feet, while the main portion of the organ lies in the body-cavity 
 in the base of the leg, and extends into the trunk to about the 
 level of the outer edge of the nerve-cord of its side. The ex- 
 ternal opening (p s) leads into a narrow tube (s d], which 
 gradually dilates into a large sack (s). 
 
OF PERIPATUS CAPENSIS. QOI 
 
 The narrow part is lined by small epithelial cells, which are 
 directly continuous with and perfectly similar to those of the 
 epidermis (fig. 20). It is provided with a superficial coating 
 of longitudinal muscular fibres, which thins out where it passes 
 over the sack, along which it only extends for a short distance. 
 
 The sack itself, which forms a kind of bladder or collecting 
 vesicle for the organ, is provided with an extremely thin wall, 
 lined with very large flattened cells. These cells are formed of 
 granular protoplasm, and each of them is provided with a large 
 nucleus, which causes a considerable projection into the lumen 
 of the sack (figs. 20, 29 .$). The epithelial wall of the sack is 
 supported by a membrana propria, over which a delicate layer 
 of the peritoneal epithelium is reflected. 
 
 The coiled tube forming the second section of the nephridium 
 varies in length, and by the character of the epithelium lining 
 it may be divided into four regions. It commences with a region 
 lined by a fairly columnar epithelium with smallish nuclei (fig. 
 28 s c i). The boundaries of the cells of this epithelium are 
 usually very indistinct, and the protoplasm contains numerous 
 minute granules, which are usually arranged in such a manner 
 as to give to optical or real sections of the wall of this part of 
 the tube a transversely striated appearance. These granules are 
 very probably minute balls of excretory matter. 
 
 The nuclei of the cells are placed near their free extremities, 
 contrary to what might have been anticipated, and the inner 
 ends of the cells project for very different lengths into the inte- 
 rior, so causing the inner boundary of the epithelium of this 
 part of the tube to have a very ragged appearance. This por- 
 tion of the coiled tube is continuous at its outer end with the 
 thin-walled vesicle. At its inner end it is continuous with region 
 No. 2 of the coiled tube (fig. 28 s c 2), which is lined by small 
 closely-packed columnar cells. This portion is followed by 
 region No. 3, which has a very characteristic structure (fig. 
 28 s c 3). The cells lining this part are very large and flat, and 
 contain large disc-shaped nuclei, which are usually provided 
 with large nucleoli, and often exhibit a beautiful reticulum. 
 They may frequently be observed in a state of division. The 
 protoplasm of this region is provided with similar granules to 
 that in the first region, and the boundaries of the cells are usually 
 
902 ANATOMY AND DEVELOPMENT 
 
 very indistinct. The fourth region is very short (fig. 28 s c 4), 
 and is formed of small columnar cells. It gradually narrows 
 till it opens suddenly into the terminal section (s o t\ which 
 ends by opening into the body-cavity, and constitutes the most 
 distinct portion of the whole organ. Its walls are formed of 
 columnar cells almost filled by oval nuclei, which absorb 
 colouring matters with very great avidity, and thus renders 
 this part extremely conspicuous. The nuclei are arranged in 
 several rows. 
 
 The study of the internal opening of this part gave me some 
 trouble. No specimens ever shew it as rounded off in the 
 characteristic fashion of tubes ending in a cul-de-sac. It is 
 usually somewhat ragged and apparently open. In the best 
 preserved specimens it expands into a short funnel-shaped 
 mouth, the free edge of which is turned back. Sections confirm 
 the results of dissections. Those passing longitudinally through 
 the opening prove its edges are turned back, forming a kind of 
 rudimentary funnel. This is represented in fig. 29, from the last 
 leg of a female. I have observed remains of what I consider 
 to be cilia in this section of the organ. The fourth region of the 
 organ is always placed close to the thin-walled collecting vesicle 
 (figs. 28 and 29). In the whole of the coiled tube just de- 
 scribed the epithelium is supported by a membrana propria, 
 which in its turn is invested by a delicate layer of peritoneal 
 epithelium. 
 
 The fourth and fifth pairs are very considerably larger than 
 those behind, and are in other respects peculiar. The great 
 mass of each organ is placed behind the leg, on which the ex- 
 ternal opening is placed, immediately outside one of the lateral 
 nerve-cords. Its position is shewn in fig. 8. 
 
 The external opening, instead of being placed near the base 
 of the leg, is placed on the ventral side of the third ring (count- 
 ing from the outer end) of the thicker portion of the leg. It 
 leads (fig. 27) into a portion which clearly corresponds with the 
 collecting vesicle of the hinder nephridia. This part is not, 
 however, dilated into a vesicle in the same sort of way, and the 
 cells which form the lining epithelium have not the same charac- 
 teristic structure, but are much smaller. Close to the point 
 where the vesicle joins the coiled section of the nephridium the 
 
OF PERIPATUS CAPENSIS. 903 
 
 former has a peculiar nick or bend in it. At this nick it is firmly 
 attached to the ventral side of the foot by muscles and tracheae, 
 and when cut away from its attachment the muscles and trachese 
 cannot easily be detached from it. The main part of the coils 
 are formed by region No. i, and the epithelial cells lining this 
 part present very characteristically the striated appearance which 
 has already been spoken of. The large-celled region of the 
 coiled tube (fig. 27) is also of considerable dimensions, and the 
 terminal portion is wedged in between this and the commencing 
 part of the coiled tube. The terminal portion with its internal 
 opening is in its histological characters exactly similar to the 
 homologous region in the hinder nephridia. 
 
 The three pairs of nephridia in the three foremost pairs of 
 legs are very rudimentary, consisting, so far as I have been 
 able to make out, solely of the collecting vesicle and the duct 
 leading from them to the exterior. The external opening is 
 placed on the ventral side of the base of the feet, in the same 
 situation as that of the posterior nephridia, but the histological 
 characters of the vesicle are similar to those of the fourth and 
 fifth pairs. 
 
 GENERATIVE ORGANS. 
 
 [The sexes are distinct, and the average size of the females 
 appears to be greater than that of the males. 
 
 The only outward characteristic by which the males can be 
 distinguished from the females is the presence in the former of 
 a small white papilla on the ventral side of the I7th pair of legs 
 (PI. 47, fig. 4). At the extremity of this papilla the modified 
 crural gland of the last leg opens by a slit-like aperture. 
 
 The generative orifice in both sexes is placed on the ventral 
 surface of the body, close to the anus, and between the two anal 
 papillae, which are much more marked in small specimens than 
 in large ones, and in two cases (of females) were observed to 
 bear rudimentary claws. 
 
 i. The Male Organs. PI. 53, fig. 43. 
 
 The male organs consist of a pair of testes (te), a pair of 
 prostrates (pr) and vasa deferentia (vd) and accessory glandular 
 tubules (/). 
 
904 ANATOMY AND DEVELOPMENT 
 
 All the above parts lie in the central compartment of the 
 body-cavity. In addition, the accessory glandular bodies or 
 crural glands of the last (i/th) pair of lens are enlarged and 
 prolonged into an elongated tube placed in the lateral com- 
 partment of the body-cavity (a g}. 
 
 The arrangement of these parts represented in the figure 
 appears essentially that which Moseley has already described 
 for this species. The dilatations on the vasa deferentia, which 
 he calls vesiculse seminales, is not so marked ; nor can the 
 peculiar spiral twisting of this part of the vas deferens which he 
 figures (No. 13) be made out in this specimen. The testes are 
 placed at different levels in the median compartment of the body- 
 cavity, and both lie on the same side of the intestine (right side). 
 
 The arrangement of the terminal portions of the vas deferens 
 is precisely that described by Moseley. The right vas deferens 
 passes under both nerve-cords to join the left, and from the 
 enlarged tube (/), which, passing beneath the nerve-cord of its 
 side, runs to the external orifice. The enlarged terminal portion 
 possesses thick muscular walls, and possibly constitutes a sper- 
 matophore maker, as has been shewn to be the case in P. N. 
 Zealandiae, by Moseley. 
 
 In some specimens a different arrangement obtains, in that 
 the left vas deferens passes under both nerve-cords to join the 
 right. 
 
 In addition to the above structures, which are all described 
 by Moseley, there are a pair of small glandular tubes (/), 
 which open with the unpaired terminal portion of the vas 
 deferens at the generative orifice. 
 
 2. Female Organs. PL 52, fig. 33. 
 
 The female organs consist of a median unpaired ovary and 
 a pair of oviducts, which are dilated for a great part of their 
 course to perform a uterine function, and which open behind 
 into a common vestibule communicating directly with the 
 exterior. 
 
 Ovary. In the specimen figured the following is the arrange- 
 ment : 
 
 The ovary lies rather to the dorsal side in the central com- 
 partment of the bodj'-cavity, and is attached to one of the 
 
OF PERIPATUS CAPENSIS. 905 
 
 longitudinal septa separating this from the lateral compart- 
 ment. It lies between the penultimate and antepenultimate pair 
 of legs. 
 
 The oviducts cross before opening to the exterior. The 
 right oviduct passes under the rectum, and the left over the 
 rectum. They meet by opening into a common vestibule, 
 which in its turn opens to the exterior immediately ventral to 
 the anus. It has not been ascertained how far this arrange- 
 ment, which differs from that observed by Moseley, is a normal 
 one. The young undergo nearly the whole of their develop- 
 ment within the uterus. They possess at birth the full number 
 of appendages, and differ from the parent only in size and 
 colour.] 
 
 NOTES ON ADDITIONAL GLANDULAR BODIES IN THE LEGS 
 [CRURAL GLANDS]. 
 
 1. They are present in all except the first. 
 
 2. They open externally to the nephridia (PI. 51, fig. 20), 
 except in the fourth and fifth pairs of legs, in which they are 
 internal. 
 
 3. A muscular layer covers the whole gland, consisting, I 
 believe, of an oblique circular layer. 
 
 4. The accessory gland in the male (fig. 43, ag) is probably 
 a modification of one of these organs. 
 
 [The structure and relations of these glands may be best 
 understood by reference to PI. 51, fig. 20. Each consists of a 
 dilated vesicular portion (fgl] placed in the lateral compart- 
 ment of the body cavity in the foot, and of a narrow duct 
 leading to the exterior, and opening on the ventral surface 
 amongst the papillae of the second row (counting from the in- 
 ternal of the three foot pads fig. 20 P). 
 
 The vesicular portion is lined by columnar cells, with very 
 large oval nuclei, while the duct is lined by cells similar to 
 the epidermic cells, with which they are continuous at the 
 opening. 
 
 In the last (i/th) leg of the males of this species, this gland 
 (i'ittc above, note 4) possesses a slit-like opening placed at the 
 
906 ANATOMY AND DEVELOPMENT 
 
 apex of a well-developed white papilla (PI. 47, fig. 4). It is 
 enormously enlarged, and is prolonged forward as a long tubular 
 gland, the structure of which resembles that of the vesicles of 
 the crural glands in the other legs. This gland lies in the 
 lateral compartment of the body cavity, and extends forward to 
 the level of the Qth leg (PI. 48, fig- 8, and PL 53, fig. 43)- It is 
 described by Professor Balfour as the accessory gland of the 
 male, and is seen in section lying immediately dorsal to the 
 nerve-cord in fig. 20, ag.] 
 
 PART III. 
 
 THE DEVELOPMENT OF PERIPATUS CAPENSIS. 
 
 [The remarkable discoveries about the early development of 
 Peripatus, which Balfour made in June last, shortly before 
 starting for Switzerland, have already been the subject of a 
 short communication to the Royal Society (Proc. Roy. Soc. 
 No. 222, 1882). They relate (i) to the blastopore, (2) to the 
 origin of the mesoblast. 
 
 Balfour left no manuscript account or notes of his discovery 
 in connection with the drawings which he prepared in order to 
 illustrate it, but he spoke about it to Professor Ray Lankester 
 and also to us, and he further gave a short account of the matter 
 in a private letter to Professor Kleinenberg. 
 
 In this letter, which by the courtesy of Professor Kleinenberg 
 we have been permitted to see, he describes the blastopore as an 
 elongated slit-like structure extending along nearly the whole 
 ventral surface ; and further states, as the result of his examin- 
 ation of the few and ill-preserved embryos in his possession, 
 that the mesoblast appears to originate as paired outgrowths 
 from the lips of the blastopore. 
 
 The drawings left by Balfour in connection with the dis- 
 coveries are four in number: one of the entire embryo, shewing 
 the slit-like blastopore and the mesoblastic somites, the other 
 three depicting the transverse sections of the same embryo. 
 
OF PERIPATUS CAPENSIS. 907 
 
 The first drawing (fig. 37), viz. that of the whole embryo, 
 shews an embryo of an oval shape, possessing six somites, 
 whilst along the middle of its ventral surface there are two slit- 
 like openings, lying parallel to the long axis of the body, and 
 placed one behind the other. The mesoblastic somites are ar- 
 ranged bilaterally in pairs, six on either side of these slits. The 
 following note in his handwriting is attached to this drawing: 
 
 "Young larva of Pcripatus capcnsis. I could not make out 
 for certain which was the anterior end. Length r34 milli- 
 metres." 
 
 Balfour's three remaining drawings (figs. 40 42) are, as 
 already stated, representations of transverse sections of the 
 embryo figured by him as a whole. They tend to shew, as 
 he sfated in the letter referred to above, that the mesoblast 
 originates as paired outgrowths from the hypoblast, and that 
 these outgrowths are formed near the junction of the hypoblast 
 with the epiblast at the lips of the blastopore. 
 
 In fig. 42 the walls of the mesoblastic somites appear con- 
 tinuous with those of the mesenteron near the blastopore. 
 
 In fig. 40, which is from a section a little in front of fig. 42, 
 the walls of the mesoblastic somites are independent of those of 
 the mesenteron. 
 
 Fig. 41 is from a section made in front of the region of the 
 blastopore. 
 
 In all the sections the epiblast lying over the somites is 
 thickened, while elsewhere it is formed of only one layer of 
 cells; and this thickening subsequently appears to give rise to 
 the nervous system. Balfour in his earlier investigations on 
 the present subject found in more advanced stages of the em- 
 bryo the nerve-cords still scarcely separated from the epiblast 1 . 
 
 We have since found, in Balfour's material, embryos of a 
 slightly different age to that just described. Of these, three 
 (figs. 34, 35, 36) arc younger, while one (fig. 38) is older than 
 Balfour's embryo. 
 
 Stage A. The youngest (fig. 34) is of a slightly oval form, 
 and its greatest length is '48 mm. It possesses a blastopore, 
 
 1 Comparative Embryology, original edition, Vol. I. p. 318. [This edition, Vol. II. 
 P. 385-] 
 
 582 
 
ANATOMY AND DEVELOPMENT 
 
 which is elongated in the direction of the long axis of the em- 
 bryo, and is slightly narrower in its middle than at either end. 
 From one end of the blastopore there is continued an opaque 
 band. This we consider to be the posterior end of the blasto- 
 pore of the embryo. The blastopore leads into the archenteron. 
 
 Stage B. In the next stage (fig. 35) the embryo is elongate- 
 oval in form. Its length is 7 mm. The blastopore is elongated 
 and slightly narrowed in the middle. At the posterior end of 
 the embryo there is a mass of opaque tissue. On each side of 
 the blastopore are three mesoblastic somites. The length of the 
 blastopore is '45 mm. 
 
 Stage C. In the next stage (fig. 36) the features are much 
 the same as in the preceding. The length of the whole embryo 
 is '9 mm. 
 
 The following were the measurements of an embryo of this 
 stage with five somites, but slightly younger than that from 
 which fig. 36 was drawn. 
 
 Length of embryo ........ 74 mm. 
 
 blastopore ..... . . '46 
 
 Distance between hind end of blastopore and hind end 
 
 of body ......... '22 
 
 Distance between front end of body and front end of 
 
 blastopore ........ '06 
 
 The somites have increased to five, and there are indications 
 of a sixth being budded off from the posterior mass of opaque 
 tissue. The median parts of the lips of the blastopore have 
 come together preparatory to the complete fusion by which the 
 blastopore becomes divided into two parts. 
 
 Stage D. The next stage is Balfour's stage, and has been 
 already described. 
 
 The length is i'34. 
 
 It will be observed, on comparing it with the preceding em- 
 bryos, that while the anterior pair of somites in figs. 35 and 36 
 lie at a considerable distance from what we have called the 
 anterior end of the embryo (a), in the embryo now under con- 
 sideration they are placed at the anterior end of the body, one 
 on each side of the middle line. We cannot speak positively 
 as to how they come there, whether by a pushing forward of 
 
OF PERIPATUS CAPENSIS. 909 
 
 the anterior somites of the previous stage, or by the formation 
 of new somites anteriorly to those of the previous stage. 
 
 In the next stage it is obvious that this anterior pair of 
 somites has been converted into the prasoral lobes. 
 
 The anterior of the two openings to which the blastopore 
 gives rise is placed between the second pair of somites ; we 
 shall call it the embryonic mouth. The posterior opening 
 formed from the blastopore is elongated, being dilated in front 
 and continued back as a narrow slit (?) to very near the hind 
 end of the embryo, where it presents a second slight dilatation. 
 The anterior dilatation of the posterior open region of the 
 blastopore we shall call the embryonic anus. 
 
 Lately, but too late to be figured with this memoir, we have 
 been fortunate enough to find an embryo of apparently precisely 
 the same stage as fig. 37. We are able, therefore, to give a few 
 more details about the stage. 
 
 The measurements of this embryo were : 
 
 Length of whole embryo . . . . . i -32 mm. 
 
 Distance from front end of body to front end of mouth '32 
 
 Distance from embryonic mouth to hind end of em- 
 bryonic anus -52 
 
 Distance from hind end of embryonic anus to hind end 
 
 of body "45 
 
 Length of embryonic anus - 2 
 
 part of blastopore behind embryonic anus . '2 
 
 Greatest width of embryo '64 
 
 Stage E. In the next stage (figs. 38 and 39) the flexure 
 of the hind end of the body has considerably increased. The 
 anterior opening of the blastopore, the embryonic mouth, has 
 increased remarkably in size. It is circular, and is placed 
 between the second pair of mesoblastic somites. The anterior 
 dilatation of the posterior opening of the blastopore, the em- 
 bryonic anus, has, like the anterior opening, become much 
 enlarged. It is circular, and is placed on the concavity of the 
 ventral flexure. From its hind end there is continued to the 
 hind end of the body a groove (shewn in fig. 39 as a dotted 
 line), which we take to be the remains of the posterior slit- like 
 part of the posterior opening of the blastopore of the pre- 
 ceding stage. The posterior dilatation has disappeared. The 
 
ANATOMY AND DEVELOPMENT 
 
 embryo has apparently about thirteen somites, which are still 
 quite distinct from one another, and apparently do not com- 
 municate at this stage with the mesenteron. 
 
 The epiblast lying immediately over the somites is, as in the 
 earlier stages, thickened, and the thickenings of the two sides 
 join each other in front of the embryonic mouth, where the 
 anterior pair of mesoblastic somites (the praeoral lobes) are 
 almost in contact. 
 
 The median ventral epiblast, i.e. the epiblast in the area, 
 bounded by the embryonic mouth and anus before and behind 
 and by the developing nerve-cords laterally, is extremely thin, 
 and consists of one layer of very flat cells. Over the dorsal 
 surface of the body the epiblast cells are cubical, and arranged 
 in one layer. 
 
 Measurements of Embryo of Stage E. 
 
 Length of embryo 1*12 mm. 
 
 Greatest width . '64 
 
 Distance from front end of embryonic mouth to hind 
 
 end of embryonic anus ...... '48 
 
 Greatest length of embryonic mouth . . . . '16 
 
 Length between hind end of embryonic mouth and 
 
 front end of embryonic anus ..... '29 
 
 These measurements were made with a micrometer eyepiece, 
 with the embryo lying on its back in the position of fig. 38, so 
 that they simply indicate the length of the straight line connect- 
 ing the respective points. 
 
 This is the last embryo of our series of young stages. The 
 next and oldest embryo was 3'2 mm. in length. It had ringed 
 antennae, seventeen (?) pairs of legs, and was completely doubled 
 upon itself, as in Moseley's figure. 
 
 The pits into the cerebral ganglia and a mouth and anus 
 were present. There can be no doubt that the mouth and anus 
 of this embryo become the mouth and anus of the adult. 
 
 The important question as to the connection between the 
 adult mouth and anus, and the embryonic mouth and anus of 
 the Stage E, must, considering the great gap between Stage E 
 and the next oldest embryo, be left open. Meanwhile, we may 
 point out that the embryonic mouth of Stage E has exactly the 
 
OF PERIPATUS CAPENSIS. 911 
 
 same position as that of the adult ; but that the anus is consider- 
 ably in front of the hind end of the body in Stage E, while it is 
 terminal in the adult. 
 
 If the embryonic mouth and anus do become the adult mouth 
 and anus, there would appear to be an entire absence of stomo- 
 dasum and proctodseum in Peripatus, unless the buccal cavity 
 represents the stomodaeum. The latter is formed, as has been 
 shewn by Moseley, by a series of outgrowths round the simple 
 mouth-opening of the embryo, which enclosing the jaws give rise 
 to the tumid lips of the adult. 
 
 For our determination of the posterior and anterior ends of 
 each of these embryos, Stage A to E, we depend upon the 
 opaque tissue seen in each case at one end of the blastopore. 
 
 In Stage A it has the form of a band, extending backwards 
 from the blastopore. 
 
 In Stages B D, it has the form of an opaque mass of tissue 
 occupying the whole hind end of the embryo, and extending a 
 short distance on either side of the posterior end of the blas- 
 topore. 
 
 This opacity is due in each case to a proliferation of cells of 
 the hypoblast, and, perhaps, of the epiblast (?). 
 
 There can be no doubt that the mesoblast so formed gives 
 rise to the great majority of the mesoblastic somites. 
 
 This posterior opacity is marked in Stage C by a slight 
 longitudinal groove extending backwards from the hind end 
 of the blastopore. This is difficult to see in surface views, and 
 has not been represented in the figure, but is easily seen in 
 sections. 
 
 But in Stage D this groove has become very strongly marked 
 in surface views, and looks like a part of the original blastopore 
 of Stage C. 
 
 Sections shew that it does not lead into the archenteron, but 
 only into the mass of mesoblast which forms the posterior 
 opacity. It presents an extraordinary resemblance to the pri- 
 mitive streak of vertebrates, and the ventral groove of insect 
 embryos. 
 
 We think that there can be but little doubt that it is a part 
 of the original blastopore, which, on account of its late appear- 
 ance (this being due to the late development of the posterior 
 
912 ANATOMY AND DEVELOPMENT 
 
 part of the body to which it belongs), does not acquire the 
 normal relations of a blastopore, but presents only those 
 rudimentary features (deep groove connected with origin of 
 mesoblast) which the whole blastopore of other tracheates 
 presents. 
 
 We think it probable that the larval anus eventually shifts 
 to the hind end of the body, and gives rise to the adult anus. 
 We reserve the account of the internal structure of these em- 
 bryos (Stages A E) and of the later stages for a subsequent 
 memoir. 
 
 We may briefly summarise the more important facts of the 
 early development of Peripatus capensis, detailed in the preceding 
 account. 
 
 1. The greater part of the mesoblast is developed from the 
 walls of the archenteron. 
 
 2. The embryonic mouth and anus are derived from the 
 respective ends of the original blastopore, the middle part of the 
 blastopore closing up. 
 
 3. The embryonic mouth almost certainly becomes the 
 adult mouth, i.e. the aperture leading from the buccal cavity 
 into the pharynx, the two being in the same position. The 
 embryonic anus is in front of the position of the adult anus, but 
 in all probability shifts back, and persists as the adult anus. 
 
 4. The anterior pair of mesoblastic somites gives rise to the 
 swellings of the praeoral lobes, and to the mesoblast of the 
 head 1 . 
 
 There is no need for us to enlarge upon the importance of 
 these facts. Their close bearing upon some of the most im- 
 portant problems of morphology will be apparent to all, and 
 we may with advantage quote here some passages from Bal- 
 four's Comparative Embryology, which shew that he himself 
 long ago had anticipated and in a sense predicted their dis- 
 covery. 
 
 "Although the mesoblastic groove of insects is not a gas- 
 trula, it is quite possible that it is the rudiment of a blasto- 
 pore, the gastrula corresponding to which has now vanished 
 
 1 We have seen nothing in any of our sections which we can identify as of so- 
 called mesenchymatous origin. 
 
OF PERIPATUS CAPENSIS. 913 
 
 from development." (Comparative Embryology, Vol. I. p.- 378, 
 the original edition 1 .) 
 
 "TRACHEATA. Insecta. It (the mesoblast) grows inwards 
 from the lips of the germinal groove, which probably represents 
 the remains of a blastopore." (Comparative Embryology, Vol. II. 
 p. 291, the original edition 2 .) 
 
 " It is, therefore, highly probable that the paired ingrowths 
 of the mesoblast from the lips of the blastopore may have been, 
 in the first instance, derived from a pair of archenteric diver- 
 ticula." (Comparative Embryology, Vol. n. p. 294, the original 
 edition 3 .) 
 
 The facts now recorded were discovered in June last, only 
 a short time before Balfour started for Switzerland ; we know 
 but little of the new ideas which they called up in his mind. 
 We can only point to passages in his published works which 
 seem to indicate the direction which his speculations would have 
 taken. 
 
 After speculating as to the probability of a genetic connec- 
 tion between the circumoral nervous system of the Ccelenterata, 
 and the nervous system of Echinodermata, Platyelminthes, Chae- 
 topoda, Mollusca, &c., he goes on to say : 
 
 " A circumoral nerve-ring, if longitudinally extended, might 
 give rise to a pair of nerve-cords united in front and behind 
 exactly such a nervous system, in fact, as is present in many 
 Ncmertines (the Enopla and Pelagonemertes), in Peripatus and 
 in primitive molluscan types (Chiton, Fissurella, &c.). From 
 the lateral parts of this ring it would be easy to derive the ventral 
 cord of the Chaitopoda and Arthropoda. It is especially de- 
 serving of notice, in connection with the nervous system of the 
 above-mentioned Nemertines and Peripatus, that the commis- 
 sure connecting the two nerve-cords behind is placed on the 
 dorsal side of the intestines. As is at once obvious, by referring 
 to the diagram (fig. 231 B), this is the position this commissure 
 ought, undoubtedly, to occupy if derived from part of a nerve- 
 ring which originally followed more 6r less closely the ciliated 
 edge of the body of the supposed radiate ancestor." (Compara- 
 tive Embryology, Vol. II. pp. 311, 312, the original edition 4 .) 
 
 1 This edition, Vol. n. p. 457. * This edition, Vol. III. p. 352. 
 
 3 This edition, Vol. in. p. 356. 4 This edition, Vol. ill. pp. 378, 379. 
 
ANATOMY AND DEVELOPMENT OF PER1PATUS CAPENSIS. 
 
 The facts of development here recorded give a strong addi- 
 tional support to this latter view, and seem to render possible 
 a considerable extension of it along the same lines.] 
 
 LIST OF MEMOIRS ON PERIPATUS. 
 
 1. M. Lansdown Guilding. "An Account of a New Genus of 
 Mollusca," Zoological Journal, Vol. ll. p. 443, 1826. 
 
 2. M. Andouin and Milne-Edwards. " Classific. des Annelides et 
 description de celles qui habitent les cotes de France," p. 411, Ann. Scien. 
 Nat. ser. I. Vol. xxx. 1833. 
 
 3. M. Gervais. "Etudes p. servir a 1'histoire naturelle des Myria- 
 podes," Ann. Scien. Nat. ser. n. Vol. vn. 1837, p. 38. 
 
 4. Wiegmann. Wiegmann's Archiv, 1837. 
 
 5. H. Milne-Edwards. "Note sur le Peripate juluforme? Ann. 
 Scien. Nat. ser. n. Vol. xvm. 1842. 
 
 6. Blanchard. "Sur 1'organisation des Vers/' chap. IV. pp. 137 141, 
 Ann. Scien. Nat. ser. ill. Vol. VI 1 1. 1847. 
 
 7. Quatrefages. "Anat. des Hermelles, note on," p. 57, Ann. Scien. 
 Nat. ser. in. Vol. x. 1848. 
 
 8. Quatrefages. Hist. Nat. des Annele's, 1865, Appendix, pp. 6756. 
 
 9. De Blainville. SiippL an Diet, des Sc. Nat. Vol. I. 
 
 10. Ed. Grube. " Untersuchungen lib. d. Bau von Peripatus Ed- 
 wardsii" Archiv filr Anat. und Physiol. 1853. 
 
 11. Saenger. " Moskauer Naturforscher Sammlung," Abth. Zool. 
 1869. 
 
 12. H. N. Moseley. "On the Structure and Development of Peripatus 
 capensis? Proc. Roy. Soc. No. 153, 1874. 
 
 13. H. N. Moseley. "On the Structure and Development of Peripatus 
 capensis," Phil. Trans. Vol. CLXIV. 1874. 
 
 14. H. N. Moseley. "Remarks on Observations by Captain Hutton, 
 Director of the Otago Museum, on Peripatus nova; zealandicc," Ann. and 
 Mag. of Nat. History, Jan. 1877. 
 
 15. Captain Hutton. " Observations on Peripatiis nova: sealandice" 
 Ann. and Mag. of Nat. History, Nov. 1876. 
 
 16. F. M. Balfour. "On Certain Points in the Anatomy of Peripatns 
 capensis,'" Quart. Journ. of Micr. Science, Vol. xix. 1879. 
 
 17. A. Ernst. Nature, March loth, 1881. 
 
EXPLANATION OF PLATES. 915 
 
 EXPLANATION OF PLATES 46 53 . 
 
 COMPLETE LIST OF REFERENCE LETTERS. 
 
 A. Anus. a. Dorso-lateral horn of white matter in brain, a.g. Accessory gland 
 of male (modified accessoiy leg gland), at. Antenna, at. n. Antennary nerve, b. 
 Ventro-lateral horn of white matter of brain. b. c. Body-cavity. bl. Blastopore. 
 c. Cutis. c. Postero-dorsal lobe of white matter of brain. e.g. Supraoesophageal 
 ganglia, cl. Claw. c. m. Circular layer of muscles, co. Commissures between the 
 ventral nerve-cords, co. i. Second commissure between the ventral nerve-cords. 
 co 1 . 2. Mass of cells developed on second commissure, cor. Cornea, c. s. d. Com- 
 mon duct for the two salivary glands, at. Cuticle, d. Ventral protuberance of 
 brain. d.l.m. Dorsal longitudinal muscle of pharynx. d. . Median dorsal nerve 
 to integument from supraoesophageal ganglia, d. o. Muscular bands passing from the 
 ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity. 
 E. Eye. E. Central lobe of white matter of brain, e. n. Nerves passing outwards from 
 the ventral cords, ep. Epidermis, ep. c . Epidermis cells. F. i, F. 2,&c. First and 
 second pair of feet, &c. f. Small accessory glandular tubes of the male generative 
 apparatus. F.^. Ganglionic enlargement on ventral nerve-cord, from which a pair of 
 nerves to foot pass off. /. gl. Accessory foot-gland, v. n. Nerves to feet. g. co. 
 Commissures between the ventral nerve-cords containing ganglion cells, g. o. Gene- 
 rative orifice. H. Heart, h. Cells in lateral division of body-cavity. hy. Hypo- 
 blast, i.j. Inner jaw. j. Jaw. j. n. Nerves to jaws. L. Lips. /. Lens. /. b. c. 
 Lateral compartment of body-cavity, le. Jaw lever (cuticular prolongation of inner 
 jaw lying in a backwardly projecting diverticulum of the buccal cavity). /. m. Bands 
 of longitudinal muscles. M. Buccal cavity. M 1 . Median backward diverticulum of 
 mouth or common salivary duct which receives the salivary ducts, me. Mesenteron. 
 tnes. Mesoblaslic somite. ;//./. Muscles of jaw lever, m. s. Sheets of muscle passing 
 round the side walls of pharynx to dorsal body wall. od. Oviduct. <K. Oesophagus. 
 a's. co. GZsophageal commissures, o.f. g. Orifice of duct of foot-gland, o.j. Outer 
 jaw. op. Optic ganglion, op. n. Optic nerve, or.g. Ganglionic enlargements for 
 oral papillae, or. n. Nerves to oral papillae, or. p. Oral papilla:, o. s. Orifice of 
 duct of segmental organ, ov. Ovary. P. Pads on ventral side of foot. /. Common 
 duct into which the vasa deferentia open. /. c. Posterior lobe of brain, p. d. c. 
 Posterior commissure passing dorsal to rectum. /./. Internal opening of nephridium 
 into body cavity, ph. Pharynx, pi. Pigment in outer ends of epidermic cells, pi. r. 
 Retinal pigment, p.n. Nerves to feet. /./. Primary papilla, pr. Prostate. R. 
 Rectum. Re. Retinal rods. R. ;//. Muscle of claw. s. Vesicle of nephridium. j 1 . 
 Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia. 
 
 1 The explanations of the figures printed within inverted commas are by Professor 
 Balfour, the rest are by the Editors. 
 
gi6 EXPLANATION OF PLATES. 
 
 s. c. i. Region No. i of coiled tube of nephridium. s. c. 2. Region No. 2 of ditto. 
 s. c. 3. Region No. 3 of ditto. s. c. 4. Region No. 4 of ditto, s. d. Salivary duct. 
 s. g. Salivary gland, si. d. Reservoir of slime gland, sl.g. Tubules of slime gland. 
 s. o. i, i, 3, &c. Nephridia of ist, 2nd, &c., feet. s. o.f. Terminal portion of nephri- 
 dium. s.p. Secondary papilla, st. Stomach, st. e. Epithelium of stomach, sy. 
 Sympathetic nerve running in muscles of tongue and pharynx, sy 1 . Origin of pharyn- 
 geal sympathetic nerves. T. Tongue, t. Teeth on tongue, te. Testis. tr. Tracheae. 
 tr. c. Cells found along the course of the tracheae. tr. o. Tracheal stigma. tr. p. 
 Tracheal pit. ut. Uterus. v. c. Ventral nerve cord. v. d. Vas deferens. z/. g. 
 Imperfect ganglia of ventral cord. 
 
 PLATE 46. 
 
 Fig. i. Peripatus capensis, x 4 ; viewed from the dorsal surface. (From a 
 drawing by Miss Balfour.) 
 
 PLATE 47. 
 
 Fig. 2. A left leg of Peripatus capensis, viewed from the ventral surface ; x 30. 
 (From a drawing by Miss Balfour.) 
 
 Fig. 3. A right leg of Peripatus capensis, viewed from the front side. (From a 
 drawing by Miss Balfour.) 
 
 Fig. 4. The last left (i;th) leg of a male Peripatns capensis, viewed from the 
 ventral side to shew the papilla at the apex of which the accessory gland of the male, 
 or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.) 
 Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is 
 appended the following note. " Figure shewing the accessory genital gland of male, 
 which opens on the last pair of legs by a papilla on the ventral side. The papilla has 
 got a slit-like aperture at its extremity. " 
 
 Fig- 5- Ventral view of head and oral region of Peripatus capensis. (From a 
 drawing by Miss Balfour.) 
 
 PLATE 48. 
 
 Figs. 6 and 7 are from one drawing. 
 
 Fig. 6. Peripatus capensis dissected so as to shew the alimentary canal, slime 
 glands, and salivary glands ; x 3. (From a drawing by Miss Balfour.) 
 
 Fig. 7. The anterior end of Fig. 6 enlarged ; x 6. (From a drawing by Miss 
 Balfour.) The dissection is viewed from the ventral side, and the lips, L., have been 
 cut through in the middle line behind and pulled outwards, so as to expose the jaws, 
 /., which have been turned outwards, and the tongue, T., bearing a median row of 
 chitinous teeth, which branches behind into two. The junction of the salivary ducts, 
 s. d. , and the opening of the median duct so formed into the buccal cavity is also 
 shewn. The muscular pharynx, extending back into the space between the ist and 
 2nd pairs of legs, is followed by a short tubular oesophagus. The latter opens into 
 the large slomach with plicated walls, extending almost to the hind end of the animal. 
 The stomach at its point of junction with the rectum presents an S-shaped ventro- 
 dorsal curve. 
 
EXPLANATION OF PLATES. 917 
 
 A. Anus. at. Antenna. F. i, K. 2. First and second feet. j. Jaws.~~ r. Lips. 
 a. (Esophagus. or. p. Oral papilla. ph. Pharynx. R. Rectum. s. d. Salivary 
 duct. s. g. Salivary gland, si. d. Slime reservoir, si. g. Portion of tubules of slime 
 gland, st. Stomach. T. Tongue in roof of mouth. 
 
 Fig. 8. Peripatus capensis, x 4 ; male. (From a drawing by Miss Balfour.) 
 Dissected so as to shew the nervous system, slime glands, ducts of the latter passing 
 into the oral papilla, accessory glands opening on the last pair of legs (enlarged crural 
 glands), and segmental organs, viewed from dorsal surface. The first three pairs of 
 scgmental organs consist only of the vesicle and duct leading to the exterior. The 
 fourth and fifth pairs are larger than the succeeding, and open externally to the crural 
 glands. The ventral nerve-cords unite behind dorsal to the rectum. 
 
 A. Anus. a. g. Accessory generative gland, or enlarged crural gland of the lyth 
 leg. at. Antenna, c. g. Supra-oesophageal ganglia with eyes. co. Commissures 
 between the ventral nerve-cords, d. n. Large median nerve to dorsal integument from 
 hinder part of brain. F. i, 2, &c. Feet. g. o. Generative orifice, ee. Oesophagus. 
 as. co. QEsophageal commissures, or. p. Oral papilla, p.d.c. Posterior dorsal com- 
 missure between the ventral nerve-cords, ph. Pharynx, p. n. Nerves to feet, one 
 pair from each ganglionic enlargement. si. d. Reservoir of slime gland. si. g. 
 Tubules of slime gland. s. o. i, 2, 3, &^. Segmental organs. v. c. Ventral nerve- 
 cords, v. g. Imperfect ganglia of ventral cords. 
 
 Figs. 9 and 10. Left jaw of Peripatus capensis (male), shewing reserve jaws. 
 (From a drawing by Miss Balfour.) 
 
 Fig. 9. Inner jaw. 
 Fig. ro. Outer jaw. 
 
 PLATE 49. 
 
 Figs, ii 16. A series of six transverse sections through the head of Peripatus 
 capensis. 
 
 Fig. ir. The section is taken immediately behind the junction of the supra- 
 oesophageal ganglia, e.g., and passes through the buccal cavity, M., and jaws, o.j. 
 and i.j. 
 
 Fig. 12. The section is taken through the hinder part of the buccal cavity at the 
 level of the opening of the mouth into the pharynx and behind the jaws. The cuti- 
 cular rod-like continuation (le.) of the inner jaw lying in a backwardly directed pit of 
 the buccal cavity is shewn; on the right hand side the section passes through the 
 opening of this pit. 
 
 Fig. 13. The section passes through the front part of the pharynx, and shews the 
 opening into the latter of the median backward diverticulum of the mouth (M 1 ), 
 which receives the salivary ducts. It also shews the commencement of the ventral 
 nerve-cords, and the backwardly projecting lobes of the brain. 
 
 Fig. 14. The section passes through the anterior part of the pharynx at the level 
 of the second commissure (co. 2), between the ventral nerve-trunks, and shews the 
 mass of cells developed on this commissure, which is in contact with the epithelium of 
 the backward continuation of the buccal cavity (M 1 ). 
 
91 8 EXPLANATION OF PLATES. 
 
 Fig. 15. Section through the point of junction of the salivary ducts with the 
 median oral diverticulum. 
 
 Fig. 1 6. Section behind the pharynx through the oesophagus. 
 
 b. c. Body-cavity, c. Cutis. c. b. c. Central compartment of body-cavity, c. g. 
 Supra-cesophageal ganglia, c. m. Layer of circular muscles, co. Commissure between 
 ventral nerve-cords. co. i. Second commissure between the ventral nerve-cords. 
 co 1 . 2. Mass of cells developed on second commissure (probably sensory), c. s. d. 
 Common duct for the two salivary glands, d. I. m. Dorsal longitudinal muscles of 
 pharynx, d. o. Muscles serving to dilate the opening of the pharynx. Ep. Epider- 
 mis, e. n. Nerve passing outwards from ventral nerve-cord. H. Heart, i.j. Inner 
 jaw. j.p. Jaw papillae. L. Lips of buccal cavity. /. b. c. Lateral compartment of 
 body-cavity, le. Rod-like cuticular continuation of inner jaw, lying in a pit of the 
 buccal cavity. /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median 
 backward continuation of buccal cavity. m.L Muscles of jaw lever, m. s. Muscular 
 sheets passing from side walls of pharynx to dorsal body wall. ft'. Oesophagus. 
 a's. co. OZsophageal commissures. o.j. Outer jaw. ph. Pharynx. s. d. Salivary 
 duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sy. Sympathetic nerves 
 running in muscles of tongue or pharynx, sy 1 . Origin of sympathetic nerves to 
 pharynx. T. Tongue, v. c. Ventral nerve-cords. 
 
 Figs. 17, 1 8. Two longitudinal horizontal sections through the head of Peripattis 
 capensis. Fig. 17 is the most ventral. They are both taken ventral to the cerebral 
 ganglia. In Fig. 17 dorsal tracheal pits are shewn with tracheae passing off from 
 them. (Zeiss a a, Hartnack's camera.) c. Cutis. c. s. d. Common salivary duct. 
 ep. Epidermis, i.j. Inner jaw. M. Buccal cavity. M 1 . Median backward diverti- 
 culum of mouth, o.j. Outer jaw. s. d. Salivary ducts. T. Tongue. /. Teeth on 
 tongue, tr. Tracheae, tr. p. Tracheal pits. 
 
 PLATE 50. 
 
 Fig. 19. "A, B, c, D, E, F, G. Seven transverse sections illustrating the structure 
 of the supra-oesophageal ganglia. (Zeiss A, Hartnack's camera.) a. Dorso-lateral 
 horn of white matter. b. Ventro-latefral horn of white matter, c. Postero-dorsal 
 lobe of white matter, d. Ventral protuberance of brain, e. Central lobe of white 
 matter, o.p. Optic ganglion. 
 
 " A. Section through anterior portions of ganglia close to the origin of the anten- 
 nary nerve. B. Section a little in front of the point where the two ganglia unite, c. 
 Section close to anterior junction of two ganglia. D. Section through origin of optic 
 nerve on the right side. E. Section shewing origin of the optic nerve on the left side. 
 F. Section through the dorso-median lobe of white matter. G. Section near the termi- 
 nation of the dorsal tongue of ganglion cells." 
 
 PLATE 51. 
 
 Fig. 20. Portion of a transverse section through the hinder part of Peripatm 
 capensis (male). The section passes through a leg, and shews the opening of the 
 segmental organ (o. s.), and of a crural gland, o.f.g., and the forward continuation of 
 
EXPLANATION OF PLATES. 919 
 
 the enlarged crural gland of the lyth leg (f. gl.). (Zeiss a a, Hartnack's carmrn.} a-g. 
 accessory gland of male (modified crural gland of last leg). C. Cutis. cl. Claw. 
 eu. Cuticle, ep. Epidermis, f-gl- Crural gland, h. Cells in lateral compartment of 
 body cavity, o.f. g. Orifice of accessory foot gland, o. s. Opening of segmental 
 organ. P. Three spinous pads on ventral surface of foot. pr. Prostate. R. M. 
 Retractor muscle of claw. s. Vesicle of nephridium. s. c. i. Region No. i of coiled 
 part of nephridium. si. g. Tubule of slime gland, s. o. t. Terminal portion of nephri- 
 dium. st. Stomach, st. e. Epithelium of stomach, v. c. Ventral nerve-cord, v. d. 
 Vas deferens. 
 
 Fig. 21. "Longitudinal vertical section through the supra-cesophageal ganglion 
 and cesophageal commissures of Peripatus capensis. (Zeiss a a, Hartnack.)" at. An- 
 tenna, e. Central lobe of white matter. /. Part of jaw. s.g. Salivary gland. 
 
 Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral nerve- 
 cords of Peripatus capensis enlarged and viewed from the ventral surface. The paired 
 appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic 
 nerves (sy 1 ) arising from the ventral surface of the hinder part. 
 
 From the commencement of the cesophageal commissures (as. co. ) pass off on each 
 side a pair of nerves to the jaws (/. .). 
 
 The three anterior commissures between the ventral nerve-cords are placed close 
 together; immediately behind them the nerve-cords are swollen, to form the ganglionic 
 enlargements from which pass off to the oral papilla; a pair of large nerves on each 
 side (or. n.) 
 
 FiL-hind this the cords present a series of enlargements, one pair for each pair of 
 feet, from which a pair of large nerves pass off on each side to the feet (p. n). at. n. 
 Antennary nerves, co. Commissures between ventral cords, d. Ventral appendages 
 of brain. E. Eye. e. n. Nerves passing outwards from ventral cord. F.g. Gan- 
 glionic enlargements from which nerves to feet pass off. j. n. Nerves to jaws. or. g. 
 Ganglionic enlargement from which nerves to oral papilla; pass off. or. n. Nerves to 
 oral papillae.' p.c. Posterior lobe of brain, p. n. Nerves to feet. s.y. Sympathetic 
 
 Fig- 23. "Longitudinal horizontal section through the head of Peripatus capensis ; 
 shewing the structure of the brain, the antennary and optic nerves, &c. (Zeiss a a, 
 Hartnack's camera.)" at. Antenna, at. n. Antennary nerve, cor. Cornea, e. 
 Central mass of white matter. /. Lens. op. n. Optic nerve, ph. Pharynx. /./. 
 Primary papilla covered with secondary papillae and terminating in a long spine. Sy. 
 Pharyngeal sympathetic nerves. 
 
 Fig. 24. "Eye of Peripatus capensis, as shewn in a longitudinal horizontal section 
 through the head. The figure is so far diagrammatic that the lens is represented as 
 filling up the whole space between the rods and the cornea. In the actual section 
 there is a considerable space between the parts, but this space is probably artificial, 
 being in part caused by the shrinkage of the lens and in part by the action of the 
 razor. (Zeiss c, Hartnack's camera.)" (It appears that the ganglionic region of the 
 eye is covered by a thin capsule, which is omitted in the figure. ) 
 
 cor. Cornea. /. Lens. op. Optic ganglion, op. n. Optic nerve. //'. r. Pigment. 
 Re. rods. s. p. Secondary papillae. 
 
920 EXPLANATION OF PLATES. 
 
 Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing the 
 peculiar arrangement of the circular muscular fibres. (Zeiss A, Hartnack's camera.) 
 
 PLATE 52. 
 
 Fig. 26. Portion of ventral cord of Pcripatus capensis enlarged, shewing two 
 ganglionic enlargements and the origin of the nerves and commissures. (From a 
 drawing by Miss Balfour.) 
 
 co. Commissures. E. n. Nerves passing out from ventral cords. F. n. Nerves to 
 feet. g. co. Commissures between the ventral cords containing ganglion cells, v. g. 
 Ganglionic enlargements. 
 
 Fig. 27. Segmental organ from the 5th pair of legs of Peripatus capensis. This 
 nephridium resembles those of the 4th legs, and differs from all the others in its large 
 size and in the absence of any dilatation giving rise to a collecting vesicle on its external 
 portion (enlarged). The terminal portion has the same histological characters as in 
 the case of the hinder segmental organs. (From a drawing by Miss Balfour.) 
 
 Fig. 28. Segmental organ or nephridium from the gth pair of legs of Peripatus 
 capensis, shewing the external opening, the vesicle, the coiled portion and the 
 terminal portion with internal opening (enlarged). (From a drawing by Miss 
 Balfour.) 
 
 o. .?. External opening of segmental organ, p.f. Internal opening of nephridium 
 into the body-cavity (lateral compartment). s. Vesicle of segmental organ. j 1 . 
 Portion of segmental organ of 4th and 5th. legs, corresponding to vesicle of the other 
 nephridia. s. c. i. First or external portion of coiled tube of nephridium, lined by 
 columnar epithelium with small nuclei ; the cells project for very different distances, 
 giving the inner boundary of this region a ragged appearance, s. c. 2. Region No. 2 
 of coiled tube of nephridium, lined by small closely-packed columnar cells, s. c. 3. 
 Region No. 3 of coiled tube of segmental organ, lined by large flat cells with 
 large disc-shaped nuclei, s. c. 4. Region No. 4 of coiled tube of nephridium ; this 
 region is very short and lined by small columnar cells, s. o. t. Terminal portion of 
 nephridium. 
 
 Fig. 29. " Portion of nephridium of the hindermost leg of Peripattis capensis, seen 
 in longitudinal and vertical section. The figure is given to shew the peritoneal funnel 
 of the nephridium. Portions of the collecting sack (s.) and other parts are also repre- 
 sented. (Zeiss B, Hartnack's camera.)" 
 
 /./. Peritoneal funnel, s. Vesicle, s. c. i, s. c . i, s.c.$. Portions of coiled tube. 
 
 Fig. 30. "Section through a tracheal pit and diverging bundles of tracheal tubes" 
 taken transversely to the long axis of the body. (Zeiss E, oc. 2.) (From a rough 
 drawing by Prof. Balfour.) 
 
 tr. Tracheae, shewing rudimentary spiral fibre, tr. c. Cells resembling those 
 lining the tracheal pits, which occur at intervals along the course of the trachece. 
 tr. s. Tracheal stigma, tr. p. Tracheal pit. 
 
 Fig. 31. "Sense organs and nerves attached from antenna of Peripatus capensis 
 (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof. Balfour.) The figure 
 shews the arrangement of the epidermis cells round the base of the spine. The spine 
 is seen to be continuous with the inner layer of the cuticle. 
 
EXPLANATION OF PLATE 53. 92 1 
 
 Fig- 32. Section through the skin of Peripatus capensis ; it shews the secondary 
 papillae covered with minute spinous tubercles and the relation of the epidermis to 
 them. (The cuticle in the process of cutting has been torn away from the subjacent 
 cells.) The cells of the epidermis are provided with large oval nuclei, and there is a 
 deposit of pigment in the outer ends of the cells. The granules in the protoplasm of 
 the inner ends of the cells are arranged in lines, so as to give a streaked appearance. 
 (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.) 
 
 c. Dermis. cu. Cuticle, ep. c. Epidermis cells, pi. Deposit of pigment in outer 
 ends of epidermis cells, s.p. Secondary papillae. 
 
 Fig- 33- Female generative organs of Peripatus capensis, x 5. (From a rough 
 drawing by Prof. Balfour.) The following note was appended to this drawing: 
 "Ovary rather to dorsal side, lying in a central compartment of body-cavity and 
 attached to one of the longitudinal septa, dividing this from the lateral compartment 
 between the penultimate pair of legs and that next in front. The oviducts cross 
 before opening to the exterior, the right oviduct passing under the rectum and the 
 left over it. They meet by opening into a common vestibule, which in its turn opens 
 below the anus. On each side of it are a pair of short papillae (aborted feet ?)." 
 
 F. 16, 17. Last two pairs of legs, od. Oviduct, .ov. Ovary, itt. Uterus, v. c. 
 Nerve-cord. 
 
 PLATE 53. 
 
 Figs. 34 39. Five young embryos of Peripatus capensis ; ventral view. All, 
 excepting Fig. 37, from drawings by Miss Balfour. In Figures 34 to 38 a denotes 
 what is probably the anterior extremity. 
 
 Fig. 34, Stage A. Youngest embryo found, with slightly elongated blastopore. 
 
 Fig. 35, Stage B. Embryo with three mesoblastic somites and elongated blasto- 
 pore. The external boundaries of the somites are not distinct. 
 
 Fig. 36, Stage C. Embryo with five somites. The blastopore is closing in its 
 middle portion. 
 
 Fig. 37, Stage D. The blastopore has completely closed in its middle portion, 
 and given rise to two openings, the future mouth and anus. (From a rough drawing 
 left by Professor Balfour.) (Zeiss A, Camera Oberhaus. on level of stage.) 
 
 The following note was appended to this drawing in his handwriting : " Young 
 larva of Peripatus capensis. I could not tell for certain which was the anterior end. 
 Length, 1-34 mm." 
 
 Fig. 38, Stage E. Embryo with about thirteen mesoblastic somites in which the 
 flexure of the hind part of the body has commenced. The remains of the original 
 blastopore are present as the mouth, placed between the second pair of mesoblastic 
 somites, and the anus placed on the concavity of the commencing flexure of the hind 
 part of the body. 
 
 B. 59 
 
922 EXPLANATION OF PLATE 53. 
 
 Fig. 39. Side view of same embryo. 
 
 Figs. 4042. Drawings by Professor Balfour of three transverse sections through 
 the embryo from which fig. 36 was taken. (Zeiss c, Camera.) Figs. 40 and 42 pass 
 through the region of the blastopore. 
 
 bl. Blastopore. ep. Epiblast. hy. Hypoblast. me. Mesenteron. mes. Meso- 
 blastic somite. 
 
 Fig. 43. Male generative organs of Peripatus capensis, viewed from the dorsal 
 surface. (From a drawing by Miss Balfour.) 
 
 a, g. Enlarged crural glands of last pair of legs. F. 16, 17. Last pairs of legs. 
 /. Small accessory glandular tubes, p. Common duct into which vasa deferentia 
 open. p. r. Prostate, te. Testes. v. c. Nerve-cord, v. d. Vas deferens. 
 
 CAMBRIDGE : PRINTED BY C. J. CLAY, M.A., AND SON, AT THE UNIVERSITY PRESS. 
 
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