(a) A nest of primitive ova makes its appearance, either by continued division of a single primitive ovum or otherwise. The bodies of all the ova of the nest fuse together, and a polynuclear mass is formed, which increases in size concomitantly with the division of its nuclei. The nuclei, moreover, pass through a series of transformations. They increase in size and form delicate vesicles filled with a clear fluid, but contain close to one side a granular mass which stains very deeply with colouring reagents. The granular mass becomes somewhat stellate, and finally assumes a reticulate form with one more highly refracting nucleoli at the nodal points of the reticulum. When a nucleus has reached this condition the protoplasm around it has become slightly granular, and with the enclosed nucleus is segmented off from the nest as a special cell—a permanent ovum (figs. 13, 14, 15, 16). Not all the nuclei in a nest undergo the whole of the above changes; certain of them, on the contrary, stop short in their development, atrophy, and become employed as a kind of pabulum for the remainder. Thus it happens that out of a large nest perhaps only two or three permanent ova become developed.
(b) In the second mode of development of ova the nuclei and protoplasm undergo the same changes as in the first mode; but the ova either remain isolated and never form part of a nest, or form part of a nest in which no fusion of the protoplasm takes place, and all the primitive ova develop into permanent ova. Both the above modes of the formation continue through a great part of life.
(5) The follicle.—The cells of the germinal epithelium arrange themselves as a layer around each ovum, almost immediately after its separation from a nest, and so constitute a follicle. They are at first flat, but soon become more columnar. In Scyllium they remain for a long time uniform, but in large eggs they become arranged in two or three layers, while at the same time some of them become large and flask-shaped, and others small and oval (fig. 29). The flask-shaped cells have probably an important function in the nutrition of the egg, and are arranged in a fairly regular order amongst the smaller cells. Before the egg is quite ripe both kinds of follicle cells undergo retrogressive changes (Pl. 25, fig. 23).
In Raja a great irregularity in the follicle cells is observable at an early stage, but as the ovum grows larger the cells gradually assume a regular arrangement more or less similar to that in Scyllium (Pl. 25, figs. 30-33).
(6) The egg membranes.—Two membranes are probably always present in Elasmobranchii during some period of their growth. The first formed and outer of these arises in some instances before the formation of the follicular epithelium, and would seem to be of the nature of a vitelline membrane. The inner one is the zona radiata with a typical radiately striated structure. It is formed from the vitellus at a much later period than the proper vitelline membrane. It is more developed in Scyllium than in Raja, but atrophies early in both genera. By the time the ovum is nearly ripe both membranes are very much reduced, and when the egg (in Scyllium and Pristiurus) is laid, no trace of any membrane is visible.
(7) The vitellus.—The vitellus is at first faintly granular, but at a later period exhibits a very distinct (protoplasmic) network of fibres, which is still present after the ovum has been laid.
The yolk arises, in the manner described by Gegenbaur, in ova of about 0.06 mm. as a layer of fine granules, which stain deeply with osmic acid. They are at first confined to a stratum of protoplasm slightly below the surface of the ovum, and are most numerous at the pole furthest removed from the germinal vesicle. They are not regularly distributed, but are aggregated in small masses. They gradually grow into vesicles, in the interior of which oval solid bodies are developed, which form the permanent yolk-spheres. These oval bodies in the later stages exhibit a remarkable segmentation into plates, which gives them a peculiar appearance of transverse striation.
Certain bodies of unknown function are occasionally met with in the vitellus, of which the most remarkable are those figured at x on Pl. 25, fig. 25A.
(8) The germinal vesicle.—A reticulum is very conspicuous in the germinal vesicle in the freshly formed ova, but becomes much less so in older ova, and assumes, moreover, a granular appearance. At first one to three nucleoli are present, but they gradually increase in number as the germinal vesicle grows older, and are frequently situated in close proximity to the membrane.