The formation of cells in the yolk, apart from the general process of segmentation, has been recognised by many observers. Kupffer (_Archiv. für Micr. Anat._, Bd. IV. 1868) and Owsjannikow ("Entwicklung der Coregonus," _Bulletin der Akad. St Petersburgh_, Vol. XIX.) in osseous fishes[13], Ray Lankester (_Annals and Mag. of Nat. Hist._ Vol. XI. 1873, p. 81) in Cephalopoda, Götte (_Archiv. für Micr. Anat._ Vol. X.) in the chick, have all described a new formation of cells from the so-called food-yolk. The organized nature of the whole or part of this, previous to the formation of the cells from it, has not, however, as a rule, been distinctly recognised. In the majority of cases, as, for instance, in Loligo, the nucleus is not the first thing to be formed, but a plastide is first formed, in which a nucleus subsequently makes its appearance. 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. _Formation of the Layers._ 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.