IRLF 
 
 QL 
 
 949 
 
 F7 
 
 VITREOUS BODY 
 
 ITS ORIGIN, DEVELOPMENT, AND STRUCTURE 
 AS OBSERVED IN THE EYE OF THE PIG 
 
 BY 
 
 ALOISIUS WILLIAM^FROMM, o. F. M, 
 
 sis submitted to the Faculty of Sciences of the Catholic 
 University of America, in partial fulfillment of the 
 requirements for the degree of 
 Doctor of Philosophy 
 
 WASHINGTON, D. C, 
 
EXCHANGE 
 
- 
 
THE 
 
 VITREOUS BODY 
 
 ITS ORIGIN, DEVELOPMENT, AND STRUCTURE 
 AS OBSERVED IN THE EYE OF THE PIG 
 
 BY 
 
 ALOISIUS WILLIAM FROMM, o. F. M. 
 
 t 
 
 liesis submitted to the Faculty of Sciences of the Catholic 
 
 University of America, in partial fulfillment of the 
 
 requirements for the degree of 
 
 Doctor of Philosophy 
 
 V 
 
 WASHINGTON, D. C. 
 
 1021 
 
TABLE OF CONTENTS 
 
 Page 
 
 Introduction 1 
 
 Historical Sketch 2 
 
 Methods ! 4 
 
 Investigation : 
 
 I. Primitive Vitreous Body 6 
 
 II. Period of Mesodermal Invasion of Vitreous Body .... 11 
 
 III. Permanent Vitreous Body 18 
 
 Conclusions 25 
 
 Literature , 26 
 
 Explanation of the Figures 31 
 
INTRODUCTION 
 
 The origin of the vitreous body of the eye has long been in doubt. 
 Even the numerous and thorough investigations of the first decade 
 of the present century, although clearing up many difficulties and 
 correcting false notions, have failed to bring a satisfactory answer 
 to the question, i ' Is the vitreous body of the eye a derivative of the 
 outer or middle germ layer ; is it an ectodermal or mesodermal for- 
 mation?" A glance at some of the latest and most widely used 
 textbooks, chosen at random, reveals the uncertainty existing among 
 modern authors as to the origin of this interesting structure of the 
 eye. Says Parker (page 113), "Mesoderm also makes its way into 
 the optic cup, through the choroid fissure, and becomes the vitreous 
 humour." Lillie, on the other hand, maintains (page 275) the 
 "researches of the last few years have demonstrated that the vitre- 
 ous body is primarily of ectodermal origin, its fibers arising as pro- 
 cesses of cells of the inner layer of the optic cup and the matrix 
 as secretion." Again, Prentiss and Arey assert (page 381) that 
 "the vitreous body may be regarded as a derivative both of the 
 ectoderm and the mesoderm. ' ' 
 
 The reasons for this diversity of opinion among biologists may be 
 reduced to the following : 1. The very delicate nature of the vitre- 
 ous body, which differs so widely from all other tissues, renders its 
 study extremely difficult. It requires special methods of technique 
 not ordinarily employed in histological investigations. The diffi- 
 culties of obtaining perfect sections of the eye in all its stages of 
 development have been regarded by some investigators as almost 
 insurmountable ; 2. The extreme complexity of the mammalian eye, 
 its very rapid development, especially in early embryonic life, the 
 appearance and disappearance of an intricate vascular system in the 
 course of development with its concomitant radical changes all 
 this obscures the origin and growth of the vitreous body and renders 
 its study as difficult as it is interesting. The vast changes, which 
 follow one another in rapid succession, make it wellnigh impossible 
 
 Contribution from the Biological Laboratory of the Catholic University of 
 America, No. 4. 
 
 1 
 
 448419 
 
l ^ijK &y^<Jegree of certainty later developments. The 
 study of the entire embryonic history of the eye alone can solve the 
 mysteries of the origin, the development, and the structure of the 
 vitreous body. It goes without saying that the study of the fully 
 developed eye does not throw much light upon the origin and de- 
 velopment of its parts. 
 
 To attempt a solution of the problem by methods not heretofore 
 employed in this matter, is the purpose of this work. If further 
 justification for reopening the discussion were needed, we might 
 point to the not very creditable fact that, to our knowledge, no 
 special treatise on this subject exists in the English language. 
 
 It was at first planned to make this study comparative, but a 
 closer acquaintance with the work already done made it appear more 
 advisable to investigate the development of the vitreous body of one 
 species from the earliest beginning to its adult condition. The 
 species that was finally selected is the pig, principally on account of 
 the facility of obtaining a complete series of embryonic eyes. More 
 than one hundred and fifty specimens, representing all stages of 
 development, were examined. The material was obtained from the 
 local abattoir. A few specimens were given me by my former 
 teacher, Dr. Carl R. Moore, of the University of Chicago, to whom 
 I extend here my sincere thanks for his interest in the success of 
 this work. Acknowledgment of indebtedness is also made to Pro- 
 fessor J. B. Parker, Ph. D., and Mr. G. J. Brilmyer, M. S., of the 
 Catholic University of America, under whose guidance these investi- 
 gations were made ; to Fr. Mahan, S. J., and Dr. T. T. Job, of Loyola 
 Medical School, Chicago, who kindly permitted me the use of their 
 laboratory in the summer of 1920, and to Fr. Victor Herring. 
 0. F. M., for preparing most of the sketches. 
 
 HISTORICAL SKETCH 
 
 The history of the origin of the vitreous body of the eye dates 
 back to the year 1848, when H. Scholer submitted to the University 
 of Dorpat an inaugural dissertation embodying his observations on 
 the development of the chick's eye. Scholer was the first to notice 
 that very early in the development of the chick a very delicate 
 tissue of mesoderm, pars systematis cutis, enters the optic cup 
 through the choroid fissure. Finding no other elements present, 
 Scholer naturally attributed to this portion of the middle germ 
 
layer the origin of the vitreous body. Scholer, therefore, must 
 be credited with the enunciation of the theory which later was to 
 be known as the Theory of the Mesodermic Origin of the Vitreous 
 Body. 
 
 This theory became more generally known when R. Virchow, 
 in 1852, reviewed Scholer 's work, approved it, and classified the 
 vitreous body, which had always defied classification, as a special 
 kind of connective tissue. A. v. Kolliker, in 1861, also accepted the 
 new theory, and his authority blazed the way for Scholer 's views, 
 Which thenceforth dominated biological circles almost to the end of 
 the last century and which find defenders even now. 
 
 While all these authors agree as to the mesodermic origin of the 
 vitreous body, they differ widely in determining more precisely 
 the portion of the mesoblast genetically responsible for this tissue. 
 Most of them, following Scholer, derive the vitreous body from 
 the mesoderm entering the optic cup through the choroid fissure. 
 This view seems to be based largely on the observations made in the 
 development of the chick's eye, in which other mesoblastic elements 
 are very scarce. Other investigators have called attention to the 
 relatively large circular opening between the lens and the inner 
 layer of the retina and the continuity of the extra-ocular mesen- 
 chyme with the vitreous body. Thus Van Pee attributes the origin 
 of a portion of the vitreous fibers to the mesenchyme surrounding the 
 optic cup on all sides and entering through the perilenticular open- 
 ing. But the most conflicting statements are made by the defend- 
 ers of the mesodermic origin regarding the very thin layer of mesen- 
 chyme originally found between the optic vesicle and the body ecto- 
 derm. Kolliker at first thought it probable that during the forma- 
 tion of the lens a part of this mesoderm is carried into the optic 
 cup and gives rise to the tunica vasculosa lentis. Later, however, 
 he began to doubt the correctness of this view. Arnold is more em- 
 phatic in attributing the origin of the vitreous body exclusively to 
 this portion of the mesoderm. On the other hand, Cirincione, the 
 most ardent defender of the mesodermic origin of the vitreous 
 body, rejects these explanations and returns to Scholer 's more gen- 
 erally accepted views. 
 
 The first to challenge the theory of the mesodermic origin of the 
 vitreous body was L. Kessler, who, in 1871, made an exhaustive study 
 of the development of the chick's eye, and came to the conclusion 
 that the vitreous body is an amorphous gelatinous mass, formed as 
 
4 
 
 an exudation from the blood vessels, into which a few migratory 
 cells or leucocytes had found their way. The theory of Kessler, 
 known as the Transudation Theory, found little favor with sci- 
 entists. 
 
 A more generous reception was accorded to a new theory pro- 
 posed by Tornatola at the International Congress of Anatomists, 
 held in Moscow, in 1897. Tornatola asserted that the vitreous body 
 in vertebrates is essentially a fibrous substance, derived exclusively 
 from the retinal cells, and hence an ectodermal formation. His 
 Theory of the Ectodermic Origin of the Vitreous Body found many 
 defenders, such as Rabl, Fischel, Addario, Wolf rum, v. Szily, Mavas, 
 Magitot, Seefelder, and others. According to this theory, the 
 structural parts of the vitreous body consist of a network of very 
 delicate fibers, that have their origin in the supporting cells of the 
 retina and hence are ectodermal. The mesoderm elements are 
 thought to be either leucocytes or true mesoderm cells, having, 
 however, only vaso-formative or nutritive functions and in no way 
 contributing to the production of the vitreous body. 
 
 Lenhossek, in 1902, while admitting the ectodermic origin of the 
 vitreous body, attributed it exclusively to the lens cells and pro- 
 posed his Theory of the Lenticular Origin of the Vitreous Body. 
 
 To reconcile these conflicting opinions, Van Pee and v. Kolliker 
 almost simultaneously expressed the view that the fibrous portions 
 of the vitreous body must be regarded as a complex tissue, to which 
 both ectoderm and mesoderm contribute. According to both these 
 investigators, some of the vitreous fibers originate from the cells 
 of the retina, while the rest are the product either of the extra-ocular 
 mesenchyme (Van Pee) or of the extensive vascular system which is 
 found at certain stages in the development of all mammalian eyes 
 (Kolliker). 
 
 METHODS 
 
 The extreme delicacy of the structure of the vitreous body makes 
 the question of method all-important. Indeed, diversity of opinion 
 regarding the origin, the development, and the structure of this 
 tissue is owing, in large measure, to the methods used. 
 
 In the earlier stages of development, including embryos of 25 
 mm length, most of the ordinary killing fluids were found service- 
 able, Zenker's, Bouin's, and Schaffner's being most frequently used. 
 
The embryos were removed from the uterus and immediately placed 
 into the killing fluid, which had been warmed to about body tempera- 
 ture. Smaller embryos were fixed in toto ; in larger specimens the 
 head was removed and sometimes cut longitudinally to permit uni- 
 form penetration. The material was left in the fluids at least 
 twenty-four hours. Dehydration was effected by means of alcohoi. 
 the material being passed through the usual grades of 25%, 35%, 
 etc., to absolute alcohol. It was then cleared in xylol or cedar oil, 
 the latter being preferred, because it clears readily from 95% alco- 
 hol and because it prevents the- tissues from becoming too brittle. 
 After being cleared, the tissue was embedded in paraffin. Sections 
 were cut in various thicknesses, 6, 8, 10, 15, and 25 microns. For 
 the study of the fibrous parts of the vitreous body, thick sections are 
 generally to be preferred. A number of sections were mounted 
 without the aid of a fixative to eliminate, as far as possible, all for- 
 eign material from the sections, a perfectly clean slide and careful 
 handling being necessary to bring about the desired results. The 
 fibers of the vitreous body are susceptible of almost all stains. 
 Good results were obtained with fuchsin, borax-carmine, Mallory's 
 connective tissue stain, Delafield's haematoxylin, Heidenhain's iron- 
 alum-haematoxylin with or without an additional slight stain of 
 eosin or erythrosin. Overstaining was resorted to in a few cases 
 to bring out the finer connections between the fibers. A heavy stain 
 greatly facilitates the study of this delicate tissue. No material 
 was stained in bulk, thus insuring a uniform stain. 
 
 This method was found unsuited to embryos of more than 35 mm. 
 The action of the alcohol used in dehydration, and the heat neces- 
 sary for paraffin embedding caused greater or smaller shrinkage 
 of the vitreous fibers, while the subsequent removal of the paraffin 
 almost invariably led to a collapse of the delicate fibrous frame- 
 work of the vitreous body. For larger embryos a new method, first 
 described by Szent-Gyorgyi, with some modifications, was found to 
 give excellent results. 
 
 As killing and fixing fluid the following mixture was used : 
 acetone 125 cc, formalin 40 cc, acetic acid 5 cc, water 100 cc, mer- 
 curic chloride 4 g. The eyes were carefully removed from their 
 sockets, freed of all adhering tissue, plunged into the killing fluid, 
 which had been heated to body temperature, and left in the fluid 
 from five to seven days, according to their size. Then to every 100 
 cc of the fluid 50 cc of concentrated acetone were added and the eyes 
 
left for an additional two to four days. Without being washed, 
 they were then transferred to concentrated acetone for two to three 
 days. To facilitate dehydration, a layer of calcium chloride was 
 spread in the bottom of the vessel. To prevent a too violent action 
 of the calcium chloride on the eyes, direct contact between the two 
 must be avoided. The material was then placed for twenty-four 
 hours in ether-alcohol, and thereafter embedded. Embedding was 
 done in celloidin or paralodion, dissolved in ether-alcohol, smaller 
 eyes being placed successively in 2, 5, 8, and 10% solution, larger 
 eyes in 1, 3, 5, and 8% solution, remaining in each about three days. 
 Parts of the coats of the eyes were removed before embedding to 
 insure perfect penetration. Hardening was done by means of 
 chloroform and terpineol. Sections were cut very thick, 100, 150, 
 200, 250, and 300 microns. They were placed into a 95% alcoholic 
 solution of iodin to remove the mercuric chloride crystals, then into 
 75% alcohol and, immediately before staining, into distilled water. 
 Various stains were tried, the best results being obtained with 
 Held's molybdic haematoxylin (haematox. cryst. 1 g., molybdic 
 acid 10 g., 70% alcohol 100 cc). The stain was permitted to ripen 
 about three weeks, carefully decanted and, before use, diluted with 
 ten times its volume of distilled water. The sections were then 
 passed through the various grades of alcohol to 95%, cleared in 
 carbol-xylol, and mounted in balsam. The structure and arrange- 
 ment of the fibrous part of the vitreous body were brought out with 
 remarkable clearness in this way, but, unfortunately, the remain- 
 ing structures were deeply overstained. To remedy this defect, 
 some sections were stained in Lyons Blue, but with indifferent 
 success. 
 
 INVESTIGATION 
 
 I PRIMITIVE VITREOUS BODY 
 
 The period of development forming the object of this part of 
 our investigation, includes embryos ranging from 4 to 11 mm in 
 length and shows the origin and growth of the eye from its begin- 
 ning to the completion of the optic cup and the lens vesicle, before 
 the appearance of blood vessels in the cavity of the vitreous body. 
 It shows the latter in its primitive and simplest form, not obscured 
 by the numerous mesodermal elements, which later render its study 
 more difficult. 
 
In embryos of 4, 5, and 6 mm, we witness the origin of the eye. 
 The primary optic vesicle gradually invaginates to form the optic 
 cup, the overlying body ectoderm thickens and bends inward to give 
 rise to the lens vesicle. Cup and lens are in close contact through- 
 out; no mesoderm is found between them (Figure 6). The cavity 
 of the vitreous body, the posterior chamber of the eye, is absent in 
 the younger specimens, but appears as a narrow slit between future 
 retina and lens in the 6 mm embryo. The development thenceforth 
 is very rapid, and in the 11 mm embryo, the structure of the eye 
 may be described thus : the optic cup is complete, consisting of an 
 inner layer, the retina, and an outer, the pigment layer. The former 
 shows two distinct regions, a cellular, and the so-called mantle layer. 
 Nerve fibers and pigment are still absent. The lens vesicle has sepa- 
 rated from the body ectoderm. The choroid fissure is wide open, 
 and the surrounding mesoderm is pressing up into the fissure (Fig- 
 ure 3), but has not yet penetrated into the optic cup. The optic 
 stalk, an open tube with wide lumen, can easily be traced to the 
 diencephalon. 
 
 The cavity of the vitreous body is still narrow. It contains no 
 blood vessels and only a few cells scattered here and there. The 
 origin and the nature of these cells have not been satisfactorily ex- 
 plained. Some authors, as v. Lenhossek, consider them the rem- 
 nants of the thin layer of mesoderm originally found between the 
 optic vesicle and the body ectoderm, and carried into the optic cup 
 by the invaginating lens placode. Others, among whom are See- 
 felder, Mavas, and Magitot, maintain that these cells have migrated 
 from the retinal mantle layer into the vitreous body. Seefelder 
 proposes to call them ectodermal vitreous body cells (ektodermale 
 Glasskorperzellen), and believes them to be identical with similar 
 cells found later in large numbers in the mantle layer of the retina. 
 Mavas and Magitot, moreover, are of opinion that, passing through 
 a process of degeneration and disintegration, these cells contribute 
 to the production of the liquid parts of the vitreous body. Wolf rum 
 believes some to be real mesoderm cells with vaso-formative function, 
 others he prefers to call ectodermal cells. The difference of opinion 
 may, to some extent, be owing to the difference in the material used. 
 In the material which formed the subject of our investigation, the 
 cellular elements are exceedingly rare, scarcely half a dozen being 
 counted in as many complete series of sections. In the eye of the 
 pig, in which no mesoderm is carried into the optic cup by the lens 
 
8 
 
 placode (Figure 6), and in which at this time no mesoderm has 
 entered through the choroid fissure (Figure 1), it would be difficult 
 to account for the presence of mesoderm in the vitreous body. 
 In structure, the cells resemble the retinal cells. For these 
 reasons, we incline to the belief, that they are of retinal origin and 
 not mesenchyme. But whatever their origin and their nature may 
 be, they show no connection with the vitreous body, nor do they at 
 this stage of development contribute to the production of the vitre- 
 ous fibers. 
 
 The vitreous body is represented in all embryos by a great mass 
 of fibers, of which the larger portion extend radially between the 
 retina and the lens, while, especially in embryos of 10 and 11 mm, 
 some very prominent fibers are seen running approximately parallel 
 to the retina and lens. It is not difficult to determine the origin of 
 both. The radial fibers arise as long slender outgrowths from the 
 conelike bases of the inner layer of retinal cells (Miiller's support- 
 ing cells). Some can be traced directly from the retina to the lens, 
 others are seen to send off branches in various directions which, 
 dividing and subdividing, anastomose freely with branches from 
 neighboring fibers, forming a dense reticulum in which further 
 tracing becomes impossible. The fibers are slightly granular and 
 react to the various stains in a manner not unlike that of the 
 cytoplasm of their mother cells. In some sections, larger granules 
 are found at the junction or branching points of the fibers, but their 
 absence in the more perfect preparations, especially in the celloidin 
 preparations, seems to show them to be artefacts, produced by 
 precipitation. In the more perfect sections, the fibers appear almost 
 homogeneous, slightly granular, tapering gradually from their 
 broad bases at the retina to a slender thread near the lens. In the 
 narrow isthmus between the anterior part of the retina and the 
 lens, a very regular structure of the fibers is frequently found. 
 
 The radial fibers, however, are not restricted to the retina; the 
 basal cells of the lens also send out a large number of them. These 
 fibers differ widely from the retinal fibers. They are short and 
 never traceable to the retina. At a short distance from the lens, 
 they split into several branches which run almost parallel to the 
 posterior part of the lens. At this point they are met by the more 
 numerous and more massive retinal fibers, and together with them 
 they form a thick, feltlike mass, especially observable in equatorial 
 sections (Figures 1 and 8). The entire surface of the lens con- 
 
9 
 
 tributes to the formation of these fibers, especially the cells oppo- 
 site the region where retina and pigment layer meet. On the ante- 
 rior surface of the lens, some of these fibers penetrate deeply into 
 the surrounding mesoderm with which they frequently unite ; others 
 even find their way to the overlying body ectoderm. In the more 
 advanced stages of development, when the lens capsule begins to be 
 formed as a secretion of the lens cells, the lenticular fibers gradually 
 lose their connection with the basal cells of the lens. The light 
 space between the lens and the feltlike mass of fibers described above 
 (Figure 1) signalizes the region of the future tunica vasculosa 
 lent-is, or vascular capsule of the lens. 
 
 The vitreous body at this stage of development, therefore, con- 
 sists of fibers, both radial and parallel, which, springing from the 
 conelike projections of the basal cells of the retina and lens, form 
 with frequent anastomoses a dense network, or rather framework, 
 constituting the solid or structural part of the vitreous body. These 
 fibers arise solely as protoplasmic prolongations of the retinal and 
 lenticular cells and are, therefore, ectodermal in their origin. No 
 mesodermal elements contribute to their production. 
 
 We can not agree, therefore, with Scholer and his followers in 
 their contention, that the vitreous body takes its origin from the 
 mesoderm entering through the choroid fissure ; for in all embryos, 
 examined at the various stages of development up to 12 mm length, 
 no mesoderm was found to have entered into the optic cup through 
 the choroid fissure. The cells found here and there in the vitreous 
 body, whatever their origin, nature, and ultimate fate may be, take 
 no part in the production of the primitive vitreous body fibers, nor 
 do they, at this stage, enter into any connection with them. The 
 fibers can be traced so easily to the basal cells of the retina and 
 lens that their retinal and lenticular origin respectively, at this 
 stage of the development of the eye, admits of no doubt. 
 
 Cirincione maintains that these fibers are only temporary, and 
 that their sole purpose is to fill out the cavity of the optic cup until 
 the permanent vitreous body is formed by the mesoderm entering 
 later through the choroid fissure. He admits the presence of the 
 retinal and the lenticular fibers, and concedes their origin from the 
 basal cells of the retina and lens, but regards them only as a 
 sostanza di replezione destined to disappear in the same degree as 
 the mesodermal elements advance into the cavity of the secondary 
 optic vesicle to form the vitreous body (page 1,358). But further 
 
10 
 
 investigation will show that this primitive condition of the vitreous 
 body, although obscured and modified by the ingrowing mesoderm, 
 remains essentially the same. A comparison of figures 2 and 19 
 shows rather that the mesodermal elements are only temporary 
 structures in the vitreous body, which both in its primitive and in 
 its final stage of development is purely ectodermal. 
 
 Neither do our observations agree with the views of Kessler that 
 the vitreous body is an amorphous, gelatinous mass, the product of a 
 process of transudation from the blood vessels (page 81). Its 
 fibrous structure is so uniform, regular, and constant in all sections, 
 whatever method is used in their preparation, that we can not regard 
 the vitreous body as an amorphous mass. To dispose of the fibers as 
 artificial productions of the reagents used in the preparation of the 
 material, would be to do violence to fact. 
 
 By a careful study of the vitreous body in the sheep 's eye, Van 
 Pee arrives at the conclusion, that the radial fibers are, indeed, of 
 retinal or ectodermal origin, but that the more prominent parallel 
 fibers, which constitute the bulk of the vitreous body, are mesodermal 
 in their origin, arising from the mesenchyme between the optic cup 
 and the body wall. These fibers then pass through the narrow peri- 
 lenticular opening and form the major part of the vitreous body. 
 It is true, as will be seen from figure 2, that at a later stage of 
 development numerous mesoderm cells with long protoplasmic out- 
 growths enter into the vitreous body, and the significance of this 
 formation will be considered in its place, but it is equally true, that 
 in the earlier stages, which are under consideration here, and which 
 show the vitreous body in its primitive and simplest form, both the 
 radial and the parallel fibers are exclusively the product of the basal 
 cells of the retina and lens. In the eye of the pig, mesodermal 
 vitreous fibers are hot found at this stage of development. 
 
 Lenhossek, in a lengthy monograph, attempts to demonstrate 
 that all the fibers constituting the structural parts of the vitreous 
 body, are the product of the basal cells of the lens, and that the 
 retina has no share in their production. All our preparations show 
 the lenticular fibers first described by v. Lenhossek and later by his 
 pupil, v. Szily. They are especially prominent in embryos of 10 
 and 11 mm length, radiating from all parts of the lens, and enter- 
 ing into communication with the retinal fibers, the extra-ocular 
 mesoderm, and even with the body ectoderm. But we can not regard 
 them as the exclusive structural parts of the vitreous body. It is 
 
11 
 
 surprising that v. Lenhossek should have overlooked the more 
 numerous, more massive, and more prominent fibers arising from the 
 retina. The retinal fibers, moreover, are a permanent formation, 
 as will appear from further study; the lenticular fibers are very 
 early separated from their mother cells by the formation of the lens 
 capsule, as v. Lenhossek himself points out. Wolfrum is of the 
 opinion that the lenticular fibers serve as an apparatus for retaining 
 the lens vesicle in its place after it has separated from the body 
 ectoderm. The peculiar shape of the lens vesicle at this time seems 
 to lend color to this theory. Whether or not the lens fibers contrib- 
 ute to the formation of the primitive vitreous body, their influence 
 is neither predominant over, nor equal to, that of the retina, while 
 after the formation of the lens capsule, no further connection exists 
 between the lens and the vitreous body. Kolliker says of v. Len- 
 hossek 's theory : The lenticular vitreous body of v. Lenhossek does 
 not exist (page 18). 
 
 The purely retinal, i. e., ectodermal origin of the primitive 
 vitreous body, as described in this chapter, has found defenders 
 in many modern investigators. It may suffice to mention Tornatola, 
 Addario, Wolfrum, v. Szily, v. Kolliker, Mavas, Magitot, and See- 
 felder. Mavas and Magitot thus sum up the results of their in- 
 vestigation on the origin and the development of the vitreous body 
 in the human eye : The primitive vitreous body is of retinal original. 
 It consists of a very delicate fibrous mass, arising from the marginal 
 zone of the embryonic retina. This marginal layer is formed by 
 protoplasmic prolongations of the supporting cells, which are the 
 first to differentiate in the inner layer of the optic vesicle. The 
 primitive vitreous body is, therefore, an exoplastic formation of this 
 layer (page 127). 
 
 II PERIOD OF MESODERMAL INVASION OF THE VITREOUS BODY 
 The primitive simple condition of the vitreous body soon under- 
 goes a radical change, brought about by the entrance of the complex 
 hyaloid vascular system and numerous mesodermal elements. The 
 relationship of these structures to the vitreous body is of paramount 
 importance in judging of the true character and the further develop- 
 ment of the latter. 
 
 The hyaloid artery, a branch of the arteria centralis retinae, 
 entering the optic cup through the still partly open choroid fissure, 
 at first appears as a single solid trunk, pushing its way far into the 
 
12 
 
 vitreous body (Figure 4). It soon gives rise to a number of branches 
 which fill out the larger part of the space between the retina and the 
 lens (Figure 2). Their tendency, however, is always towards the 
 lens, which finally is completely surrounded by blood vessels furnish- 
 ing it with nourishment during the period of its most rapid growth. 
 Later there arises from the hyaloid artery near its entrance into the 
 optic cup a second group of branches which radiate in all directions 
 through the outer layer of the vitreous body near the retina. This 
 condition is observable especially in embryos ranging from 35 to 100 
 mm in length (Figure 14). The larger part of the vitreous body of 
 such embryos is again free from blood vessels. 
 
 A closer examination of the structure of the blood vessels shows 
 that their walls are generally thin, consisting of a layer of endothelial 
 cells to which a connective tissue cell is attached here and there. This 
 structure is common to the main trunk as well as to all the branches 
 of the hyaloid system at this time. Later, however, the main trunk of 
 the hyaloid artery seems to be surrounded by an additional layer of 
 cells enveloping the vascular endothelium like a mantle. This mantle 
 layer has been classed by some authors among the neuroglia tissues 
 of the central nervous system. Its intimate connection with the sup- 
 porting tissues of the optic nerve, the structure and the epithelial ar- 
 rangement of its cells, and the reaction of the latter to stains in a 
 manner not unlike the neuroglia cells of the optic nerve, are said to 
 prove the identity of this cell layer with the supporting tissues of the 
 optic nerve (Mavas and Magitot, p. 129) . For the eye of the pig, this 
 structure has been described at length by Wolf rum (p. 249) , to whom 
 the interested reader is referred. 
 
 While Seefelder at first observed the neuroglia mantle around 
 only the main trunk of the hyaloid artery, Mavas and Magitot found 
 it enveloping not only the main trunk, but all the branches of the 
 hyaloid system, thus interposing an ectodermal sheath of neuroglia 
 tissue between the mesodermal elements of the blood vessels and 
 the vitreous body and effectively preventing their union. Ac- 
 cording to these authors, therefore, the vitreous body is at this time 
 free from all mesodermal admixture, and represents a purely ecto- 
 dermal formation consisting of a framework of fibers, the product 
 partly of the supporting tissues of the retina and partly of the 
 neuroglia cells of the optic nerve. The vitreous body would then 
 have to be regarded as a purely neuroglia tissue derived ultimately 
 from the central nervous system (Mavas and Magitot, p. 132). 
 
13 
 
 This simple solution of the complex vitreous body problem, how- 
 ever, meets with some difficulties in the subject under consideration 
 here. The above-mentioned investigators arrived at their conclusion 
 by the study of human material almost exclusively. But in the eye 
 of the pig, the neuroglia mantle has been observed around only 
 about one-third of the length of the main trunk of the hyaloid ar- 
 tery ; it has never been shown to surround any of its branches. Our 
 own observations agree fully with those of Wolfrum. A number 
 of sections show the neuroglia mantle surrounding a portion of the 
 hyaloid artery proper ; but the branches exhibit only the usual vas- 
 cular endothelium, which, as further investigation will show, enters 
 into a close relationship with the vitreous body. This complex 
 relationship between the vitreous body and the mesodermal ele- 
 ments of the blood vessels prevents our accepting, for the eye of 
 the pig at least, the simple and attractive theory of the neuroglia 
 origin and nature of the vitreous body. 
 
 But there are other elements found in the optic cup at this time 
 of development which claim our attention. Together with the hyaloid 
 artery, some free mesoderm cells likewise enter the optic cup 
 and almost simultaneously we witness the invasion of the 
 vitreous body by a large mass of mesoderm through the perilenticu- 
 lar opening (Figure 2). The function of these mesodermal ele- 
 ments has never been satisfactorily accounted for. Several cir- 
 cumstances, to which, as far as we know, attention has not yet been 
 called, incline us to the belief that these mesoderm cells are pri- 
 marily vaso-formative, aiding in the building up of the complex 
 vascular system of the embryonic mammalian eye : 1. the invasion 
 of the vitreous body by mesoderm is restricted to a comparatively 
 short period of time, which corresponds to the period of the most 
 rapid growth and expansion of the vascular system, and ceases as 
 soon as the latter has attained its highest development (Figures 2 
 and 12) ; 2. while at first the mesodermal elements are found scat- 
 tered throughout the larger part of the vitreous body, most of them 
 soon congregate in the vicinity of the blood vessels, the rest of the 
 vitreous body being at that time almost devoid of mesoderm ; 3. that 
 there exists some relationship between the vascular system of the 
 vitreous body and the mesodermal elements, seems to be indicated 
 also by the fact that in the eye of the birds, which even during em- 
 bryonic development never contains blood vessels, the number of 
 mesoderm cells is exceedingly small. Many authors even doubt the 
 
14 
 
 presence of mesoderm in the chick's eye. Lillie says of these cells 
 that "in character they resemble embryonic blood-cells and not 
 mesenchyme, and disappear entirely by the eighth day" (p. 275). 
 
 That not all the mesoderm cells are, however, used in the forma- 
 tion of the blood vessels is evident from the fact that at all stages 
 of development a number of them are found isolated in the vitreous 
 body. It is not improbable, as some authors maintain, that these 
 free mesoderm cells by a process of disintegration and degeneration 
 contribute to the production of the fluid parts of the vitreous body. 
 Accordingly, we have at this period of development two distinct 
 mesodermal elements in the vitreous body, the mesoderm tissue of 
 the blood vessels and a number of free mesoderm cells (Figure 13). 
 
 The question now arises as to what influence the hyaloid system 
 and the free mesoderm cells have on the further development of 
 the vitreous body. Do they modify the originally ectodermal nature 
 of this tissue in such wise that we are forced to admit a mixed struc- 
 ture consisting of various parts, derivatives of both the outer and 
 the middle germ layer ? 
 
 It can not be denied that a close relationship arises between the 
 fibers of the vitreous body and the walls of the blood vessels. The 
 endothelium of the latter sends out a large number of fibers which 
 penetrate into the surrounding vitreous body. They are short and 
 delicate and never attain the massiveness of the vitreous fibers. 
 Their purpose may be solely to secure a hold for the blood vessels 
 in the loose surrounding tissue, a phenomenon not infrequently 
 observed in embryonic structures. The retinal fibers, on the other 
 hand, enter into direct union with the blood vessels. It is not diffi- 
 cult to trace many fibers in their entire course from the retina to the 
 endothelial lining of the blood vessels, forming a protoplasmic 
 bridge between the two. While the phenomenon can be observed 
 at almost any stage of development, it is especially prominent in 
 embryos ranging from 50 to 130 mm in length. Here at times we 
 see the 'fibers apparently radiating from the blood vessels like the 
 radii of a wheel (Figure 14). It is probable that in this way nutri- 
 ment is carried from the blood vessels to the vitreous body and also 
 to the' retina, which for a long time is deprived of a vascular system 
 of its own. 
 
 A. v. Szily, who has devoted much labor to the study of this 
 peculiar phenomenon of the concrescence of ectodermal vitreous 
 
15 
 
 body fibers and mesodermal endothelial cells, maintains that the 
 retinal fibers after entering into a union with the mesoderm, lose 
 all connection with their mother cells of the retina, and become 
 structurally as well as functionally dependent on the endothelium 
 of the blood vessels. We should thus have the unique case of an 
 ectodermal structure separating from its parental tissue and enter- 
 ing into functional dependence on a mesodermal structure, a phe- 
 nomenon unparalleled in embryonic development. While it is true, 
 that in the inner portions of the optic cup, with progressive differ- 
 entiation, the retina loses the faculty of producing vitreous fibers, 
 this does not hold of the retina in its whole extent. In embryos of 
 25 and even 35 mm length, which marks the height of development 
 of the vascular system, the greater part of the vitreous fibers is still 
 in connection with the retina, while throughout the whole course 
 of embryonic development, and even in the fully developed eye, 
 the vitreous body fibers remain united with the retina in the region 
 of the pars ciliaris sive coeca retinae. There the vitreous body never 
 separates from the basal cells of the retina to which it owes its ori- 
 gin. This may be readily seen by comparing the corresponding 
 sections of the eyes of embryos of 25, 35, 60, 80, 100, 130, 175, and 
 250 mm length. As the study of this part of the retina, at various 
 stages of development, throws much light on the development and 
 the ultimate structure of the vitreous body, we have reproduced a 
 number of these both by sketches and by photographs. A 
 close study of these also refutes the theory of v. Lenhossek that the 
 vitreous body, separating from its place of origin, the basal cells of 
 the crystalline lens, forms a syncytium, deprived of cellular ele- 
 ments, and capable of independent growth, development, and nutri- 
 tion. Even in those regions where the retina loses the faculty of 
 producing vitreous fibers, there is no clear separation between the 
 two. The vitreous body even there retains its intimate connection 
 with the internal limiting membrane of the retina by a great number 
 of very delicate fibrils, which form the ragged edge of the vitreous 
 body in sections where shrinkage has pulled the two apart. 
 
 That the vitreous body fibers also enter into close relationship 
 with the free mesodermal elements found in the vitreous body is 
 made clear by many sections. After the manner of embryonic con- 
 nective tissue cells, many of the mesoderm cells assume a stellate 
 or fusiform shape and send out protoplasmic processes of greater or 
 shorter length. From these not infrequently long slender threads 
 
16 
 
 arise which attach themselves to any structure within reach, whether 
 other mesoderm cells or blood vessels or vitreous body fibers. There 
 seems to be no doubt that the latter also enter into direct union with 
 the mesoderm cells. We have thus an extremely complex relation- 
 ship between retinal fibers, blood vessels and free mesoderm cells. 
 We would emphasize this fact in view of the contention of Mavas, 
 Magitot, and Seefelder, that the mesodermal elements present in the 
 vitreous body are hermetically shut off (Seefelder) from the latter, 
 thus excluding the possibility of mesodermal contributions to the 
 vitreous body. 
 
 In view of the complex relationship between ectodermal and 
 mesodermal elements in the vitreous body, we agree with v. Szily 
 that the "vitreous body problem" is not so much to decide its ori- 
 gin, which at the present time is almost universally regarded as 
 ectodermal, but to determine the influence of various mesodermal 
 structures upon its further development. Szily puts the question 
 on a wider basis in so far as he maintains the real point at issue is 
 the union of two structures, primarily derived from different germ 
 layers, into one tissue. While all our preparations of many stages 
 of development of the eye of the pig, reveal this complex union 
 of mesodermal and ectodermal derivatives in a clear and unmis- 
 takable manner, we are not prepared to follow v. Szily in asserting 
 that the ectodermal vitreous body fibers become also functionally 
 dependent on the mesoderm. It is the retina which gives rise to the 
 primitive vitreous body ; it is the retina which directs and controls 
 the development of its structural parts ; it is the retina which in the 
 fully developed eye contributes exclusively to the formation of the 
 vitreous body. The retina alone never loses its original relationship 
 to the vitreous fibers; upon the retina they are structurally and 
 functionally dependent in their origin, development, and final ar- 
 rangement. Though this intimate relationship between retina and 
 vitreous body is found at first in the whole extent of the retina, it is, 
 however, later restricted to a narrow circular strip of the retina 
 between the ora serrata and the pars ciliaris retinae proper. There 
 the controlling influence of the retina upon the vitreous body fibers 
 is manifest at every stage of development as well as in the eye of 
 the adult animal. 
 
 Our conception of the relationship between retina and vitreous 
 body finds a singular confirmation in the results of experiments 
 made by Haemers to determine whether regeneration of the vitreous 
 
17 
 
 body is possible and how it takes place. After a number of experi- 
 ments on the eyes of various animals, Haemers asserts that ''the 
 vitreous body regenerates at the expense of the retinal neuroglia" 
 (page 114) . But surely, the regeneration of one tissue from another 
 presupposes a relationship of dependence between them. It is to 
 be. regretted that Haemers has not found any followers in his in- 
 genious attack upon the vitreous body problem by a careful study of 
 the process of regeneration. No doubt, a repetition and extension of 
 Haemers' investigations would tend to clear up other difficulties 
 and point the way for a final solution of this interesting and difficult 
 problem. Regeneration of the vitreous body has been observed also 
 by Sauri. 
 
 During the period of development described in this chapter, 
 the general appearance of the vitreous body has greatly changed 
 even in the regions not directly affected by the invasion of the meso- 
 derm. While during the earlier stages, including embryos of 35 mm 
 length, the radial fibers still form the larger part of its structure, 
 we soon observe that they are more and more replaced by fibers run- 
 ning approximately parallel to retina and lens. The parallel fibers 
 are at first found in the exterior regions of the vitreous body near 
 the retina, where they gradually assume the appearance of a special 
 membrane, distinct from the internal limiting membrane of the 
 retina, and forming the outer covering of the vitreous body. It has 
 been generally called the hyaloid membrane. Later a number of 
 fibers are found to take their course from the region anterior to the 
 ora serrata to the posterior part of the lens and then turn inward 
 in the direction of the optic nerve. They represent the first traces 
 of a more solid portion of the anterior surface of the vitreous body, 
 and they have sometimes been called the anterior hyaloid membrane. 
 The discussion of the significance of these structures will be reserved 
 for the next chapter. 
 
 Our conclusions, derived from the study of this most important 
 period in the development of the vitreous body, may be summed up 
 thus: The original purely ectodermal structure of the vitreous 
 body is radically changed by the accession of various mesodermal 
 elements in the form of blood vessels and free mesoderm cells with 
 their outgrowths. The ectodermal vitreous fibers enter into a close 
 relationship with the mesoderm, the relationship being one not only 
 of contiguity but of protoplasmic continuity, consisting of a pro- 
 topi j^mic union between them. This union, however, does not de- 
 
18 
 
 prive the retina of its controlling influence in the further develop- 
 ment of the vitreous body, which throughout remains functionally 
 dependent on it. The vitreous body, therefore, at this stage of de- 
 velopment is a very complex structure, the constituent parts of 
 which are partly ectodermal and partly mesodermal in their origin. 
 
 
 III THE PERMANENT VITREOUS BODY 
 
 The structure of -the vitreous body described in the preceding 
 chapter, is not a permanent one. The hyaloid vascular system, hav- 
 ing fulfilled its function of supplying nourishment to the crystalline 
 lens, soon shows signs of degeneration and gradually disappears. 
 The remnants of the mesodermal cells begin to disintegrate and 
 dissolve. The vitreous body then appears again in its original 
 purity, modified indeed and greatly changed, but in its essentials 
 like the primitive ectodermal vitreous body. An excellent repre- 
 sentation of its general appearance and internal structure is found 
 on plate VI, showing various portions of the eye of a fetus 220 mm 
 in length. 
 
 Figure 17 represents . a horizontal section of the eye, passing 
 through the optic nerve and the lens in a plane parallel to the hya- 
 loid artery, and bisecting the hyaloid canal. The lens has been 
 slightly displaced in the act of sectioning, causing some disarrange- 
 ment of the adjacent structures, especially of the fibers of the 
 zonula ciliaris. The vitreous body, however, is in perfect condition. 
 The section was stained in H eld's molybdic haematoxylin, which 
 shows very clearly the delicate fibers, but unfortunately overstains 
 the remaining structures. The small crystals found scattered here 
 and there are owing to mercuric chloride used in killing and fixing 
 the material. 
 
 The salient feature of the vitreous body at this period of de- 
 velopment, compared with figure 5, is the absence of the large meso- 
 dermal ingredient of the previous stages of development. All that 
 remains of the extensive hyaloid arterial system is the main trunk, 
 the hyaloid artery, extending through the entire posterior chamber 
 of the eye, from the optic nerve to the lens, to which it still ad- 
 heres. Its various branches have already disappeared. A closer 
 examination of the vitreous body, however, reveals a large number 
 of cells in various stages of disintegration. The methods used in the 
 preparation of the material and the heavy stain make it impossible 
 
19 
 
 to determine the structure and the nature of these cells, but nowhere 
 does there appear any connection between them and the vitreous 
 body fibers. 
 
 The vitreous body proper is represented by a great mass of very 
 delicate fibers, arising exclusively from a small region of the anterior 
 portion of the retina, the pars coeca retince. From here they descend 
 in fairly heavy bundles towards the center of the bulbus in the 
 region of the optic nerve. Several of these fiber bundles pass closely 
 along the retina, while the others grow slightly inward in the direc- 
 tion of the lens and then turning backward join the first near the 
 optic nerve (Figure 18). Between these two heavier portions of 
 the vitreous body we have a rather large space filled with an irregu- 
 lar mass of fibrous tissue. The vitreous body, therefore,- is com- 
 posed of two rather prominent masses or layers of fibers, the one 
 forming the outer portions near the retina, and the other lining the 
 hyaloid canal, while the intervening region contains a very loosely 
 arranged fibrous mass. The outer portion of the vitreous fibers has 
 been quite generally held to be a special membrane and called the 
 hyaloid membrane. Careful investigation shows that the vitreous 
 fibers are, indeed, more closely arranged in this region, but they do 
 not coalesce to form a distinct membrane. The internal limiting 
 membrane of the retina seems rather to be common to both the retina 
 and the vitreous body. This view is held by most recent investiga- 
 tors (Kolliker, Szily, Wolf rum, Mavas, Magitot, Seef elder, Szent- 
 Gyorgyi), who insist that the supposition of the existence of a mem- 
 brane enveloping the vitreous body and distinct from the internal 
 limiting membrane of the retina is not based upon fact. In this 
 connection we may call attention to the peculiar fact that, when 
 shrinkage of the vitreous body takes place, the retinal membrane 
 quite generally adheres to the vitreous body, which shows the inti- 
 mate union of the two. The inner mass of vitreous body fibers 
 separates the vitreous body proper from the so-called hyaloid canal. 
 This portion of the vitreous body, the existence, size, and real struc- 
 ture of which have been the subject of much controversy, is clearly 
 shown in figure 18. 
 
 Figure 17 shows its relation to the other parts of the eye, its rela- 
 tive size and its shape. It roughly resembles a funnel, with its mouth 
 towards the lens, and the stem surrounding the optic papilla. While 
 quite narrow in the inner portion of the eye, it widens very much 
 
20 
 
 and extends not to the lens, but to the ciliary portion of the retina, 
 which gives rise to the vitreous fibers. Its vast size, compared with 
 the diameter of the hyaloid artery, seems to dispose at once of the 
 theory that the hyaloid canal simply represents the portion of the 
 vitreous body formerly occupied by that blood vessel. Its walls are 
 formed by the vitreous fibers which in many sections have all the 
 appearances of a real membrane (Figure 18). This impression- is 
 heightened by closer study. Still we hesitate to describe this por- 
 tion as a special membrane. We rather incline to the belief that the 
 membranous appearance is owing to the thickness of the section, 
 which in this case is not less than 400 microns. The fibrous nature 
 of this structure seems to be quite plainly shown in the upper left- 
 hand portions of figure 18. It is not improbable, however, that the 
 fibers are held together by the thickened fluids of the vitreous* body, 
 giving it the firm membranous appearance it possesses. This portion 
 of the vitreous body, although generally called the hyaloid canal, is 
 in reality not a canal. It is not an open tube without structural 
 elements, because its interior is filled with an irregular mass of 
 tissue, not unlike that of the central portions of the vitreous body 
 enclosed between the outer and inner fiber bundles already described 
 and illustrated in figure 18. 
 
 To complete the description of the eye at this period we call 
 attention to another new structure which has appeared in the mean 
 time, the zonula ciliaris (Figure 15). The sketch is prepared from 
 a section of the left eye of the same embryo which furnished the 
 material for the illustrations on plate. The zonula ciliaris consists 
 of a number of rather strong fibers between the pars ciliaris retina? 
 and the lens. They arise, as is clearly shown in many sections, from 
 the region of the retina, that produces the vitreous fibers, only 
 slightly anterior to the latter. Retina, vitreous body, and zonula 
 ciliaris, therefore, are genetically most intimately related. 
 
 Keeping in mind the structure of the eye at this rather advanced 
 stage of embryonic life, we may now attempt to trace the develop- 
 ment of the various parts of the vitreous body up to this time. In 
 embryos of 80 mm length, the hyaloid vascular system consists of 
 the main trunk and a number of branches, some of which surround 
 the posterior half of the lens, while the rest form a complex system 
 in the outer portion of the vitreous body near the retina. The 
 entire system is only a temporary formation, whose function is to 
 furnish nourishment to the rapidly developing lens, and no doubt, 
 
21 
 
 also to the vitreous body and to part of the retina, which for a long- 
 time has no blood vessels of its own. This functional interde- 
 pendence of blood vessels, vitreous fibers, and retina might offer a 
 teleological explanation for the intimate protoplasmic union of these 
 parts described in chapter II. As soon as the hyaloid system has 
 discharged this nutritive function, a process of resorption sets in. 
 This process does not always begin from the extremities of the 
 branches; it first manifests itself by a general decrease in the dia- 
 meter of the blood vessels, which may thus be cut up into a number 
 of parts found scattered in various places of the vitreous body. 
 These parts gradually disintegrate and give rise to many cellular 
 elements which are met with in more advanced stages of develop- 
 ment and sometimes even in the eyes of adult animals. They are 
 the remnants of the hyaloid vascular system. The gradual resorp- 
 tion of the blood vessels does not proceed in a uniform manner in 
 all embryos, as a comparison of embryos of 130, 150, and 180 mm 
 length shows, but in the eyes of embryos of 200 mm, only the main 
 trunk, the hyaloid artery proper, remains (Figure 17). It may be 
 added here that the main trunk was found also in a practically 
 mature fetus 12 inches in length, which makes it probable that com- 
 plete resorption of the hyaloid artery takes place at the time of 
 birth or even later. In the eye of a full grown animal, a piece of 
 the hyaloid artery, about 6 mm long, was still present attached to 
 the lens. Together with the hyaloid system the remaining meso- 
 dermal elements also undergo a process of disintegration. 
 
 What then is the fate of the fibrous network which we found in 
 such intimate union with the mesoderm in the previous stages of 
 development f It is probable that a portion of it also disintegrates 
 and is resorbed. But it is no less probable that the remaining por- 
 tions make up the loose fibrous tissue found in the hyaloid canal and 
 in the more fluid parts of the vitreous body. The reasons for this 
 assumption will be given below. 
 
 We must first turn our attention to another structure, a clear 
 conception of the formation and the significance of which will throw 
 more light on the present investigation. This structure is the hya- 
 loid canal. We have already called attention to the wide difference 
 in the diameter of the hyaloid artery and the so-called hyaloid canal, 
 and expressed our doubts that the latter represents nothing but the 
 cavity left for a time after the former has been resorbed. This 
 
22 
 
 explanation is defended most persistently by Wolfrum, who main- 
 tains moreover that the hyaloid canal is not found in the eyes of 
 adult animals, except in connection with remnants of the hyaloid 
 artery. The discussion of the latter statement we reserve until 
 later. For the embryonic eye, however, we can not accept 
 Wolf rum's explanation. 
 
 Equatorial sections prepared for the purpose of ascertaining 
 the precise time of the first appearance of the hyaloid canal show 
 that it is formed in embryos between 130 and 150 mm length. From 
 the first it shows its characteristic funnel-like shape, its size consid- 
 erably exceeding the diameter of the blood vessel (Figure 20). The 
 latter most frequently appears in the center of the canal and shows 
 no relation to its walls (Figure 18). In a number of longitudinal 
 sections of the eyes of embryos ranging from 150 to 180 mm length, 
 the slowly degenerating hyaloid system consists of the main trunk, 
 the hyaloid artery, and several branches surrounding it in a manner 
 not unlike the supporting framework of a tent. At this time there 
 is observed likewise an increased growth of the vitreous fibers from 
 the ciliary region of the retina. A portion of these fibers, as was 
 shown above, takes its course toward the lens and then turns inward 
 toward the centre of the eye. It is this portion which is so prom- 
 inent in figure 18. Now it seems to us very probable that this 
 peculiar direction is given to the fibers by the branches of the blood 
 vessels. In many sections, parts of the latter are almost invariably 
 found in a plane parallel to the vitreous fiber bundles (Figure 21). 
 But whereas the blood vessels finally disappear, the direction given 
 to the fibers remains, thus determining the outline and shape of the 
 hyaloid canal of which they form the membrane-like wall. 
 
 This denser formation of the vitreous fibers, descending in a cir- 
 cular band from the ciliary retina encloses a large portion of the 
 original vitreous body which forms the fibrous contents of the hya- 
 loid canal. This irregular fibrous mass may, therefore, be regarded 
 as the remnant of the central portion of the vitreous body of the 
 preceding period of development. A similar explanation is offered 
 for the irregular fibrous tissue between the two heavier layers of 
 vitreous fibers in the lateral parts of the eye. Thus the hyaloid 
 canal is not a canal in the generally accepted meaning of the word, 
 yet it possesses such a definite structure, and it is so sharply marked 
 off from the rest of the vitreous body that it deserves a special 
 
23 
 
 name. ' * Hyaloid portion of the' vitreous body ' ' might be more de- 
 scriptive of its real structure. Szent-Gyorgyi suggests tractus hya- 
 loideus corporis vitrei. Its essential structure is given at this stage 
 of development; only minor modifications are observed in the eye 
 of the adult animal. 
 
 The description of the formation of the hyaloid canal leads us 
 also to a better appreciation of the structure of the vitreous body 
 as a whole. The complex structure of the preceding period, owing 
 to the invasion by great masses of mesoderm, has disappeared 
 through a process of disintegration and resorption, and has given 
 place to a much simpler and more regular arrangement of the fibers. 
 The rather strong fibers of the pars ciliaris, already observed in 
 embryos of 60 and 80 mm length, have in the meantime assumed 
 the appearance of fiber bundles (Figure 19), the arrangement of 
 which has already been described at length. There is no longer 
 evidence of an extensive union between these fibers, each bundle be- 
 ing more or less independent of the others. But their place of 
 origin is the same, the cells of that portion of the ciliary retina, 
 immediately anterior to the ora serrata. The ectodermal origin 
 and nature of the permanent vitreous body is, therefore, no less 
 certain than that of the primitive vitreous body. The mesoderm 
 has no share in its production. 
 
 The vitreous body in the eye of the adult animal differs only 
 in minor details from that described so far. Comparing figures 17 
 and 19, we notice that the hyaloid artery has entirely disappeared. 
 The fibers of the vitreous body and the zonula ciliaris have essen- 
 tially the same arrangement and structure (Figures 15 and 16). 
 The shape of the hyaloid canal, however, has slightly changed. By 
 a further ingrowth of the vitreous fibers in the region of the lens, 
 the funnel-like shape of the preceding stages (Figures 17 and 18) 
 has given place to a structure of almost uniform diameter through- 
 out its entire length (Figures 19 and 22). This tissue, however, 
 shows no tendency to condensation or growth which might lead to 
 a gradual obliteration of the canal. The latter was found in the 
 eyes of every specimen examined, forty in all. Wolf rum's state- 
 ment, therefore, that the hyaloid canal is not a constant structure 
 of the eye of the adult pig, but is found only occasionally and then 
 always in connection with remnants of the hyaloid artery, is not 
 in accordance with the facts. Only in two eyes, out of a total of 
 
24 
 
 forty, were traces of the hyaloid artery found, and even in these 
 two the independent formation of the hyaloid canal was plainly 
 shown. 
 
 Our own results may be summed up thus : the hyaloid canal, or 
 the hyaloid portion of the vitreous body, formed by a denser ar- 
 rangement of the vitreous fibers and including a loose and irregular 
 fibrous tissue, is a constant structure in the eye of the adult pig. It 
 is of almost uniform width throughout, and extends through the 
 entire posterior chamber of the eye from the optic disc to the lens, 
 from which it is separated by the thin membranelike lining of the 
 fossa patellaris. In rare cases it contains remnants of the hyaloid 
 artery, which, however, show no relationship to its walls. The hya- 
 loid canal in the eye of the adult pig, is, therefore, not a temporary 
 formation nor is it to be attributed to the hyaloid artery. 
 
 Szent-Gyorgyi, who has made extensive investigations of the 
 structure of the hyaloid canal in the eyes of adult animals, including 
 the pig, maintains that the relationship of the vitreous body fibers 
 to the ciliary portion of the retina is a secondary formation, attrib- 
 utable to the tendency of the vitreous fibers to attach themselves to 
 any structure near them, and that in consequence this relationship 
 does not warrant a conclusion as to the origin of these fibers, which 
 must be sought rather in the vitreous body itself with its native 
 faculty of growth and differentiation. But it is plain that the 
 structure of the adult eye does not of itself reveal whether the con- 
 ditions found there are primary or secondary. Nor is it always 
 safe to attempt the reconstruction of the ontogeny of a species from 
 its supposed phylogenetic development. Only the careful study 
 of the various phases of embryonic development shows whether the 
 union between the vitreous fibers and the ciliary retina is a secon- 
 dary formation or has genetic significance. A comparison of figures 
 8 and 19 reveals at once the untenability of Szent-Gyorgyi 's 
 opinion, and disposes at the same time effectively of the view of 
 Lenhossek, adopted and modified by his brilliant followers, v. Szily 
 and Szent-Gyorgyi, that the vitreous body, separating from its place 
 of origin, becomes a syncytium, capable of independent nutrition, 
 growth, differentiation and regeneration. The union between the 
 vitreous body and the ciliary retina can be traced uninterruptedly 
 from the first appearance of the vitreous fibers to the formation of 
 the permanent vitreous body. The latter, therefore, no less than 
 
25 
 
 the primitive, and the essential portions of the secondary vitreous 
 body, are a purely ectodermal derivative. 
 
 CONCLUSIONS 
 
 1. The vitreous body in its origin is a purely ectodermal struc- 
 ture and consists of delicate fibers, the protoplasmic outgrowths of 
 the basal cells of the retina. 
 
 2. With the development of ,the hyaloid vascular system and 
 the accession of other mesodermal elements, an intimate union 
 arises between the vitreous body fibers and the mesoderm, so that 
 the vitreous body at that stage of development must be regarded as 
 a tissue of derivatives of both the outer and the middle germ layer. 
 
 3. The union between the vitreous body and the mesoderm does 
 not destroy the dependence of the vitreous fibers upon the retina, 
 which during the entire embryonic life controls the development of 
 the vitreous body. 
 
 4. The union of the vitreous body and the mesoderm is not 
 permanent. It is dissolved with the gradual resorption of the hya- 
 loid vascular system and the disintegration of the remaining meso- 
 dermal elements. 
 
 5. The vitreous body in its final development is again a purely 
 ectodermal structure, consisting of great masses of delicate fibers, 
 which have their origin in the basal cells of the ciliary portion of 
 the retina. 
 
 6. There is no hyaloid membrane distinct from the internal lim- 
 iting membrane of the retina. 
 
 7. The hyaloid canal, or rather the hyaloid portion of the vitre- 
 ous body, is found in all advanced stages of development and in the 
 eye of the adult animal. 
 
 8. The hyaloid canal is formed by a thick, membranelike mass 
 of vitreous fibers and contains a loosely arranged fibrous tissue, 
 which may be regarded as the remnant of the secondary vitreous 
 body. 
 
LITERATURE 
 
 List of authors, to whose works special reference is made in this 
 dissertation : 
 Addario, C., Sulla struttura del vitreo embrionale e dei neonati; 
 
 sulla matrice del vitreo e suH'origine della zonula. Pavia, 1902. 
 
 tiber die Matrix des Glaskorpers im menschlichen und 
 tierischen Auge. Anat. Anz., Bd. XXI, 1902. 
 
 Sulla matrice del vitreo nell'occhio umano et degli ani- 
 mali. Riforma medic., an. 18, 1902. 
 
 Sull 'apparente membrane limitante della retina ciliare. 
 Monit. Zool. Ital., an. 13, 1902. 
 
 La matrice ciliare delle fibrille del vitreo, loro forma et 
 disposizione, nonche loro rapporti colla neuroglia della 
 retina visiva perif erica nell'occhio umano adulto. Arch. 
 Ottalm., vol. XII, 1904. 
 
 Arnold, J., Die Linse und das Strahlenblattchen. Graefe u. Saemich 
 Handb. d. ges. Augenheilk., Bd. Ill, Kap. II, 1874. 
 
 Bach, L., und Seefelder, R., Atlas zur Entwicklungsgeschichte des 
 menschlichen Auges. Leipzig und Berlin, 1914. 
 
 Bribach, R., tiber den Zentralkanal des Glaskorpers. Arch. Opht., 
 Bd. LXXVI, 1910. 
 
 Cirincione, G., Sullo stato odierno della questione riguardante la 
 genesi del vitreo. Siena, 1905. 
 
 tiber die Genese des Glaskorpers bei Wirbeltieren. Cen- 
 tralbl. prakt. Augenheilk., 27 Jahrg., 1903. 
 
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 Haemers, A., Regeneration du corps vitre. Arch. d'Opht., t. 
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 Held, H., tiber die Neuroglia marginalis der menschlichen Gross- 
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 Die Entwicklung des Nervengewebes bei Wirbeltieren. 
 Leipzig, 1909. 
 
27 
 
 Kessler, L., Untersuchungen iiber die Entwicklung des Auges, an- 
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 Tiber die Entwicklung des Glaskorpers. Dorpat mediz. 
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 Zur Entwicklung des Wirbeltierauges. Leipzig, 1877. 
 
 Kolliker, A. v., Entwicklungsgeschichte des Menschen und der 
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 Die Entwicklung und Bedeutung des Glaskorpers. Zeit- 
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 Lenhossek, M. v., Die Entwicklung des Glaskorpers. Leipzig, 1903 
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 Untersuchungen am Hiihnchen. Arch. mikr. Anat., Bd. 
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 Lillie, Frank E., The Development of the Chick. New York, 1908. 
 
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 Recherches sur 1 'anatomie et la physiologic de la region 
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 Recherches cytologiques et physiologiques sur la retine 
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28 
 
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29 
 
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 and LXX, 1909. 
 
31 
 
 EXPLANATION OF THE FIGURES 
 
32 
 
 PLATE I. 
 
 A. W. Fromm photog. 
 
33 
 
 PLATE I. 
 
 Figures 1, 2, 4 and 5 were magnified 100 diameters, figure 3 
 was magnified 150 diameters, the entire plate was then reduced 
 to two-thirds in reproduction. 
 
 Fig. 1. Longitudinal section of eye of 9 mm embryo. The sec- 
 tion is slightly posterior to the optic stalk, a part of which is shown 
 between pigment layer and portion of the diencephalon visible to 
 right. 
 
 Fig. 2. Longitudinal section of eye of 20 mm. embryo. The 
 section shows plainly the connection between the vitreous body and 
 the extraocular mesenchyme, which is invading the optic cup 
 through the perilenticular opening; also extensive vascular system 
 and considerable differentiation of central portion of retina. 
 
 Fig. 3. Longitudinal section of eye of 8 mm embryo. Lens 
 vesicle complete, choroid fissure wide open, mesenchyme pressing 
 into optic cup through choroid fissure. 
 
 Fig. 4. Horizontal section of eye of 12 mm embryo. Hyaloid 
 artery has penetrated to lens vesicle. 
 
 Fig. 5. Longitudinal section of eye of 25 mm embryo. Hya- 
 loid artery entering through optic stalk, has given rise to extensive 
 vascular system which penetrates larger part of vitreous body. 
 
34 
 
 
 PLATE II. 
 
 Fromm & Herring del. 
 
35 
 
 i 
 
 PLATE II. 
 
 Figures 6-9 were drawn with a camera lucida, Bausch and 
 Lomb microscope, ocular 10, objective 4. Plate reduced to two- 
 thirds in reproduction. 
 
 Fig. 6. Longitudinal section of eye of 5 mm embryo. First 
 stage in the formation of the optic cup and the lens vesicle. 
 
 Fig. 7. Longitudinal section of eye of 3.5 mm embryo, showing 
 primary optic vesicle. Note layer of mesenchyme between optic 
 vesicle and body ectoderm. 
 
 Fig. 8. Detail of section of eye of 9 mm embryo. 
 
 Fig. 9. Detail of longitudinal section of eye of 11 mm embryo, 
 showing general arrangement of vitreous fibers. 
 
mK?2i<WK 
 
 Frorart & Herring del. 
 
PLATE III. 
 
 Figures 10-12 were drawn with a camera lucida, Bausch and 
 Lomb microscope, ocular 5, objective 16. Plate reduced to one-half. 
 
 Fig. 10. Detail of section of eye of 25 mm embryo. 
 Fig. 11. Detail of section of eye of 80 mm embryo. 
 Fig. 12. Detail of section of eye of 35 mm embryo. 
 
38 
 
 14 
 
 PLATE IV. 
 
 A. W. Fromm photog. 
 
39 
 
 PLATE IV. 
 
 Fig. 13. Cross-section of vitreous body of 23 mm embryo, 
 showing the many fibrous outgrowths of the hyaloid arterial system 
 and of the numerous free mesoderm cells, also the union of these 
 mesodermal elements with the vitreous body. Magnification 350 
 diameters. 
 
 Fig. 14. Section of eye of 100 mm embryo. Blood vessels form 
 two distinct systems, one surrounding the lens, and the other lying 
 close to the retina. Numerous fibers of blood vessels. Note espe- 
 cially fibers running parallel to retina. Magnification 95 diameters. 
 
40 
 
 16 
 
 PLATE V. 
 
 Fromm & Herring del. 
 
41 
 
 PLATE V. 
 
 Figures 15 and 16. Zonula ciliaris of eye of 225 mm embryo 
 and of adult animal. Drawn with camera lucida, Bausch and Lomb 
 microscope, ocular 5, objective 16. Plate reduced to two-thirds. 
 Portion of lens (below) and of ciliary retina. 
 
PLATE VI. 
 
 A. W. Fromm photog. 
 
43 
 
 PLATE VI. 
 
 Fig. 17. Horizontal section of eye of 225 mm fetus, showing 
 hyaloid canal with hyaloid artery, and origin of vitreous fibers from 
 ciliary retina. Magnification 6 diameters. 
 
 Fig. 18. Central portion of fig. 17 further enlarged. 
 
 Fig. 19. Horizontal section of eye of adult pig, showing hya- 
 loid canal, vitreous fibers, and zonula ciliaris. Magnification 2.5 
 diameters. 
 
 Fig. 20. Cross-section of hyaloid canal of 250 mm fetus, with 
 hyaloid artery in lower centre. Magnification 20 diameters. 
 
 Fig. 21. Cross-section of hyaloid canal of 150 mm embryo. 
 Magnification 20 diameters. 
 
 Fig. 22. Cross-section of hyaloid canal of eye of adult pig. 
 Magnified 20 diameters. 
 
VITA AUCTORIS 
 
 The writer of this dissertation was born in Volkerode, in the 
 Province of Saxony, Germany, March 26, 1883. After attending 
 the public school of his native town for seven years, he came to this 
 country in 1895 and entered St. Joseph Seminary, Teutopolis, Illi- 
 nois, from which he was graduated in June, 1902. In the same 
 month he entered the Order of Friars Minor in the Province of the 
 Sacred Heart, pursuing his rhetorical and philosophical studies in 
 Chicago, Illinois, from 1903 to 1906, and his theological studies in 
 St. Louis, Missouri, from 1906 to 1910. He was ordained to the 
 holy priesthood on June 25, 1909. From 1910 to 1917 he was a 
 member of the faculty of St. Joseph Seminary, Teutopolis, Illinois. 
 He then spent two years at the University of Chicago in the study 
 of biology, physiology, and psychology, and three semesters at the 
 Catholic University of America, Washington, D. C. 
 
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