From this period they rapidly increase in number, up to the commencement of the formation of the embryo as a body distinct from the germ. Though before this period they probably become the nuclei of veritable cells which enter the germ, it is not till this period, when the growth of the blastoderm becomes very rapid and it commences to spread over the yolk, that these new cells are formed in large numbers. I have many specimens of this age which shew the formation of these new cells with great clearness. This is most distinctly to be seen immediately below the embryo, where the yolk-spherules are few in number. At the opposite end of the blastoderm I believe that more of these cells are formed, but, owing to the presence of numerous yolk-spherules, it is much more difficult to make certain of this.
As to the final destination of these cells, my observations are not yet completed. Probably a large number of them are concerned in the formation of the vascular system, but I will give reasons later on for believing that some of them are concerned in the formation of the walls of the digestive canal and of other parts.
I will conclude my account of these nuclei by briefly summarizing the points I have arrived at in reference to them.
A portion, or more probably the whole, of the _yolk_ of the Dog-fish consists of _organized material_, in which nuclei appear and increase either by _division_ or by a process of _independent_ formation, and a great number of these subsequently become the nuclei of cells formed around them, frequently at a distance from the germ, which then travel up and enter it.
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.