Chapter VII: Introduction: 1 (6)

Another point to be noticed is the different relation of epiblast cells to the hypoblast cells at the upper and lower side of the mouth of the involution. Above it, on its dorsal side, the epiblast and hypoblast are continuous with one another. On its ventral side they are primitively not so continuous. This is due to the epiblast, as was before mentioned, growing round the lower layer cells on the ventral side, vide B II, and merely remaining continuous with them on the dorsal. The importance of these two points will appear when we come to speak of other vertebrates.

The next animal whose development it is necessary to speak of is the Frog, and its differences from the mode of development are quite easy to follow and interpret. Segmentation is again not uniform, and results in the formation of an upper layer of smaller cells and a lower one of larger; in the centre is a segmentation cavity. The stage at the close of segmentation is represented in C I. From the diagram it is apparent that the lower layer cells occupy a larger bulk than they did in the previous animal (Plate 5, B I), and tend to encroach still more upon the segmentation cavity, otherwise the differences between the two are unimportant. There are, however, two points to be noted. In the first place, although the cells of the upper pole are distinguished in the diagrams from the lower by their colour, it is not possible at this stage to say what will become epiblast and what hypoblast. In the second place the cells of the upper pole or epiblast consist of two layers--an outer called the epidermic layer and an inner called the nervous. In the previous cases the epiblast consisted of a single layer of cells. The presence of these two layers is due to a distinction which, arising in most other vertebrates late, in the Frog arises early. In most other vertebrates in the later stages of development the epiblast consists of an outer layer of passive and an inner of active cells. In the Frog and other Batrachians these two layers become distinguished at the commencement of development.

In the next stage (C II) we find that the involution to form the alimentary canal has commenced (_al_), but that it is of a very different character to the involution in the previous case. It consists in the growing inwards of a number of cells from the point _x_ (C I) towards the segmentation cavity. The cells which grow in this way are partly the blue cells and partly the smaller yellow ones. At first this involuted layer of cells is only separated by a slit from the remainder of the lower layer cells; but by the stage represented in C II this has widened into an elongated cavity (_al_). In its formation this involution pushes backwards the segmentation cavity, which finally disappears in the stage C III. The point _x_ remains practically stationary, but by the general growth of the epiblast, mesoblast and hypoblast, becomes further removed from the segmentation cavity in C II than in C I. On the opposite side of the embryo to that at which the involution occurs the epiblast cells as before, grow round the lower layer cells. The commencement of this is already apparent in C I, and in C II the process is nearly completed, though there is still a small mass of yolk filling up the blastopore. The features of this involution are in the main exaggerations of what was supposed to occur in the previous animal. The asymmetry of the involution is so great that it is completely one-sided and results, in the first instance, in a mere slit; and the whole process of enclosing the yolk by epiblast is effected by the epiblast cells on the side of the egg opposite to the involution.

The true mesoblast and hypoblast are formed precisely as in the previous case. The involuted cells become separated into two layers, one forming the dorsal epithelium of the alimentary canal, and a layer between this and the epiblast forming the mesoblast. There is also a layer of mesoblast accompanying the epiblast which encloses the yolk, which is derived from the smaller yellow cells at _y_ (C I). The edge of this mesoblast, _m´_, forms a thickened ridge, a feature which persists in other vertebrates.

It is a point of some importance for understanding the relation between the mode of formation of the alimentary canal in the Frog and other vertebrates to notice that on the ventral surface the cells which are to form the epithelium of the alimentary canal become distinguished as such very much later than do those to form its dorsal epithelium, and are derived not from the involuted cells but from the primitive large yolk-cells. It is indeed probable that only a very small portion of epithelium of the ventral wall of the mid-gut is in the end derived from these larger yolk-cells. The remainder of the yolk-cells (C III, and C II, _yk_) form the yolk mass and do not become directly formed into the tissues of the animal.

In the last stage I have represented for the frog, C III, there are several features to be noticed.

The direct connection at their hind-ends between the cavities of the neural and alimentary canals is the most important of these. This is a result of the previous continuity of the epiblast and hypoblast at the point _x_, and is a feature almost certainly found in Amphioxus, but which I will speak of more fully in my account of the Selachian's development. The opening of the blastopore called the anus of Rusconi is now quite narrowed, it does not become the anus of the adult. It may be noticed that at the front end of the embryo the primitive dorsal epithelium of the alimentary canal is growing in such a way as to form the epithelium both of the dorsal and ventral surfaces of the fore-gut.

In spite of various features rendering the development of the Frog more difficult of comprehension than that of most other vertebrates, it is easy to see that the step between it and Amphioxus is not a very great one, and will very likely be bridged over at some future time, when our knowledge of the development of other forms becomes greater.

From the Frog to the Selachian is a considerable step, but I have again hypothetically sketched a type intermediate between them whose development agrees in some important points with that of _Pelobates fuscus_ as described by Bambeke. The points of agreement, though not obvious at first sight, I shall point out in the course of my description.

The first stage (D I), at the close of segmentation, deserves careful attention. The segmentation cavity by the increase of the food yolk is very much diminished in size, and, what is still more important, has as it were sunk down so as to be completely within the _lower layer cells_. The roof of the segmentation cavity is thus formed of epiblast and lower layer cells, a feature which Bambeke finds in _Pelobates fuscus_ and which is certainly found in the Selachians. In the Frog we found that the segmentation cavity began to be encroached on by the lower layer cells, and from this it is only a small step to find these cells creeping still further up and forming the roof of the cavity. In the lower layer cells themselves we find an important new feature, viz. that during segmentation they become divided in two distinct parts--one of these where the segments owing to the presence of much food yolk are very large, and the other where the segments are much smaller.

The separation between these two is rather sharp. Even this separation was foreshadowed in the Frog's egg, in which a number of lower layer cells were much smaller and more active at the two sides of the segmentation cavity than elsewhere. The segmentation cavity at first lies completely within the region of the small spheres. The larger cells serve almost entirely as food yolk. The epiblast, as is normal with vertebrates, consists of a single layer of columnar cells.

In the next stage (D II) the formation of the alimentary canal (_al_) has commenced, but it is to be observed that there is in this case _no true involution_.

As an accompaniment to the encroachment upon the segmentation cavity, which was a feature of the last stage, the cells to form the walls of the alimentary canal have come to occupy their final position during segmentation and without the intermediation of an involution, and traces only of the involution, are to be found in (1) a split in the lower layer cells which passes along the line separating the small and the large lower layer cells; and (2) in the epiblast becoming continuous with the hypoblast on the dorsal side of the mouth of this split. It is even possible that at this point a few cells (though certainly only a very small number) of those marked blue in D I become involuted. This point in this, as in all other cases, is the tail end of the embryo. The other features of this stage are as follows:--(1) The segmentation cavity has become smaller and less conspicuous than it was. (2) The epiblast cells have begun to grow round the yolk even in a more conspicuous manner than they did in the Frog, and are accompanied by a layer of mesoblast cells which again becomes thickened at its edge. The mesoblast cells in the region of the body are formed in the same way as before, viz. by the separation of a layer to form the epithelium of the alimentary canal, the other cells remaining as mesoblast; and as in the Frog, or in a more conspicuous manner, we find that the dorsal surface only of the alimentary cavity has a wall formed of a _distinct layer of cells_, but on the ventral side the cavity is at first closed in by the large spheres of the yolk only. The formation of the alimentary canal by a split and not by an involution is exactly what Bambeke finds in _Pelobates_.

The next stage, D III, is about an equivalent age to C III in the Frog. It exhibits the same connection between the neural and the alimentary canals as was found there.

The alimentary canal is beginning to become closed in below, and this occurs near the two ends earlier than in the middle. The cells to form the ventral wall are derived from the large yolk-cells. The non-formation of the ventral wall of the alimentary canal so soon in the middle as at the ends is an early trace of the umbilical canal found in Birds and Selachians, by which the alimentary tract is placed in communication with the yolk-sac. The segmentation cavity has by this stage completely vanished, and the epiblast with its accompanying mesoblast has spread completely round the yolk material so as to form the ventral wall of the body.

Though in some points this manner of development may seem to differ from that of the Frog, there is really a fundamental agreement between the two, and between this mode of development and that of the Selachians we shall find the agreement to be very close.

After segmentation we find that the egg of a Selachian consists of two parts--one of these called the germinal disc or blastoderm, and the other the yolk. The former of these corresponds with the epiblast and the part of the lower pole composed of smaller segments in the last-described egg, and the latter to the larger segments of the lower pole. This latter division, owing to the quantity of _yolk_ which it contains, has not undergone segmentation, but its homology with the larger segments of the previous eggs is proved (1) by its containing a number of nuclei (E I, _n_), which become the nuclei of true cells and enter the blastoderm, and (2) by the presence in it of a number of lines forming a network similar to that of many cells. The segmentation cavity, as before, lies completely within the lower layer cells.

The next stage, E II, is almost precisely similar to the second stage of the last egg. As there, the primitive involution is merely represented by a split separating the yolk and the germinal disc, and on the dorsal side alone is there a true cellular wall for this split, and at the dorsal mouth of the split the alimentary epithelium becomes continuous with the epiblast.

The segmentation cavity has become diminished, and round the yolk the epiblast, accompanied by a layer of mesoblast, is commencing to grow. In this growth all parts of the blastoderm take a share except that part where the epiblast and hypoblast are continuous. This manner of growth is precisely what occurs in the Frog, though there it is not so easily made out; and not all the investigators who have studied the Frog have understood the exact meaning of the appearances they have seen and drawn. This similarity of relation of the epiblast to the yolk in the two cases is a further confirmation of the identity of the Selachian's yolk with the large yolk-spheres of the previous eggs.

The next stage, E III, is in many ways identical with the corresponding stage in the last-described egg, and in the same way as in that case the neural and alimentary canals are placed in communication with each other.

The mode in which this occurs will be easily gathered from a comparison of E II and E III. It is the same for the Selachians and Batrachians. The neural canal (_nc_) is by the stage figured E III, completely formed in the way so well known in the Bird, and between the roof of the canal and the external epiblast a layer of mesoblast has already grown in. The floor of the neural canal is the same layer marked _ep_ in E II, and therefore remains continuous with the hypoblast at _x_; and when by a simultaneous process the roof of the neural canal and the ventral wall of the alimentary become formed by the folding over of one continuous layer (the epiblast and hypoblast continuous at the point _x_), the two canals, viz. the neural and alimentary, are necessarily placed in communication at their hind-ends, as is seen in the diagram.

There are several important points of difference between E III and D III. In the first place, owing to the larger size of the yolk mass in E III, the epiblast, accompanied by mesoblast, has not proceeded nearly so far round it as in the previous case. It is also worth notice that at the right as well as at the left end of the germinal disc the epiblast is commencing to grow round the yolk. The yolk has, however, become surrounded to a much smaller extent on the right hand than on the left. Since, in the earlier stage, the epiblast became continuous with the hypoblast at _x_, it is not from sections obvious how this occurs. I have therefore appended a diagram to explain it (E´). The blastoderm rests like a disc on the yolk and grows over it on all sides, except at the point where the epiblast and hypoblast are continuous (_x_). This point becomes as it were left in a bay. Next the two sides of the bay coalesce, the bay becomes obliterated, and the effect produced is exactly as if the blastoderm had grown round the yolk at the point _x_ (corresponding with the tail of the embryo) as well as everywhere else. It thus comes about that the final point where the various parts of the blastoderm meet and completely enclose the yolk mass does not correspond with the anus of Rusconi of the Frog, but is at some little distance from the hind-end of the embryo. In other words, the position of the blastopore in the Selachian is not the same as in the Frog.

Another point deserving attention is the formation of the ventral wall of the alimentary canal. This takes place in two ways--partly by a folding-in at the sides and end, and partly from cells formed around the nuclei (_n_) in the yolk. From these a large portion of the ventral wall of the mid-gut is formed.

The folding-in of the sheet of hypoblast to assist in the closing-in of the ventral wall of the alimentary canal is a consequence of the flattened form of the original alimentary slit which is far too wide to form the cavity of the final canal. In the Bird whose development must next be considered this folding-in is a still more prominent feature in the formation of the alimentary canal. As in the last case, the alimentary canal is widely open in the middle to the yolk at the time when its two ends are closed below and shut off from it; still later this opening becomes very narrow and forms the duct of the so-called umbilical cord which places the yolk-sac in communication with the alimentary canal. As the young animal becomes larger the yolk-sac ceases to communicate directly with the alimentary canal, and is carried about by it for some time as an appendage and only at a later period shrivels up.

The mesoblast is formed in a somewhat different way in the Sharks than in other vertebrates. It becomes split off from the hypoblast, not in the form of a single sheet as in other vertebrates, but as two lateral sheets, one on each side of the middle line and separated from one another by a considerable interval; whilst the notochord is derived not as in other vertebrates from the mesoblast, but from the hypoblast (vide F. M. Balfour, "Development of Selachians[21]," _Journal of Microscopical Science_, Oct., 1874).

Footnote 21: Paper No. V, p. 82 _et seq._ in this edition.

Between the Selachians and the Aves there is a considerable gulf, which it is more difficult satisfactorily to bridge over than in the previous cases; owing to this I have not attempted to give any intermediate stage between them.

The first stage of the Bird (F I) is very similar in many respects to the corresponding stage in the Selachian. The segmentation cavity is, however, a less well-defined formation, and it may even be doubted whether a true segmentation cavity, homologous with the segmentation cavity in the previously described eggs, is present. On the floor of the cavity which is formed by the yolk are a few larger cells known as formative cells which, according to Götte's observations, are derived from the yolk, in a somewhat similar manner to the cells which were formed around the nuclei in the Selachian egg, and which helped to form the ventral wall of the alimentary canal. Another point to be noticed is that the segmentation cavity occupies a central position, and not one to the side as in the Selachian.

The yolk is proportionately quite as large as in the Selachian's egg, but, as in that case, there can be little or no doubt of its being homologous with the largest of the segmentation spheres of the previous eggs. It does not undergo segmentation. The epiblast is composed of columnar cells, and extends a short way beyond the edge of the lower layer cells.

In the next stage the more important departures from the previous type of development become visible.

The epiblast spreads uniformly over the yolk-sac and not on the one side only as in the former eggs.

This is due to the embryo (indicated in F II by a thickening of the cells) lying in the centre and not at the edge of the blastoderm. A necessary consequence of this is, that the epiblast does not, as in the previous cases, become continuous with the hypoblast at the tail end of the embryo. This continuity, being of no functional importance, could easily be dispensed with, and the central position of the embryo may perhaps be explained by supposing the process, by which in the Selachian egg the blastopore ceases to correspond in position with the opening of the alimentary slit or anus of Rusconi (vide E´), to occur quite early during segmentation instead of at a late period of development. For the possibility of such a change in the date of formation, the early appearance of the nervous and epidermic layers in the Frog affords a parallel.

The epiblast in its growth round the yolk is only partially accompanied by mesoblast, which, however, is thickened at its extreme edge as in the Frog. Owing to the epiblast not becoming continuous with the hypoblast at the tail end of the embryo, the alimentary slit is not open to the exterior. The hypoblast is formed by some of the lower layer cells becoming distinguished as a separate layer; the remainder of the lower layer cells become the mesoblast.

The formation of the mesoblast and hypoblast out of the lower layer cells has been accepted for the Bird by most observers, but has been disputed by several, and recently by Kölliker. These have supposed that the mesoblast is derived from the epiblast. I feel convinced that these observers are in the wrong, and that the mesoblast is genuinely derived from the lower layer cells.

The greater portion of the alimentary cavity consists of the original segmentation cavity (vide diagrams). This feature of the segmentation cavity of Birds sharply distinguishes it from any segmentation cavity of other eggs, and renders it very doubtful whether the similarly named cavities of the Bird and of other vertebrates are homologous. On the floor of the cavity are still to be seen some of the formative cells, but observers have not hitherto found that they take any share in forming the ventral wall of the alimentary canal.

The features of the next stage are the necessary consequences of those of the last.

The ventral wall of the alimentary canal is entirely formed by a folding-in of the sheet of hypoblast.

The more rapid folding-in at the head still indicates the previous more vigorous growth there, otherwise there is very little difference between the forms of the fold at the head and tail. The alimentary canal does not of course, at this or any period, communicate with the neural tube, since the epiblast and hypoblast are never continuous. The other features, such as the growth of the epiblast round the yolk-sac, are merely continuations of what took place in the last stage.

In the development of a yolk-sac as a distinct appendage, and its absorption within the body, at a later period, the bird fundamentally resembles the Dog-fish.

Although there are some difficulties in deriving the type of development exhibited by the Bird directly from that of the Selachian, it is not very difficult to do so directly from Amphioxus. Were the alimentary involution to remain symmetrical as in Amphioxus, and the yolk-containing part of the egg to assume the proportions it does in the Bird, we should obtain a mode of development which would not be very dissimilar to that of the Bird. The epiblast would necessarily overgrow the yolk uniformly on all sides and not in the unsymmetrical fashion of the Selachian egg. A confirmation of this view might perhaps be sought for in the complete difference between the types of circulation of the yolk-sac in Birds and Selachians; but this is not so important as might at first sight appear, since it is not from the Selachian egg but from some Batrachian that it would be necessary to derive the Reptiles' and Birds' eggs.

If this view of the Bird's egg be correct, we are compelled to suppose that the line of ancestors of Birds and Reptiles did not include amongst them the Selachians and the Batrachians, or at any rate Selachians and Batrachians which develop on the type we now find.

The careful investigation of the development of some Reptiles might very probably throw light upon this important point. In the meantime it is better to assume that the type of development of Birds is to be derived from that of the Frog and Selachians.

_Summary._--If the views expressed in this paper are correct, all the modes of development found in the higher vertebrates are to be looked upon as modifications of that of Amphioxus. It is, however, rather an interesting question whether it is possible to suppose that the original type was _not_ that of Amphioxus, but of some other animal, say, for instance, that of the Frog, and that this varied in two directions,--on the one hand towards Amphioxus, in the reverse direction to the course of variation presupposed in the text; and on the other hand in the direction towards the Selachians as before.

The answer to this question must in my opinion be in the negative. It is quite easy to conceive the food material of the Frog's egg completely vanishing, but although this would entail simplifications of development and possibly even make segmentation uniform, there would, as far as I can see, be no cause why the essential features of difference between the Frog's mode of development and that of Amphioxus should change. The asymmetrical and slit-like form of involution on the one side and the growth of the epiblast over the mesoblast on the other side, both characteristics of the present Frog's egg, would still be features in the development of the simplified egg.

In the Mammal's egg we probably have an example of a Reptile's egg simplified by the disappearance of the food material; and when we know more of Mammalian embryology it will be very interesting to trace out the exact manner in which this simplification has affected the development. It is also probable that the eggs of Osseous fish are fundamentally simplified Selachian eggs; in which case we already know that the diminution of food material has affected but very slightly the fundamental features of development.

One common feature which appears prominently in reviewing the embryology of vertebrates as a whole is the derivation of the mesoblast from the hypoblast; in other words, we find that it is from the layer corresponding to that which becomes involuted in Amphioxus so as to line the alimentary cavity that the mesoblast is split off.

That neither the hypoblast or mesoblast can in any sense be said to be derived from the epiblast is perfectly clear. When the egg of Amphioxus is in the blastosphere stage we cannot speak of either an epiblast or hypoblast. It is not till the involution or what is equivalent has occurred, converting the single-walled vesicle into a double-walled one, that we can speak of these two layers. It might seem scarcely necessary to insist upon this point, so clear is it without explanation, were it not that certain embryologists have made a confusion about it.

The derivation of the mesoblast from the hypoblast is the more interesting, since it is not confined to the vertebrates, but has a very wide extension amongst the invertebrates. In the cases (whose importance has been recently insisted upon by Professor Huxley), of the Asteroids, the Echinoids, Sagitta, and others, in which the body-cavity arises as an outgrowth of the alimentary canal and the somatopleure and splanchnopleure are formed from that outgrowth, it is clear without further remark that the mesoblast is derived from the hypoblast. For the Echinoderms in which the water-vascular system and muscular system arise as a solid outgrowth of the wall of the alimentary canal there can also be no question as to the derivation of the mesoblast from the hypoblast.

Amongst other worms, in addition to Sagitta, the investigations of Kowalevsky seem to shew that in Lumbricus the mesoblast is derived from the hypoblast.

Amongst Crustaceans, Bobretsky's[22] observations on Oniscus (_Zeitschrift für wiss. Zoologie_, 1874) lead to the same conclusion.

Footnote 22: He says, p. 182: "Bevor aber die Hälfte der
Eioberfläche von den Embryonalzellen bedeckt ist, kommt die
erste gemeinsame Anlage des mittleren und unteren Keimblattes
zum Vorschein."

In Insects Kowalevsky's observations lead to the conclusion that mesoblast and hypoblast arise from a common mass of cells; Ulianin's observations bring out the same result for the abnormal Poduridæ, and Metschnikoff's observations shew that this also holds for Myriapods.

In Molluscs the point is not so clear.

In Tunicates, even if we are not to include them amongst vertebrates[23], the derivation of mesoblast from hypoblast is without doubt.

Footnote 23: Anton Dohrn, _Der Ursprung des Wirbelthieres_.
Leipzig, 1875.

Without going further into details it is quite clear that the derivation of the mesoblast from the hypoblast is very general amongst invertebrates.

It will hardly be disputed that primitively the muscular system of the body-wall could not have been derived from the layer of cells which lines the alimentary canal. We see indeed in Hydra and the Hydrozoa that in its primitive differentiation, as could have been anticipated beforehand, the muscular system of the body is derived from the epiblast cells. What, then, is the explanation of the widespread derivation of the mesoblast, including the muscular system of the body, from the hypoblast?

The explanation of it may, I think, possibly be found, and at all events the suggestion seems to me sufficiently plausible to be worth making, in the fact that in many cases, and probably this applies to the ancestors of the vertebrates, the body-cavity was primitively a part of the alimentary.

Mr Lankester, who has already entered into this line of speculation, even suggests (_Q. J. of Micr. Science_, April, 1875) that this applies to all higher animals. It might then be supposed that the muscular system of part of the alimentary canal took the place of the primitive muscular system of the body; so that the whole muscular system of higher animals would be primitively part of the muscular system of the digestive tract.

I put this forward merely as a suggestion, in the truth of which I feel no confidence, but which may perhaps induce embryologists to turn their attention to the point. If we accept it for the moment, the supplanting of the body muscular system by that of the digestive tract may hypothetically be supposed to have occurred in the following way.

When the diverticulum or rather paired diverticula were given off from the alimentary canal they would naturally become attached to the body-wall, and any contractions of their intrinsic muscles would tend to cause movements in the body-wall. So far there is no difficulty, but there is a physiological difficulty in explaining how it can have happened that this secondary muscular system can have supplanted the original muscular system of the body.

The following suggestions may lessen this difficulty, though perhaps they hardly remove it completely. If we suppose that the animal in which these diverticula appeared had a hard test and was not locomotive, the intrinsic muscular system of the body would naturally completely atrophy. But since the muscular system of the diverticula from the stomach would be required to keep up the movement of the nutritive fluid, it would not atrophy, and were the test subsequently to become soft and the animal locomotive, would naturally form the muscular system of the body. Or even were the animal locomotive in which the diverticula appeared, it is conceivable that the two systems might at first coexist together; that either (1) subsequently owing to the greater convenience of early development, the two systems might acquire a development from the same mass of cells and those the cells of the inner or hypoblast layer, so that the derivation of the body muscles from the hypoblast would only be apparent and not real, or (2) owing to their being better nourished as they would necessarily be, and to their possibly easier adaptability to some new form of movement of the animal, the muscle-cells of the alimentary canal might become developed exclusively whilst the original muscular system atrophied.

I only hold this view provisionally till some better explanation is given of the cases of Sagitta and the Echinoderms, as well as of the nearly universal derivation of the mesoblast from the hypoblast. The cases of this kind may be due to some merely embryonic changes and have no meaning in reference to the adult condition, but I think that we have no right to assume this till some explanation of the embryonic can be suggested.

For vertebrates, I have shewn that in Selachians the body-cavity at first extends quite to the top of what becomes the muscle plate, so that the line or space separating the two layers of the muscle plate (vide Balfour, 'Development of Elasmobranch Fishes[24],' _Quart. Journ. of Micro. Science_ for Oct., 1874. Plate XV, fig. 11,_a_, 11_b_, 12_a_, _mp_.) is a portion of the original body-cavity. If this is a primitive condition, which is by no means certain, we have a condition which we might expect, in which both the inner and the outer wall of the primitive body-cavity assists in forming the muscular system of the body.

Footnote 24: Paper No. V, p. 60 _et seq._ of this edition,
pl. 4, figs. 11_a_, 11_b_, 12_a_, _mp_.

It is very possible that the formation of the mesoblast as two masses, one on each side of the middle line as occurs in Selachians, and which as I pointed out in the paper quoted above also takes place in some worms, is a remnant of the primitive formation of the body-cavity as paired outgrowth of the alimentary canal. This would also explain the fact that in Selachians the body-cavity consists at first of two separate portions, one on each side of the alimentary canal, which only subsequently become united below and converted into a single cavity (vide _loc. cit._[25], Plate XIV, fig. 8_b_, _pp_).

Footnote 25: Pl. 3 of this edition, fig. 8_b_, _pp_.

In the Echinoderms we find instances where the body-cavity and water-vascular system arise as an outgrowth from the alimentary canal, which subsequently becomes constricted off from the latter (Asteroids and Echinoids), together with other instances (Ophiura, Synapta) where the water-vascular system and body-cavity are only secondarily formed in a solid mass of mesoblast originally split off from the walls of the alimentary canal.

These instances shew us how easily a change of this kind may take place, and remove the difficulty of understanding why in vertebrates the body-cavity never communicates with the alimentary.

The last point which I wish to call attention to is the blastopore or anus of Rusconi.

This is the primitive opening by which the alimentary canal communicates with the exterior, or, in other words, the opening of the alimentary involution. It is a distinctly marked structure in Amphioxus and the Batrachians, and is also found in a less well-marked form in the Selachians; in Birds no trace of it is any longer to be seen. In all those vertebrates in which it is present, it closes up and does not become the anus of the adult. The final anus nevertheless corresponds very closely in position with the anus of Rusconi. Mr Lankester has shewn (_Quart. Journ. of Micro. Science_ for April, 1875) that in invertebrates as well as vertebrates the blastopore almost invariably closes up. It nevertheless corresponds as a rule very nearly in position either with the mouth or with the anus.

If this opening is viewed, as is generally done, as really being the mouth in some cases and the anus in others, it becomes very difficult to believe that the blastopore can in all cases represent the same structure. In a single branch of the animal kingdom it sometimes forms the mouth and sometimes the anus: thus for instance in Lumbricus it is the mouth (according to Kowalevsky), in Palæmon (Bobretzky) the anus. Is it credible that the mouth and anus have become changed, the one for the other?

If, on the other hand, we accept the view that the blastopore never becomes either the one or the other of these openings, it is, I think, possible to account for its corresponding in position with the mouth in some cases or the anus in others.

That it would soon come to correspond either with the mouth or anus (probably with the earliest formed of these in the embryo), wherever it was primitively situated, follows from the great simplification which would be effected by its doing so. This simplification consists in the greater facility with which the fresh opening of either mouth or anus could be made where the epiblast and hypoblast were in continuity than elsewhere. Even a change of correspondence from the position of the mouth to that of the anus or _vice versa_ could occur. The mode in which this might happen is exemplified by the case of the Selachians. I pointed out in the course of this paper how the final point of envelopment of the yolk became altered in Selachians so as to cease to correspond with the anus of Rusconi; in other words, how the position of the blastopore became changed. In such a case, if the yolk material again became diminished, the blastopore would correspond in position with neither mouth nor anus, and the causes which made it correspond in position with the anus before, would again operate, and make it correspond in position perhaps with the mouth. Thus the blastopore might absolutely cease to correspond in position with the anus and come to correspond in position with the mouth.

It is hardly possible to help believing that the blastopore primitively represented a mouth. It may perhaps have lost this function owing to an increase of food yolk in the ovum preventing its being possible for the blastopore to develop directly into a mouth, and necessitating the formation of a fresh mouth. If such were the case, there would be no reason why the blastopore should ever again serve functionally as a mouth in the descendants of the animal which developed this fresh mouth.

EXPLANATION OF PLATE 5.

COMPLETE LIST OF REFERENCES.

_al._ Cavity of alimentary canal. _bl._ Blastoderm. _ch._ Notochord. _ep._ Epiblast. _em._ Embryo. _f._ Formative cells. _hy._ Hypoblast. _ll._ Lower layer cells. _m._ Mesoblast. _n._ Nuclei of yolk of Selachian egg. _nc._ Neural canal. _sg._ Segmentation cavity. _x._ Point where epiblast and hypoblast are continuous at the mouth of the alimentary involution. This point is always situated at the tail end of the embryo. _yk._ Yolk.

Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower layer cells before their separation into hypoblast and mesoblast are also coloured green.

A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages (founded upon Kowalevsky's observations).

B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical animal, intermediate between Amphioxus and Batrachians, in its early stages.

C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus in its early stages (founded upon Götte's observations). In C III the neural canal is completed, which was not the case in B III. The epiblast in C III has been diagrammatically represented as a single layer.

D I, D II, D III. Diagrammatic longitudinal sections of an animal, intermediate between Batrachians and Selachians, in its early stages.

E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early stages.

E´. Surface view of the yolk of a Selachian's egg to shew the manner in which it is enclosed by the Blastoderm. The yolk is represented yellow and the Blastoderm blue.

F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early stages.

VII. ON THE ORIGIN AND HISTORY OF THE URINOGENITAL ORGANS OF
VERTEBRATES[26].

Footnote 26: From the _Journal of Anatomy and Physiology_,
Vol. X. 1875.

Recent discoveries[27] as to the mode of development and anatomy of the urinogenital system of Selachians, Amphibians, and Cyclostome fishes, have greatly increased our knowledge of this system of organs, and have rendered more possible a comparison of the types on which it is formed in the various orders of vertebrates.

Footnote 27: The more important of these are:--

Semper--Ueber die Stammverwandtschaft der Wirbelthiere u.
Anneliden. _Centralblatt f. Med. Wiss._ 1874, No. 35.

Semper--Segmentalorgane bei ausgewachsenen Haien. _Centralblatt
f. Med. Wiss._ 1874, No. 52.

Semper--Das Urogenitalsystem der höheren Wirbelthiere.
_Centralblatt f. Med. Wiss._ 1874, No. 59.

Semper--Stammesverwandtschaft d. Wirbelthiere u. Wirbellosen.
_Arbeiten aus Zool. Zootom. Inst._ Würzburg. II Band.

Semper--Bildung u. Wachstum der Keimdrüsen bei den
Plagiostomen. _Centralblatt f. Med. Wiss._ 1875, No. 12.

Semper--Entw. d. Wolf. u. Müll. Gang. _Centralblatt f. Med.
Wiss._ 1875, No. 29.

Alex. Schultz--Phylogenie d. Wirbelthiere. _Centralblatt f.
Med. Wiss._ 1874, No. 51.

Spengel--Wimpertrichtern i. d. Amphibienniere. _Centralblatt f.
Med. Wiss._ 1875, No. 23.

Meyer--Anat. des Urogenitalsystems der Selachier u. Amphibien.
_Sitzb. Naturfor. Gesellschaft._ Leipzig, 30 April, 1875.

F. M. Balfour--Preliminary Account of development of
Elasmobranch fishes. _Quart. Journ. of Micro. Science_, Oct.
1874. (This edition, Paper V. p. 60 _et seq._)

W. Müller--Persistenz der Urniere bei Myxine glutinosa.
_Jenaische Zeitschrift_, 1873.

W. Müller--Urogenitalsystem d. Amphioxus u. d. Cyclostomen.
_Jenaische Zeitschrift_, 1875.

Alex. Götte--_Entwicklungsgeschichte der Unke (Bombinator
igneus)._

The following paper is an attempt to give a consecutive history of the origin of this system of organs in vertebrates and of the changes which it has undergone in the different orders.

For this purpose I have not made use of my own observations alone, but have had recourse to all the Memoirs with which I am acquainted, and to which I have access. I have commenced my account with the Selachians, both because my own investigations have been directed almost entirely to them, and because their urinogenital organs are, to my mind, the most convenient for comparison both with the more complicated and with the simpler types.

On many points the views put forward in this paper will be found to differ from those which I expressed in my paper (_loc. cit._) which give an account of my original[28] discovery of the segmental organs of Selachians, but the differences, with the exception of one important error as to the origin of the Wolffian duct, are rather fresh developments of my previous views from the consideration of fresh facts, than radical changes in them.

Footnote 28: These organs were discovered independently by
Professor Semper and myself. Professor Semper's preliminary
account appeared prior to my own which was published (with
illustrations) in the _Quarterly Journal of Mic. Science_.
Owing to my being in South America, I did not know of Professor
Semper's investigations till several months after the
publication of my paper.

* * * * *

In Selachian embryos an intermediate cell-mass, or middle plate of mesoblast is formed, as in birds, from a partial fusion of the somatic and splanchnic layers of the mesoblast at the outer border of the protovertebræ. From this cell-mass the whole of the urinogenital system is developed.

At about the time when three visceral clefts have appeared, there arises from the intermediate cell-mass, opposite the fifth protovertebra, a solid knob, from which a column of cells grows backwards to opposite the position of the future anus (Fig. 1. _pd._).

The sections are to shew the development of the segmental duct (_pd_) or primitive duct of the kidneys. In _A_ (the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. In _B_ is seen a section of the column which has grown backwards from the knob in _A_.

_spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord; _X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._ specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._ segmental duct; _so._ somatopleura; _sp._ splanchnopleura; _pp._ pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]

This knob projects outwards toward the epiblast, and the column lies at first between the mesoblast and epiblast. The knob and column do not long remain solid. The knob becoming hollow acquires a wide opening into the pleuro-peritoneal or body-cavity, and the column a lumen; so that by the time that five visceral clefts have appeared, the two together form a duct closed behind, but communicating in front by a wide opening with the pleuro-peritoneal cavity.

Before these changes are accomplished, a series of _solid_[29] outgrowths of elements of the 'intermediate cell-mass' appear at the uppermost corner of the body-cavity. These soon become hollow and appear as involutions from the body-cavity, curling round the inner and dorsal side of the previously formed duct.

Footnote 29: These outgrowths are at first solid in both
Pristiurus, Scyllium and Torpedo, but in Torpedo attain a
considerable length before a lumen appears in them.

One involution of this kind makes its appearance for each protovertebra, and the first belongs to the protovertebra immediately behind the anterior end of the duct whose development has just been described. In Pristiurus there are in all 29 of these at this period. The last two or three arise from that portion of the body-cavity, which at this stage still exists behind the anus. The first-formed duct and the subsequent involutions are the rudiments of the whole of the urinary system. The duct is the primitive duct of the kidney[30]; I shall call it in future _the segmental duct_; and the involutions are the commencements of the segmental tubes which constitute the body of the kidney. I shall call them in future _segmental tubes_.

Footnote 30: This duct is often called either Müller's duct,
the oviduct, or the duct of the primitive kidneys
'Urnierengang.' None of these terms are very suitable. A
justification of the name I have given it will appear from the
facts given in the later parts of this paper. In my previous
paper I have always called it oviduct, a name which is very
inappropriate.

Soon after their formation the segmental tubes become convoluted, and their blind ends become connected with the segmental duct of the kidney. At the same time, or rather before this, the blind posterior termination of each of the segmental ducts of the kidneys unites with and opens into one of the horns of the cloaca. At this period the condition of affairs is represented in Fig. 2.

_pd._ segmental duct. It opens at _o_ into the body-cavity and at its other extremity into the cloaca; _x._ line along which the division appears which separates the segmental duct into the Wolffian duct above and the Müllerian duct below; _st._ segmental tubes. They open at one end into the body-cavity, and at the other into the segmental duct.]

There is at _pd_, the segmental duct of the kidneys, opening in front (_o_) into the body-cavity, and behind into the cloaca, and there are a series of convoluted segmental tubes (_st_), each opening at one end into the body-cavity, and at the other into the duct (_pd_).

The next important change which occurs is the longitudinal division of the segmental duct of the kidneys into Müller's duct, or the oviduct, and the duct of the Wolffian bodies or Leydig's duct. The splitting[31] is effected by the growth of a wall of cells which divides the duct into two parts (fig. 3, _wd._ and _md._). It takes place in such a way that the front end of the segmental duct, anterior to the entrance of the first segmental tube, together with the ventral half of the rest of the duct, is split off from its dorsal half as an independent duct (vide fig. 2, _x_).

Footnote 31: This splitting was first of all discovered and an
account of it published by Semper (_Centralblatt f. Med. Wiss._
1875, No. 29). I had independently made it out for the female a
few weeks before the publication of Semper's account--but have
not yet made observations about the point for the male.

My own previous account of the origin of the Wolffian duct
(_Quart. Journ. of Micros. Science_, Oct. 1874, and this
edition, Paper V.), is completely false, and was due to my not
having had access to a complete series of my sections when I
wrote the paper.

_mc._ medullary canal; _mp._ muscle-plate; _ch._ notochord; _ao._ aorta; _cav._ cardinal vein; _st._ segmental tube. On the one side the section passes through the opening of a segmental tube into the body-cavity. On the other this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; _wd._ Wolffian duct; _md._ Müllerian duct. The Müllerian duct and the Wolffian duct together constitute the primitive segmental duct; _gr._ The germinal ridge with the thickened germinal epithelium; _l._ liver; _i._ intestine with spiral valve.]

The dorsal portion also forms an independent duct, and into it the segmental tubes continue to open. Such at least is the method of splitting for the female--for the male the splitting is according to Professor Semper, of a more partial character, and consists for the most part in the front end of the duct only being separated off from the rest. The result of these changes is the formation in both sexes of a fresh duct which carries off the excretions of the segmental involutions, and which I shall call the Wolffian duct--while in the female there is formed another complete and independent duct, which I shall call the Müllerian duct, or oviduct, and in the male portions only of such a duct.

The next change which takes place is the formation of another duct from the hinder portion of the Wolffian duct, which receives the secretion of the posterior segmental tubes. This secondary duct unites with the primary or Wolffian duct near its termination, and the primary ducts of the two sides unite together to open to the exterior by a common papilla.

Slight modifications of the posterior terminations of these ducts are found in different genera of Selachians (vide Semper, _Centralblatt für Med. Wiss_. 1874, No. 59), but they are of no fundamental importance.

These constitute the main changes undergone by the segmental duct of the kidneys and the ducts derived from it; but the segmental tubes also undergo important changes. In the majority of Selachians their openings into the body-cavity, or, at any rate, the openings of a large number of them, persist through life; but the investigations of Dr Meyer[32] render it very probable that the small portion of each segmental tube adjoining the opening becomes separated from the rest and becomes converted into a sort of lymph organ, so that the openings of the segmental tubes in the adult merely lead into lymph organs and not into the gland of the kidneys.

Footnote 32: _Sitzun. der Naturfor. Gesellschaft_, Leipzig, 30
April, 1875.

These constitute the whole changes undergone in the female, but in the male the open ends of a varying number (according to the species) of the segmental tubes become connected with the testis and, uniting with the testicular follicles, serve to carry away the seminal fluid[33]. The spermatozoa have therefore to pass through a glandular portion of the kidneys before they enter the Wolffian duct, by which they are finally carried away to the exterior.

Footnote 33: We owe to Professor Semper the discovery of the
arrangement of the seminal ducts. _Centralblatt f. Med. Wiss._
1875, No. 12.

In the adult female, then, there are the following parts of the urinogenital system (fig. 4):

(1) The oviduct, or Müller's duct (fig. 4, _md._), split off from the segmental duct of the kidneys. Each oviduct opens at its upper end into the body-cavity, and behind the two oviducts have independent communications with the cloaca. The oviducts serve simply to carry to the exterior the ova, and have no communication with the glandular portion of the kidneys.

_md._ Müllerian duct; _wd._ Wolffian duct; _st._ segmental tubes; _d._ duct of the posterior segmental tubes; _ov._ ovary.]

(2) The Wolffian ducts (fig. 4, _wd._) or the remainder of the segmental ducts of the kidneys. Each Wolffian duct ends blindly in front, and the two unite behind to open by a common papilla into the cloaca.

This duct receives the secretion of the whole anterior end of the kidneys[34], that is to say, of all the anterior segmental tubes.

Footnote 34: This upper portion of the kidneys is called
Leydig's gland by Semper. It would be better to call it the
Wolffian body, for I shall attempt to shew that it is
homologous with the gland so named in Sauropsida and Mammalia.

(3) The secondary duct (fig. 4, _d._) belonging to the lower portion of the kidneys opening into the former duct near its termination.

(4) The segmental tubes (fig. 4, _st_) from whose convolutions and outgrowths the kidney is formed. They may be divided into two parts, according to the duct by which their secretion is carried off.

In the male the following parts are present:

(1) The Müllerian duct (fig. 5, _md._), consisting of a small remnant, attached to the liver, which represents the foremost end of the oviduct of the female.

(2) The Wolffian duct (fig. 5, _wd_), which precisely corresponds to the Wolffian duct of the female, except that, in addition to functioning as the duct of the anterior part of the kidneys, it also serves to carry away the semen. In the female it is straight, but has in the adult male a very tortuous course (vide fig. 5).

_md._ rudiment of Müllerian duct; _wd._ Wolffian duct, which also serves as vas deferens; _st._ segmental tubes. The ends of three of those which in the female open into the body-cavity, have in the male united with the testicular follicles, and serve to carry away the products of the testis; _d._ duct of the posterior segmental tubes; _t._ testis.]