CHAPTER VI.

DEVELOPMENT OF THE TRUNK DURING STAGES G TO K.

By the stage when the external gills have become conspicuous objects, the rudiments of the greater number of the important organs of the body are definitely established.

Owing to this fact the first appearance of the external gills forms a very convenient break in the Elasmobranch development; and in the present chapter the history is carried on to the period of this occurrence.

While the last chapter dealt for the most part with the formation of the main organic systems from the three embryonic layers, the present one has for its subject the gradual differentiation of these systems into individual organs. In treating of the development of the separate organs a divergence from the plan of the last chapter becomes necessary, and the following arrangement has been substituted for it. First of all an account is given of the development of the external epiblast, which is followed by a description of the organs derived from the mesoblast and of the notochord.

_External Epiblast._

During stages G to I the epiblast[192] is formed of a single layer of flattened cells; and in this, as in the earlier stages, it deserves to be especially noticed that the epiblast is never more than _one cell deep_, and is therefore incapable of presenting any differentiation into nervous and epidermic layers. (Pl. 11, figs. 1-5.)

Footnote 192: Unless the contrary is stated, the facts recorded in this chapter apply only to the genera Scyllium and Pristiurus.

The cells which compose it are flattened and polygonal in outline, but more or less spindle-shaped in section. They present a strong contrast to the remaining embryonic cells of the body in possessing a considerable quantity of clear protoplasm, which in most other cells is almost entirely absent. Their granular nucleus is rounded or oval, and typically contains a single nucleolus. Frequently, however, two nucleoli are present, and when this is the case an area free from granules is to be seen around each nucleolus, and a dark line, which could probably be resolved into granules by the use of a sufficiently high magnifying power, divides the nucleus into two halves. These appearances probably indicate that nuclei, in which two nucleoli are present, are about to divide.

The epiblast cells vary in diameter from .022 to .026 Mm. and their nuclei from .014 to .018 Mm. They present a fairly uniform character over the greater part of the body. In Torpedo they present nearly the same characters as in Pristiurus and Scyllium, but are somewhat more columnar. (Pl. 11, fig. 7.)

Along the summit of the back from the end of the tail to the level of the anus, or slightly beyond this, epiblast cells form a fold--the rudiment of the embryonically undivided dorsal fin--and the cells forming this, unlike the general epiblast cells, are markedly columnar; they nevertheless, here as elsewhere, form but a single layer. (Pl. 11, fig. 3 and 5, _df._) Although at this stage the dorsal fin is not continued as a fold anteriorly to the level of the anus, yet a columnar thickening or ridge of epiblast, extending along the median dorsal line nearly to the level of the heart, forms a true morphological prolongation of the fin.

On the ventral side of the tail is present a rudiment of the ventral unpaired fin, which stops short of the level of the anus, but, though less prominent, is otherwise quite similar to the dorsal fin and continuous with it round the end of the tail. At this stage the mesoblast has no share in forming either fin.

In many sections of the tail there may be seen on each side two folds of skin, which are very regular, and strongly simulate the rudimentary fins just described. The cells composing them are, however, not columnar, and the folds themselves are merely artificial products due to shrinking.

At a stage slightly younger than K an important change takes place in the epiblast.

From being composed of a single layer of cells it becomes two cells deep. The two layers appear first of all anteriorly, and subsequently in the remaining parts of the body. At first, both layers are formed of flattened cells (Pl. 11, figs. 8, 9); but at a stage slightly subsequent to that dealt with in the present chapter, the cells of the inner of the two layers become columnar, and thus are established the two strata always present in the epidermis of adult vertebrates, viz. an outer layer of flattened cells and an inner one of columnar cells[193].

Footnote 193: The layers are known as epidermic (horny) and mucous layers by English writers, and as Hornschicht and Schleimschicht by the Germans. For their existence in all Vertebrates, vide Leydig _Ueber allgemeine Bedeckungen der Amphibien_, p. 20. Bonn, 1876.

The history of the epiblast in Elasmobranchii is interesting, from the light which it throws upon the meaning of the nervous and epidermic layers into which the epiblast of Amphibians and some other Vertebrates is divided. The Amphibians and Elasmobranchii present the strongest contrast in the development of their epiblast, and it is worth while shortly to review and compare the history of the layer in the two groups.

In Amphibians the epiblast is from the first divided into an outer stratum formed of a single row of flattened cells, and an inner stratum composed of several rows of more rounded cells. These two strata were called by Stricker the nervous and epidermic layers, and these names have been very generally adopted.

Both strata have a share in forming the general epiblast, and though eventually they partially fuse together, there can be but little doubt that the horny layer of the adult epiblast, where such can be distinguished[194], is derived from the epidermic layer of the embryo, and the mucous layer of the epiblast from the embryonic nervous layer. Both layers of the epiblast assist in the formation of the cerebro-spinal nervous system, and there also at first fuse together[195], though the epidermic layer probably separates itself again, as the central epithelium of the spinal canal. The lens and auditory sac are derived exclusively from the nervous layer of the epidermis, while this layer also has the greater share in forming the olfactory sac.

Footnote 194: Vide Leydig, _loc. cit._

Footnote 195: Vide Götte, _Entwicklungsgeschichte der Unke_.

In Elasmobranchii the epiblast is at first uniformly composed of a single row of cells. The part of the layer which will form the central nervous system next becomes two or three cells deep, but presents no distinction into two layers; the remaining portions of the layer remain, as before, one cell deep. Although the epiblast at first presents this simple structure, it eventually, as we have seen, becomes divided throughout into two layers, homologous with the two layers which arise so early in Amphibians. The outer one of the two forms the horny layer of the epidermis and the central epithelium of the neural canal. The inner one, the mucous layer of the epidermis and the nervous part of the brain and spinal cord. Both layers apparently enter into the formation of the organs of sense.

While there is no great difficulty in determining the equivalent parts of the epidermis in Elasmobranchii and Amphibians, it still remains an open question in which of these groups the epiblast retains its primitive condition.

Though it is not easy to bring conclusive proofs on the one side or the other, the balance of argument appears to me to be decidedly in favour of regarding the condition of the epiblast in Elasmobranchii, and most other Vertebrates, as the primitive one, and its condition in Amphibians as a secondary one, due to the throwing back of the differentiation of their epiblast into two layers to a very early period in their development.

In favour of this view are the following points: (1) That a _primitive_ division of the epiblast into two layers is unknown in the animal kingdom, except amongst Amphibians and (?) Osseous Fish. (2) That it appears more likely for a particular feature of development to be thrown back to an earlier period, than for such an important feature as a distinction between two primary layers to be absolutely lost during an early period of development, and then to reappear again in later stages.

The fact of the epiblast of the neural canal being divided, like the remainder of the layer, into nervous and epidermic parts, cannot, I think, be used as an argument in favour of the opposite view to that here maintained.

It seems probable that the central canal of the nervous system arose as an involution from the exterior, and therefore that the epidermis lining it is in reality merely a part of the external epidermis, and as such is naturally separated from the true nervous structures adjacent to it[196].

Footnote 196: Vide Self, "Development of Spinal Nerves in Elasmobranchii." _Phil. Transact._ 1876. [This Edition, No. VIII.]

Leaving the general features of the external skin, I pass to the special organs derived from it during the stage just anterior to K.

_The unpaired Fins._ The unpaired fins have grown considerably, and the epiblast composing them becomes, like the remainder of the layer, divided into two strata, both however composed of more or less columnar cells. The ventral fin has now become more prominent than the dorsal fin; but the latter extends forward as a fold quite to the anterior part of the body.

_The paired Fins._ Along each side of the body there appears during this stage a thickened line of epiblast, which from the first exhibits two special developments: one of these just in front of the anus, and a second and better marked one opposite the front end of the segmental duct. These two special thickenings are the rudiments of the paired fins, which thus arise as special developments of a continuous ridge on each side, precisely like the ridges of epiblast which form the rudiments of the unpaired fins.

Similar thickenings to those in Elasmobranchii are found at the ends of the limbs in the embryos of both Birds and Mammals, in the form of caps of columnar epiblast[197].

Footnote 197: For Birds, vide _Elements of Embryology_, Foster and Balfour, pp. 144, 145, and for Mammals, Kölliker, _Entwicklungsgeschichte_, p. 283.

The ridge, of which the limbs are special developments, is situated on a level slightly ventral to that of the dorsal aorta, and extends from just behind the head to the level of the anus. It is not noticeable in surface views, but appears in sections as a portion of the epiblast where the cells are more columnar than elsewhere; precisely resembling in this respect the forward continuation of the dorsal fin. At the present stage the posterior thickenings of this ridge which form the abdominal fins are so slight as to be barely visible, and their real nature can only be detected by a careful comparison between sections of this and the succeeding stages. The rudiments of the anterior pair of limbs are more visible than those of the posterior, though the passage between them and the remainder of the ridges is most gradual. Thus at first the rudiments of both the limbs are nothing more than slight thickenings of the epiblast, where its cells are more columnar than elsewhere. During stage K the rudiments of both pairs of limbs, but especially of the anterior pair, grow considerably, while at the same time the thickened ridge of epiblast which connects them together rapidly disappears. The thoracic limbs develop into an elongated projecting fold of epiblast, in every way like the folds forming the unpaired fins; while at the same time the cells of the subjacent mesoblast become closely packed, and form a slight projection, at the summit of which the fold of the epiblast is situated (Pl. 11, fig. 9). The maximum projection of the thoracic fin is slightly in advance of the front end of the segmental duct. The abdominal fins do not, during stage K, develop quite so fast as the thoracic, and at its close are merely elongated areas where the epiblast is much thickened, and below which the mesoblast is slightly condensed. In the succeeding stages they develop into projecting folds of skin, precisely as do the thoracic fins.

The features of the development of the limbs just described, are especially well shewn in Torpedo; in the embryos of which the passage from the general linear thickening of epiblast into the but slightly better marked thickening of the thoracic fin is very gradual, and the fact of the limb being nothing else than a special development of the linear lateral thickening is proved in a most conclusive manner.

If the account just given of the development of the limbs is an accurate record of what really takes place, it is not possible to deny that some light is thrown by it upon the first origin of the vertebrate limbs. The facts can only bear one interpretation, _viz.: that the limbs are the remnants of continuous lateral fins_.

The unpaired dorsal fin develops as a continuous thickening, which then grows up into a projecting fold of columnar cells. The greater part of this eventually atrophies, but three separate lobes are left which form the two dorsal fins and the upper lobe of the caudal fin.

The development of the limbs is almost identically similar to that of the dorsal fins. There appears a lateral linear thickening of epiblast, which however does not, like the similar thickening of the fins, grow into a distinct fold. Its development becomes confined to two special points, at each of which is formed a continuous elongated fold of columnar cells precisely like the fold of skin forming the dorsal fins. These two folds form the paired fins. If it be taken into consideration that the continuous lateral fin, of which the rudiment appears in Elasmobranchii, does not exist in any adult Vertebrate, and also that a continuous dorsal fin exists in many Fishes, the small differences in development between the paired fins and the dorsal fins will be seen to be exactly those which might have been anticipated beforehand. Whereas the continuous dorsal fin, which often persists in adult fishes, attains a considerable development before vanishing, the originally continuous lateral one has only a very ephemeral existence.

While the facts of development strongly favour a view which would regard the limbs as remnants of a primitively continuous lateral fin, there is nothing in the structure of the limbs of adult Fishes which is opposed to this view. Externally they closely resemble the unpaired fins, and both their position and nervous supply appear clearly to indicate that they do not belong to one special segment of the body. They appear rather to be connected with a varying number of segments; a fact which would receive a simple explanation on the hypothesis here adopted[198].

Footnote 198: For the nervous supply in fishes, vide Stannius, _Peripher. Nerv. System d. Fische_. In Osseous Fishes he states that the thoracic fin is supplied by branches from the first three though sometimes from the first four spinal nerves. In Acipenser there are branches from the first six nerves. In Spinax the limb is supplied by the rami anteriores of the fourth and succeeding ten spinal nerves. In the Rays not only do the sixteen anterior spinal nerves unite to supply the fin, but in all there are rami anteriores from thirty spinal nerves which pass to the thoracic limb.

My researches throw no light on the nature of the skeletal parts of the limb, but the suggestion which has been made by Günther[199] with reference to the limb of Ceratodus (the most primitive known), that it is a modification of a series of parallel rays, would very well suit the view here proposed.

Footnote 199: _Philosophical Transactions_, 1871.

Dr Dohrn[200] in speaking of the limbs, points out the difficulties in the way of supposing that they can have originated _de novo_, and not by the modification of some pre-existing organ, and suggests that the limbs are modified gill-arches; a view similar to which has been hinted at by Professor Gegenbaur[201].

Footnote 200: _Ursprung d. Wirbelthiere and Functionswechsels._

Footnote 201: _Grundriss d. Vergleichenden Anat._ p. 494.

Dr Dohrn has not as yet given the grounds for his determination, so that any judgment on his views is premature.

None of my observations on Elasmobranchii lends any support to these views; but perhaps, while regarding the limbs as the remains of a continuous fin, it might be permissible to suppose that the pelvic and thoracic girdles are altered remnants of the skeletal parts of some of the gill-arches which have vanished in existing Vertebrates.

The absence of limbs in the Marsipobranchii and Amphioxus, for reasons already insisted upon by Dr Dohrn[202], cannot be used as an argument against limbs having existed in still more primitive Vertebrates.

Footnote 202: _Loc. cit._

Though it does not seem probable that a dorsal and ventral fin can have existed contemporaneously with lateral fins (at least not as continuous fins), yet, judging from such forms as the Rays, there is no reason why small balancing dorsal and caudal fins should not have co-existed with fully developed lateral fins.

_Mesoblast. G-K._

The mesoblast in stage F forms two independent lateral plates, each with a splanchnic and somatic layer, and divided, as before explained, into a vertebral portion and a parietal portion. At their peripheral edge these plates are continuous with the general mesoblastic tissue of the non-embryonic part of the blastoderm; except in the free parts of the embryo, where they are necessarily separated from the mesoblast of the yolk-sac, and form completely independent lateral masses of cells.

During the stages G and H, the two layers of which the mesoblast is composed cease to be in contact, and leave between them a space which constitutes the commencement of the body-cavity (Pl. 10, fig. 1). From the very first this cavity is more or less clearly divided into two distinct parts; one of them in the vertebral portion of the plates of mesoblast, the other in the parietal. The cavity in the parietal part of the plates alone becomes the true body-cavity. It extends uninterruptedly through the anterior parts of the embryo, but does not appear in the caudal region, being there indicated only by the presence of two layers in the mesoblast plates. Though fairly wide below, it narrows dorsally before becoming continuous with the cavity in the vertebral plates. The line of junction of the vertebral and parietal plates is a little ventral to the dorsal summit of the alimentary canal (Pl. 10, fig. 5). Owing to the fact that the vertebral plates are split up into a series of segments (protovertebræ), the section of the body-cavity they enclose is necessarily also divided into a series of segments, one for each protovertebra.

Thus the whole body-cavity consists of a continuous parietal space which communicates by a series of apertures with a number of separate cavities enclosed in the protovertebræ. The cavity in each of the protovertebræ is formed of a narrowed dorsal and a dilated ventral segment, the latter on the level of the dorsal aorta (Pl. 11, fig. 5). Cavities are present in all the vertebral plates with the exception of a few far back in the tail; and exist in part of the caudal region posterior to that in which a cavity in the parietal plate is present.

_Protovertebræ._ Each protovertebra[203] or vertebral segment of the mesoblast plate forms a flattened rectangular body, ventrally continuous with the parietal plate of mesoblast. During stage G the dorsal edge of the protovertebræ is throughout on about a level with the ventral third of the spinal cord. Each vertebral plate is composed of two layers, a somatic and a splanchnic, and encloses the already-mentioned section of the body-cavity. The cells of both layers of the plate are columnar, and each consists of a very large nucleus, invested by a delicate layer of protoplasm.

Footnote 203: No attempt has been made to describe in detail the different appearances presented by the protovertebræ in the various parts of the body, but in each stage a protovertebra from the dorsal region is taken as typical.

Before the end of stage H the inner or splanchnic wall of the protovertebra loses its simple constitution, owing to the middle part of it, opposite the dorsal two-thirds of the notochord, undergoing peculiar changes. These changes are indicated in transverse sections (Pl. 11, figs. 5 and 6, _mp´_), by the cells in the part we are speaking of acquiring a peculiar angular appearance, and becoming one or two deep; and the meaning of the changes is at once shewn by longitudinal horizontal sections. These prove (Pl. 12, fig. 10) that the cells in this situation have become elongated in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a protovertebra, and in the present stage, or at any rate in stage I, acquires a peculiar granulation, which clearly foreshadows transverse striation (Pl. 12, figs. 11-13).

Thus by stage H a small portion of the splanchnopleure which forms the inner layer of each protovertebra, becomes differentiated into a distinct band of longitudinal striated muscles; these almost at once become functional, and produce the peculiar serpentine movements of the embryo, spoken of in a previous chapter, p. 291.

It may be well to say at once that these muscles form but a very small part of the muscles which eventually appear; which latter are developed at a very much later period from the remaining cells of the protovertebræ. The band developed at this stage appears to be a special formation, which has arisen through the action of natural selection, to enable the embryo to meet its respiratory requirements, by continually moving about, and so subjecting its body to fresh oxydizing influences; and as such affords an interesting example of an important structure acquired during and for embryonic life.

Though the cavities in the protovertebræ are at first perfectly continuous with the general body-cavity, of which indeed they merely form a specialized part, yet by the close of stage H they begin to be constricted off from the general body-cavity, and this process is continued rapidly, and completed shortly after stage I, and considerably before the commencement of stage K (Pl. 11, figs. 6 and 8). While this is taking place, part of the splanchnic layer of each protovertebra, immediately below the muscle-band just described, begins to proliferate, and produce a number of cells, which at once grow in between the muscles and the notochord. These cells are very easily seen both in transverse and longitudinal sections, and form the commencing vertebral bodies (Pl. 11, fig. 6, and Pl. 12, figs. 10 and 11, _Vr_).

At first the vertebral bodies have the same segmentation as the protovertebræ from which they sprang; that is to say, they form masses of embryonic cells separated from each other by narrow slits, continuous with the slits separating the protovertebræ. They have therefore at their first appearance a segmentation completely different from that which they eventually acquire (Pl. 12, fig. 11).

After the separation of the vertebral bodies from the protovertebræ, the remaining parts of the protovertebræ may be called muscle-plates; since they become directly converted into the whole voluntary muscular system of the trunk. At the time when the cavity of the muscle-plates has become completely separate from the body-cavity, the muscle-plates themselves are oblong structures, with two walls enclosing the cavity just mentioned, in which the original ventral dilatation is still visible. The outer or somatic wall of the plates retains its previous simple constitution. The splanchnic wall has however a somewhat complicated structure. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity of the plates the epithelium forming the remainder of the layer commences to insert itself; so that between the first-formed muscle and the cavity of the muscle-plate there appears a thin layer of cells, not however continuous throughout.

At the end of the period K the muscle-plates have extended dorsally two-thirds of the way up the sides of the spinal cord, and ventrally to the level of the segmental duct. Their edges are not straight, but are bent into an angular form, with the apex pointing forwards. Vide Pl. 12, fig. 17, _mp_.

Before the end of the period a number of connective-tissue cells make their appearance, and extend upwards from the dorsal summit of the muscle-plates around the top of the spinal cord. These cells are at first rounded, but become typical branched connective-tissue cells before the close of the period (Pl. 11, figs. 7 and 8).

Between stages I and K the bodies of the vertebræ rapidly increase in size and send prolongations downwards and inwards to meet below the notochord.

These soon become indistinguishably fused with other cells which appear in the area between the alimentary cavity and the notochord, but probably serve alone to form the vertebral bodies, while the cells adjoining them form the basis for the connective tissue of the kidneys, &c.

The vertebral bodies also send prolongations dorsalwards between the sides of the spinal cord and the muscle-plates. These grow round till they meet above the spinal and enclose the dorsal nerve-roots. They soon however become fused with the dorsal prolongations from the muscle-plates, at least so far as my methods of investigation enable me to determine; but it appears to me probable that they in reality remain distinct, and become converted into the neural arches, while the connective-tissue cells from the muscle-plates form the adjoining subcutaneous and inter-muscular connective tissue.

All the cells of the vertebral rudiments become stellate and form typical embryonic connective-tissue. The rudiments however still retain their primitive segmentation, corresponding with that of the muscle-plates, and do not during this period acquire their secondary segmentation. Their segmentation is however less clear than it was at an earlier period, and in the dorsal part of the vertebral rudiments is mainly indicated by the dorsal nerve-roots, which always pass out in the interval between two vertebral rudiments. Vide Pl. 12, fig. 12, _pr_.

_Intermediate Cell-mass._ At about the period when the muscle-plates become completely free, a fusion takes place between the somatopleure and splanchnopleure immediately above the dorsal extremity of the true body-cavity (Pl. 11, fig. 6). The cells in the immediate neighbourhood of this fusion form a special mass, which we may call the intermediate cell-mass--a name originally used by Waldeyer for the homologous cells in the Chick. Out of it are developed the urinogenital organs and the adjoining tissues. At first it forms little more than a columnar epithelium, but by the close of the period is divided into (1) An epithelium on the free surface; from this are derived the glandular parts of the kidneys and functional parts of the genital glands; and (2) a subjacent stroma which forms the basis for the connective-tissue and vascular parts of these organs.

To the history of these parts a special section is devoted; and I now pass to the description of the mesoblast which lines the body-cavity and forms the connective tissue of the body-wall, and the muscular and connective tissue of the wall of the alimentary canal.

_Body-cavity and Parietal Plates._ By the close of stage H, as has been already mentioned, a cavity is formed between the somatopleure and splanchnopleure in the anterior part of the trunk, which rapidly widens during the succeeding stages. Anteriorly, it invests the heart, which arises during stage G, as a simple space between the ventral wall of the throat and the splanchnopleure (Pl. 11, fig. 4). Posteriorly it ends blindly.

This cavity forms in the region of the heart the rudiment of the pericardial cavity. The remainder of the cavity forms the true body-cavity.

Immediately behind the heart the alimentary canal is still open to the yolk-sac, and here naturally the two lateral halves of the body-cavity are separated from each other. In the tail of the embryo no body-cavity has appeared by stage I, although the parietal plates of mesoblast are distinctly divided into somatic and splanchnic layers. In the caudal region the lateral plates of mesoblast of the two sides do not unite ventrally, but are, on the contrary, quite disconnected. Their ventral edge is moreover much swollen (Pl. 11, fig. 1). At the caudal swelling the mesoblast plates cease to be distinctly divided into somatopleure and splanchnopleure, and more or less fuse with the hypoblast of the caudal vesicle (Pl. 11, fig. 2).

Between stages I and K the body-cavity extends backwards behind the point where the anus is about to appear, though it never reaches quite to the extreme end of the tail. The backward extension of the body-cavity, as is primitively the case everywhere, is formed of two independent lateral halves (Pl. 11, fig. 9_a_). Anteriorly, opposite the hind end of the small intestine, these two lateral halves unite ventrally to form a single cavity in which hangs the small intestine (Pl. 11, fig. 8) suspended by a very short mesentery.

The most important change which takes place in the body-cavity during this period is the formation of a septum which separates off a pericardial cavity from the true body-cavity.

Immediately in front of the liver the splanchnic and somatic walls of the body come into very close contact, and I believe unite over the greater part of their extent. The septum so formed divides the original body-cavity into an anterior section or pericardial cavity, and a posterior section or true body-cavity. There is left, however, on each side dorsally a rather narrow passage which serves to unite the pericardial cavity in front with the true body-cavity behind.

In Pl. 11, fig. 8_a_, there is seen on one side a section through this passage, while on the other side the passage is seen to be connected with the pericardial cavity.

It is not possible from transverse sections to determine for certain whether the septum spoken of is complete. An examination of longitudinal horizontal sections from an embryo belonging to the close of the stage K has however satisfied me that this septum, by that stage at any rate, is fully formed.

The two lateral passages spoken of above probably unite in the adult to form the passage connecting the pericardial with the peritoneal cavity, which, though provided with but a single orifice into the pericardial cavity, divides into two limbs before opening into the peritoneal cavity.

The body-cavity undergoes no further changes of importance till the close of the period.

_Somatopleure and Splanchnopleure._ Both the somatic and splanchnic walls of the body-cavity during stage I exhibit a simple uniform character throughout their whole extent. They are formed of columnar cells where they line the dorsal part of the body-cavity, but ventrally of more rounded and irregular cells (Pl. 11, fig. 5).

In them may occasionally be seen aggregations of very peculiar and large cells with numerous highly refracting spherules; the cells forming these are not unlike the _primitive ova_ to be described subsequently, but are probably large cells derived from the yolk.

It is during the stage intermediate between I and K that the first changes become visible which indicate a distinction between an epithelium (endothelium) lining the body-cavity and the connective tissue adjoining this.

There are at first but very few connective-tissue cells between the epithelium of the somatic layer of the mesoblast and the epiblast, but a connection between them is established by peculiar protoplasmic processes which pass from the one to the other (Pl. 11, fig. 8). Towards the end of stage K, however, there appears between the two a network of mesoblastic cells connecting them together. In the rudimentary outgrowth to form the limbs the mesoblast cells of the somatic layer are crowded in an especially dense manner.

From the first the connective-tissue cells around the hypoblastic epithelium of the alimentary tract are fairly numerous (Pl. 11, fig. 8), and by the close of this period become concentrically arranged round the intestinal epithelium, though not divided into distinct layers. A special aggregation of them is present in the hollow of the rudimentary spiral valve.

Behind the anal region the two layers of the mesoblast (somatic and splanchnic) completely fuse during stage K, and form a mass of stellate cells in which no distinction into two layers can be detected (Pl. 11, figs. 9_c_, 9_d_).

The alimentary canal, which at first lies close below the aorta, becomes during this period gradually carried further and further from this, remaining however attached to the roof of the body-cavity by a thin layer of the mesoblast of the splanchnopleure formed of an epithelium on each side, and a few interposed connective-tissue cells. This is the mesentery, which by the close of stage K is of considerable length in the region of the stomach, though shorter elsewhere.

* * * * *

The above account of the protovertebræ and body-cavity applies solely to the genera Pristiurus and Scyllium. The changes of these parts in Torpedo only differ from those of Pristiurus in unimportant though fairly noticeable details. Without entering into any full description of these it may be pointed out that both the true body-cavity and its continuations into the protovertebræ appear later in Torpedo than in Pristiurus and Scyllium. In some cases even the muscle-plates become definitely separated and independent before the true body-cavity has appeared. As a result of this the primitive continuity of the body-cavity and cavity of the muscle-plates becomes to a certain extent masked, though its presence may easily be detected by the obvious continuity which at first exists between the somatic and splanchnic layers of mesoblast and the two layers of the muscle-plate. In the muscle-plate itself the chief point to be noticed is the fact that the earlier formed bands of muscles (_mp´_) arise very much later, and are less conspicuous, in Torpedo than in the genera first described. They are however present and functional.

The anatomical relations of the body-cavity itself are precisely the same in Torpedo as in Pristiurus and Scyllium, and the pericardial cavity becomes separated from the peritoneal in the same way in all the genera; the two lateral canals connecting the two cavities being also present in all the three genera. The two independent parietal plates of mesoblast of the posterior parts of the body have ventrally a swollen edge, as in Pristiurus, and in this a cavity appears which forms a posterior continuation of the true body-cavity.

_Resumé._ The primitive independent mesoblast plates of the two sides of the body become divided into two layers, a somatic and a splanchnic (Hautfaserblatt and Darmfaserblatt). At the same time in the dorsal part of the mesoblast plate a series of transverse splits appear which mark out the limits of the protovertebræ and serve to distinguish a dorsal or vertebral part of the plate from a ventral or parietal part.

Between the somatic and splanchnic layers of the mesoblast plate a cavity arises which is continued quite to the summit of the vertebral part of the plate. This is the primitive body-cavity; and at first the cavity is divided into two lateral and independent halves.

The next change which takes place is the complete separation of the vertebral portion of the plate from the parietal; thereby the upper segmented part of the body-cavity becomes isolated and separated from the lower and unsegmented part. In connection with this change in the constitution of the body-cavity there are formed a series of rectangular plates, each composed of two layers, a somatic and a splanchnic, between which is the cavity originally continuous with the body-cavity. The splanchnic layer of the plates buds off cells to form the rudiments of the vertebral bodies which are originally segmented in the same planes as the protovertebræ. The plates themselves remain as the muscle-plates and develop a special layer of muscle (_mp´_) in their splanchnic layer.

In the meantime the parietal plates of the two sides unite ventrally throughout the intestinal and cardiac regions of the body, and the two primitively isolated cavities contained in them coalesce. Posteriorly however the plates do not unite ventrally, and their contained cavities remain distinct.

At first the pericardial cavity is quite continuous with the body-cavity; but by the close of the period included in the present chapter it becomes separated from the body-cavity by a septum in front of the liver, which is however pierced dorsally by two narrow channels.

The parts derived from the two layers of the mesoblast (not including special organs or the vascular system) are as follow:--

From the somatic layer are formed

(1) A considerable part of the voluntary muscular system of the body. (2) The dermis. (3) A large part of the intermuscular connective tissue. (4) Part of the peritoneal epithelium.

From the splanchnic layer are formed

(1) A great part of the voluntary muscular system. (2) Part of the intermuscular connective tissue (?). (3) The axial skeleton. (4) The muscular and connective-tissue wall of the alimentary tract. (5) A great part of the peritoneal epithelium.

_General Considerations._ In the history which has just been given of the development of the mesoblast, there are several points which appear to me to throw light upon the primitive origin of that layer. Before entering into these it is however necessary to institute a comparison between the history of the mesoblast in Elasmobranchii and in other Vertebrates, in order to distinguish as far as possible the primitive and the secondary characters present in the various groups.

Though the Mammals are to be looked on as the most differentiated group amongst the Vertebrates, yet in their embryonic history they retain many very primitive features, and, as has been recently shewn by Hensen[204], present numerous remarkable approximations to the Elasmobranchii. We find accordingly[205] that the primitive lateral plates of mesoblast undergo nearly the same changes in these two groups. In Mammals there is at first a continuous cavity extending through both the parietal and vertebral portions of each plate, and dividing the plates into a somatic and a splanchnic layer: this cavity is the primitive body-cavity. The vertebral portion of each plate with its contained cavity then becomes divided off from the parietal. The later development of these parts is not accurately known, but it seems that the outer portion of each vertebral plate, composed of two layers (somatic and splanchnic) enclosing between them a remnant of the primitive body-cavity, becomes separated off as a muscle-plate. The remainder forms a vertebral rudiment, &c. Thus the extension of the body-cavity into the vertebral portion of the mesoblast, and the constriction of the vertebral portion of the cavity from the remainder, are as distinctive features of Mammals as they are of the Elasmobranchii.

Footnote 204: _Zeitschrift f. Anat. Entwicklungsgeschichte_, Vol. 1.

Footnote 205: Hensen _loc. cit._

In Birds[206] the horizontal splitting of the mesoblast into somatic and splanchnic layers appears, as in Mammals, to extend at first to the summit of the protovertebræ, but these bodies become so early separated from the parietal plates that this fact has usually been overlooked or denied; but even on the second day of incubation the outer layer of the protovertebræ is continuous with the somatic layer of the lateral plates, and the inner layer and kernel of the protovertebræ with the splanchnic layer of the lateral plates[207]. After the isolation of the protovertebræ the primitive position of the split which separated their somatic and splanchnic layers becomes obscured, but when on the third day the muscle-plates are formed they are found to be _constituted of two layers, an inner and an outer, which enclose between them a central cavity_. This remarkable fact, which has not received much attention, though noticeable in most figures, receives a simple explanation as a surviving rudiment on Darwinian principles. The central cavity of the muscle-plate is, in fact, a remnant of the vertebral extension of the body-cavity, and is the same cavity as that found in the muscle-plates of Elasmobranchii. The two layers of the muscle-plate also correspond with the two layers present in Elasmobranchii, the one belonging to the somatic, the other to the splanchnic layer of mesoblast. The remainder of the protovertebræ internal to the muscle-plates is very large in Birds, and is the equivalent of that portion of the protovertebræ which in Elasmobranchii is split off to form the vertebral bodies[208] (Pl. 11, figs. 6, 7, 8, _Vr_). Thus, though the history of the development of the mesoblast is not precisely the same for Birds as for Elasmobranchii, yet the differences between the two groups are of such a character as to prove in a striking manner that the Avian development is a derivation from a more primary form, like that of the Elasmobranchii.

Footnote 206: For the history of protovertebræ and muscle-plates in Birds, vide _Elements of Embryology_, Foster and Balfour. The statement there made that the horizontal splitting of the mesoblast does not extend to the summit of the vertebral plate, must however be regarded as doubtful.

Footnote 207: Vide _Elements of Embryology_, p. 56.

Footnote 208: Dr Götte, _Entwicklungsgeschichte der Unke_, p. 534, gives a different account of the development of the protovertebræ from that in the text. He states that the muscle-plates do not give rise to the main dorso-lateral muscles, but only to some superficial ventral muscles, while the dorso-lateral muscles are according to him formed from part of the kernel of the protovertebræ internal to the muscle-plates. The account given in the text is the result of my own investigations, and accords precisely with the recent statements of Professor Kölliker, _Entwicklungsgeschichte_, 1876.

According to the statements of Bambeke and Götte, the Amphibians present rather remarkable peculiarities in the development of their muscular system. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (protovertebræ) of the plates soon become separated from the parietal, and form an independent mass of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates. The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a central kernel of cells and an inner single layer. This central kernel is the first portion of the vertebral body to undergo any change, and it becomes converted into the main dorso-lateral muscles of the body, which apparently correspond with the muscles derived from the whole muscle-plate of the Elasmobranchii. From the inner layer of the splanchnic division there are next formed the main internal ventral muscles, rectus abdominis, &c., as well as the chief connective-tissue elements of the parts surrounding the spinal cord. The outer layer of the vertebral plates forms the dermis and subcutaneous connective tissue, as well as some of the superficial muscles of the trunk and the muscles of the limbs.

Dr Götte appears to think that the vertebral plates in Amphibians present a perfectly normal development very similar to that of other Vertebrates. The divergences between Amphibians and other Vertebrates appear, however, to myself, to be very great, and although the very careful account given by Dr Götte is probably to be relied on, yet some further explanation than he has offered of the development of these parts amongst the Amphibians would seem to be required.

A primary stage in which the two layers of the vertebral plates are continuous with the somatic and splanchnic layers of the body-wall is equally characteristic of Amphibians, Elasmobranchii and Mammals. In the subsequent development, however, a great difference between the types becomes apparent, for whereas in Elasmobranchii both layers of the vertebral plates combine to form the muscle-plates, out of which the great dorso-lateral muscles are formed, in Amphibians what appear to be the equivalent muscles are derived from a few of the cells (the kernel) of the inner layer of the vertebral plates only. The cells which form the lateral muscles in Amphibians might be thought to correspond in position with the cells which become, in Elasmobranchii, converted into the special early formed band of muscles (_m.p´._), rather than, as their development seems to indicate, with the whole Elasmobranch muscle-plates[209].

Footnote 209: The type of development of the muscle-plates of Amphibians would become identical with that of Elasmobranchii if their first-formed mass of muscle corresponded with the early-formed muscles of Elasmobranchii, and the remaining cells of both layers of the protovertebræ became in the course of development converted into muscle-cells indistinguishable from those formed at first. Is it possible that, owing to the distinctness of the first-formed mass of muscle, Dr Götte can have overlooked the fact that its subsequent growth is carried on at the expense of the adjacent cells of the somatic layer?

Osseous Fishes are stated to agree with Amphibians in the development of their protovertebræ and muscular system[210], but further observations on this point are required.

Footnote 210: Ehrlich, "Ueber den peripher. Theil d. Urwirbel." _Archiv f. Mic. Anat._ Vol. XI.

Though the development of the general muscular system and muscle-plates does not, according to existing statements, take place on quite the same type throughout the Vertebrate subkingdom, yet the comparison which has been instituted between Elasmobranchii and other Vertebrates appears to prove that there are one or two common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (1) The extension of the body-cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscle-plates; (2) The primitive division of the vertebral plate into a somatic and a splanchnic layer, and the formation of a large part of the voluntary muscular system out of the splanchnic layer.

* * * * *

The ultimate derivation of the mesoblast forms one of the numerous burning questions of modern embryology, and there are advocates to be found for almost every one of the possible views the question admits of.

All who accept the doctrine of descent are agreed that primitively only two embryonic layers were present--the epiblast and the hypoblast--and that the mesoblast subsequently appeared as a distinct layer, after a certain complexity of organization had been attained.

The general agreement stops, however, at this point, and the greatest divergence of opinion exists with reference to all further questions which bear on the development of the mesoblast. There appear to be four possibilities as to the origin of this layer.

It may be derived:

(1) entirely from the epiblast, (2) partly from the epiblast, and partly from the hypoblast, (3) entirely from the hypoblast, (4) or may have no fixed origin.

The fourth of these possibilities may for the present be dismissed, since it can be only maintained should it turn out that all the other views are erroneous. The first possibility is supported by the case of the Coelenterata, and we might almost say by that of this group only[211].

Footnote 211: The most important other instances in addition to that of the Coelenterata which can be adduced in favour of the epiblastic origin of the mesoblast are the Bird and Mammal, in which according to the recent observations of Hensen for the Mammal, and Kölliker for the Mammal and Bird, the mesoblast is split off from the epiblast. If the views I have elsewhere put forward about the meaning of the primitive groove be accepted, the derivation of the mesoblast from the epiblast in these instances would be apparent rather than real, and have no deep morphological significance for the present question.

Other instances may be brought forward from various groups, but none of these are sufficiently well confirmed to be of any value in the determination of the present question.

Amongst the Coelenterata the mesoblast, when present, is unquestionably a derivative of the epiblast, and when, as is frequently the case, a distinct mesoblast is not present, the muscle-cells form a specialized part of the epidermic cells.

The condition of the mesoblast in these lowly organized animals is exactly what might _à priori_ have been anticipated, but the absence throughout the group of a true body-cavity, or specially developed muscular system of the alimentary tract, prevents the possibility of generalizing for other groups, from the condition of the mesoblast in this one.

In those animals in which a body-cavity and muscular alimentary tract are present, it would certainly appear reasonable to expect the mesoblast to be derived from both the primitive layers: the voluntary muscular system from epiblast, and the splanchnic system from the hypoblast. This view has been taken and strongly advocated by so distinguished an embryologist as Professor Haeckel, and it must be admitted, that on _à priori_ grounds there is much to recommend it; there are, however, so far as I am aware of, comparatively few observed facts in its favour.

Professor Haeckel's own objective arguments in support of his view are as follows:

(1) From the fact that some investigators derive the mesoblast with absolute confidence from the hypoblast, while others do so with equal confidence from the epiblast, he concludes that it is really derived from both these layers.

(2) A second argument is founded on the supposed derivation of the mesoblast in Amphioxus from both epiblast and hypoblast. Kowalevsky's account (on which apparently Prof. Haeckel's[212] statements are based) appears to me, however, too vague, and his observations too imperfect, for much confidence to be placed in his statements on this head. It does not indeed appear to me that the formation of the layers in Amphioxus, till better known, can be used as an argument for any special view about this question.

Footnote 212: Vide _Anthropogenie_, p. 197.

(3) Professor Haeckel's own observations on the development of Osseous fish form a third argument in support of his views. These observations do not, however, accord with those of the majority of investigators, and not having been made by means of sections, require further confirmation before they can be definitely accepted.

(4) A fourth argument rests on the fact that the various embryonic layers fuse together to form the primitive streak or axis-cord in higher vertebrates. This he thinks proves that the mesoblast is derived from both the primitive layers. The primitive streak has, however, according to my views, quite another significance to that attributed to it by Professor Haeckel[213]; but in any case Professor Kölliker's researches, and on this point my own observations accord with his, appear to me to prove that the fusion which there takes place is only capable of being used as an argument in favour of an epiblastic origin of the mesoblast, and not of its derivation from both epiblast and hypoblast.

Footnote 213: Vide Self, "Development of Elasmobranch Fishes," _Journal of Anat. and Phys._ Vol. X. note on p. 682, and also Review of Professor Kölliker's "Entwicklungsgeschichte des Menschen u. d. höheren Thiere," _Journal of Anat. and Phys._ Vol. X.

The objective arguments in favour of Professor Haeckel's views are not very conclusive, and he himself does not deny that the mesoblast as a rule apparently arises as a single and undivided mass from one of the two primary layers, and only subsequently becomes split into somatic and splanchnic strata. This original fusion and subsequent splitting of the mesoblast is explained by him as a secondary condition, a possibility which cannot by any means be thrown on one side. It seems therefore worth while examining how far the history of the somatic and splanchnic layers of the mesoblast in Elasmobranchii and other Vertebrates accords with the supposition that they were primitively split off from the epiblast and the hypoblast respectively.

It is well to consider first of all what parts of the mesoblast of the body might be expected to be derived from the somatic and splanchnic layers on this view of their origin[214].

Footnote 214: Professor Haeckel speaks of the splitting of the mesoblast in Vertebrates into a somatic and splanchnic layer as a secondary process (_Gastrula u. Eifurchung d. Thiere_), but does not make it clear whether he regards this secondary splitting as taking place along the old lines. It appears to me to be fairly certain that even if the original unsplit condition of the mesoblast is to be regarded as a secondary condition, yet that the splitting of this must take place along the old lines, otherwise a change in the position of the body-cavity in the adult would have to be supposed--an unlikely change producing unnecessary complication. The succeeding argument is based on the assumption that the unsplit condition is a secondary condition, but that the split which eventually appears in this occurs along the old lines, separating the primitive splanchnopleure from the primitive somatopleure.

From the somatic layer of the mesoblast there would no doubt be formed the whole of the voluntary muscular system of the body, the dermis, the subcutaneous connective tissue, and the connective tissue between the muscles. It is probable also, though this point is less certain, that the skeleton would be derived from the somatic layer. From the splanchnic layer would be formed the connective tissue and muscular layers of the alimentary tract, and possibly also the vascular system.

Turning to the actual development of these parts, the discrepancy between theory and fact becomes very remarkable. From the somatic layer of the mesoblast, part of the voluntary muscular system and the dermis is no doubt derived, but the splanchnic layer supplies the material, not only for the muscular wall of the digestive canal and the vascular system, but also for the whole of the axial skeleton _and a great part of the voluntary muscular system of the body, including the first-formed muscles_. Though remarkable, it is nevertheless not inconceivable, that the skeleton might be derived from the splanchnic mesoblast, but it is very difficult to understand how there could be formed from it a part of the voluntary muscular system of the body indistinguishably fused with part of the muscular system derived from the somatopleure. No fact in my investigations comes out more clearly than that a great part of the voluntary muscular system is formed from the splanchnic layer of the mesoblast, yet this fact presents a most serious difficulty to the view that the somatic and splanchnic layers of the mesoblast in Vertebrates are respectively derived from the epiblast and hypoblast.

In spite, therefore, of general _à priori_ considerations of a very convincing kind which tell in favour of the double origin of the mesoblast, this view is supported by so few objective facts, and there exists so powerful an array of facts against it, that at present, at least, it seems impossible to maintain it. The full strength of the facts against it will appear more fully in a review of the present state of our knowledge as to the development of the mesoblast in the different groups.

To this I now pass.

In a paper on the "Early stages of Development in Vertebrates[215]" a short _resumé_ was given of the development of the mesoblast throughout the animal kingdom, which it may be worth while repeating here with a few additions. So far as we know at present, the mesoblast is derived from the hypoblast in the following groups:

Echinoderms (Hensen, Agassiz, Metschnikoff, Selenka, Götte), Nematodes (Bütschli), Sagitta (Kowalevsky, Bütschli), Lumbricus and probably other Annelids (Kowalevsky), Brachiopoda (Kowalevsky), Crustaceans (Bobretzky), Insects (Kowalevsky, Ulianin, Dohrn), Myriapods (Metschnikoff), Tunicates (Kowalevsky, Kuppfer), Petromyzon (Owsjanikoff), Osseous fishes (Oellacher, Götte, Kowalevsky), Elasmobranchii (Self), Amphibians (Remak, Stricker, Götte).

Footnote 215: _Quart. Jl. of Micros. Science_, July, 1875. [This Edition, No. VI.]

The list includes members from the greater number of the groups of the animal kingdom; the most striking omissions being the Coelenterates, Mollusks, and the Amniotic Vertebrates. The absence of the Coelenterates has been already explained and my grounds for regarding the Amniotic Vertebrates as apparent rather than real exceptions have also been pointed out. The Mollusks, however, remain as a large group, in which we as yet know very little as to the formation of the mesoblast.

Dr Rabl[216], who seems recently to have studied the development of Lymnæus by means of sections, gives some figures shewing the origin of the mesoblast; they are, however, too diagrammatic to be of much service in settling the present question, and the memoirs of Professor Lankester[217] and Dr Fol[218] are equally inconclusive for this purpose, for, though they contain figures of elongated and branched mesoblast cells passing from the epiblast to the hypoblast, no satisfactory representations are given of the origin of these cells. I have myself observed in embryos of Turbo or Trochus similar elongated cells to those figured by Lankester and Fol, but was unable clearly to determine whence they arose. The most accurate observations which we have on this question are those of Professor Bobretzky[219]. In Nassa he finds that the three embryonic layers are all established during segmentation. The outermost and smallest cells form the epiblast, somewhat larger cells adjoining these the mesoblast, and the large yolk-cells the hypoblast. These observations do not, however, demonstrate from which of the primary layers the mesoblast is derived.

Footnote 216: _Jenaische Zeitschrift_, Vol. IX.

Footnote 217: _Quart. Jl. of Micros. Science_, Vol. XXV. 1874, and _Phil. Trans._ 1875.

Footnote 218: _Archives de Zoologie_, Vol. IV.

Footnote 219: _Archiv f. Micr. Anat._ Vol. XIII.

The evidence at present existing is clearly in favour of the mesoblast being, in almost all groups of animals, developed from the hypoblast, but strong as this evidence is, it has not its full weight unless the actual manner in which the mesoblast is in many groups derived from the hypoblast, is taken into consideration. The most important of these are the Echinoderms, Brachiopods and Sagitta.

In the Echinoderms the mesoblast is in part formed by cells budded off from the hypoblast, _the remainder, however, arises as one or more diverticula of the alimentary tract_. From the separate cells first budded off there are formed the cutis, part of the connective tissue and the calcareous skeleton[220]. The diverticula from the alimentary cavity form the water-vascular system and the somatic and splanchnic layers of mesoblast. _The cavity of the diverticula after the separation of the water-vascular system, forms the body-cavity. The outer lining layer of the cavity forms the somatic layer of mesoblast and the voluntary muscles; the inner lining layer the splanchnic mesoblast which unites with the epithelium of the alimentary tract._ Though this fundamental arrangement would seem to be universal amongst Echinoderms, considerable variations of it are exhibited in different groups.

Footnote 220: The recent researches of Selenka, _Zeitschrift f. Wiss. Zoologie_, Vol. XXVII. 1876, demonstrate that in Echinoderms the muscles are derived from the cells first split off from the hypoblast, and that the diverticula only form the water-vascular system and the epithelial lining of the body-cavity.

There is _one_ outgrowth from the alimentary tract in Synapta; _two_ in Echinoids, Asteroids and Ophiura; _three_ in Comatula, and four (?) in Amphiura. The cavity of the outgrowth usually forms the body-cavity, but sometimes in Ophiura and Amphiura (Metschnikoff) the outgrowths are from the first or soon become solid, and only secondarily acquire a cavity, which is however homologous with the body-cavity of the other groups.

In Sagitta[221] the formation of the mesoblast and the alimentary tract takes place in nearly the same fashion as in the Echinoderms. The simple invaginate alimentary cavity becomes divided into three lobes, a central and two lateral. The two lateral lobes are gradually more and more constricted off from the central one, and become eventually quite separated from it; their cavities remain independent, _and form in the adult the body-cavity_, divided by a mesentery into two distinct lateral sections. _The inner layer of each of the two lateral lobes forms the mesoblast of the splanchnopleure, the outer layer the mesoblast of the somatopleure._ The central division of the primitive gastræa cavity remains as the alimentary tract of the adult.

Footnote 221: Kowalevsky, "Würmer u. Arthropoden," _Mém. Acad. Pétersbourg_, 1871.

The remarkable observations of Kowalevsky[222] on the development of the Brachiopoda have brought to light the unexpected fact that in two genera at least (Argiope and Terebratula) the mesoblast and body-cavity develop as paired constrictions from the alimentary tract in a manner almost identically the same as in Sagitta.

Footnote 222: "Zur Entwicklungsgeschichte d. Brachiopoden", Protokoll d. ersten Session der Versammlung Russischer Naturforscher in Kasan, 1873. Published in _Kaiserliche Gesellschaft Moskau_, 1874 (Russian). Abstracted in Hoffmann and Schwalbe, _Jahresbericht f._ 1873.

It thus appears that, so far as can be determined from the facts at our disposal, the mesoblast in almost all cases is derived from the hypoblast, and in three widely separated groups it arises as a pair of diverticula from the alimentary tract, each diverticulum containing a cavity which eventually becomes the body-cavity. I have elsewhere suggested[223] that the origin of the mesoblast from alimentary diverticula is to be regarded as primitive for all higher animals, and that the more general cases in which the mesoblast becomes split off, as an undivided layer, from the hypoblast, are in reality derivates from this. The chief obstacle in the way of this view arises from the difficulty of understanding how the whole voluntary muscular system can have been derived at first from the alimentary tract. That part of a voluntary system of muscles might be derived from the contractile diverticula of the alimentary canal attached to the body-wall is not difficult to understand, but it is not easy to believe that the secondary system so formed could completely replace the primitive muscular system, derived, as it must have been, from the epiblast. In my paper above quoted will be found various speculative suggestions for removing this difficulty, which I do not repeat here. If it be granted, however, that in Sagitta, Brachiopods, and Echinoderms we have genuine examples of the formation of the whole mesoblast from alimentary diverticula, it is easy to see how the formation of the mesoblast in Vertebrates may be a secondary derivate from an origin of this nature.

Footnote 223: Comparison of Early Stages, _Quart. Jl. Micros. Science_, July, 1875. [This Edition, No. VI.]

An attempt has been already made to shew that the mesoblast in Elasmobranchii is formed in a very primitive fashion, and for this reason the Elasmobranchii appear to be especially adapted for determining whether any signs are exhibited of a derivation of the mesoblast as paired diverticula of the alimentary tract. There are, it appears to me, several such features. In the first place, the mesoblast is split off from the hypoblast not as a single mass but as a pair of distinct masses, comparable with the paired diverticula already alluded to. Secondly, the body-cavity when it appears in the mesoblast plates, _does not arise as a single cavity, but as a pair of cavities, one for each plate of mesoblast_, and these cavities remain permanently distinct in some parts of the body, and nowhere unite till a comparatively late period. Thirdly, the primitive body-cavity of the embryo is not confined to the region in which a body-cavity exists in the adult, _but extends to the summit of the muscle-plates_, at first separating parts which become completely fused in the adult to form the great lateral muscles of the body. It is difficult to understand how the body-cavity could have such an extension as this, on the supposition that it represents a primitive split in the mesoblast between the wall of the gut and the body-wall; but its extension to this part is quite intelligible, on the supposition that it represents the cavities of two diverticula of the alimentary tract, from whose muscular walls the voluntary muscular system has been derived. Lastly, I would point out that the derivation of part of the muscular system from what appears as the splanchnopleure is quite intelligible on the assumed hypothesis, but, as far as I see, on no other.

Such are the main features presented by the mesoblast in Elasmobranchii, which favour the view of its having originally formed the walls of the alimentary diverticula. Against this view of its nature are the facts (1) of the mesoblast plates being at first solid, and (2), as a consequence of this, of the body-cavity never communicating with the alimentary canal. These points, in view of our knowledge of embryological modifications, cannot be regarded as great difficulties to my view. We have many examples of organs, which, though in most cases arising as involutions, yet appear in other cases as solid ingrowths. Such examples are afforded by the optic vesicle, auditory vesicle, and probably also by the central nervous system, of Osseous Fish. In most Vertebrates these organs are formed as hollow involutions from the exterior; in Osseous Fish, however, as solid involutions, in which a cavity secondarily appears.

The segmental duct of Elasmobranchii or the Wolffian duct (segmental duct) of Birds are cases of a similar kind, being organs which must originally have been formed as hollow involutions, but which now arise as solid bodies.

Only one more instance of this kind need be cited, taken from the Echinoderms.

The body-cavity and the mesoblast investing it arise in the case of most Echinoderms as hollow involutions of the alimentary tract, but in some exceptional groups, Ophiura and Amphiura, are stated to be solid at first and only subsequently to become hollow. Should the accuracy of Metschnikoff's account of this point be confirmed, an almost exact parallel to what has been supposed by me to have occurred with the mesoblast in Elasmobranchii, and other groups, will be supplied.

The tendency of our present knowledge appears to be in favour of regarding the body-cavity in Vertebrates as having been primitively the cavity of alimentary diverticula, and the mesoblast as having formed the walls of the diverticula.

This view, to say the least of it, suits the facts which we know far better than any other theory which has been proposed, and though no doubt the _à priori_ difficulties in its way are very great, yet it appears to me to be sufficiently strongly supported to deserve the attention of investigators. In the meantime, however, our knowledge of invertebrate embryology is so new and imperfect that no certainty on a question like that which has just been discussed can be obtained; and any generalizations made at present are not unlikely to be upset by the discovery of fresh facts.

The only other point in connection with the mesoblast which I would call attention to is the formation of the vertebral bodies.

My observations confirm those of Remak and Gegenbaur, shewing that there is a primary segmentation of the vertebral bodies corresponding to that of the muscle-plates, followed by a secondary segmentation in which the central lines of the vertebral bodies are opposite the partitions between the muscle-plates.

The explanation of these changes is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which must have been present at a time when the vertebral bodies had no existence. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments.

The conditions under which the lateral muscles can cause the flexure of the vertebral column are clearly that each muscle-segment shall be capable of acting on two vertebræ; and this condition can only be fulfilled when the muscle-segments are opposite the intervals between the vertebræ. Owing to this necessity, when the vertebral segments became formed, their centres corresponded, not with the centres of the muscle-plates, but with the inter-muscular septa.

These considerations fully explain the secondary segmentation of the vertebræ by which they become opposite the inter-muscular septa. On the other hand, the primary segmentation is clearly a remnant of the time when no vertebral bodies were present, and has no greater morphological significance than the fact that the cells to form the unsegmented investment of the notochord were derived from the segmented muscle-plates, and only secondarily became fused into a continuous tube.

_The Urinogenital System._

The first traces of the urinary system become visible at about the time of the appearance of the third visceral cleft. At about this period the somatopleure and splanchnopleure become more or less fused together at the level of the dorsal aorta, and thus, as has been already mentioned, each of the original plates of mesoblast becomes divided into a vertebral plate and lateral plate (Pl. 11, fig. 6). The mass of cells resulting from this fusion corresponds with Waldeyer's intermediate cell-mass in the Fowl.

At about the level of the fifth protovertebra the first trace of the urinary system appears.

From the intermediate cell-mass a solid knob grows outwards towards the epiblast (woodcut, fig. 4, _pd_). This knob consists at first of 20-30 cells, which agree in character with the neighbouring cells of the intermediate cell-mass, and are at this period rounded. It is mainly, if not entirely, derived from the somatic layer of the mesoblast.

From this knob there grows backwards a solid rod of cells which keeps in very close contact with the epiblast, and rapidly diminishes in size towards its posterior extremity. Its hindermost part consists in section of at most one or two cells. It keeps so close to the epiblast that it might be supposed to be derived from that layer were it not for the sections shewing its origin from the knob above mentioned. We have in this rod the commencement of what I have elsewhere[224] called the segmental duct.

Footnote 224: "Urinogenital Organs of Vertebrates," _Journ. of Anat. and Phys._ Vol. X. [This Edition, No. VII.]

My observations shew that the segmental duct is developed in the way just described in both Pristiurus and Torpedo. Its origin in Pristiurus is shewn in the adjoining woodcut, and in Torpedo in Pl. 11, fig. 7, _sd_.

At a stage somewhat older than I, the condition of the segmental duct has not very materially altered. It has increased considerably in length, and the knob at its front end is both absolutely smaller, and also consists of fewer cells than before (Pl. 11, fig. 7, _sd_). These cells have become more columnar, and have begun to arrange themselves radially; thus indicating the early appearance of the lumen of the duct. The cells forming the front part of the rod, as well as those of the knob, commence to exhibit a columnar character, but in the hinder part of the rod the cells are still rounded. In no part of it has a lumen appeared.

At this period also the knob, partly owing to the commencing separation of the muscle-plate from the remainder of the mesoblast, begins to pass inwards and approach the pleuro-peritoneal cavity.

At the same stage the first not very distinct traces of the remainder of the urinary system become developed. These appear in the form of solid outgrowths from the intermediate cell-mass just at the most dorsal part of the body-cavity.

The outgrowths correspond in numbers with the vertebral segments, and are at first quite disconnected with the segmental duct. At this stage they are only distinctly visible in the first few segments behind the front end of the segmental duct. A full description of them will come more conveniently in the next stage.

By a stage somewhat earlier than K important changes have taken place in the urinary system.

The segmental duct has acquired a lumen in its anterior portion, which opens at its front end into the body-cavity. (Pl. 11, fig. 9, _sd._) The lumen is formed by the columnar cells spoken of in the last stage, acquiring a radiating arrangement round a central point, at which a small hole appears. After the lumen has once become formed, it rapidly increases in size.

The duct has also grown considerably in length, but its hind extremity is still as thin, and lies as close to the epiblast, as at first. The segmental involutions which commenced to be formed in the last stage, have now appeared for every vertebral segment along the whole length of the segmental duct, and even for two or three segments behind this.

They are simple independent outgrowths arising from the outer and uppermost angle of the body-cavity, and are at first almost without a trace of a lumen; though their cells are arranged as two layers. They grow in such a way as to encircle the oviduct on its inner and upper side (Pl. 11, fig. 8 and Pl. 12, fig. 14_b_, _st_). When the hindermost ones are formed, a slight trace of a lumen is perhaps visible in the front ones. At a stage slightly subsequent to this, in Scyllium canicula, I noticed 29 of them; the first of them arising in the segment immediately behind the front end of the oviduct (Pl. 12, fig. 17, _st_), and two of them being formed in segments just posterior to the hinder extremity of the oviduct.

Pl. 12, figs. 16 and 18 represent two longitudinal sections shewing the segmental nature of the involutions and their relation to the segmental duct.

Many of the points which have been mentioned can be seen by referring to Pl. 11 and 12. Anteriorly the segmental duct opens into the pleuro-peritoneal cavity. In the sections behind this there may be seen the segmental duct with a distinct lumen, and also a pair of segmental involutions (Pl. 12, fig. 14_a_). In the still posterior sections the segmental duct would be quite without a lumen, and would closely adjoin the epiblast.

It seems not out of place to point out that the modes of the development of the segmental duct and of the segmental involutions are strikingly similar. Both arise as solid involutions, from homologous parts of the mesoblast. The segmental duct arises in the vertebral segment immediately in front of that in which the first segmental involution appears; _so that the segmental duct appears to be equivalent to a single segmental involution_.

The next stage corresponds with the first appearance of the external gills. The segmental duct now communicates by a wide opening with the body-cavity (Pl. 11, fig. 9, _sd_). It possesses a lumen along its whole length up to the extreme hind end (Pl. 11, fig. 9_a_). It is, however, at this hinder extremity that the most important change has taken place. This end has grown downwards towards that part of the alimentary canal which still lies behind the anus. This downgrowth is beginning to shew distinct traces of a lumen, and will appear in the next stage as one of the horns by which the segmental ducts communicate with the cloaca (Pl. 11, fig. 9_b_). All the anterior segmental involutions have now acquired a lumen. But this is still absent in the posterior ones (Pl. 11, fig. 9_a_).

Owing to the disappearance of the body-cavity in the region behind the anus, the primitive involutions there remain as simple masses of cells still disconnected with the segmental duct (Pl. 11, figs. 9_b_, 9_c_ and 9_d_).

_Primitive Ova._ The true generative products make their first appearance as the _primitive ova_ between stages I and K.

In the sections of one of my embryos of this stage they are especially well shewn, and the following description is taken from those displayed in that embryo.

They are confined to the region which extends posteriorly nearly to the end of the small intestine and anteriorly to the abdominal opening of the segmental duct.

Their situation in this region is peculiar. There is no trace of a distinct genital ridge, but the ova mainly lie in the dorsal portion of the mesentery, and therefore in a part of the mesoblast which distinctly belongs to the splanchnopleure (Pl. 12, fig. 14_a_). Some are situated external to the segmental involutions; and others again, though this is not common, in a part of the mesoblast which distinctly belongs to the body-wall (Pl. 12, fig. 14_b_).

The portion of mesentery, in which the primitive ova are most densely aggregated, corresponds to the future position of the genital ridge, but the other positions occupied by ova are quite outside this. Some ova are in fact situated on the outside of the segmental duct and segmented tubes, and must therefore effect a considerable migration before reaching their final positions in the genital ridge on the inner side of the segmental duct (Pl. 12, fig. 14_b_).

The condition of the tissue in which the ova appear may at once be gathered from an examination of the figures given. It consists of an irregular epithelium of cells partly belonging to the somatopleure and partly to the splanchnopleure, but passing uninterruptedly from one layer to the other. The cells which compose it are irregular in shape, but frequently columnar (Pl. 12, figs. 14_a_ and 14_b_).

They are formed of a nucleus which stains deeply, invested by a _very delicate_ layer of protoplasm. At the junction of somatopleure and splanchnopleure they are more rounded than elsewhere. Very few loose connective-tissue cells are present. The cells just described vary from .008 Mm. to .01 Mm. in diameter.

The primitive ova are situated amongst them and stand out with extraordinary clearness, to which justice is hardly done in my figures.

The normal full-sized ova exhibit the following structure. They consist of a mass of somewhat granular protoplasm of irregular, but more or less rounded, form. Their size varies from .016 - .036 Mm. In their interior a nucleus is present, which varies from .012 - .016 Mm., but its size as a rule bears _no_ relation to the size of the containing cell.

This is illustrated by the subjoined list of measurements.

Size of Primitive ova in Size of nucleus of Primitive degrees of micrometer scale ova in degrees of micrometer with F. ocul 2. scale with F. ocul 2.

10 8 13 8 13 8 14 7 15 7 13 7-1/2 11 8 16 5-1/2 12 7 10 7 15 6 13 6 12 7

The numbers given refer to degrees on my micrometer scale.

Since it is the ratio alone which it is necessary to call attention to, the numbers are not reduced to decimals of a millimeter. Each degree of my scale is equal, however, with the object glass employed, to .002 Mm.

This series brings out the result I have just mentioned with great clearness.

In one case we find a cell has three times the diameter of the nucleus 16 : 5-1/2; in another case 10 : 8, the nucleus has only a slightly smaller diameter than the cell. The irrationality of the ratio is fairly shewn in some of my figures, though none of the largest cells with very small nuclei have been represented.

The nuclei are granular, and stain fairly well with hæmatoxylin. They usually contain a single deeply stained nucleolus, but in many cases, especially where large (and this independently of the size of the cell), they contain two nucleoli (Pl. 12, figs. 14_c_ and 14_d_), and are at times so lobed as to give an apparent indication of commencing division.

A multi-nucleolar condition of the nuclei, like that figured by Götte[225], does not appear till near the close of embryonic life, and is then found equally in the large ova and in those not larger than the ova which exist at this early date.

Footnote 225: _Entwicklungsgeschichte der Unke_, Pl. 1, fig. 8.

As regards the relation of the primitive ova to each other and the neighbouring cells, there are a few points which deserve attention. In the first place, the ova are, as a rule, collected in masses at particular points, and not distributed uniformly (fig. 14_a_). The masses in some cases appear as if they had resulted from the division of one primitive ovum, but can hardly be adduced as instances of a commencing coalescence; since if the ova thus aggregated were to coalesce, an ovum would be produced of a very much greater size than any which is found during the early stages. Though at this stage no indication is present of such a coalescence of cells to form ova as is believed to take place by Götte, still the origin of the primitive ova is not quite clear. One would naturally expect to find a great number of cells intermediate between primitive ova and ordinary columnar cells. Cells which may be intermediate are no doubt found, but not nearly so frequently as might have been anticipated. One or two cells are shewn in Pl. 12, fig. 14_a_, _x_, which are perhaps of an intermediate character; but in most sections it is not possible to satisfy oneself that any such intermediate cells are present.

In one case what appeared to be an intermediate cell was measured, and presented a diameter of .012 Mm. while its nucleus was .008 Mm. Apart from certain features of the nucleus, which at this stage are hardly very marked, the easiest method of distinguishing a primitive ovum from an adjacent cell is the presence of a large quantity of protoplasm around the nucleus. The nucleus of one of the smallest primitive ova is not larger than the nucleus of an ordinary cell (being about .008 Mm. in both). It is perhaps the similarity in the size of the nuclei which renders it difficult at first to distinguish developing primitive ova from ordinary cells. Except with the very thinnest sections a small extra quantity of protoplasm around a nucleus might easily escape detection, and the developing cell might only become visible when it had attained to the size of a small typical primitive ovum.

It deserves to be noticed that the nuclei even of some of the largest primitive ova scarcely exceed the surrounding nuclei in size. This appears to me to be an argument of some weight in shewing that the great size of primitive ova is not due to the fact of their having been formed by a coalescence of different cells (in which case the nucleus would have increased in the same proportion as the cell); but to an increase by a normal method of growth in the protoplasm around the nucleus.

It appears to me to be a point of great importance certainly to determine whether the primitive ova arise by a metamorphosis of adjoining cells, or may not be introduced from elsewhere. In some of the lower animals, _e.g._ Hydrozoa, there is no question that the ova are derived from the epiblast; we might therefore expect to find that they had the same origin in Vertebrates. Further than this, ova are frequently capable in a young state of executing amoeboid movements, and accordingly of migrating from one layer to another. In the Elasmobranchii the primitive ova exhibit in a hardened state an irregular form which might appear to indicate that they possess a power of altering their shape, a view which is further supported by some of them being at the present stage situated in a position very different from that which they eventually occupy, and which they can only reach by migration. If it could be shewn that there were no intermediate stages between the primitive ova and the adjoining cells (their migratory powers being admitted) a strong presumption would be offered in favour of their having migrated from elsewhere to their present position. In view of this possibility I have made some special investigations, which have however led to no very satisfactory results. There are to be seen in the stages immediately preceding the present one, numerous cells in a corresponding position to that of the primitive ova, which might very well be intermediate between the primitive ova and ordinary cells, but which offer no sufficiently well marked features for a certain determination of their true nature.

In the particular embryo whose primitive ova have been described these bodies were more conspicuous than in the majority of cases, but at the same time they presented no special or peculiar characters.

In a somewhat older embryo of Scyllium the cells amongst which the primitive ova lay had become very distinctly differentiated as an epithelium (the germinal epithelium of Waldeyer) well separated by what might almost be called a basement membrane from the adjoining connective-tissue cells. Hardly any indication of a germinal ridge had appeared, but the ova were more definitely confined than in previous embryos to the restricted area which eventually forms this. The ova on the average were somewhat smaller than in the previous cases.

In several embryos intermediate in age between the embryo whose primitive ova were described at the commencement of this section and the embryo last described, the primitive ova presented some peculiarities, about the meaning of which I am not quite clear, but which may perhaps throw some light on the origin of these bodies.

Instead of the protoplasm around the nucleus being clear or slightly granular, as in the cases just described, it was filled in the most typical instances with numerous highly refracting bodies resembling yolk-spherules. In osmic acid specimens (Pl. 12, fig. 15) these stain very darkly, and it is then as a rule very difficult to see the nucleus; in specimens hardened in picric acid and stained with hæmatoxylin these bodies are stained of a deep purple colour, but the nucleus can in most cases be distinctly seen. In addition to the instances in which the protoplasm of the ova is quite filled with these bodies, there are others in which they only occupy a small area adjoining the nucleus (Pl. 12, fig. 15_a_), and finally some in which only one or two of these bodies are present. The protoplasm of the primitive ova appears in fact to present a series of gradations between a state in which it is completely filled with highly refracting spherules and one in which these are completely absent.

This state of things naturally leads to the view that the primitive ova, when they are first formed, are filled with these spherules, which are probably yolk-spherules, but that they gradually lose them in the course of development. Against this interpretation is the fact that the primitive ova in the younger embryo first described are completely without these bodies; this embryo however unquestionably presented an abnormally early development of the ova; and I am satisfied that embryos present considerable variations in this respect.

If the primitive ova are in reality in the first instance filled with yolk-spherules, the question arises as to whether, considering that they are the only mesoblast cells filled at this period with yolk-spherules, we must not suppose that they have migrated from some peripheral part of the blastoderm into their present position. To this question I can give no satisfactory answer. Against a view which would regard the spherules in the protoplasm as bodies which appear subsequently to the first formation of the ova, is the fact that hitherto no instances in which these spherules were present have been met with in the late stages of development; and they seem therefore to be confined to the first stages.

_Notochord._

The changes undergone by the notochord during this period present considerable differences according to the genus examined. One type of development is characteristic of Scyllium and Pristiurus; a second type, of Torpedo.

My observations being far more complete for Scyllium and Pristiurus than for Torpedo, it is to the two former genera only that the following account applies, unless the contrary is expressly stated. Only the development of the parts of the notochord in the trunk are here dealt with; the cephalic section of the notochord is treated of in a subsequent section.

During stage G the notochord is composed of flattened cells arranged vertically, rendering the histological characters of the notochord difficult to determine in transverse sections. In longitudinal sections, however, the form and arrangement of the cells can be recognised with great ease. At the beginning of stage G each cell is composed of a nucleus invested by granular protoplasm frequently vacuolated and containing in suspension numerous yolk-spherules. It is difficult to determine whether there is only one vacuole for each cell, or whether in some cases there may not be more than one.

Round the exterior of the notochord there is present a distinct though delicate cuticular sheath.

The vacuoles are at first small, but during stage G rapidly increase in size, while at the same time the yolk-spherules completely vanish from the notochord.

As a result of the rapid growth of the vacuoles, the nuclei, surrounded in each case by a small amount of protoplasm, become pushed to the centre of the notochord, the remainder of the protoplasm being carried to the edge. The notochord thus becomes composed during stages H and I (Pl. 11, fig. 4-6) of a central area mainly formed of nuclei with a small quantity of protoplasm around them, and of a thin peripheral layer of protoplasm without nuclei, the widish space between the two being filled with clear fluid. The exterior of the cells is indurated, so that they may be said to be invested by a membrane[226]; the cells themselves have a flattened form, and each extends from the edge to the centre of the notochord, the long axis of each being rather greater than half the diameter of the cord.

Footnote 226: This membrane is better looked upon, as is done by Gegenbaur and Götte, as intercellular matter.

The nuclei of the notochord are elliptical vesicles, consisting of a membrane filled with granular contents, amongst which is situated a distinct nucleolus. They stain deeply with hæmatoxylin. Their long diameter in Scyllium is about 0.02 Mm.

The diameter of the whole notochord in Pristiurus during stage I is about 0.1 Mm. in the region of the back, and about 0.08 Mm. near the posterior end of the body.

Owing to the form of its constituent cells, the notochord presents in transverse sections a dark central area surrounded by a lighter peripheral one, but its true structure cannot be unravelled without the assistance of longitudinal sections. In these (Pl. 12, fig. 10) the nuclei form an irregular double row in the centre of the cord. Their outlines are very clear, but those of the individual cells cannot for certain be made out. It is, however, easy to see that the cells have a flattened and wedge-shaped form, with the narrow ends overlapping and interlocking at the centre of the notochord.

By the close of stage I the cuticular sheath of the notochord has greatly increased in thickness.

During the period intermediate between stages I and K the notochord undergoes considerable transformations. Its cells cease to be flattened, and become irregularly polygonal, and appear but slightly more compressed in longitudinal sections than in transverse ones. The vacuolation of the cells proceeds rapidly, and there is left in each cell only a very thin layer of protoplasm around the nucleus. Each cell, as in the earlier stages, is bounded by a membrane-like wall.

Accompanying these general changes special alterations take place in the distribution of the nuclei and the protoplasm. The nuclei, accompanied by protoplasm, gradually leave the centre and migrate towards the periphery of the notochord. At the same time the protoplasm of the cells forms a special layer in contact with the investing sheath.

The changes by which this takes place can easily be followed in longitudinal sections. In Pl. 12, fig. 11 the migration of the nuclei has commenced. They are still, however, more or less aggregated at the centre, and very little protoplasm is present at the edges of the notochord. The cells, though more or less irregularly polygonal, are still somewhat flattened. In Pl. 12, fig. 12 the notochord has made a further progress. The nuclei now mainly lie at the side of the notochord, where they exist in a somewhat shrivelled state, though still invested by a layer of protoplasm.

A large portion of the protoplasm of the cord forms an almost continuous layer in close contact with the sheath, which is more distinctly visible in some cases than in others.

While the changes above described are taking place the notochord increases in size. At the age of fig. 11 it is in the anterior part of the body of Pristiurus about 0.11 Mm. At the age of fig. 12 it is in the same species 0.12 Mm., while in Scyllium stellare it reaches about 0.17 Mm.

During stage K (Pl. 11, fig. 8) the vacuolation of the cells of the notochord becomes even more complete than during the earlier stages, and in the central cells hardly any protoplasm is present, though a starved nucleus surrounded by a little protoplasm may be found in an occasional corner.

The whole notochord becomes very delicate, and can with great difficulty be conserved whole in transverse sections.

The layer of protoplasm which appeared during the last stage on the inner side of the cuticular membrane of the notochord becomes during the present stage a far thicker and more definite structure. It forms a continuous layer with irregular prominences on its inner surface; and contains numerous nuclei. The layer sometimes presents in transverse sections hardly any indication of a division into a number of separate cells, but in longitudinal sections this is generally very obvious. The cells are directed very obliquely forwards, and consist of an oblong nucleus invested by protoplasm. The layer formed by them is very delicate and very easily destroyed. In one example its thickness varied from .004 to .006 Mm., in another it reached .012 Mm. The thickness of the cuticular membrane is about .002 Mm. or rather less.

The diameter of a notochord in the anterior part of the body of a Pristiurus embryo of this stage is about 0.21 Mm. Round the exterior of the notochord the mesoblast cells are commencing to arrange themselves as a special sheath.

In Torpedo the notochord at first presents the same structure as in Pristiurus, _i.e._ it forms a cylindrical rod of flattened cells.

The vacuolation of these cells does not however commence till a relatively very much later period than in Pristiurus, and also presents a very different character (Pl. 11, fig. 7).

The vacuoles are smaller, more numerous, and more rounded than in the other genera, and there can be no question that in many cases there is more than one vacuole in a cell. The most striking point in which the notochord of Torpedo differs from that of Pristiurus consists in the fact that in Torpedo there is never any aggregation of the nuclei at the centre of the cord, but the nuclei are always distributed uniformly through it. As the vacuolation proceeds the differences between Torpedo and the other genera become less and less marked. The vacuoles become angular in form, and the cells of the cord cease to be flattened, and become polygonal.

At my final stage for Torpedo (slightly younger than K) the only important feature distinguishing the notochord from that of Pristiurus, is the absence of any signs of nuclei or protoplasm passing to the periphery. Around the exterior of the cord there is early found in Torpedo a special investment of mesoblastic cells.

EXPLANATION OF PLATES 11 AND 12.

COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary tract. _an._ Point where anus will be formed. _ao._ Dorsal aorta. _ar._ Rudiment of anterior root of spinal nerve. _b._ Anterior fin. _c._ Connective-tissue cells. _cav._ Cardinal vein. _ch._ Notochord. _df._ Dorsal fin. _ep._ Epiblast. _ge._ Germinal epithelium. _ht._ Heart. _l._ Liver. _mp._ Muscle-plate. _mp´._ Early formed band of muscles from the splanchnic layer of the muscle-plates. _nc._ Neural canal. _p._ Protoplasm from yolk in the alimentary tract. _pc._ Pericardial cavity. _po._ Primitive ovum. _pp._ body-cavity. _pr._ Rudiment of posterior root of spinal nerve. _sd._ Segmental duct. _sh._ Cuticular sheath of notochord. _so._ Somatic layer of mesoblast. _sp._ Splanchnic layer of mesoblast. _spc._ Spinal cord. _sp.v._ Spiral valve. _sr._ Interrenal body. _st._ Segmental tube. _sv._ Sinus venosus. _ua._ Umbilical artery. _um._ Umbilical cord. _uv._ Umbilical vein. _V._ Splanchnic vein. _v._ Blood-vessel. _vc._ Visceral cleft. _Vr._ Vertebral rudiment. _W._ White matter of spinal cord. _x._ Subnotochordal rod (except in fig. 14_a_). _y._ Passage connecting the neural and alimentary canals.

PLATE 11.

Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews (1) the constriction of the subnotochordal rod (_x_) from the summit of the alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the ventral thickening of the parietal plate.

Fig. 1_a_. Portion of alimentary wall of the same embryo, shewing the formation of the subnotochord rod (_x_).

Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1.

It shews the bilobed condition of the alimentary vesicle and the fusion of the mesoblast and hypoblast at the caudal vesicle.

Fig. 3_a_. Sections from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Picric acid specimen.

It shews the communication which exists posteriorly between the neural and alimentary canals, and also by comparison with 3_b_ it exhibits the dilatation undergone by the alimentary canal in the caudal vesicle.

Fig. 3_b_. Section from the caudal region of an embryo slightly younger than 3_a_. Zeiss C, ocul. 1. Osmic acid specimen.

Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews the formation of the heart (_ht_) as a cavity between the splanchnopleure and the wall of the throat.

Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews the general features of an embryo of stage H, more especially the relations of the body-cavity in the parietal and vertebral portions of the lateral plate, and the early-formed band of muscle (_mp´_) in the splanchnic layer of the vertebral plate.

Fig. 6. Section from the oesophageal region of Scyllium embryo belonging to stage I. Zeiss C, ocul. 1. Chromic acid specimen.

It shews the formation of the rudiments of the posterior nerve-roots (_pr_) and of the vertebral rudiments (_Vr_).

Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. Zeiss C, ocul. 1, reduced 1/3. Osmic acid specimen.

It shews (1) the formation of the anterior and posterior nerve-roots. (2) The solid knob from which the segmental duct (_sd_) originates.

Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a stage intermediate between I and K. Zeiss C, ocul. 1. Chromic acid specimen.

It illustrates the structure of the primitive ova, segmental tubes, notochord, etc.

Fig. 8_a_. Section from the caudal region of an embryo of the same age as 8. Zeiss A, ocul. 1.

It shews (1) the solid oesophagus. (2) The narrow passage connecting the pericardial (_pc_) and body cavities (_pp_).

Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul. 1. Osmic acid specimen.

It shews the formation of the liver (_l_), the structure of the anterior fins (_b_), and the anterior opening of the segmental duct into the body-cavity (_sd_).

Figs. 9_a_, 9_b_, 9_c_, 9_d_. Four sections through the anterior region of the same embryo as 9. Osmic acid specimens.

The sections shew (1) the atrophy of the post-anal section of the alimentary tract (9_b_, 9_c_, 9_d_). (2) The existence of the segmental tubes behind the anus (9_b_, 9_c_, 9_d_). With reference to these it deserves to be noted that the segmental tubes behind the anus are quite disconnected, as is proved by the fact that a tube is absent on one side in 9_c_ but reappears in 9_d_. (3) The downward prolongation of the segmental duct to join the posterior or cloacal extremity of the alimentary tract (9_b_).

PLATE 12.

Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of stage H. Zeiss C, ocul. 1. Reduced by 1/3. Picric acid specimen.

It shews (1) the structure of the notochord; (2) the appearance of the early formed band of muscles (_mp´_) in the splanchnic layer of the protovertebra.

Fig. 11. Longitudinal and horizontal sections of an embryo belonging to stage I. Zeiss C, ocul. 1. Chromic acid specimen. It illustrates the same points as the previous section, but in addition shews the formation of the rudiments of the vertebral bodies (_Vr_) which are seen to have the same segmentation as the muscle-plates.

Fig. 12.[227] Longitudinal and horizontal section of an embryo belonging to the stage intermediate between I and K. Zeiss C, ocul. 1. Osmic acid specimen illustrating the same points as the previous section.

Footnote 227: The apparent structure in the sheath of the notochord in this and the succeeding figure is merely the result of an attempt on the part of the engraver to represent the dark colour of the sheath in the original figure.

Fig. 13. Longitudinal and horizontal section of an embryo belonging to stage K. Zeiss C, ocul. 1, and illustrating same points as previous section.

Figs. 14_a_, 14_b_, 14_c_, 14_d_. Figures taken from preparations of an embryo of an age intermediate between I and K, and illustrating the structure of the primitive ova. Figs. 14_a_ and 14_b_ are portions of transverse sections. Zeiss C, ocul. 3 reduced 1/3. Figs. 14_c_ and 14_d_ are individual ova, shewing the lobate form of nucleus. Zeiss F, ocul. 2.

Fig. 15. Osmic acid preparation of primitive ova belonging to stage K. Zeiss immersion No. 2, ocul. 1. The protoplasm of the ova is seen to be nearly filled with bodies resembling yolk-spherules: and one ovum is apparently undergoing division.

Fig. 15_a_. Picric acid preparation shewing a primitive ovum partially filled with bodies resembling yolk-spherules.

Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging to stage K. Zeiss A, ocul. 1. Picric acid preparation. The connective-tissue cells are omitted.

The section shews that there is one segmental tube to each vertebral segment.

Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a transparent object.

It shews the segmental duct and the segmental involutions--two of which are seen to belong to segments behind the end of the alimentary tract.

Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging to stage K. Zeiss A, ocul. 1. Hardened in a mixture of osmic and chromic acid. It shews

(1) the commissures connecting together the posterior roots of the spinal nerves;

(2) the junction of the anterior and posterior roots;

(3) the relations of the segmental ducts to the segmental involutions and the alternation of calibre in the segmental tube;

(4) the germinal epithelium lining the body-cavity.