On the other hand, from this as well as other sections, I have satisfied myself that there is a distinct ingrowth of cells from the embryonic swelling. It is therefore most probable that both these processes, viz. a fresh formation and an ingrowth, have a share in the formation of the layer of cells on the floor of the segmentation cavity.
In the next stage we find the embryo rising up as a distinct body from the blastoderm, and I shall in future speak of the body, which now becomes distinct as the embryo. It corresponds with what Kupffer (loc. cit.) in his paper on the “Osseous Fishes” has called the “embryonic keel.”This starting-point for speaking of the embryo as a distinct body is purely arbitrary and one merely of convenience. If I wished to fix more correctly upon a period which could be spoken of as marking the commencing formation of the embryo, I should select the time when structures first appear to mark out the portion of the germ from which the embryo becomes formed; this period would be in the Elasmobranchii, as in the Osseous fish, at the termination of segmentation, when the want of symmetry between the embryonic end of the germ and the opposite end first appears.
I described in the last stage the appearance of the “embryonic rim.” It is in the middle point of this, where it projects most, that the formation of the embryo takes place. There appear two parallel folds extending from the edge of the blastoderm towards the centre, and cut off at their central end by another transverse fold. These three folds raise up, between them, a flat broadish ridge, “the embryo” (Pl. 3, fig. 5). The head end of the embryo is the end nearest the centre of the blastoderm, the tail end being the one formed by its (the blastoderm's) edge.
Almost from its first appearance this ridge acquires a shallow groove—the medullary groove (Pl. 3, fig. 5, mg)—along its middle line, where the epiblast and hypoblast are in absolute contact (vide fig. 6a, 7a, 7b, &c.) and where the mesoblast (which is already formed by this stage) is totally absent. This groove ends abruptly a little before the front end of the embryo, and is deepest in the middle and wide and shallow behind.
On each side of it is a plate of mesoblast equivalent to the combined vertebral and lateral plates of the Chick. These, though they cannot be considered as entirely the cause of the medullary groove, may perhaps help to make it deeper. In the parts of the germ outside the embryo the mesoblast is again totally absent, or, more correctly, we might say that outside the embryo the lower layer cells do not become differentiated into hypoblast and mesoblast, and remain continuous only with the lower of the two layers into which the lower layer cells become differentiated in the body of embryo. This state of things is not really very different from what we find in the Chick. Here outside the embryo (i.e. in the opaque area) there is a layer of cells in which no differentiation into hypoblast and mesoblast takes place, but the layer remains continuous rather with the hypoblast than the mesoblast.
There is one peculiarity in the formation of the mesoblast which I wish to call attention to, i.e. its formation as two lateral masses, one on each side of the middle line, but not continuous across this line (vide figs. 6a and 6b, and 7a and 7b). Whether this remarkable condition is the most primitive, i.e. whether, when in the stage before this the mesoblast is first formed, it is only on each side of the middle line that the differentiation of the lower layer cells into hypoblast and mesoblast takes place, I do not certainly know, but it is undoubtedly a very early condition of the mesoblast. The condition of the mesoblast as two plates, one on each side of the neural canal, is precisely similar to its embryonic condition in many of the Vermes, e.g. Euaxes and Lumbricus. In these there are two plates of mesoblast, one on each side of the nervous cord, which are known as the Germinal streaks (Keimstreifen) (vide Kowalevsky “Würmern u. Arthropoden”; Mém. de l'Acad. Imp. St Pétersbourg, 1871).
From longitudinal sections I have found that the segmentation cavity has ceased by this stage to have any distinct existence, but that the whole space between the epiblast and the yolk is filled up with a mass of elongated cells, which probably are solely concerned in the formation of the vascular system. The thickened posterior edge of the blastoderm is still visible.
At the embryonic end of the blastoderm, as I pointed out in an earlier stage, the epiblast and the lower layer cells are perfectly continuous.
Where they join the epiblast, the lower layer cells become distinctly divided, and this division commenced even in the earlier stage, into two layers; a lower one, more directly continuous with the epiblast, consisting of cells somewhat resembling the epiblast-cells, and an upper one of more flattened cells (Pl. 3, fig. 4, m). The first of these forms the hypoblast, and the latter the mesoblast. They are indicated by hy and m in the figures. The hypoblast, as I said before, remains continuous with the whole of the rest of lower layer cells of the blastoderm (vide fig. 7b). This division into hypoblast and mesoblast commences at the earlier stage, but becomes much more marked during this one.
In describing the formation of the hypoblast and mesoblast in this way I have assumed that they are formed out of the large mass of lower layer cells which underlie the epiblast at the embryonic end of the blastoderm. But there is another and, in some ways, rather a tempting view, viz. to suppose that the epiblast, where it becomes continuous with the hypoblast, in reality becomes involuted, and that from this involuted epiblast are formed the whole mesoblast and hypoblast.