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. IX. 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.
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.