Footnote 13: Götte, at the end of a paper on "The Development of the Layers in the Chick" (_Archiv. für Micr. Anat._, Vol. X. 1873, p. 196), mentions that the so-called cells in Osseous fishes which Oellacher states to have migrated into the yolk, and which are clearly the same as those mentioned by Owsjannikow, are really not _cells_, but large _nuclei_. If this statement is correct the phenomena in Osseous fishes are precisely the same as those I have described in the Dog-fish.
Leaving these nuclei, I will now pass on to the formation of the layers.
At the close of segmentation the surface of the blastoderm is composed of cells of a uniform size, which, however, are too small to be seen by the aid of the simple microscope.
The cells of this uppermost layer are somewhat columnar, and can be distinguished from the remainder of the cells of the blastoderm as a separate layer. This layer forms the epiblast; and the Dog-fish agree with Birds, Batrachians, and Osseous fish in the very early differentiation of it.
The remainder of the cells of the blastoderm form a mass, many cells deep, in which it is impossible as yet or till a very considerably later period to distinguish two layers. They may be called the _lower layer cells_. Some of them near the edge of this mass are still considerably larger than the rest, but they are, as a whole, of a fairly uniform size. Their nuclei are of the same character as the nuclei in the yolk.
There is one point to be noticed in the shape of the blastoderm as a whole. It is unsymmetrical, and a much larger number of its cells are found collected at one end than at the other. This absence of symmetry is found in all sections which are cut parallel to the long axis of the egg-capsule. The thicker end is the region where the embryo will subsequently appear.
This very early appearance of distinction in the blastoderm between the end at which the embryo will appear, and the non-embryonic end is important, especially as it shews the affinity of the modes of development of Osseous fishes and the Elasmobranchii. Oellacher (_Zeitschrift für Wiss. Zoologie_, Vol. XXXIII. 1873) has shewn, and, though differing from him on many other points, on this point Götte (_Arch. für Micr. Anat._ Vol. IX. 1873) agrees with him, that a similar absence of symmetry by which the embryonic end of the germ is marked off, occurs almost immediately after the end of segmentation in Osseous fishes. In the early stages of development there are a number of remarkable points of agreement between the Osseous fish and the Dog-fish, combined with a number of equally remarkable points of difference. Some of these I shall point out as I proceed with my description.