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BIOLOGY 
 LIBRARY 
 
it*!* 
 
Reprinted from the 'Quarttrly Journal of Microscopical Science,' July, 1893.] 
 
 The Structure of the Pharyngeal Bars of 
 Amphioxus. 
 
 By 
 
 W. Blaxland Beiiliam, D.Sc.Loiicl., lion. M.A.Oxou., 
 
 Aldrichian Demonstrator of Comparative Anatomy in the University of 
 
 Oxford. 
 
 With Plates 6 and 7. 
 
 IT may be thought that the structure of the pharyngeal bars 
 of Amphioxus is sufficiently known, after the description 
 by Lankester, and more recently by Spengel; but there 
 still remains a certain amount of doubt as to some points in 
 the structure of the tongue or secondary bar, although recent 
 authors are in agreement as to the general structure of the 
 primary bar. It is to the tongue bar, therefore, that I have 
 more particularly directed my attention. 
 
 Spengel contradicted Professor Lankester on several points, 
 both with regard to matters of observation, and more espe- 
 cially with regard to certain interpretations, in a very dog- 
 matic and, indeed, discourteous manner. I was surprised to 
 find that Professor Spengel himself is by no means correct 
 in sundry matters of mere observation. 
 
 It will be remembered that Lankester, in addition to his 
 account of the structure of the gill bars which was, like his 
 figures, in great advance of the work of previous writers on 
 the subject made certain statements with regard to the spaces 
 within these bars; he attempted to distinguish, not only in the 
 
 1 
 
 4-27201 
 
BIOLOGY 
 
 2 W. BLAXLAND BENHAM. 
 
 bars, but in Am phi ox us as a whole, coelomic spaces from 
 blood-vessels. It is chiefly with regard to the interpretation 
 of the spaces within the tongue bars that Spengel joins issue 
 with him, and it was to this part of the subject that I directed 
 my attention last summer in the endeavour more especially to 
 decide whether the cavity traversing the "chitinoid" rod of the 
 tongue bar be coelom (Lankester) or blood-vessel (Spengel). 
 
 I hoped to decide the question by the examination of speci- 
 mens which had been fed with carmine; and for this purpose 
 Professor Lankester requested Mr. A. Willey, who was then at 
 Naples, to feed some animals and preserve them for me. My 
 thanks are due to Mr. Willey for so doing. Unfortunately, 
 however, these carmine-fed specimens have not been of much 
 value for my purpose, and I was compelled to fall back upon 
 carefully preserved specimens (in picro-sulphuric acid) stained 
 in various media. This was the method followed both by 
 Lankester and by Spengel careful tracing of spaces from 
 section to section in order to ascertain their communication 
 with other cavities about whose nature there is no doubt. 
 
 Before I had completed my work, Boveri's most interesting 
 paper on the uephridia was published. Boveri found, as I had, 
 that Spengel, both in incomplete observation and in certain 
 interpretations, had fallen into errors similar to those which 
 with an accompaniment of gratuitous insolence he had attri- 
 buted to Lankester. 
 
 This short note may conveniently be divided as follows : 
 
 A. Description of the tongue bar, according to my own ob- 
 servations, and comparison of it with the primary bar. 
 
 B. Interpretations of the parts of the tongue bar. 
 c. The observations of recent observers. 
 
 D. Certain abnormalities in the pharyngeal bars. 
 
 The authors who have described and figured the gill bars of 
 Am phi ox us, to whom I shall have occasion to refer, are 
 
 1. STIEDA. " Studien lib. d. Amphioxus lanceolatus," ' Mem. Ac. Sci. 
 Petersbourg,' xix, 1873. 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS 3 
 
 2. LANGERHANS. "Zur Anat. d. Amphioxus lanceolatus," ' Arch. f. 
 
 mikr. Anat.,' xii, 1876. 
 
 3. SCHNEIDER. ' Beitrage zur Vergleich. Anat. und Entwickel. der Wir- 
 
 belthiere,' 1879. 
 
 4. LANKFSTER. "Contributions to the Knowledge of Amphioxus lanceo- 
 
 latus," 'Quart. Journ. Micr. Sci.,' xxix. 
 
 5. SPENGEL. "Beit. z. Kenntiiiss d. Kiemen des Amphioxus," 'Zool. 
 
 Jahrbuch ' (Anat.), iv, 1891. 
 
 6. BOVERI. "Die Nierencanalchen des Amphioxus," 'Zool. Jahrbuch ' 
 
 (Anat.), v, 1892. 
 
 In addition to these, a bibliography rela ting to Amphioxus 
 will be found in Lankester's paper. 
 
 A. The Tongue Bar. 
 
 The tongue (or secondary) bar is usually distinguished from 
 the primary bar (a) in being supported by a tubular skeletal 
 rod in place of the double rod of the primary bar, (b) and in 
 being without any coelom between the rod and the atrial epi- 
 thelium. It is unnecessary to describe the relations of the 
 bars to one another or to neighbouring parts of the animal, as 
 these matters have been fully described and illustrated by re- 
 cent writers on the subject. 
 
 The structure of the bar is most readily seen in its transverse 
 sections, but such sections accurately transverse to the bar 
 are not so easily obtained; in sections transverse to the long 
 axis of Amphioxus, only one or two bars on each side of the 
 pharynx will be cut transversely, though in the pre-hepatic 
 region more bars are so cut, and still more are very nearly 
 transversely cut, than is the case posteriorly. But by varying 
 the obliquity of the plane of section to that of the long axis of 
 the body, I was able, ultimately, to obtain sections which cut 
 nearly the whole series of bars in any section almost accu- 
 rately at right angles to their length. 
 
 It appears to me that this is most important, for the discre- 
 pancies in various descriptions and figures of the bar are doubt- 
 less due to the more or less obliquity of the sections. 
 
 Another matter which must be taken into serious account is 
 the mode of preparation and the character of the stain ; for I 
 
4 W. BLAXLAND BENHAM. 
 
 liave noted in my series treated differently in both these 
 respects various differences due to shrinkage and such effects, 
 which suggest the cause of certain omissions by some authors, 
 and of wrong interpretations and other errors. 
 
 I have found that Kleinenberg's fluid (picro-sulphuric acid) 
 is the preservative which produces less distortion than other 
 reagents. Cochineal stains, especially Mayer's alcoholic cochi- 
 neal, serves best for the demonstration of blood in the vessels 
 and for the examination of the skeletal tissues. 
 
 Hsematoxylin is, of course, useful for the nuclei, but cochi- 
 neal gives better results in the case of cell-bodies. 
 
 Some of the animals were stained in bulk ; in other cases 
 the sections were stained on the slide. 
 
 I proceed now to a description of a transverse section of a 
 tongue bar, as elucidated by the examination of sections 
 treated in different ways, as well as of isolated portions of 
 pharyngeal walk 
 
 Such a section is a narrow bar (about three times as long as 
 it is wide), presenting two ends and two sides : one end, the 
 Outer end, projects into the atrium, and is covered by atrial 
 epithelium ; the opposite end, or Inner end, projects into the 
 cavity of the pharynx, and is lined by hypoblast. The sides 
 are directed (roughly) anteriorly and posteriorly. 
 
 The bar consists almost entirely of columnar epithelial 
 c e 1 1 s, the inner ends of which abut upon a membrane, the c u t i s 
 (I follow here Professor Lankester), throughout the greater part 
 of the bar; whilst at the outer end the cells rest upon the 
 skeletal rod, which I regard as merely a specialised part of 
 this cutis. 
 
 The character of the epithelium, however, differs at the two 
 ends and at the sides. At the Inner or pharyngeal end the 
 nuclei are arranged in three groups, a central group and a 
 lateral group on each side, as Lankester was the first to point 
 out. The nuclei of the central group are arranged in two 
 more or less curved rows (as in fig. 1), but this arrangement 
 is due not to the existence of two layers of cells, but to the 
 fact that alternately the nuclei are situated nearer to or 
 
STRUCTURE OF THE PHARTNGEAL BARS OF AMPHIOXUS. 5 
 
 further from the base of the cell. The nuclei are elongated 
 and oval ; they are placed much closer to one another than my 
 figure represents, and as they stain deeply are very conspi- 
 cuous. In thick sections it is difficult to determine whether 
 one has to do with a single row of very elongated cells (as 
 Lankester believed), or with several rows of them; this diffi- 
 culty is emphasised when the sections are not accurately 
 transverse. But in thin sections, successfully cut, it is easily 
 seen that the nuclei are arranged as I describe them. 
 
 The cells to which these nuclei belong are, therefore, as 
 long as the epithelium is thick ; they are very narrow peri- 
 pherally and swollen at the nuclear level, producing the flask- 
 shaped appearance of the whole group. These cells carry 
 cilia which are considerably longer than those carried by the 
 lateral groups of cells at this inner end ; and it is curious that 
 of previous observers, only Spengel and Boveri have noted 
 this special bundle of cilia. The free ends of the cells are pro- 
 vided with a very finely striated border, which comes out well 
 in cochineal preparations, but which is riot differentiated by 
 other stains used. 
 
 Each lateral group of nuclei consists of a single row curving 
 downwards from the central group towards each side, some- 
 what in the way represented by Lankester; but I find that 
 these nuclei, which are long and narrow, are not arranged 
 quite in the fan-shaped manner represented by him. 
 
 The cells containing these nuclei carry quite short cilia 
 entirely overlooked by Spengel and Boveri, and their free 
 ends are not provided with a striated border. The shape of 
 this inner end has been very variously represented, as the 
 copies of previous figures on PL 6 will show. 
 
 The side of the bar presents some four or five rows of small 
 nuclei forming a broad band, about two thirds the whole width 
 of the epithelium. These nuclei are not circular, as most ob- 
 servers have represented them (owing to the obliquity of the 
 section, as I know from experience), but are oval, with the 
 long axis directed vertically to the plane of the surface. 
 Langerhans appears to have noted their oval shape. 
 
6 W. BLAXLAND BENHAM. 
 
 Here, again, these rows of nuclei do not represent as many 
 layers of cells, for there is but a single layer of very long and 
 very narrow cells, with the nuclei at different levels in neigh- 
 bouring cells. In some of my preparations these cells are more 
 or less macerated, so that I was able to confirm the description 
 of them given by Langerhans (see fig. 19). Each cell, or at 
 any rate most cells for it is impossible to be absolutely certain 
 that all the cells reach the surface, though I believe this to be 
 the case carries a single very long cilium, and presents a 
 finely striated border. The nuclei of the lowermost row ap- 
 pear to be slightly longer than those of other rows, and are 
 close to the septal membrane or cutis, along which they form 
 a very well-marked series. Towards the extremity of the side, 
 both outwards and inwards, i. e. towards the atrial end and the 
 pharyngeal end, the lowest row of nuclei curve upwards to- 
 wards the surface, so that the epithelial cells become shorter 
 and shorter, and the number of rows less and less, till finally 
 there is but a single nucleus contained in a cell not much longer 
 than itself (see fig. 1). At the inner end of the bar the thin 
 membrane-like cutis curves outwards towards the surface, and 
 naturally the row of nuclei take on this curvature. At the 
 opposite end of the bar the row of nuclei follow the curve of 
 the chitinoid rod; but at this end, for a short space, the epi- 
 thelium is overlapped, as it were, by a row of three or four 
 cubical cells containing pigment, and not carrying cilia. These 
 cells are part of the atrial epithelium, the invaginated epiblast 
 of the pharyngeal slits. These pigmented cells differ from 
 those hitherto mentioned in containing circular (spherical) 
 nuclei, and herein agree with the cells constituting the epithe- 
 lium of the Outer end of the bar. 
 
 This atrial epithelium is characterised by the larger size of 
 its cells, the absence of cilia, and the presence of relatively 
 small round nuclei, which are placed at different levels in the 
 cells (fig. 1). These cells are highest in the middle of this end 
 i. e. at the end of the long axis of the section through the bar 
 and decrease in length at each side of this point till they 
 graduate into the cubical pigmented cells which overlap the 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 7 
 
 cells of the sides of the bar. In the tongue bar these cells are 
 all of one kind (fig. 18), whereas in the primary bar two kinds 
 of cell (fig. 17, a, b) are present, one (a) being vacuolated, the 
 other (b) more granular. Langerhans noted this fact, though 
 he exaggerated the difference. So much for the epithelium of 
 the bar. I shall point out later how far these statements of 
 fact differ from those of my predecessors. 1 
 
 Turning now to the cutis of the bar, i.e. the septal mem- 
 brane and the chitinoid rod: the septal membrane forms 
 a very thin sheet of tissue traversing the greater part of the 
 long axis of the section, and separating the epithelium of the 
 two sides. At the base of the epithelium of the Inner or 
 pharyngeal end of the bar the septal membrane splits 
 into two, and each of the two branches curves outwards to- 
 wards the surface ; this forking of the membrane leaves a 
 V-shaped space, which is converted into a triangle by a mem- 
 brane (cutis) at the base of the pharyngeal epithelium. 
 
 In this triangular space is a blood-vessel, as Spengel and 
 Boveri have already described (figs. 7, 8). It may be called the 
 internal or Visceral vessel. At the opposite end the septal mem- 
 brane similarly divides into two, each half of which appears to 
 be continuous with the corresponding side of the rod (fig. 1). 
 This space, which differs somewhat in shape according to the 
 shape of the rod, but which is, on the whole, triangular, also 
 contains a blood-vessel the external, or somatic vessel. This 
 vessel was observed by Lankester (fig. 6), but overlooked by 
 Spengel (fig. 7), although he represents the space here, whilst 
 Boveri described it as existing only in the primary bar. From 
 the fact that, at each extremity, this septal membrane forks, 
 and from theoretical considerations, I believed this membrane 
 to be in reality double, as indeed it is represented by Stieda's 
 figure (PI. 6, fig. 3). But I was for a long time unable to 
 assure myself of this fact. I was unable to satisfy myself as to 
 the presence of two membranes here, for, owing to the refran- 
 gibility of the structure, it is difficult to make certain whether 
 
 1 I have not observed the " muscle-cells " in the bar described by Rohon 
 and by Langerhans, J. Miiller and Schneider. 
 
8 W. BLAXLAND BENHAM. 
 
 one sees a double outline to the single membrane (such as 
 Boveri represents, PI. 6, fig. 8) or two closely apposed mem- 
 branes. 
 
 Spengel vehemently animadverts on Lankester's interpre- 
 tation of this membrane as a mesoblastic cutis, and insists on 
 its being a "basement membrane," i.e. epiblastic. Lankester 
 was, it seems to me, in error in referring the origin of this 
 membrane to the deepest layer of nuclei in the sides of the 
 bar ; at the same time, if we consider the relations of this 
 membrane to the rod and to the vessels at each end, we cannot 
 doubt that the rod and the membrane have the same origin. 
 The rod would scarcely, I presume, be referred by Spengel 
 to the epiblast. I believe, however, that I have decided that 
 the rod is mesoblastic by the discovery of the flattened nuclei 
 pressed against its inner surface (PI. 7, fig. 13); and we may 
 conclude that the rods in both bars are produced by the 
 flattened epithelium which, as Hatschek has pointed out, forms 
 the " connective tissue " throughout the body of Amphioxus. 
 If the rod is mesoblastic, then a priori we may believe the 
 eptal membrane, which is absolutely continuous with it, to 
 be also mesoblastic; but further, I have detected flattened 
 nuclei in this axis of the bar, i. e. between the two halves 
 of the closely apposed membranes. 1 searched long and care- 
 fully for any nuclei in relation to the septal membrane, and 
 ultimately, in my series of accurately transverse sections, I 
 was able, with the aid of Zeiss's apochromatic, to observe some 
 structures which I believed to be nuclei. However, I was not 
 absolutely certain of their existence, owing to the refrangibility 
 of the membrane and the denseness with which the epithelial 
 nuclei are packed ; but in a series of sections intended to be 
 horizontal, and stained in haematoxylin, some of the bars were 
 cut longitudinally for a considerable distance some six or 
 seven times the length of an ordinary transverse section of a bar, 
 and here I saw distinctly elongated and much compressed 
 nuclei (fig. 21) of fair size, lying between the two membranes. 
 These, like the nuclei surrounding ccelomic spaces, are not 
 rich in chromatin, and do not take the stain easily. We may, 
 
STRUCTURE OP THE PHARTNGEAL BARS OF AMPHIOXUS. 9 
 
 therefore, conclude as, from a priori reasoning, we should 
 be led to believe that this septal membrane is double, 
 and is mesoblastic, and not a basement membrane. 
 
 The chitinoid rod of the tongue bar is distinguished, as is 
 well known, from that in the primary bar by the presence of a 
 canal it is a perforated rod. 
 
 The shape of the section of the rod varies considerably, both 
 in different bars and even in different parts of the same bar, 
 but that represented in this figure may be regarded as the 
 most general shape. Not only the general outline, but the 
 shape and extent of the contained space are included in the 
 above statement as to the variability of the rod. Professor 
 Lankester has already figured several such variations (loc. cit., 
 pi. xxxvi, B), and others will be found on the plate illus- 
 trating this note (PL 7, figs. 1316). But most gene- 
 rally the rod presents a somewhat triangular section, with 
 rounded angle at the base, and a notch more or less pro- 
 found at the apex. This notch forms a part of the internal 
 triangular cavity partially bounded by the septal membrane 
 and occupied by the somatic blood-vessel. 
 
 The chief cavity the canal of the rod is similarly more or 
 less triangular in section, rarely round, as Lankester represents 
 it, though this shape does occur. The thickness of the outer wall 
 presents very interesting variations : more usually it is nearly 
 as thick as the sides, but it may be very much thinner (as in 
 fig. 14); it may be represented merely by a thin membrane little 
 thicker than the septal membrane, and much thinner than the 
 cutis (basement membrane Spengel) below the atrial epithe- 
 lium of the primary bar. Further, this rod may be represented 
 by a couple of curved pieces, which do not quite meet externally, 
 so that the curtained cavity is without an outer wall (fig. 15). 
 This is always the case at the points where a synapticulum 
 joins the tongue bar (fig. 31), but it also occasionally occurs 
 elsewhere. 
 
 This cavity frequently appears quite empty, invariably so in 
 my hsematoxylin preparations ; but in sections stained with 
 cochineal, granular matter is very frequently present in greater 
 
10 W. BLAXLAKD BENHAM. 
 
 or less abundance ; x and I have sometimes noticed an apparent 
 division of this cavity by a transverse partition, the granula- 
 tions having a slightly different appearance in the two sides of 
 this partition (fig. 14). Moreover in this same series of 
 sections, as also in sections stained with Weigert's picro- 
 carmine, the blood in undoubted blood-vessels, such as hepatic 
 vessels, dorsal and subendostylar vessels, takes on a character- 
 istic deep red colour deeper than the tint taken by the 
 skeletal tissues, so that I am able most definitely to state that 
 there are three blood-vessels traversing the tongue 
 bar, not two, only as Spengel and Boveri believed (c.f. fig. 1 
 with figs. 7, 8). 
 
 Of these three vessels, two occur always at opposite ex- 
 tremities of the septal membrane, in the triangular spaces 
 already mentioned ; the third lies inside the rod, and may 
 be called the skeletal vessel. Usually it has the position 
 represented in fig. 1 in the chief canal of the rod, but it does 
 not appear to fill this canal; lean generally detect a 
 slight space around it. This may, of course, be due to shrinkage 
 of the clotted blood ; but the apparent existence in some cases 
 of a partition (see fig. 14 and the explanation of it) favours my 
 view, as also does the condition of things represented in fig. 13, 
 where the vessel is passing out of the cavity, that this cavity 
 of the rod is coelom, which contains a blood-vessel. 
 This view is further strengthened by the fact that, both in my 
 hsematoxylin preparations (where the blood-vessels are not 
 evident) and in my cochineal sections, I have detected flattened 
 nuclei pressed against the inner surface of the rod, as repre- 
 sented diagrammatically in fig. 1, and accurately drawn from 
 the object in fig. 13. This, I may say, has not been an occa- 
 sional occurrence, but can be seen in many accurately trans- 
 verse sections of the bar. 
 
 In some of the variations from the normal the rod presents 
 a small cavity about midway between the main canal and the 
 apical notch (figs. 13, 14), and this cavity is usually connected 
 
 1 Spengel mentions the presence of finely granular material in the canal of 
 the hollow rod (loc. cit., p. 278), 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 11 
 
 with the former by a narrow channel; in this accessory cavity 
 I have sometimes seen a vessel in addition to that in the chief 
 canal, and sometimes I have not detected the latter. I take it 
 that there may be occasional anastomosisbetween the " somatic" 
 vessel in the notch and the skeletal vessel in the canal. 
 
 Comparison of the Tongue Bar and Primary Bar. 
 
 I wish now to compare such a transverse section of a tongue 
 bar with that of a primary bar, so far as regards the Outer 
 end. Compare my figures 1 and 2 and that copied from Boveri 
 (fig. 12) . In the primary bar Boveri describes, as I myself 
 find, three vessels (a) the inner, or visceral, and (b) outer, or 
 somatic, as is the case in the tongue bar, and (c) the third or 
 skeletal (first observed by Spengel) outside the ccelom of the 
 primary bar, between the atrial epiblast and the "cutis" (base- 
 ment membrane of Spengel). This last vessel may project 
 through the cutis into the coelom, and at the base of the bar, 
 where it springs from the endostyle, and where the rod forks, 
 this blood-vessel lies in the angle of the fork, i. e. in the coelom 
 itself (fig. 30, a), as Spengel has figured. In the tongue bar the 
 first two of these vessels are identical with those of the primary, 
 and one can scarcely resist the idea- that the third, or skeletal 
 vessel inside the rod, may correspond with the subepidermal 
 vessel of the primary bar ; it differs from it, however, in one 
 very important point, namely, in the absence of any connection 
 with the subendostylar vessel. But now turn to the rod itself. 
 This is, in the primary bar, made up of two pieces, more or less 
 fused according to the region of the bar (see fig. 30, , b, c], 
 so as to form a triangle, usually with a more or less pro- 
 nounced notch, or linear channel, arising from near the base 
 (see fig. 9) ; or again, as Schneider figured and as I have fre- 
 quently observed, a triradiate split in its centre 1 (fig. 30, c). 
 Outside the rod comes the coelom every one is agreed about 
 that lined by flattened cells; and outside them the cutis, 
 
 1 This usually is due to the softer nature of the central part of this rod, 
 and is not really a cavity : the rod presents irregularly concentric markings, 
 as if shrunk, and is firmer externally. 
 
12 W. BLAXLAND BENHAM. 
 
 which varies considerably in thickness, and can be traced, as 
 Boveri's figure shows, into the rod at each extremity (PI. 6, 
 fig. 12). This cutis stains exactly like the rod in cochineal, 
 and also like the rod is unstained in hsematoxylin. Sometimes 
 the cleft in the rod is more pronounced, and gives rise to a more 
 definite channel (see Lankester, xxxvi B, fig. 3,/) open to the 
 ccelom. Such a condition of things is represented in my PL 
 7, fig. 31, which passes through a primary rod (P. 1) at the 
 level of a synapticulum, 1 and should be compared with certain 
 variations in the rod of the tongue bar in the same figure and'in 
 fig. 15, and one is struck with the resemblance between the two. 
 
 I would suggest that the distinction between the two rods, 
 viz. that of the primary bar and that of the tongue bar, is not 
 so profound as one would be led to think from the use of the 
 terms solid (or bifid) rod and hollow rod. We have seen that 
 not infrequently the rod of the tongue bar is formed of two 
 pieces (fig. 15), whilst, on the other hand, the rod of the 
 primary bar may enclose a cavity. But I think the real dis- 
 tinction between them is that in the tongue rod the subepi- 
 thelial portion is usually and typically as thick as the sides, 
 and distinctly continuous therewith; whereas in the primary 
 rod the extra-coelomic piece (subepithelial) is thinner, and, 
 owing to the greater development of the ccelom here, is more 
 widely separated from the rest of the rod. This suggestion 
 occurred to me forcibly in examining the connections of the 
 synapticula with the two bars. 
 
 In a lucky series of sections, cutting the bars very accu- 
 rately transversely, one often gets the whole synapticulum 
 in sections, passing from one primary bar to the next, and 
 showing the connection of the rod in the transverse bar with 
 those of the main bars (fig. 31). Starting with the tongue 
 bar (T.), the rod forks, so that its contained cavity is no longer 
 bounded by a chitinoid wall ; one branch of the fork passes 
 towards each of the adjacent primary bars, and is con- 
 tinuous, not with the main part of the rod itself, but with 
 
 1 Spengel gives a figure very similar to this one in pi. xvii, fig. 13, illus- 
 trating his paper. 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 13 
 
 the cutis (basement membrane of Spengel), or extra-ccelomic 
 portion of the rod, as I would regard it. But this apparent 
 forking of the tongue rod is due merely to the passage of the 
 contained blood-vessel out of the rod to the vessels of the 
 adjacent primary bars; so that in reality, as many sections 
 show, the rod of the connecting bar, i. e. the synapticulum, is 
 connected on the one hand with the extra-coelomic portion of 
 the tongue rod, and on the other with the extra-coelomic 
 " cutis " of the primary bar. 
 
 A. second difference between the rods is that the skeletal 
 vessel is inside the ccelom in the tongue bar, but outside 
 it in the primary bar over the greater part of its extent, 
 though, in the lower part of the latter bar, it is intra-coelomic, 
 as in the tongue bar (fig. 30, #). I have sometimes seen a 
 small cavity outside the rod in the tongue bar, but I have 
 not been able to satisfy myself as to its nature ; it may be 
 merely artifact. 
 
 B. Summary of my Observations and Interpre- 
 
 tations. 
 
 1. The epithelium of the bar is everywhere only one cell 
 in thickness. 
 
 2. The arrangement of the cells at the pharyngeal end of 
 the bar, both in the primary and in the tongue, is much more 
 definite than previous observers, except Lankester, have 
 figured and described; the central group, contrary to 
 Lankester's opinion, presents two rows of nuclei, and carries 
 a bundle of very long cilia ; the lateral groups carry quite 
 short cilia. 1 
 
 3. The nuclei at the sides of the bar are oval and not 
 round, and the lowest row has nothing to do with the septal 
 membrane. 
 
 4. There are three blood-vessels in the tongue, as in the 
 1 This differentiation of the cilia round the bar may be compared with that 
 
 occurring in the gill filaments of Lamellibranchs, and in the cirri of Bra- 
 chiopods, where there are similarly bundles of long cilia, situated at the sides 
 or angles, and shorter cilia elsewhere. The existence of a skeletal tissue in 
 these cases is a further analogous resemblance. 
 
14 W. BLAXLAND BEN HAM. 
 
 primary, bar; (a) the visceral vessel at the pharyngeal end of 
 the bar j (b) the somatic vessel in the apical notch of the rod ; 
 and (c) the skeletal vessel inside the rod : the last two anasto- 
 mose here and there. 
 
 5. The cavity of the rod is crelom, is lined by flat cells, 
 and contains this skeletal blood-vessel. 
 
 6. The outer wall of this coelom is homologous with the 
 extra-coelomic cutis (Spengel's basement membrane) of the 
 primary bar. 
 
 7. The septal membrane and the rod are mesoblastic, and 
 the nuclei of the cells forming them are here recorded for the 
 first time. 
 
 c. The Observations of Recent Observers. 
 
 I will now pass on to a brief survey of the various descrip- 
 tions and figures of the tongue bar of previous writers, copies 
 of whose figures are represented on PI. 6, and in the expla- 
 nation of these, the references whence the figures are taken. 
 Stieda, Langerhans, and Schneider saw no modifications of 
 the epithelium at the pharyngeal end of the bar. Lankester 
 was the first to observe this, and his figures represent more 
 accurately than do those of his successors, Spengel and Boveri, 
 the actual arrangement. As I have already pointed out, how- 
 ever, he missed the fact that the middle group presents two 
 rows of nuclei. This is equally overlooked by Boveri, while 
 Spengel's drawing is not quite clear on this point, and neither 
 of these latter indicate the much greater length of the nuclei 
 in this part of the bar. So far as the general shape of the 
 pharyngeal end of the bar is concerned, Spengel's and Boveri's 
 drawings show it fairly. Fig. 12 from Boveri is taken from 
 quite the upper end of the bar, hence the peculiar shape of 
 this end. Lankester is alone in regarding as I believe 
 wrongly the epithelium of the sides of the bar as in several 
 layers. Spengel points this out ; but he, like all my prede- 
 cessors, with the doubtful exception of Schneider and Langer- 
 hans, represents small round instead of oval nuclei here. 
 
 The authors, as will be seen from the reproductions of 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 15 
 
 their figures, represent the characters of the atrial epithelium at 
 the upper end of the bar correctly, with the exception of Lan- 
 gerhans, who describes each cell here as bearing a flagellum. 
 
 Stieda and Langerhans did not recognise the differences in 
 the length of the cilia round the bar. Schneider is the first to 
 figure the long cilia at the sides, and the remaining authors 
 follow him. Spengel is the first to note this tuft of long 
 cilia at the pharyngeal end ; but both he and Boveri l over- 
 looked the short cilia carried by the lateral groups of cells at 
 this end. 
 
 "With regard to the contained vessels in the primary bar, 
 Langerhans entirely missed the vessel at each end of the 
 septal membrane. Schneider and Lankester saw only one 
 vessel that at the outer end of the bar at the apex of the rod 
 the somatic vessel. Spengel most unaccountably missed 
 this vessel, although he states that he looked carefully for one 
 here, and his figure of the tongue bar (PI. 6, fig. 7) shows a 
 very narrow cleft in its position, whilst he describes the inner 
 or visceral vessel (vn.), which is usually less noticeable than the 
 other. In this respect his figure shows no advance upon that 
 of Stieda, who also saw, but did not describe, the space at the 
 inner end of the bar. 
 
 Boveri, however, observed this vessel (and figures it) in the 
 primary bar (fig. 12, ve. 1), in the position already given to it 
 by Lankester, but overlooked it in the tongue bar (fig. 8). As 
 a matter of fact, as I have stated above, there are three 
 vessels in each bar. Schneider and Lankester found one, the 
 "outer or somatic" vessel; Spengel found one, the "inner or 
 visceral vessel/' but denied the somatic vessel. Boveri found 
 both these in the primary bar, but overlooked the (( outer 
 vessel" in the tongue bar. 
 
 As to the nature of the rod cavity, Stieda and Langerhans 
 leave it aside. 2 Schneider represents (at A, fig. 5) " a blood- 
 
 1 It should be borne in mind, however, that Boveri did not pretend to 
 describe the bars except in so far as they are related to the nephridia. 
 
 2 Stieda regards the granular substance in the centre of the rod (Gg. 2, a) 
 as an axial part of the rod itself. 
 
16 W. B LAX LAND BENHAM. 
 
 vessel communicating with the branchial artery. 5 ' Lankester, 
 from its relation to the subendostylar coelom, regarded this 
 canal in the rod as "coelom;" whereas Spengel and Boveri 
 look upon it as a blood-vessel, and deny its ccelomic character. 
 Lankester gives half a dozen figures (loc. cit., pi. xxxvi B, figs. 
 5 9) of as many consecutive sections representing the canal 
 communicating with the subendostylar coelom. Spengel denies 
 this altogether, and states that the rod becomes solid before 
 it reaches the endostyle, and there becomes continuous with 
 the subendostylar skeleton. He further denies any communi- 
 cation of this rod cavity with the dorso-pharyngeal coelom. 
 
 Now the tracing of these cavities is an extremely difficult 
 matter, owing to the difficulty of making sections in the right 
 plane, and I searched through section after section before I 
 could satisfy myself as to the mode of termination of these 
 rods. Time after time it seemed to me that Speugel was right, 
 so far as the non-communication of the rod cavity was con- 
 cerned ; but in certain lucky sections I found that the plane 
 was convenient for this purpose, and I represent five consecu- 
 tive sections which show, as I believe, the continuation of the 
 subendostylar coelom into the cavity of the rod (PI. 7, figs. 
 22 26). The series shows most certainly no continuity be- 
 tween the rod and the subendostylar skeleton on which Spengel 
 insists. 
 
 I was not successful in tracing the rod cavity into the dorso- 
 pharyngeal coelom ; but I attribute this to the difficulty of ob- 
 servation, and am by no means inclined to conclude that such 
 a connection does not exist. 
 
 The " skeletal vessel " ceases some little way before the rod 
 does, being connected with the vessels in. the neighbouring 
 primary bars at the lowest synapticulum. 
 
 If the hinder gill-slits be examined in a preparation of the 
 pharynx, flattened out on a slide, and the mode of development 
 of the " tongue " observed, it will be seen that the rod in the 
 tongue is double at its upper end; the two pieces diverge 
 and constitute the arcade connecting the series of rods. The 
 tongue bar, as is known, is a dowugrowth from the upper 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 17 
 
 boundary of a primary slit ; the ccelom is here in the form of a 
 canal giving off shoots to each bar, and fro them appearance 
 presented in such a preparation, it is not an inconsistent inter- 
 pretation to regard the co2lom as sharing in the downgrowth 
 of its ventral wall. The appearance presented by the rod in 
 the hinder gill-slits is thus : M. 
 
 D. Certain Abnormal Bars. 
 
 In one series of sections the pharynx presented a few bars 
 containing sometimes three and sometimes two rods. In all 
 cases these abnormal bars are " primary " bars, i. e. the rods 
 contained within them are cleft rods, similar to those found in 
 normal primary bars; and on each side of such abnormal bars 
 there lies a normal tongue bar, separated by a pharyngeal slit 
 from the abnormal one. On Plate 7, fig. 27, I represent 
 one such bar cut at about the middle of its length, containing 
 three rods, which are in all respects similar to one another. 
 The bar itself is otherwise normal, and presents the usual 
 characters of the epithelium in its various parts. Naturally 
 the bar is much wider than an ordinary one, especially at its 
 atrial end. The septal membrane (s.m.) is very short, but the 
 two usual blood-vessels (vix.v. and som.v.) are present. There is 
 not a vessel at the apex of each rod, as one might expect ; the 
 coelom is very extensive, and dips in between the rods as the 
 nuclei show. The usual subepidermic blood-vessel (skel. v.) is 
 also present. But this bar, if traced upwards towards the 
 epibranchial groove and downwards towards the endostyle, 
 presents variations of this condition in different regions. 
 
 At its origin dorsally the three rods are fused together, but 
 this occurs for only a very short space, and we soon tind three 
 rods. Towards its lower end two of these (Nos. 1 and 3) are 
 fused, so that the bar has but two rods ; and still further a 
 junction between these two is effected (fig. 28), and there 
 appear to be but one rod, rather larger than usual, but evidently 
 consisting of two united rods. Soon, however, these separate 
 again, and I take this union as representing a synapticulum. 
 
 But now the relative position of the two rods changes ; 
 
 2 
 
18 W. BLAXLAND BENHAM. 
 
 hitherto they have been side by side, or rather anterior and 
 posterior ; but now (fig. 29) one (No. 2) lies outside the other 
 (Nos. 1 and 3), which occupies the normal position. I was 
 unable to track the rods quite to their junction with the floor 
 of the pharynx owing to the imperfection of some of the sec- 
 tions in this series. 
 
 Most of the other abnormal bars contained two rods, but I 
 did not trace them all out from top to bottom, so that I am 
 unable to state whether they always contain, at some part of 
 their length, three rods. But I am inclined to answer this in 
 the negative ; so frequently are there only two rods that I 
 think this is the more " usual" abnormality. There is another 
 triple-rod bar on the opposite side of the pharynx, at about the 
 same level as the one described, and usually, as far as I could 
 observe, the abnormal bars are opposite. 
 
 A bar with a double rod might conceivably be- produced by 
 closure of a primary slit between two primary bars; but there 
 is no evidence of any formation of a slit and subsequent closure 
 and fusion of the bounding bars. One would expect if this 
 had happened that the ccelom and subepidermic vessel would 
 be doubled, and that the pharyngeal end of the bar would 
 exhibit some peculiarity ; this I did not find to be the case. A 
 triple-rod bar might be explained by an extension of this sug- 
 gestion, namely, that two slits had remained imperforate, 
 whilst the rods had been formed nevertheless. 
 
 EXPLANATION OF PLATES 6 and 7, 
 
 Illustrating Dr. W. Blaxland Benham's paper on " The 
 Structure of the Pharyngeal Bars of Amphioxus." 
 
 In most of the figures the blood-vessels are represented black, but in fig. 14 
 I have drawn the actual appearance presented by the sections. 
 
 FIGS. 1 and 2. Transverse sections of a tongue anl a primary bar, so far 
 diagrammatic in that each figure is a combination of several drawings of 
 
STRUCTURE OP THE PHARYNGEAL BARS OF AMPHIOXUS. 19 
 
 different sections, which have been treated in different ways. The two figures 
 are drawn to scale, and therefore represent the true relative sizes of the bars. 
 
 Fig. 1. A transverse section of a tongue bar of the pharynx of Amphi- 
 oxus (Branchiostoma lubricum, Costa). It represents, accu- 
 rately as I believe, a typical section of the bar. Most of the parts 
 are fully named on the plate. The rod contains a cavity the coelom, 
 lined by an epithelium, whose nuclei are shown (see also Fig. 13). 
 Within this crelom lies a blood-vessel the skeletal vessel. Two other 
 blood-vessels are present in the bar, the "visceral" (Vise. Bl. vessel) 
 and the "somatic" blood-vessel; these lie at either end of the septal 
 membrane, between the two layers of which are shown three nuclei 
 (see also Figs. 20, 21). The atrial epithelium consists of only one 
 kind of cell. The grouping of the epithelial nuclei at the pharyngeal 
 end of the bar may be noted. 
 
 Fig. 2. A transverse section of a primary bar (see explanation of Fig. 1). 
 Here the coelom is more extensive, and the "cutis" or outer wall of 
 the coelom is thinner than in the tongue bar. The rod itself is made 
 up of two pieces, meeting along the middle line of the bar, and a 
 third piece wedged between. The skeletal blood-vessel is outside the 
 coelom, but is in reality surrounded by the rod, i.e. "cutis." The 
 atrial epithelium consists of two kinds of cells !(see Fig. 17), viz. 
 a, the vacuolated, and b, the granular cells. 
 
 FIGS. 3 to 12 are tracings of figures published by previous observers; the 
 only alterations that have been made are (1) colouring the rod yellow, and (2) 
 filling in with black the spaces regarded as blood-vessels by the authors. 
 
 Fig. 3. Copied from Stieda's fig. 6, pi. i. " Transverse section of a 
 branchial plate, a. The rod. b. Deep layer of the epithelium, c. 
 Superficial layer with cilia." 
 
 Fig. 4. Copied from Langerhans, fig. 24, pi. xiii. " Transverse section 
 through a gill bar. m. Epithelium, k. Nuclei of branchial epithelium 
 p.e. Pigmented epithelium, h. The atrial epithelium, s. Hollow gill rod." 
 
 Fig. 5. Copied from Schneider's fig. 4, pi. xiv. "Transverse section 
 of a thin (that is, tongue) gill bar. a. Triangular space, a blood- 
 vessel. A. A blood-vessel in communication with the branchial artery. 
 '. The rod. p.p. Peritoneal (i. e. atrial) plate." 
 
 Fig. 6. Copied from Lankester, pi. xxxvi B, fig. 2. " Transverse 
 section of a tongue bar. al. Left inner epithelial band. ar. Right 
 inner epithelial band. am. Median inner epithelial band. col. Columnar 
 lateral cells, with long cilia, n. Superficial nuclei. i. Deeper nuclei. 
 sept. Clear septal tissue. Bl. v. Supposed blood-vessel, ending blindly 
 at the ventral extremity, pig. Lateral groups of pigment in the atrial 
 epithelium, at. ep. Atrial epithelium (epidermic)." 
 
 Fig. 7. Copied from Spengel, fig. 19, pi. xviii. "Transverse section of 
 a tongue bar. vz. Chief vessel of the bar. vn. Accessory vessel." 
 
20 W. BLAXLAND BENHAM. 
 
 Fig. 8. Copied from Boveri's fig. 14, pi. xxxiii. "Transverse section 
 of tongue bar at the level of the nephridium. neph. Position of nephri- 
 dium. vi 11 . Inner axial vessel. ve n . Outer axial vessel." 
 Fig. 9. Copied from Schneider, pi. xiv, fig. 3. "Transverse section of 
 a thick (i. e. primary) gill bar. A 1 , A^. Blood-vessels which are in 
 communication with the branchial artery, a. Triangular space, blood- 
 vessel, p. p. Peritoneal (i.e. atrial) plate. V. Vein in communication 
 with the subvertebral vein, and the venous space around the branchial 
 artery. 2 . The thick rod." 
 
 Fig. 10. Copied from Lankester, pi. xxxvi B, fig. 1. " Transverse 
 section of a primary bar. B.v. Blood-vessel connected with the lateral 
 branches of the median endostylar vessel, fas. Fissure due to the 
 bilateral origin of the rod. cos. ep. Ccelomic epithelium, pg. Pigmented 
 atrial epithelium." 
 
 Fig. 11. Copied from Spengel's fig. 12, pi. xvii. "Outer part of a 
 transverse section through a primary bar. b. m. Basal membrane. 
 cbk. Coelomic canal of the bar. sp. Skeletal rod. vk. Chief vessel in 
 the primary bar." 
 
 Fig. 12. Copied from Boveri, fig. 6, pi. xxxii. " Transverse section of 
 a primary gill bar, vi 1 . Inner axial vessel. ve l . Outer axial vessel. 
 vc. Coelomic vessel, coe. Ccelom." 
 
 FIG. 13. A tongue rod in section, to show the skeletal blood-vessel (sk. b. 
 v.} running in a special canal ; it is probably about to anastomose with the 
 somatic vessel (som. b. v.}. Around the ccelom (ccel.) three nuclei (n. c. ep.} 
 are present, embedded in granulations (? protoplasm). In this section the 
 two blood-vessels, as is frequently the case in cochineal preparations, are 
 stained deep red. The ccelom is quite clear, cu. cutis. 
 
 FIG. 14. A rod from a tongue bar, exhibiting granulations in the cavity, 
 which are of two kinds : a coarser, less deeply stained, in the outer part, which 
 is probably the protoplasm of coelomic epithelium in that region marked 
 coe. coelom ; and a finer and more deeply staining mass (skel. b. v.), the 
 skeletal blood-vessel. The two are apparently separated by a partition. 
 n. is the nucleus of the ccelomic epithelium, som. 6. v. The outer or somatic 
 blood-vessel, cu. The " cutis" or outer wall of coelom. From a preparation 
 stained with Mayer's alcoholic cochineal. Drawn under Zeiss's homogeneous 
 immersion. 
 
 FIG. 15. A modification in the shape of the section of the rod, which is 
 not unusual, especially near the synapticula. The rod is here composed of 
 two pieces. The outer wall of the coelom is free of skeletal tissue (cf. Fig. 
 31), and the blood-vessel (skel. b. v.} is passing out of the coelom (ccel.), and 
 lies just below the atrial epithelium (at. ep.). 
 
 FIG. 16. Another modification in the tongue bar, which occurs near the 
 upper end of a bar. 
 
STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 21 
 
 FIG. 17. Cells from the atrial epithelium of a primary bar, from a partially 
 macerated section (cochineal), a. Vacuolated cells. 6. More granular, 
 narrower cells, the nucleus being near the free end. c. A portion of a cell, 
 which is probably similar to d, with a narrow peripheral portion, and dilated 
 nuclear region below. 
 
 FIG. 18. Three cells of the atrial epithelium of a tongue bar, from a 
 partially macerated section. 
 
 FIG. 19. Cells from the lateral epithelium of a gill bar, from a partially 
 macerated section. The nuclei of the cells are seen to be at. different levels. 
 Each cell carries a single long cilium. The small cell to the left may be a portion 
 of a larger cell, or may be really a basal interstitial cell. 
 
 FIG. 20. A small piece of the septal membrane, with a nucleus (#.) (from 
 a transverse section of a bar). The membrane appears to be double, and the 
 nucleus lies between the two sheets. 
 
 FIG. 21. A portion of a longitudinal section of a bar, showing the septal 
 membrane and three elongated, compressed nuclei (.). Only a small por- 
 tion of the epithelium of the bar is filled in, in order to show the relative 
 size of the nuclei of the septal membrane and those of epithelium (ep.}. 
 (Hsematoxyliu. Under Zeiss's apochromatic 4 mm., aperture '95, with com- 
 pensating ocular 8.) 
 
 FIGS. 22 26. Five consecutive sections to show the passage of a tongue 
 bar into the endostyle, and the communication of the rod cavity with the sub- 
 endostylar ccelom. 
 
 Fig. 22. Transverse section of the endostyle and a neighbouring tongue 
 bar. The details as to arrangement of nuclei are only approximately 
 accurate. It is merely desired to exhibit, the general topographical 
 relations of the structures, end. Endostyle. end. sk. Subendostylar 
 skeleton. end. coel. Subendostylar ccelom. cce. ep. Nuclei of the 
 ccelomic epithelium, at. ep. Atrial epithelium, art. Subendostylar 
 artery. P. The lowermost part of a primary bar fused with the endo- 
 style. x. The peculiar tissue of elongated vacuolated cells (not 
 reticular tissue). T. T. Tongue bars. rod. The contained hollow rod ; 
 on the left the bar has already reached and fused with the subendo- 
 stylar tissue, on the right it is still free. 
 
 Fig. 23. The next section. The rod of the tongue bar on the left, appears 
 to have a narrow cleft in its wall, so that its cavity communicates with 
 the Subendostylar coelom ; but this is not so distinct as in the case of 
 the right rod in Fig. 26. (The lithographer has exaggerated this cleft.) 
 
 Fig. 24. The right portion only of the next and following sections is 
 drawn. Here the rod of the tongue bar (T.) is distinctly smaller, and 
 approaches the corner of the Subendostylar coelorn. 
 
W. BLAXLAND BENHAM. 
 
 Fig. 25. The rod has passed further into the subendostylar tissue, and 
 
 lies above the corner of the ecelom. 
 Fig. 26. The cavity of the rod communicates with the subendostylar 
 
 coelom most distinctly. 
 
 FIGS. 27 29. Transverse sections of an abnormal primary bar at different 
 levels. 
 
 Fig. 27. Section about the middle of the length of the bar. The epi- 
 thelium and general structure of the bar is normal, but contains three 
 separate primary rods, 1, 2. 3, each partially surrounded by a layer of 
 crelomic epithelium, vise. v. The visceral blood-vessel, som. v. The 
 somatic blood-vessel, s.m. The very short septal membrane. COB. Coelom. 
 skel. v. Extra-ccelomic skeletal blood-vessel. 
 
 Fig. 28. A section of the same bar lower down. There are here only two 
 rods, partially fused; the rod to the right is formed by the fusion of 
 rods 1 and 3 (in Fig. 27). 
 
 Fig. 29. The same rod lower down, nearer the endostyle. The two rods 
 have separated again, but have shifted their relative positions. 
 
 FIG. 30. Three sections of a normal primary rod at different levels, (a) 
 Close to the endostyle, where it consists of two independent pieces. (6) Slightly 
 higher up, where a third piece (sometimes isolated) plugs the gap between 
 the two pieces, with which it is more or less fused, (c) The general condition, 
 in which the fusion of the three pieces is complete, except for a narrow axial 
 cleft (? or less refractive substance). R. The rod, in the usual sense, cu. 
 " Cutis," or outer wall of ccelom. cos. Ccelom. skel. b. v. Skeletal blood-vessel. 
 som. v. Somatic vessel. (Cf. Figs. 1316.) 
 
 FIG. 31 a. The connection of synapticulum with the primary and tongue 
 bars. The section is accurately transverse, and includes three bars and the 
 synapticulum. P 1 , P 2 . The two primary bars. T. The intervening tongue 
 bar. syn. To the left, half a synapticulum cut along its whole length, and 
 continuous, as is seen, with one half of the tongue rod and the extra-ccelomic 
 " cutis" {cu.} of the primary rod. syn 1 , syn?. The two portions of the other half 
 of the synapticulum (the intervening portion is in a section not figured), b. v 1 . 
 The subepidermic skeletal blood-vessels of the primary bar. b. v 2 . The skeletal 
 vessel of the tongue bar, which is seen to divide into a right and a left branch, 
 which fall into b. v 1 . of the primary bars. cce. Ccelom of the primary bar. 
 
 FIG. 31 . The next section of the primary bar (P 2 ) showing the passage 
 of syn?. into the cutis (cu.}. Other letters as in Fig. 31 a. 
 
SPE N G E L. 
 
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 OCT 17 1952 
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