In the region of the embryo (Pl. 7, fig. 9) more important changes have occurred. The epiblast still remains as a single row of columnar cells. The hypoblast is no longer fused with the mesoblast, and forms a distinct dorsal wall for the alimentary cavity. Though along the axis of the embryo the hypoblast is composed of a single row of columnar cells, yet in the lateral part of the embryo its cells are less columnar and are one or two deep.
Owing to the manner in which the mesoblast became split off from the hypoblast, a continuity is maintained between the hypoblast and the lower layer cells of the blastoderm (Pl. 7, fig. 9), while the two plates of mesoblast are isolated and disconnected from any other masses of cells.
The alimentary cavity is best studied in transverse sections. (Vide Pl. 7, fig. 10a, 10b and 10c, three sections from the same embryo.) It is closed in above and at the sides by the hypoblast, and below by the yolk. In its anterior part a floor is commencing to be formed by a growth of cells from the walls of the two sides. The cells for this growth are formed around the nuclei of the yolk; a feature which recalls the fact that in Amphibians the ventral wall of the alimentary cavity is similarly formed in part from the so-called yolk cells.
We left the mesoblast as two masses not completely separated from the hypoblast. During this stage the separation between the two becomes complete, and there are formed two great lateral plates of mesoblast cells, one on each side of the medullary groove. Each of these corresponds to a united vertebral and lateral plate of the higher Vertebrates. The plates are thickest in the middle and posterior regions (Pl. 7, fig. 10a and 10b), but thin out and almost vanish in the region of the head. The longitudinal section of this stage represented in Pl. 7, fig. 9, passes through one of the lateral masses of mesoblast cells, and shews very distinctly its complete independence of all the other cells in the blastoderm.
From what has been stated with reference to the development of the mesoblast, it is clear that in Elasmobranchii this layer is derived from the same mass of cells as the hypoblast, and receives none of its elements from the epiblast. In connection with its development, as two independent lateral masses, I may observe, as I have previously done[143], that in this respect it bears a close resemblance to mesoblast in Euaxes, as described by Kowalevsky[144]. This resemblance is of some interest, as bearing on a probable Annelid origin of Vertebrata. Kowalevsky has also shewn[145] that the mesoblast in Ascidians is similarly formed as two independent masses, one on each side of the middle line.
It ought, however, to be pointed out that a similar bilateral origin of the mesoblast had been recently met with in Lymnæus by Carl Rabl[146]. A fact which somewhat diminishes the genealogical value of this feature in the mesoblast in Elasmobranchii.
During the course of this stage the spherules of food-yolk immediately beneath the embryo are used up very rapidly. As a result of this the protoplasmic network, so often spoken of, comes very plainly into view. Considerable areas may sometimes be seen without any yolk-spherule whatever.
On Pl. 7, fig. 7a, and figs. 11 and 12, I have attempted to reproduce the various appearances presented by this network: and these figures give a better idea of it than any description. My observations tend to shew that it extends through the whole yolk, and serves to hold it together. It has not been possible for me to satisfy myself that it had any definite limits, but on the other hand, in many parts all my efforts to demonstrate its presence have failed. When the yolk-spherules are very thickly packed, it is difficult to make out for certain whether it is present or absent, and I have not succeeded in removing the yolk-spherules from the network in cases of this kind. In medium-sized ovarian eggs this network is very easily seen, and extends through the whole yolk. Part of such an egg is shewn in Pl. 7, fig. 14. In full-sized ovarian eggs, according to Schultz[147], it forms, as was mentioned in the first chapter, radiating striæ, extending from the centre to the periphery of the egg. When examined with the highest powers, the lines of this network appear to be composed of immeasurably small granules arranged in a linear direction. These granules are more distinct in chromic acid specimens than in those hardened in osmic acid, but are to be seen in both. There can be little doubt that these granules are imbedded in a thread or thin layer of protoplasm.
I have already (p. [252]) touched upon the relation of this network to the nuclei of the yolk[148].
During the stages which have just been described specially favourable views are frequently to be obtained of the formation of cells in the yolk and their entrance into the blastoderm. Two representations of these are given, in Pl. 7, fig. 7a, and fig. 13. In both of these distinctly circumscribed cells are to be seen in the yolk (c), and in all cases are situated near to the typical nuclei of the yolk. The cells in the yolk have such a relation to the surrounding parts, that it is quite certain that their presence is not due to artificial manipulation, and in some cases it is even difficult to decide whether or no a cell area is circumscribed round a nucleus (Pl. 7, fig. 13). Although it would be possible for cells in the living state to pass from the blastoderm into the yolk, yet the view that they have done so in the cases under consideration has not much to recommend it, if the following facts be taken into consideration. (1) That the cells in the yolk are frequently larger than those in the blastoderm. (2) That there are present a very large number of nuclei in the yolk which precisely resemble the nuclei of the cells under discussion. (3) That in some cases (Pl. 7, fig. 13) cells are seen indistinctly circumscribed as if in the act of being formed.