No. 9. Section very near the termination of the rod.

No. 10. Last section in which any trace of the rod is seen.

Series B. Sections passing through the head-kidney at our third stage. Zeiss C, ocul. 2. Our figures are representations of the following sections of the series, section 1 being the first which passes through the anterior groove of the head-kidney.

The Müllerian duct extends through eleven more sections.

The first groove (gr₁.) extends to No. 3.

The second groove (gr₂.) extends from No. 4 to No. 7.

The third groove (gr₃.) extends from No. 11 to No. 13.

The first ridge (r₁.) extends from No. 2 to No. 5.

The second ridge (r₂.) extends from No. 8 to No. 11.

The third ridge (r₃.) extends from No. 13 backwards through twelve sections, when it terminates by a pointed extremity.

Fig. C. Section through the ridge connecting the second and third grooves of the head-kidney of an embryo slightly younger than that from which Series B was taken. Zeiss C, ocul. 3 (reduced one-third).

The fold of the germinal epithelium, which gives rise to a deep groove (x.) external to the head-kidney is well marked.

Series G. Sections through the rod of cells constituting the termination of the Müllerian duct at a stage in which the head-kidney is still present. Zeiss C, ocul. 2.

Plate 28.

Series D. Sections chosen at intervals from a complete series traversing the peritoneal opening of the Müllerian duct, the remnant of the head-kidney, and the termination of the Müllerian duct. Zeiss C, ocul. 3 (reduced one-third).

Nos. 1 and 2. Sections through the persistent anterior opening of the head-kidney (abdominal opening of Müllerian duct). The approach of the Wolffian duct to the groove may be seen by a comparison of these two figures. In the sections in front of these (not figured) the two are much more widely separated than in No. 1.

No. 3. Section through the Müllerian duct, just posterior to the persistent opening.

Nos. 4 and 5. Remains of the ridges, which at an earlier stage connected the first and second grooves, are seen passing from the Müllerian duct to the peritoneal epithelium.

No. 6. Rudiment of the second groove (gr₂.) of the head-kidney.

Between 6 and 7 is a considerable interval.

No. 7. All traces of this groove (gr₂.) have vanished, and the Müllerian duct is quite disconnected from the epithelium.

No. 8. Rudiment of the third groove (gr₃.).

[424] From the Quarterly Journal of Microscopical Science, Vol. XIX. 1879.

[425] “Das Urogenital-System der Plagiostomen,” Arbeiten a. d. zool.-zoot. Institut. Würzburg.

[426] “Zur vergl. Anat. u. Entwick. d. Excretionsorgane d. Vertebraten,” Morphologisches Jahrbuch, Vol. IV.

[427] “On the Origin and History of the Urinogenital Organs of Vertebrates,” Journal of Anat. and Phys., Vol. X. [This Edition No. VII.]

[428] Arbeiten a. d. zool.-zoot. Institut. Würzburg, Vol. IV.

[429] Beitr. zur Anat. u. Entwick. d. Geschlechtsorgane, Inaug. Diss., Zürich, 1876.

[430] Proceedings of Royal Society, 1878.

[431] A deep focus of the rather thick section represented in No. 3 shewed the body much more nearly in the position it occupies in No. 4.

[432] Beiträge zur Entwicklungsgeschichte d. Allantois der Müller'schen Gange u. des Afters. Frankfurt, 1874.

[433] Loc. cit.

[434] Jenaische Zeitschrift, Vol. IX. 1875.

[435] Entwicklungsgeschichte d. Unke.

[436] Loc. cit.

[437] Balfour, “Origin and History of Urinogenital Organs of Vertebrates,” Journal of Anat. and Phys. Vol. X., and Monograph on Elasmobranch Fishes. [This edition Nos. VII. and X.]

[438] I am inclined to give up the view I formerly expressed with reference to the head-kidney and segmental duct, viz. “that they were to be regarded as the most anterior segmental tube, the peritoneal opening of which had become divided, and which had become prolonged backwards so as to serve as the duct for the posterior segmental tubes,” and provisionally to accept the Gegenbaur-Fürbringer view which has been fully worked out and ably argued for by Fürbringer (loc. cit. p. 96). According to this view the head-kidney and its duct are to be looked on as the primitive and unsegmented part of the excretory system, more or less similar to the excretory system of many Trematodes and unsegmented Vermes. The segmental tubes I regard as a truly segmental part of the excretory system acquired subsequently.—F. M. B.

[439] In a note on p. 50 of his memoir Fürbringer criticises my description of the mode of growth of the segmental duct. The following is a free translation of what he says: “In Balfour's, as in other descriptions, an account is given of a backward growth, which easily leads to the supposition of a structure formed anteriorly forcing its way through the tissues behind. This is, however, not the case, since, to my knowledge, no author has ever detected a sharp boundary between the growing point of the segmental duct (or Müllerian duct) and the surrounding tissues.” He goes on to say that “the growth in these cases really takes place by a differentiation of tissue along a line in the region of the peritoneal cavity.” Although I fully admit that it would be far easier to homologise the development of the segmental duct in Amphibia and Elasmobranchii according to this view, I must nevertheless vindicate the accuracy of my original account. I have looked over my specimens again, since the appearance of Dr Fürbringer's paper, and can find no evidence of the end of the duct becoming continuous with the adjoining mesoblastic tissues. In the section, before its disappearance, the segmental duct may, so far as I can make out, be seen as a very small but distinct rod, which is much more closely connected with the epiblast than with any other layer. From Gasser's observations on the Wolffian duct in the bird, I am led to conclude that it behaves in the same way as the segmental duct in the Elasmobranchii. I will not deny that it is possible that the growth of the duct takes place by wandering cells, but on this point I have no evidence, and must therefore leave the question an open one.—F. M. B.

[440] Fürbringer, loc. cit.

[441] Arch. für Mic. Anat. Vol. XIV.

[442] The views here expressed about the Wolffian duct are nearly though not exactly those which one of us previously put forward (“Urinogenital Organs of Vertebrates,” &c., pp. 45-46) [This edition, pp. 164, 165], and with which Fürbringer appears exactly to agree. Possibly Dr Fürbringer would alter his view on this point were he to accept the facts we believe ourselves to have discovered. Semper's view also differs from ours, in that he believes the Wolffian duct to correspond in its entirety with the segmental duct.

[443] “Urogenital-System d. Reptilien,” Arb. aus d. zool.-zoot. Inst. Würzburg, Vol. IV.

[444] Loc. cit.

[445] Loc. cit.

XIV. On the Early Development of the Lacertilia, together with some Observations on the Nature and Relations of the Primitive Streak[446].

(With Plate 29.)

Till quite recently no observations were recorded on the early developmental changes of the reptilian ovum. Not long ago Professors Kupffer and Benecke published a preliminary note on the early development of Lacerta agilis and Emys Europea[447]. I have myself also been able to make some observations on the embryo of Lacerta muralis. The number of my embryos has been somewhat limited, and most of those which I have had have been preserved in bichromate of potash, which has turned out a far from satisfactory hardening reagent. In spite of these difficulties I have been led on some points to very different results from those of the German investigators, and to results which are more in accordance with what we know of other Sauropsidan types. I commence with a short account of the results of Kupffer and Benecke.

Segmentation takes place exactly as in birds, and the resulting blastoderm, which is thickened at its edge, spreads rapidly over the yolk. Shortly before the yolk is half enclosed a small embryonic shield (area pellucida) makes its appearance in the centre of the blastoderm, which has, in the meantime, become divided into two layers. The upper of these is the epiblast, and the lower the hypoblast. The embryonic shield is mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. It is somewhat pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the narrow end an invagination takes place, which gives rise to an open sac, the blind end of which is directed forwards. The opening of this sac is regarded by the authors as the blastopore. A linear thickening of epiblast arises in front of the blastopore, along the median line of which the medullary groove soon appears. In the caudal region the medullary folds spread out and enclose between them the blastopore, behind which they soon meet again. On the conversion of the medullary groove into a closed canal the blastopore becomes obliterated. The mesoblast grows out from the lip of the blastopore as four masses. Two of these are lateral: a third is anterior and median, and, although at first independent of the epiblast, soon attaches itself to it, and forms with it a kind of axis-cord. A fourth mass applied itself to the walls of the sac formed by invagination.

With reference to the very first developmental phenomena my observations are confined to two stages during the segmentation[448]. In the earliest of these the segmentation was about half completed, in the later one it was nearly over. My observations on these stages bear out generally the statements of Kupffer and Benecke. In the second of them the blastoderm was already imperfectly divided into two layers—a superficial epiblastic layer formed of a single row of cells, and a layer below this several rows deep. Below this layer fresh segments were obviously being added to the blastoderm from the subjacent yolk.

Between the second of these blastoderms and my next stage there is a considerable gap. The medullary plate is just established, and is marked by a shallow groove which becomes deeper in front. A section through the embryo is represented in Pl. 29, Series A, fig. 1. In this figure there may be seen the thickened medullary plate with a shallow medullary groove, below which are two independent plates of mesoblast (me.p.), one on each side of the middle line, very imperfectly divided into somatopleuric and splanchnopleuric layers. Below the mesoblast is a continuous layer of hypoblast (hy.), which develops a rod-like thickening along the axial line (ch.). This rod becomes in the next stage the notochord. Although this embryo is not well preserved I feel very confident in asserting the continuity of the notochord with the hypoblast at this stage.

In fig. 5 is seen the dorsal opening of the passage (ne.). If a section behind this had been figured, as is done for the next series (B), it would have passed through the primitive streak, and, as in the chick, all the layers would have been fused together. The epiblast in the primitive streak completely coalesces with the mesoblast; but the hypoblast, though attached to the other layers in the middle line, can always be traced as a distinct stratum.

The passage itself is somewhat shorter than in the previous stage, and the whole of it is shown in a single section (fig. 4). This section must either have been taken somewhat obliquely, or else the passage have been exceptionally short in this embryo, since in an older embryo it could not all be seen in one section.

The general results of Kupffer's and Benecke's observations, with the modifications introduced by my own observations, are as follows:—After the segmentation and the formation of the embryonic shield (area pellucida) the blastoderm becomes distinctly divided into epiblast and hypoblast[449]. At the hind end of the shield a somewhat triangular primitive streak is formed by the fusion of the epiblast and hypoblast with a number of cells between them, which are probably derived from the lower rows of the segmentation cells. At the front end of the streak a passage arises, open at both extremities, leading obliquely forwards through the epiblast to the space below the hypoblast. The walls of the passage are formed of a layer of columnar cells continuous both with epiblast and hypoblast. In front of the primitive streak the body of the embryo becomes first differentiated by the formation of a medullary plate, and at the same time there grows out from the primitive streak a layer of mesoblast, which spreads out in all directions between the epiblast and hypoblast. In the axis of the embryo the mesoblast plate is stated by Kupffer and Benecke to be continuous across the middle line, but this appears very improbable. In a slightly later stage the medullary plate becomes marked by a shallow groove, and the mesoblast of the embryo is then undoubtedly constituted of two lateral plates, one on each side of the median line. In the median line the notochord arises as a ridge-like thickening of the hypoblast, which becomes very soon quite separated from the hypoblast, except at the hind end, where it is continued into the front wall of the neurenteric passage. It is interesting to notice the remarkable relation of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl (Gasser)[450], and support the conclusion deducible from the lower forms of vertebrata, that the notochord is essentially hypoblastic.

The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the succeeding stages the medullary groove becomes continued back to the opening of the passage, which then becomes enclosed in the medullary folds, and forms a true neurenteric passage. It becomes narrowed as the medullary folds finally unite to form the medullary canal, and eventually disappears.

I conclude this paper with a concise statement of what appears to me the probable nature of the much-disputed organ, the primitive streak, and of the arguments in support of my view.

In a paper on the primitive streak in the Quart. Journ. of Mic. Sci., in 1873 (p. 280) [This edition, p. 45], I made the following statement with reference to this subject:—“It is clear, therefore, that the primitive groove must be the rudiment of some ancestral feature.... It is just possible that it is the last trace of that involution of the epiblast by which the hypoblast is formed in most of the lower animals.”

At a later period, in July, 1876, after studying the development of Elasmobranch fishes, I enlarged the hypothesis in a review of the first part of Prof. Kölliker's Entwicklungsgeschichte. The following is the passage in which I speak of it[451]:

"In treating of the exact relation of the primitive groove to the formation of the embryo, Professor Kölliker gives it as his view that though the head of the embryo is formed independently of the primitive groove, and only secondarily unites with this, yet that the remainder of the body is without doubt derived from the primitive groove. With this conclusion we cannot agree, and the very descriptions of Professor Kölliker appear to us to demonstrate the untenable nature of his results. We believe that the front end of the primitive groove at first occupies the position eventually filled by about the third pair of protovertebræ, but that as the protovertebræ are successively formed, and the body of the embryo grows in length, the primitive groove is carried further and further back, so as always to be situated immediately behind the embryo. As Professor Kölliker himself has shewn it may still be seen in this position even later than the fortieth hour of incubation.

"Throughout the whole period of its existence it retains a character which at once distinguishes it in sections from the medullary groove.

"Beneath it the epiblast and mesoblast are always fused, though they are always separate elsewhere; this fact, which was originally shewn by ourselves, has been very clearly brought out by Professor Kölliker's observations.

"In comparing the blastoderm of a bird with that of any anamniotic vertebrate, we are met at the threshold of our investigations by a remarkable difference between the two. Whereas in all the lower vertebrates the embryo is situated at the edge of the blastoderm, it is in birds and mammals situated in the centre. This difference of position at once suggests the view that the primitive groove may be in some way connected with the change of position in the blastoderm which the ancestors of birds must have undergone. If we carry our investigations amongst the lower vertebrates a little further, we find that the Elasmobranch embryo occupies at first the normal position at the edge of the blastoderm, but that in the course of development the blastoderm grows round the yolk far more slowly in the region of the embryo than elsewhere. Owing to this, the embryo becomes left in a bay, the two sides of which eventually meet and coalesce in a linear fashion immediately behind the embryo, thus removing the embryo from the edge of the blastoderm and forming behind it a linear streak not unlike the primitive streak. We would suggest the hypothesis that the primitive groove is a rudiment which gives the last indication of a change made by the Avian ancestors in their position in the blastoderm, like that made by Elasmobranch embryos when removed from the edge of the blastoderm and placed in a central situation similar to that of the embryo bird. On this hypothesis the situation of the primitive groove immediately behind the embryo, as well as the fact of its not becoming converted into any embryonic organ would be explained. The central groove might probably also be viewed as the groove naturally left between the coalescing edges of the blastoderm.

At a somewhat later period Rauber arrived at a more or less similar conclusion, which, however, he mixes up with a number of opinions from which I am compelled altogether to dissent[452].

The general correctness of my view, as explained in my second quotation, appears to me completely established by Gasser's beautiful researches on the early development of the chick and goose[453], and by my own observations just recorded on the lizard. While at the same time the parallel between the blastopore of Elasmobranchii and of the Sauropsida, is rendered more complete by the discovery of the neurenteric passage in the latter group, which was first of all made by Gasser.

In Elasmobranchii the blastopore consists of the following parts:—(1), a section at the end of the medullary plate, which becomes converted into the neurenteric canal[454]; (2), a section forming what may be called the yolk blastopore, which eventually constitutes a linear streak connecting the embryo with the edge of the blastoderm (vide monograph on Elasmobranch fishes, pp. 281 and 296). In order to establish my hypothesis on the nature of the primitive streak, it is necessary to find the representatives of both these parts in the primitive streak of the amniotic vertebrates. The first section ought to appear as a passage from the neural to the enteric side of the blastoderm at the posterior end of the medullary plate. At its front edge the epiblast and hypoblast should be continuous, as they are at the hind end of the embryo in Elasmobranchii, and, finally, the passage should, on the closure of the medullary groove, become converted into the neurenteric canal. All these conditions are exactly fulfilled by the opening at the front end of the primitive streak of the lizard (vide woodcut, fig. 1, p. 647). In the chick there is at first no such opening, but, as I hope to shew in a future paper, it is replaced by the epiblast and hypoblast falling into one another at the front end of the primitive streak. At a later period, as has been shewn by Gasser[455], there is a distinct rudiment of the neurenteric canal in the chick, and a complete canal in the goose. Finally, in mammals, as has been shewn by Schäffer[456] for the guinea-pig, there is at the front end of the primitive streak a complete continuity between epiblast and hypoblast. The continuity of the epiblast and hypoblast at the hind end of the embryo in the bird and the mammal is a rudiment of the continuity of these layers at the dorsal lip of the blastopore in Elasmobranchii, Amphibia, &c. The second section of the blastopore in Elasmobranchii or yolk blastopore is, I believe, partly represented by the primitive streak. The yolk blastopore in Elasmobranchii is the part of the blastopore belonging to the yolk sac as opposed to that belonging to the embryo, and it is clear that the primitive streak cannot correspond to the whole of this, since the primitive streak is far removed from the edge of the blastoderm long before the yolk is completely enclosed. Leaving this out of consideration the primitive streak, in order that the above comparison may hold good, should satisfy the following conditions:

With reference to the third condition, I will make the following observations. The neurenteric canal, as it is placed at the extreme end of the embryo, must necessarily, with reference to the embryo, be the hindermost section of the blastopore, and therefore the part of the blastopore apparently behind this can only be so owing to the embryo not being folded off from the yolk sac; and as the yolk sac is in reality a specialised part of the ventral wall of the body, the yolk blastopore must also be situated on the ventral side of the embryo.

DESCRIPTION OF PLATE 29.

Series A. Sections through an embryo shortly after the formation of the medullary groove. x 120[457].

Fig. 1. Section through the trunk of the embryo.

Figs. 2-5. Sections through the neurenteric canal.

Fig. B. Surface view of a somewhat older embryo than that from which Series A is taken. x 30.

Series B. Sections through the embryo represented in Fig. B. x 120.

Fig. 1. Section through the trunk of the embryo.

Figs. 2, 3. Sections through the hind end of the medullary groove.

Fig. 4. Section through the neurenteric canal.

Fig. 5. Section through the primitive streak.

Fig. C. Surface view of a somewhat older embryo than that represented in Fig. B. x 30.

[446] From the Quarterly Journal of Microscopical Science, Vol. XIX. 1879.

[447] Die Erste Entwicklungsvorgänge am Ei der Reptilien, Königsberg, 1878.

[448] For these two specimens, which were hardened in picric acid, I am indebted to Dr Kleinenberg.

[449] This appears to me to take place before the formation of the embryonic shield.

[450] Gasser, Der Primitivstreifen bei Vogelembryonen, Marburg, 1878.

[451] Journal of Anat. and Phys., Vol. X. pp. 790 and 791. Compare also my Monograph on Elasmobranch Fishes, note on p. 68 [This edition, p. 281].

[452] “Primitivrinne u. Urmund,” Morphologisches Jahrbuch, Band II. p. 551.

[453] Gasser, Der Primitivstreifen bei Vogelembryonen, Marburg, 1878.

[454] I use this term for the canal connecting the neural and alimentary tract, which was first discovered by Kowalevsky.

[455] Loc. cit.

[456] “A contribution to the history of the development in the Guinea-pig,” Journal of Anat. and Phys. Vol. XI. pp. 332-336.

[457] The spaces between the layers in these sections are due to the action of the hardening reagent.

XV. On Certain Points in the Anatomy of Peripatus Capensis[458].

The discovery by Mr Moseley[459] of a tracheal system in Peripatus must be reckoned as one of the most interesting results obtained by the naturalists of the “Challenger.” The discovery clearly proves that the genus Peripatus, which is widely distributed over the globe, is the persisting remnant of what was probably a large group of forms, from which the present tracheate Arthropoda are descended.

The affinities of Peripatus render any further light on its anatomy a matter of some interest; and through the kindness of Mr Moseley I have had an opportunity of making investigations on some well preserved examples of Peripatus capensis, a few of the results of which I propose to lay before the Society.

I shall confine my observations to three organs. (1) The segmental organs, (2) the nervous system, (3) the so-called fat bodies of Mr Moseley.

In all the segments of the body, with the exception of the first two or three postoral ones, there are present glandular bodies, apparently equivalent to the segmental organs of Annelids.

These organs have not completely escaped the attention of previous observers. The anterior of them were noticed by Grube[460], but their relations were not made out. By Saenger[461], as I gather from Leuckart's Bericht for the years 1868-9, these structures were also noticed, and they were interpreted as segmental organs. Their external openings were correctly identified. They are not mentioned by Moseley, and no notice of them is to be found in the text-books. The observations of Grube and Saenger seem, in fact, to have been completely forgotten.

The following is a summary of the fresh points I have observed in the nervous system:

(2) The ventral nerve cords are covered on their ventral side by a thick ganglionic layer[462], and at each pair of feet they dilate into a small but distinct ganglionic swelling. From each ganglionic swelling are given off a pair of large nerves[463] to the feet; and the ganglionic swellings of the two cords are connected together by a pair of commissures containing ganglion cells[464]. The other commissures connecting the two cords together do not contain ganglion cells.

The chief feature in which Peripatus was supposed to differ from normal Arthropoda and Annelida, viz. the absence of ganglia on the ventral cords, does not really exist. In other particulars, as in the amount of nerve cells in the ventral cords and the completeness of the commissural connections between the two cords, &c., the organisation of the nervous system of Peripatus ranks distinctly high. The nervous system lies within the circular and longitudinal muscles, and is thus not in proximity with the skin. In this respect also Peripatus shews no signs of a primitive condition of the nervous system.

A median nerve is given off from the posterior border of the supra-œsophageal ganglion to the œsophagus, which probably forms a rudimentary sympathetic system. I believe also that I have found traces of a paired sympathetic system.

The organ doubtfully spoken of by Mr Moseley as a fat body, and by Grube as a lateral canal, is in reality a glandular tube, lined by beautiful columnar cells containing secretion globules, which opens by means of a non-glandular duct into the mouth. It lies close above the ventral nerve cords in a lateral compartment of the body-cavity, and extends backwards for a varying distance.

[458] From the Proceedings of the Cambridge Philosophical Society, Vol. III. 1879.

[459] “On the Structure and Development of Peripatus Capensis,” Phil. Trans., Vol. CLXIV. 1874.

[460] “Bau von Perip. Edwardsii,” Archiv f. Anat. u. Phys. 1853.

[461] Moskauer Naturforscher Sammlung, Abth. Zool. 1869.

[462] This was known to Grube, loc. cit.

[463] These nerves were noticed by Milne-Edwards, but Grube failed to observe that they were much larger than the nerves given off between the feet.

[464] These commissures were perhaps observed by Saenger, loc. cit.

XVI. On the Morphology and Systematic Position of the Spongida[465].

Professor Schulze's[466] last memoir on the development of Calcareous Sponges, confirms and enlarges Metschnikoff's[467] earlier observations, and gives us at last a fairly complete history of the development of one form of Calcareous Sponge. The facts which have been thus established have suggested to me a view of the morphology and systematic position of the Spongida, somewhat different to that now usually entertained. In bringing forward this view, I would have it understood that it does not claim to be more than a mere suggestion, which if it serves no other function may, perhaps, be of use in stimulating research.

To render clear what I have to say, I commence with a very brief statement of the facts which may be considered as established with reference to the development of Sycandra raphanus, the form which was studied by both Metschnikoff and Schulze. The segmentation of the ovum, though in many ways remarkable, is of no importance for my present purpose, and I take up the development at the close of the segmentation, while the embryo is still encapsuled in the parental tissues. It is at this stage lens-shaped, with a central segmentation cavity. An equatorial plane divides it into two parts, which have equal shares in bounding the segmentation cavity. One of these halves is formed of about thirty-two large, round, granular cells, the other of a larger number of ciliated clear columnar cells. While the embryo is still encapsuled a partial invagination of the granular cells takes place, reducing the segmentation cavity to a mere slit; this invagination is, however, quite temporary and unimportant, and on the embryo becoming free, which shortly takes place, no trace of it is visible; but, on the contrary, the segmentation cavity becomes larger, and the granular cells project very much more prominently than in the encapsuled state.