Chapter VI: Introduction: 1 (5)

Just before the appearance of the external gills this part of the digestive canal commences to atrophy. It begins to do so close to the terminal vesicle, which, however, still remains as or more conspicuous than it was before. The lumen of the canal becomes smaller and smaller, and finally it becomes a solid string of cells, and these also soon become indistinguishable and not a trace of the canal is left.

Almost the whole of it has disappeared before the vesicle begins to atrophy, but very shortly after all trace of the rest of the canal has vanished the terminal vesicle also vanishes. This occurs just about the time or shortly after the appearance of the external gills--there being slight differences probably in this respect in the different species.

In this history there are two points of especial interest:

(1) The terminal vesicle.

(2) The disappearance of a large and well-developed portion of the alimentary canal.

The interest in the terminal vesicle lies in the possibility of its being some rudimentary structure.

In Osseous fishes Kupffer has described the very early appearance of a vesicle near the tail end, which he doubtfully speaks of as the "allantois." The figure he gives of it in his earlier paper (_Archiv. für Micro. Anat._ Vol. II. pl. xxiv, fig. 2) bears a very strong resemblance to my figures of this vesicle at the time when the hind end of the alimentary canal is commencing to disappear; and I feel fairly confident that it is the same structure as I have found in the Dog-fish: but until the relations of the Kupffer's vesicle to the alimentary canal are known, any comparison between it and the terminal vesicle in the Dog-fish must be to a certain extent guess-work.

I have, however, been quite unsuccessful in finding any other vesicular structure which can possibly correspond to the so-called allantoic vesicle of Osseous fish.

The disappearance of a large portion of the alimentary canal behind the anus is very peculiar. In order, however, to understand the whole difficulties of the case I shall be obliged to speak of the relations of the anus of the Dog-fish to the anus of Rusconi in the Lamprey, &c.

In those vertebrates whose alimentary canal is formed by an involution, the anus of Rusconi represents the opening of this involution, and therefore the point where the alimentary canal primitively communicates with the exterior. When, however, the "anus of Rusconi" becomes _closed_, the wall of the alimentary canal still remains at that point in close juxtaposition to the surface, and the new and final anus is formed at or close to that point. In the Dog-fish, although the anus of Rusconi is not present, still, during the closing of the alimentary canal, the point which would correspond with this becomes marked out by the alimentary canal there approaching the surface, and it is at this point that the involution to form the true anus subsequently appears.

The anus in the Dog-fish has thus, more than a mere secondary significance. It corresponds with the point of closing of the primitive involution. If it was not for this peculiarity of the vertebrate anus we would naturally suppose, from the disappearance of a considerable portion of the alimentary canal lying behind its present termination, that in the adult the alimentary canal once extended much farther back than at present, and that the anus we now find was only a secondary anus, and not the primitive one. It is perhaps possible that this hinder portion of the alimentary canal is a result of the combined growth of the tail and the persisting continuity (at the end of the body) of the epiblast with the hypoblast.

Whichever view is correct, it may be well to mention, in order to shew that the difficulty about the anus of Rusconi is no mere visionary one, that Götte ("Untersuchung über die Entwicklung der Bombinator igneus," _Archiv. für Micro. Anat._, vol. V. 1869) has also described the disappearance of the hind portion of the alimentary canal in Batrachians, a rudiment (according to him) remaining in the shape of a lymphatic trunk.

It is, perhaps, possible that we have a further remnant of this "hind portion" of the alimentary canal amongst the higher vertebrates in the "allantois."

_Organs developed from the Digestive Canal._

In reference to the development of the liver, pancreas, &c., as far as my observations have at present gone, the Dog-fish presents no features of peculiar interest. The liver is developed as in the Bird, and independently of the yolk.

There are, however, two organs derived from the hypoblast which deserve more attention. Immediately under the notochord, and in contact with it (vide Pl. 3, fig. 10; Pl. 4, figs. 11 and 12, _x_), a small roundish (in section) mass of cells is to be seen in most of the sections.

Its mode of development is shewn in fig. 10, _x_. That section shows a mass of cells becoming pinched off from the top of the alimentary canal. By this process of pinching off from the alimentary canal a small rod-like body close under the notochord is formed. It persists till after the appearance of the external gills, but later than that I have not hitherto succeeded in finding any trace of it.

It was first seen by Götte (_loc. cit._) in the Batrachians, and he gave a correct account of its development, and added that it became the thoracic duct.

I have not myself worked out the later stages in the development of this body with sufficient care to be in a position to judge of the correctness of Götte's statements as to its final fate. If it is true that it becomes the thoracic duct it is very remarkable, and ought to throw some light upon the homologies of the lymphatic system.

Some time before the appearance of the external gills another mass of cells becomes, I believe, constricted off from the part of the alimentary canal in the neighbourhood of the anus, and forms a solid rod composed at first of dark granular cells lying between the Wolffian ducts. I have not followed out its development quite completely, but I have very little doubt that it is really constricted off from a portion of the alimentary canal chiefly in front of the point where the anus appears, but also, I believe, from a small portion behind this.

Though the cells of which it is composed are at first columnar and granular (fig. 12, _su_, _r_), they soon begin to become altered, and in the latter stage of its development the body forms a conspicuous rounded mass of cells with clear protoplasm, and each provided with a large nucleus. Later still it becomes divided into a number of separate areas of cells by septa of connective tissue, in which (the septa) capillaries are also present. Since I have not followed it to its condition in the adult, I cannot make any definite statements as to the fate of this body; but I think that it possibly becomes the so-called suprarenal organ, which in the Dog-fish forms a yellowish elongated body lying between the two kidneys.

_The development of the Wolffian Duct and Body and of the Oviduct._

The development of the Wolffian duct and the Oviduct in the various classes of vertebrates is at present involved in some obscurity, owing to the very different accounts given by different observers.

The manner of development of these parts in the Dog-fish is different from anything that previous investigators have met with in other classes, but I believe that it gives a clearer insight into the true constitution of these parts than vertebrate embryology has hitherto supplied, and at the same time renders easier the task of understanding the differences in the modes of development in the different classes.

I shall commence with a simple description of the observed facts, and then give my view as to their meaning. At about the time of the appearance of the third visceral cleft, and a short way behind the point up to which the alimentary canal is closed in front, the splanchnopleure and somatopleure fuse together opposite the level of the dorsal aorta.

From the mass of cells formed by this fusion a solid knob rises up towards the epiblast (Pl. 4, fig. 11_b_, _ov_), and from this knob a solid rod of cells grows backwards towards the tail (fig. 11_c_, _ov_) very closely applied to the epiblast. This description will be rendered clear by referring to figs. 11_b_ and _c_. Fig. 11_b_ is a section at the level of the knob, and fig. 11_c_ is a section of the same embryo a short way behind this point. So closely does the rod of cells apply itself to the epiblast that it might very easily be supposed to be derived from it. Such, indeed, was at first my view till I cut a section passing through the knob. In order, however, to avoid all possibility of mistake I made sections of a large number of embryos of about the age at which this appears, and _invariably found_ the large knob in front, and from it the solid string growing backwards.

This string is the commencement of the _Oviduct_ or _Müller's duct_, which in the Dog-fish as in the Batrachians is the first portion of the genito-urinary system to appear, and is in the Dog-fish undoubtedly at first solid. All my specimens have been hardened with osmic acid, and with specimens hardened with this reagent it is quite easy to detect even the very smallest hole in a mass of cells.

As a solid string or rod of cells the Oviduct remains for some time; it grows, indeed, rapidly in length, the extreme hind end of the rod being very small and the front end continuing to remain attached to the knob. The knob, however, travels inwards and approaches nearer and nearer to the true pleuro-peritoneal cavity, always remaining attached to the intermediate cell mass.

At about the time when five visceral clefts are present the Oviduct first begins to get a lumen and to open at its front end into the pleuro-peritoneal cavity. The cells of the rod are first of all arranged in an irregular manner, but gradually become columnar and acquire a radiating arrangement around a central point. At this point, where the ends of all the cells meet, a very small hole appears, which gradually grows larger and becomes the cavity of the duct (fig. 12, _ov_). The hole first makes its appearance at the anterior end of the duct, and then gradually extends backwards, so that the hind end is still without a lumen, when the lumen of the front end is of a considerable size.

At the front knob the same alteration in the cells takes place as in the rest of the duct, but the cells become deficient on the side adjoining the pleuro-peritoneal cavity, so that an opening is formed into the pleuro-peritoneal cavity, which soon becomes of a considerable size. Soon after its first formation, indeed, the opening becomes so large that it may be met in from two to three consecutive sections if these are very thin.

Thus is formed the lumen of the Oviduct. The duct still, at this age, ends behind without having become attached to the cloaca, so that at this time the Oviduct is a canal closed behind, but communicating in front by a large opening with the pleuro-peritoneal cavity.

It has during this time been travelling downwards, and is now much nearer the pleuro-peritoneal cavity than the epiblast.

It may be well to point out that the mode of development which I have described is really not very different from an involution, and must, in fact, be only looked upon as a modification of an involution. Many examples from all classes in the animal kingdom could be selected to exemplify how an involution may become simply a solid thickening. In the Osseous fish nearly all the organs which are usually formed by an involution have undergone this change in their mode of development. I shall attempt to give reasons later on for the solid form having been acquired in this particular case of the Oviduct.

At about the time when a lumen appears in the Oviduct the first traces of the Wolffian duct become visible.

At intervals along the whole length, between the front and hind ends of the Oviduct, involutions arise from the pleuro-peritoneal cavity (fig. 12_a_, _pwd_) on the inside (nearer the middle line) of the Oviduct. The upper ends of these numerous involutions unite together and form a string of cells, at first solid, but very soon acquiring a lumen, and becoming a duct which communicates (as it clearly must from its mode of formation), at numerous points with the pleuro-peritoneal cavity. It is very probable that there is one involution to each segment of the body between the front and hind ends of the Oviduct. This duct is the Wolffian duct, which thus, together with the Oviduct, is formed before the appearance of the external gills.

For a considerable period the front end of the Oviduct does not undergo important changes; the hind end, however, comes into connection with the extreme end of the alimentary canal. The two Oviducts do not open together into the cloaca, though, as my sections prove, their openings are very close together. The whole Oviduct, as might be expected, shares in the general growth, and its lumen becomes in both sexes very considerably greater than it was before.

It is difficult to define the period at which I find these changes accomplished without giving drawings of the whole embryo. The stage is one considerably after the external gills have appeared, but before the period at which the growth of the olfactory bulbs renders the head of an elongated shape.

During the same period the Wolffian duct has undergone most important changes. It has commenced to bud off diverticula, which subsequently become the tubules of the Wolffian body (vide fig. 13, _wd_). I am fairly satisfied that the tubules are really budded off, and are not formed independently in the mesoblast. The Dog-fish agrees so far with Birds, where I have also no doubt the tubules of the Wolffian body are formed as diverticula from the Wolffian duct.

The Wolffian ducts have also become much longer than the Oviduct, and are now found behind the anus, though they do not extend as far forward as does the Oviduct.

They have further acquired a communication with the Oviduct, in the form of a narrow duct passing from each of them into an Oviduct a short way before the latter opens into the cloacal dilatation of the alimentary canal.

The canals formed by the primitive involution leading from the pleuro-peritoneal cavity into the Wolffian duct have become much more elongated, and at the same time narrower. One of these is shewn in fig. 13, _pwd_.

Any doubt which could possibly be entertained as to the true character of the ducts whose development I have described is entirely removed by the development of the tubules of the Wolffian body. In the still later stage than this further proofs are furnished involving the function of the Oviduct. At the period when the olfactory lobes have become so developed as to render the head of the typical elongated shape of the adult, I find that the males and females can be distinguished by the presence in the former of the clasping appendages[16]. I find at this stage that in the female the front ends of the Oviducts have approached the middle line, dilated considerably, and commenced to exhibit at their front ends the peculiarities of the adult. In the male they are much less conspicuous, though still present.

Footnote 16: For the specimens of this age I am indebted to
Professor Huxley.

At the same time the tubules of the Wolffian body become much more numerous, the Malpighian tufts appear, and the ducts cease almost, if not entirely, to communicate with the pleuro-peritoneal cavity. I have not made out anything very definitely as to the development of the Malpighian tufts, but I am inclined to believe that they arise independently in the mesoblast of the intermediate cell mass.

The facts which I have made out in reference to the development of the Wolffian duct, especially of its arising as a _series of involutions_ from the pleuro-peritoneal cavity, will be found, I believe, of the greatest importance in understanding the true constitution of the Wolffian body. To this I will return directly, but first wish to clear the ground by insisting upon one preliminary point.

From their development the Oviduct and Wolffian body appear to stand to each other in the relation of the Wolffian duct being the equivalent to a series, so to speak, of Oviducts.

I pointed out before that the mode of development of the Oviduct could only be considered as a modification of a simple involution from the pleuro-peritoneal cavity. Its development, both in the Birds and in the Batrachians as an involution, still more conclusively proves the truth of this view.

The explanation of its first appearing as a solid rod of cells which keeps close to the epiblast is, I am inclined to think, the following. Since the Oviduct had to grow a long way backwards from its primitive point of involution, it was clearly advantageous for it not to bore its way through the mesoblast of the intermediate cell mass, but to pass between this and the epiblast. This modification having been adopted, was followed by the knob forming the origin of the duct coming to be placed at the outside of the intermediate cell mass rather than close to the pleuro-peritoneal cavity, a change which necessitated the mode of development by an involution being dropped and the solid mode of development substituted for it, a lumen being only subsequently acquired.

In support of the modification in the development being due to this cause is the fact that in Birds a similar modification has taken place with the Wolffian duct. The Wolffian duct there arises differently from its mode of development in all the lower vertebrates as a solid rod close to the epiblast[17], instead of as an involution.

Footnote 17: If Romiti's observations (_Archives für Mikr.
Anatom._ Vol. IX. p. 200) are correct, then the ordinary view
of the Wolffian duct arising in Birds as a solid rod at the
outer corner of the protovertebræ will have to be abandoned.

If the above explanation about the Oviduct be correct, then it is clear that similar causes have produced a similar modification in development (only with a different organ) in Birds; while, at the same time, the primitive mode of origin of the Oviduct (Müller's duct) has been retained by them.

The Oviduct, then, may be considered as arising by an involution from the pleuro-peritoneal cavity.

The Wolffian duct arises by a series of such involutions, all of which are behind (nearer the tail) the involution to form the Oviduct.

The natural interpretation of these facts is that in the place of the Oviduct and Wolffian body there were primitively a series of similar bodies (probably corresponding in number with the vertebral segments), each arising by an involution from the pleuro-peritoneal cavity; and that the first of these subsequently became modified to carry eggs, while the rest coalesced to form the Wolffian duct.

If we admit that the Wolffian duct is formed by the coalescence of a series of similar organs, we shall only have to extend the suggestion of Gegenbaur as to the homology of the Wolffian body in order to see its true nature. Gegenbaur looks upon the whole urinogenital system as homologous with a pair of segmental organs. Accepting its homology with the segmental organs, its development in Elasmobranchii proves that it is not one pair, but a series of pairs of segmental organs with which the urinogenital system is homologous. The first of these have become modified so as to form the Oviducts, and the remainder have coalesced to form the Wolffian ducts.

The part of a segmental organ which opens to the exterior appears to be lost in the case of all but the last one, where this part is still retained, and serves as the external opening for all.

Whether the external opening of the first segmental organ (Oviduct) is retained or not is doubtful. Supposing it has been lost, we must look upon the external opening for the Wolffian body as serving also for the Oviduct. In the case of all other vertebrates whose development has been investigated (but the Elasmobranchii), the Wolffian duct arises by a single involution, or, what is equivalent to it, the other involutions having disappeared. This even appears to be the case in the Marsipobranchii. In the adult Lamprey the Wolffian duct terminates at its anterior end by a large ciliated opening into the pleuro-peritoneal cavity. It will, perhaps, be found, when the development of the Marsipobranchii is more carefully studied, that there are _primitively_ a number of such openings[18]. The Oviduct, when present, arises in other vertebrates as a single involution, strongly supporting the view that its mode of formation in the Dog-fish is fundamentally merely an involution.

Footnote 18: While correcting the proofs of this paper I have
come across a memoir of W. Müller ("Ueber die Persistenz der
Urniere bei Myxine Glutinosa," _Jenaische Zeitschrift_,
Vol. VII. 1873), in which he mentions that in Myxine the upper
end of the Wolffian duct communicates by numerous openings with
the pleuro-peritoneal cavity; this gives to the suggestion in
the text a foundation of fact.

The duct of the testes is, I have little doubt, derived from the anterior part of the Wolffian body; if so, it must be looked upon as not precisely equivalent to the Oviduct, but rather to a series of coalesced organs, each equivalent to the Oviduct. The Oviduct is in the Elasmobranchii, as in other vertebrates, primitively developed in both sexes. In the male, however, it atrophies. I found it still visible in the male Torpedos, though much smaller than in the females near the close of intra-uterine life.

Whether or not these theoretical considerations as to the nature of the Wolffian body and Oviduct are correct, I believe that the facts I have brought to light in reference to the development of these parts in the Dog-fish will be found of service to every one who is anxious to discover the true relations of these parts.

Before leaving the subject I will say one or two words about the development of the Ovary. In both sexes the germinal epithelium (fig. 13) becomes thickened below the Oviduct, and in both sexes a knob (in section but really a ridge) comes to project into the pleuro-peritoneal cavity on each side of the mesentery (fig. 13, _pov_). In both sexes, but especially the females, the epithelium on the upper surface of this ridge becomes very much thickened, whilst subsequently it elsewhere atrophies. In the females, however, the thickened epithelium on the knob grows more and more conspicuous, and develops a number of especially large cells with large nuclei, precisely similar to Waldeyer's (_loc. cit._) "primitive ova" of the Bird. In the male the epithelium on the ridge, though containing primitive ova, is not as conspicuous as in the female. Though I have not worked out the matter further than this at present, I still have no doubt that these projecting ridges become the Ovaries.

_The Head._

The study of the development of the parts of the head, on account of the crowding of organs which occurs there, always presents greater difficulties to the investigator than that of the remainder of the body. My observations upon it are correspondingly incomplete. I have, however, made out a few points connected with it in reference to some less well-known organs, which I have thought it worth while calling attention to in this preliminary account.

_The continuation of the Pleuro-peritoneal Cavity into the Head._

In the earlier part of this paper (p. 86) I called attention to the extension of the separation between somatopleure and splanchnopleure into the head, forming a space continuous with the pleuro-peritoneal cavity (Pl. 3, fig. 8_a_, _pp_); this becomes more marked in the next stage, and, indeed, the pleuro-peritoneal cavity is present for a considerable time in the head before it becomes visible elsewhere. At the time of the appearance of the second visceral cleft it has become for the most part atrophied, but there persist two separated portions of it in front of the first cleft, and also remnants of it less well marked between and behind the two clefts. The visceral clefts necessarily divide it into separate parts.

The two portions in front of the first visceral cleft remain very conspicuous till the appearance of the external gills, and above the hinder one of the two the fifth nerve bifurcates.

These two are shewn as they appear in a surface view in fig. 14, _pp_. They are in reality somewhat flattened spaces, lined by a mesoblastic epithelium; the epithelium on the inner surface of the space corresponding to the splanchnopleure, and that on the outer to the somatopleure.

I have not followed the history of these later than the time of the appearance of the external gills.

The presence of the pleuro-peritoneal cavity in the head is interesting, as shewing the fundamental similarity between the head and the remainder of the body.

_The Pituitary Body._

All my sections seem to prove that it is a portion of the epiblastic involution to form the mouth which is pinched off to form the pituitary body, and not a portion of the hypoblast of the throat. Since Götte (_Archiv. für Micr. Anat._ Bd. IX.) has also found that the same is the case with the Batrachians and Mammalia, I have little doubt it will be found to be universally the case amongst vertebrates.

Probably the observations which lead to the supposition that it was the throat which was pinched off to form the pituitary body were made after the opening between the mouth and throat was completed, when it would naturally be impossible to tell whether the pinching off was from the epiblast of the mouth involution or the hypoblast of the throat.

_The Cranial Nerves._

The cranial nerves in their early condition are so clearly visible that I have thought it worth while giving a figure of them, and calling attention to some points about their embryonic peculiarities.

From my figure (14) it will be seen that there is behind the auditory vesicle a nervous tract, from which four nerves descend, and that each of these nerves is distributed to the front portion of a visceral arch. When the next and last arch (in this species) is developed, a branch from this nervous mass will also pass down to it. That each of these is of an equal morphological value can hardly be doubted.

The nerve to the third arch becomes the glosso-pharyngeal (fig. 14, _gl_), the nerves to the other arches become the branchial branches of the vagus nerve (fig. 14, _vg_). Thus the study of their development strongly supports Gegenbaur's view of the nature of the vagus and glosso-pharyngeal, viz. that the vagus is a compound nerve, each component part of it which goes to an arch being equivalent to one nerve, such as the glosso-pharyngeal.

Of the nerves in front of the auditory sac the posterior is the seventh nerve (fig. 14, VII). Its mode of distribution to the second arch leaves hardly a doubt that it is equivalent to one such nerve as those distributed to the posterior arches. Subsequently it acquires another branch, passing forwards towards the arch in front.

The most anterior nerve is the fifth (fig. 14, V), of which two branches are at this stage developed. The natural interpretation of its present condition is, that it is equivalent to two nerves, but the absence of relation in its branches to any visceral clefts renders it more difficult to determine the morphology of the fifth nerve than of the other nerves. The front branch of the two is the ophthalmic branch of the adult, and the hind branch the inferior maxillary branch. The latter branch subsequently gives off low down, _i.e._ near its distal extremity, another branch, the superior maxillary branch.

In its embryonic condition this latter branch does not appear like a third branch of the fifth, equivalent to the seventh or the glosso-pharyngeal nerves, but rather resembles the branch of the seventh nerve which passes to the arch in front, which also is present in all the other cranial nerves.

_Modes of Preparation._

Before concluding I will say one or two words as to my modes of preparation.

I have used picric and chromic acids, both applied in the usual way; but for the early stages I have found osmic acid by far the most useful reagent. I placed the object to be hardened, in osmic acid (half per cent.) for two hours and a half, and then for twenty four in absolute alcohol.

I then embedded and cut sections of it in the usual way, without staining further.

I found it advantageous to cut sections of these embryos immediately after hardening, since if kept for long in the absolute alcohol the osmic acid specimens are apt to become brittle.

LIST OF WORKS REFERRED TO.

Gegenbaur. _Anat. der Wirbelthiere_, III Heft, Leipzig, 1873.

A. Götte. _Archiv. für Micr. Anat._, Vol. X. 1873. "Der Keim der Forelleneies," _Archiv. für. Micr. Anat._, Vol. IX. 1873. "Untersuchung über die Entwicklung der Bombinator igneus," _Archiv. für Micr. Anat._, Vol. V. 1869. "Kurze Mittheilungen aus der Entwicklungsgeschichte der Unke," _Archiv. für Micr. Anat._, Vol. IX. 1873.

Kupffer. _Archiv. für Micr. Anat._, Vol. II. 1866, p. 473. Ibid. Vol. IV. 1868, p. 209.

Kowalevsky. "Entwicklungsgeschichte der Holothurien," _Mémoires de l'Acad. Impér. des Sciences de St Petersbourg_, vii ser. Vol. XI. 1867.

Kowalevsky, Owsjannikow, und Wagner. "Entwicklung der Störe," _Bulletin der K. Acad. St Petersbourg_, Vol. XIV. 1873.

Kowalevsky. "Embryologische Studien an Würmern und Arthropoden," _Mémoirs de l'Acad. Impér. des Sciences de St Petersbourg_, Vol. XVI. 1871.

E. Ray Lankester. _Annals and Mag. of Nat. History_, Vol. XI. 1873, p. 81.

W. Müller. "Ueber die Persistenz der Urniere bei Myxine Glutinosa," _Jenaische Zeitschrift_, Vol. VII. 1873.

Oellacher. _Zeitschrift für Wiss. Zoologie_, Vol. XXIII. 1873.

Owsjannikow. "Entwicklung der Coregonus," _Bul. der K. Akad. St Petersbourg_, Vol. XIX.

Romiti. _Archiv. für Micr. Anat._, Vol. IX. 1873.

Waldeyer. _Eierstock u. Eie._

EXPLANATION OF PLATES 3 AND 4.

COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary canal. _ao._ Dorsal aorta. _auv._ Auditory vesicle. _bd._ Formative cell probably derived from the yolk. _cav._ Cardinal vein. _ch._ Notochord. _ch´._ Thickening of hypoblast to form the notochord. _eb._ Line indicating the edge of the blastoderm. _ep._ Epiblast. _ep´._ Epidermis. _er._ Embryonic rim. _es._ Embryonic swelling. _gl._ Glosso-pharyngeal nerve. _h._ Head. _ht._ Heart. _hy._ Hypoblast. _ll._ Lower layer cells. _ly._ Line of separation between the blastoderm and the yolk. _m._ Mesoblast. _mc._ Medullary canal. _mg._ Medullary groove. _mp._ Muscle-plate. _mp´._ Early formed mass of muscles. _n._ Peculiar nuclei formed in the yolk. _n´._ Similar nuclei in the cells of the blastoderm. _na._ Cells which help to close in the alimentary canal, and which are derived from the yolk. _ny._ Network of lines present in the food-yolk. _ol._ Olfactory pit. _op._ Eye. _ov._ Oviduct. _pn._ Pineal gland. _pov._ Projection which becomes the ovary. _pp._ Pleuro-peritoneal cavity. _pp´._ Remains of pleuro-peritoneal cavity in the head. _prv._ Protovertebræ. _pwd._ Primary points of involution from the pleuro-peritoneal cavity by the coalescence of which the Wolffian duct is formed. _sg._ Segmentation cavity. _so._ Somatopleure. _sos._ Stalk connecting embryo with yolk-sac. _sp._ Splanchnopleure. _spn._ Spinal nerve. _sur._ Suprarenal body. _ts._ Caudal lobes. _v._ Blood-vessel. _vg._ Vagus nerve. V. Fifth nerve. VII. Seventh nerve. _vc_, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts. _vp._ Vertebral plates. _wd._ Wolffian duct. _x._ Peculiar body underlying the notochord derived from the hypoblast. _yk._ Yolk spherules.

All the figures were drawn with the Camera Lucida.

Plate 3.

Fig. 1. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet completely lined by cells. The roof of the segmentation cavity is broken. (Magnified 60 diam.) The section is intended chiefly to illustrate the distribution of nuclei (_n_) in the yolk under the blastoderm. One of the chief points to be noticed in their distribution is the fact that they form almost a complete layer under the floor of the segmentation cavity. This probably indicates that the cells whose nuclei they become take some share in forming the layer of cells which subsequently (vide fig. 4) forms the floor of the cavity.

Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the time of the first appearance of the medullary groove. (Magnified 300 diam.)

The specimen is taken from a portion of the blastoderm which will form part of the embryo. It shews two large nuclei of the yolk (_n_) and the network in the yolk between them; this network is seen to be closer around the nuclei than in the intervening space. The specimen further shews that there are no areas representing cells around the nuclei.

Fig. 3. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity is not yet covered by a complete layer of cells. (Magnified 60 diam.)

It illustrates (1) the characters of the epiblast, (2) the embryonic swelling (_es_), (3) the segmentation cavity (_sg_). It should have been drawn upon the same scale as fig. 4; the line above it represents its true length upon this scale.

Fig. 4. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. (Magnified 45 diam.)

It illustrates (1) the embryonic rim, (2) the continuity of epiblast and hypoblast at edge of this, (3) the continual differentiation of the lower layer cells, to form, on the one hand, the hypoblast, which is continuous with the epiblast, and on the other the mesoblast, between this and the epiblast; (4) the segmentation cavity, whose floor of cells is now completed.

N.B. The cells at the embryonic end of the blastoderm have been made rather too large.

Fig. 5. Surface view of the blastoderm shortly after the appearance of the medullary groove. To shew the relation of the embryo to the blastoderm.

Fig. 6_a_ and _b_. Two transverse sections of the same embryo, shortly after the appearance of the medullary groove. (Magnified 96 diam.)

_a._ In the region of the groove. It shews (1) the two masses of mesoblast on each side, and the deficiency of the mesoblast underneath the medullary groove; (2) the commencement of the closing in of the alimentary canal below, chiefly from cells (_na_) derived from the yolk.

_b._ Section in the region of the head where the medullary groove is deficient, other points as above.

Fig. 7_a_ and _b_. Two transverse sections of an embryo about the age or rather younger than that represented in fig. 5. (Magnified 96 diam.)

_a._ Section nearer the tail; it shews the thickening of the hypoblast to form the notochord (_ch´_).

In _b_ the thickening has become completely separated from the hypoblast as the notochord. In _a_ the epiblast and hypoblast are continuous at the edge of the section, owing to the section passing through the embryonic rim.

Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the flattened head (_h_) where the medullary groove is deficient, (2) the caudal lobes, with a groove between them; it also shews that at this point, the medullary groove has become roofed over and converted into a canal.

Fig. 8_a_. Transverse section of fig. 8, passing through the line _a_. (Magnified 90 diam.) The section shews (1) the absence of the medullary groove in the head and the medullary folds turning down at this time instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the head (_pp_); (3) the completely closed alimentary canal (_al_).

Fig. 8_b_. Transverse section of fig. 8, through the line _b_. (Magnified 90 diam.) It shews (1) the neural canal completely formed; (2) the vertebral plates of mesoblast not yet split up into somatopleure and splanchnopleure.

Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object a little older than the embryo represented in fig. 8. (Magnified 20 diam.) The internal anatomy has hardly altered, with the exception of the medullary folds having closed over above the head and the whole embryo having become more folded off from the germ.

The two caudal lobes, and the very marked groove between them, are seen at _ts_. The front end of the notochord became indistinct, and I could not see its exact termination. The epithelium of the alimentary canal (_al_) is seen closely underlying the notochord and becoming continuous with the epiblast at the hind end of the notochord.

The first visceral cleft (1_vc_) and eye (_op_) are just commencing to be formed, and the cranial flexure has just appeared.

Fig. 10. Section through the dorsal region of an embryo somewhat older than the one represented in fig. 9. (Magnified 96 diam.)

It shews (1) the formation by a pinching off from the top of the alimentary canal of a peculiar body which underlies the notochord (_x_); (2) the primitive extension of the pleuro-peritoneal cavity up to the top of the vertebral plates.

Plate 4.

Fig. 11_a_, _b_, and _c_. Three sections closely following each other from an embryo in which three visceral clefts are present; _a_ is the most anterior of the three. (Magnified 96 diam.) In all of these the muscle-plates are shewn at _mp_. They have become separated from the lateral plates in _b_ and _c_, but are still continuous with them in _a_. The early formed mass of muscles is also shewn in all the figures (_mp´_).

The figures further shew (1) the formation of the spinal nerves (_spn_) as small bodies of cells closely applied to the upper and outer edge of the neural canal.

(2) The commencing formation of the cells which form the axial skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sections _b_ and _c_ are given more especially to shew the mode of formation of the oviduct (_ov_).

In _b_ it is seen as a _solid knob (ov)_, arising from the point where the somatopleure and splanchnopleure unite, and in _c_ (the section behind _b_) as a _solid rod (ov)_ closely applied to the epiblast, which has grown backwards from the knob seen in _b_.

N.B. In all three sections only one side is completed.

Fig. 12_a_ and _b_. Two transverse sections of an embryo just before the appearance of the external gills. (Magnified 96 diam.)

In _a_ there is seen to be an involution on each side (_pwd_), while _b_ is a section from the space between two involutions from the pleuro-peritoneal cavity, so that the Wolffian duct (at first solid) (_wd_) is not connected as in _a_ with the pleuro-peritoneal cavity. The further points shewn in the sections are--

(1) The commencing formation of the spiral valve (_al_).
(2) The suprarenal body (_sur_).
(3) The oviduct (_ov_), which has acquired a lumen.
(4) The increase in length of the muscle-plates, the spinal nerves,
&c.

Fig. 13. Section through the dorsal region of an embryo in which the external gills are of considerable length. (Magnified 40 diam.) The chief points to be noticed:

(1) The formation of the Wolffian body by outgrowths from
the Wolffian duct (_wd_).
(2) One of the still continuing connections (primitive
involutions) between the Wolffian duct and the
pleuro-peritoneal cavity (_pwd_).
(3) The oviduct largely increased in size (_ov_).
N.B. On the left side the oviduct has been accidentally made
too small.
(4) The growth downwards of the muscle-plate to form the
muscles of the abdomen.
(5) The formation of an outgrowth on each side of the
mesentery (_pov_), which will become the ovary.
(6) The spiral valve (_al_).

Fig. 14. Transparent view of the head of an embryo shortly before the appearance of the external gills. (Magnified 20 diam.) The chief points to be noticed are--

(1) The relation of the cranial nerves to the visceral
clefts and the manner in which the glosso-pharyngeal (_gl_)
and vagus (_vg_) are united.
(2) The remnants of the pleuro-peritoneal cavity in the head
(_pp_).
(3) The eye (_op_). The stalk, as well as the bulb of the
eye, are supposed to be in focus, so that the whole eye has
a somewhat peculiar appearance.

VI. A COMPARISON OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES[19].

Footnote 19: From the _Quarterly Journal of Microscopical
Science_, Vol. XV. 1875.

With Plate 5.

If the genealogical relationships of animals are to be mainly or largely determined on embryological evidence, it becomes a matter of great importance to know how far evidence of this kind is trustworthy.

The dependence to be placed on it has been generally assumed to be nearly complete. Yet there appears to be no _à priori_ reason why natural selection should not act during the embryonic as well as the adult period of life; and there is no question that during their embryonic existence animals are more susceptible to external forces than after they have become full grown: indeed, an immense mass of evidence could be brought to shew that these forces do act upon embryos, and produce in them great alterations tending to obscure the genealogical inferences to be gathered from their developmental histories. Even the time-honoured layers form to this no exception. In _Elasmobranchii_, for instance, we find the notochord derived from the hypoblast and the spinal ganglia derived from the involuted epiblast of the neural canal, whilst in the higher vertebrates both of these organs are formed in the mesoblast. Such instances are leading embryologists to recognise the fact that the so-called layers are not quite constant and must not be absolutely depended upon in the determination of homologies. But though it is necessary to recognise the fact that great changes do occur in animals during their embryonic life, it is not necessary to conclude that all embryological evidence is thereby vitiated; but rather it becomes incumbent on us to attempt to determine which embryological features are ancestral and which secondary. For this purpose it is requisite to ascertain what are the general characters of secondary features and how they are produced. Many vertebrates have in the first stages of their development a number of secondary characters which are due to the presence of food material in the ovum; the present essay is mainly an attempt to indicate how those secondary characters arose and to trace their gradual development. At the same time certain important ancestral characters of the early phases of the development of vertebrates, especially with reference to the formation of the hypoblast and mesoblast, are pointed out and their meaning discussed.

There are three orders of vertebrates of which no mention has been made, viz., the _Mammals_, the _Osseous_ fishes, and the _Reptiles_. The first of these have been passed over because the accounts of their development are not sufficiently satisfactory, though as far as can be gathered from Bischoff's account of the dog and rabbit there would be no difficulty in shewing their relations with other vertebrates.

We also require further investigations on Osseous fishes, but it seems probable that they develop in nearly the same manner as the Elasmobranchii.

With reference to Reptiles we have no satisfactory investigations.

* * * * *

Amphioxus is the vertebrate whose mode of development in its earliest stages is simplest, and the modes of development of other vertebrates are to be looked upon as modifications of this due to the presence of food material in their ova. It is not necessary to conclude from this that Amphioxus was the ancestor of our present vertebrates, but merely that the earliest stages of development of this vertebrate ancestor were similar to those of Amphioxus.

The ovum of Amphioxus contains very little food material and its segmentation is quite uniform. The result of segmentation is a vesicle whose wall is formed of a single layer of cells. These are all of the same character, and the cavity of the vesicle called the segmentation cavity is of considerable size. A section of the embryo, as we may now call the ovum, is represented in Plate 5, fig. A I.

The first change which occurs is the pushing in of one half of the wall of the vesicle towards the opposite half. At the same time by the narrowing of its mouth the hollow hemisphere so formed becomes again a vesicle[20].

Footnote 20: I have been able to make at Naples observations
which confirm the account of the invagination of Amphioxus as
given by Kowalevsky, though my observations are not nearly so
complete as those of the Russian naturalist.

Owing to its mode of formation the wall of this secondary vesicle is composed of two layers which are only separated by a narrow space, the remnant of the segmentation cavity.

Two of the stages in the formation of the secondary vesicle by this process of involution are shewn in Plate X, fig. A II, and A III. In the second of these the general growth has been very considerable, rendering the whole animal much larger than before. The cavity of this vesicle, A III, is that of the commencing alimentary canal whose final form is due to changes of shape undergone by this primitive cavity. The inner wall of the vesicle becomes converted into the wall of the alimentary canal or hypoblast, and also into part or the whole of the mesoblast.

During the involution the cells which are being involuted undergo a change of form, and before the completion of the process have acquired a completely different character to the cells forming the external wall of the secondary vesicle or epiblast. This change of character in the cells is already well marked in fig. A II. It is of great importance, since we shall find that some of the departures from this simple mode of development, which characterise other vertebrates, are in part due to the distinction between the hypoblast and epiblast cells appearing during segmentation, and not subsequently as in Amphioxus during the involution of the hypoblast.

Kowalevsky (_Entwicklungsgeschichte des Amphioxus_) originally believed that the narrow mouth of the vesicle (according to Mr Lankester's terminology _blastopore_) became the anus of the adult. He has since, and certainly correctly, given up this view. The opening of the involution becomes closed up and the adult anus is no doubt formed as in all other vertebrates by a pushing in from the exterior, though it probably corresponds in position very closely with the point of closing up of the original involution.

The mode of formation of the mesoblast is not certainly known in Amphioxus; we shall find, however, that for all other vertebrates it arises from the cells which are homologous with the involuted cells of this animal.

Since food material is a term which will be very often employed, it will be well to explain exactly the sense in which it will be used. It will be used only with reference to those passive highly refractive particles which are found embedded in most ova.

In some eggs, of which the hen's egg may be taken as a familiar example, the yolk-spherules or food material form the larger portion of the ovum, and a distinction is frequently made between the germinal disc and the yolk.

This distinction is, however, apt to lead to a misconception of the true nature of the egg. There are strong grounds for believing that the so-called yolk, equally with the germinal disc, is composed of an active protoplasmic basis endowed with the power of growth, in which passive yolk-spherules are embedded; but that the part ordinarily called the yolk contains such a preponderating amount of yolk-spherules that the active basis escapes detection, and does not exhibit the same power of growth as the germinal disc.

With the exception of mammals, whose development requires to be more completely investigated, Amphioxus is as far as we know the only vertebrate whose ovum does not contain a large amount of food material.

In none of these (vertebrate) yolk-containing ova is the food material distributed uniformly. It is always concentrated much more at one pole than at the other, and the pole at which it is most concentrated may be conveniently called the lower pole of the egg.

In eggs in which the distribution of food material is not uniform segmentation does not take place with equal rapidity through all parts of the egg, but its rapidity is, roughly speaking, inversely proportional to the quantity of food material.

When the quantity of food material in a part of the egg becomes very great, segmentation does not occur at all; and even in those cases where the quantity of food yolk is not too great to prevent segmentation the resulting segmentation spheres are much larger than where the yolk-granules are more sparsely scattered.

The Frog is the vertebrate whose development comes nearest to that of Amphioxus, as far as the points we are at present considering are concerned. But it will perhaps facilitate the understanding of their relations shortly to explain the diagrammatic sections which I have given of an animal supposed to be intermediate in its development between the Frog and Amphioxus. Plate 5, fig. B I, represents a longitudinal section of this hypothetical egg at the close of segmentation. The lower pole, coloured yellow, represents the part containing more yolk material, and the upper pole, coloured blue, that with less yolk. Owing to the presence of this yolk the lower pole even at the close of segmentation is composed of cells of a different character to those of the upper pole. In this respect this egg can already be distinguished from that of Amphioxus, in which no such difference between the two poles is apparent at the corresponding period (Plate 5, fig. A I).

The segmentation cavity in this ovum is not quite so large proportionately as in Amphioxus, and the encroachment upon it is due to the larger bulk of the lower pole of the egg. In fig. B II the involution of the lower pole has already commenced; this involution is (1) not quite symmetrical, and (2) on the ventral side (the left side) the epiblast cells forming the upper part of the egg are growing round the cells of the lower pole of the egg or lower layer cells. Both of these peculiarities are founded upon what happens in the Frog and the Selachian, but it is to be noticed that the change from the lower layer cells being involuted towards the epiblast cells, to the epiblast cells growing round the lower layer cells, is a necessary consequence of the increased bulk of the latter.

In this involution not only are the cells of the lower pole pushed on, but also some of those of the upper or yellow portion; so that in this as in all other cases the true distinction between the epiblast and hypoblast does not appear till the involution to form the latter is completed. In the next stage, B III, the involution has become nearly completed and the opening to the exterior or blastopore quite constricted.

The segmentation cavity has been entirely obliterated, as would have been found to be the case with Amphioxus had the stage a little older than that on Plate 5, A III, been represented. The cavity marked (_al_), as was the case with Amphioxus, is that of the alimentary canal.

The similarities between the mode of formation of the hypoblast and alimentary canal in this animal and in Amphioxus are so striking and the differences between the two cases so slight that no further elucidation is required. One or two points need to be spoken of in order to illustrate what occurs in the Frog. When the involution to form the alimentary canal occurs, certain of the lower layer cells (marked _hy_) become distinguished from the remainder of the lower layer cells as a separate layer and form the hypoblast which lines the alimentary canal. It is to be noticed that the cells which form the ventral epithelium of the alimentary canal are not so soon to be distinguished from the other lower layer cells as those which form its dorsal epithelium. This is probably a consequence of the more active growth, indicated by the asymmetry of the involution, on the dorsal side, and is a fact with important bearings in the ova with more food material. The cells marked _m_ and coloured red also become distinguished as a separate layer from the remainder of the hypoblast and form the mesoblast. The remainder of the lower layer cells form a mass equivalent to the yolk-sac of many vertebrates, and are not converted directly into the tissues of the animal.