(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.
According to both Götte and Oellacher the roof of the segmentation cavity in Osseous fishes is in the earlier stages formed _alone_ of the two layers which correspond with the single layer forming the epiblast in the Dog-fish. In Osseous fishes it is very difficult to distinguish the various layers, owing to the similarity of their component cells. In Dog-fish this is very easy, owing to the great distinctness of the epiblast, and it appears to me, on this account, very probable that the two above-named observers may be in error as to the constitution of its roof in the Osseous fish. With both the Bird and the Frog the segmentation cavity of the Dog-fish has some points of agreement, and some points of difference, but it would take me too far from my present subject to discuss them.
When the segmentation cavity is first formed, no great changes have taken place in the cells forming the blastoderm. The upper layer--the _epiblast_--is composed of a single layer of columnar cells, and the remainder of the cells of blastoderm, forming the lower layer, are of a fairly uniform size, and polygonal from mutual pressure. The whole edge of the blastoderm is thickened, but this thickening is especially marked at its embryonic end.
This thickened edge of the blastoderm is still more conspicuous in the next and second stage (Pl. 3, fig. 3).
In the second stage the chief points of progress, in addition to the increased thickness of the edge of the blastoderm, are--
(1) The increased thickness and distinctness of the epiblast, caused by its cells becoming more columnar, though it remains as a one-cell-thick layer.
(2) The disappearance of the cells from the floor of the segmentation cavity.
The lower layer cells have undergone no important changes, and the blastoderm has increased very little if at all in size.