The embryonic end of the germ is always the one which points towards the pole of the yolk farthest removed from the egg-capsule.
The germ grows, but not very rapidly, and without otherwise undergoing any very appreciable change, for some time.
The growth at these early periods appears to be particularly slow, especially when compared with the rapid manner in which some of the later stages of the development are passed through.
The next important change which occurs is the formation of the so-called “segmentation cavity.”
This forms a very marked feature throughout the early stages. It appears, however, to have somewhat different relations to the blastoderm than the homologous structure in other vertebrates. In its earliest stage which I have observed, it appears as a small cavity in the centre of the lower layer cells. This grows rapidly, and its roof becomes composed of epiblast and only a thin lining of “lower layer” cells, while its floor is formed by the yolk (Pl. 3, fig. 3, sg). In the next and third stage (Pl. 3, fig. 4, sg) its floor is formed by a thin layer of cells, its roof remaining as before. It has, however, become a less conspicuous formation than it was; and in the last (fourth) stage in which it can be distinguished it is very inconspicuous, and almost filled up by cells.
What I have called the second stage corresponds to a period in which no trace of the embryo is to be seen. In the third stage the embryonic end of the blastoderm projects outwards to form a structure which I shall speak of as the “embryonic rim,” and in the fourth and last stage a distinct medullary groove is formed. For a considerable period during the second stage the segmentation cavity remains of about the same size; during the third stage it begins to be encroached upon, and becomes smaller both absolutely, and relatively to the increased size of the germ.
The segmentation cavity of the Dog-fish most nearly agrees with that of Osseous fishes in its mode of formation and relation to the embryo.
Dog-fish resemble Osseous fish in the fact that their embryos are entirely formed from a portion of the germ which does not form part of the roof of the segmentation cavity, so that the cells forming the roof of the segmentation cavity take no share at any time in the formation of their embryos. They further agree with Osseous fish (always supposing that the descriptions of Oellacher, loc. cit., and Götte, Archiv. für Micr. Anat. Bd. IX. are correct) in the floor of the segmentation cavity being formed at one period by yolk. Together with these points of similarity there are some important differences.
(1) The segmentation cavity in the Osseous fish from the first arises as a cavity between the yolk and the blastoderm, and its floor is never at any period covered with cells. In the Dog-fish, as we have said above, both in the earlier and later periods the floor is covered with cells.
(2) The roof in the Dog-fish is invariably formed by the epiblast and a row of flattened lower layer cells.