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. 6_a_, 7_a_, 7_b_, &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.