Fig. 19. Section through part of the germinal epithelium of the ovary of Scyllium at the time when the primitive germinal cells are becoming converted into ova.
nn. Nests formed of agglomerated germinal cells. The nuclei of these cells are imbedded in undivided protoplasm. do. developing ova. o. ovum with follicle. po. primitive germinal cell. dv. blood-vessels.
In the first mode the protoplasm of all the cells forming a nest unites into a single mass containing the nuclei of the previously independent ova ([fig. 19], nn). The nuclei in the nest increase in number, probably by division, and at the same time the nest itself increases in size. The nuclei while increasing in number also undergo important changes. A segregation of their contents takes place, and the granular part (nuclear substance) forms a mass close to one side of the membrane of the nucleus, while the remainder of the nucleus is filled with a clear fluid. The whole nucleus at the same time increases somewhat in size. The granular mass gradually assumes a stellate form, and finally becomes a beautiful reticulum, of the character so well known in nuclei ([fig. 19], do). Two or three special nucleoli are present, and form the nodal points of the reticulum, while its meshes are filled up with the clear fluid constituents of the nucleus. Not all the nuclei undergo the above changes; but some of them stop short in their development, undergo atrophy, and appear finally to be absorbed as pabulum by the protoplasm of the nest. Such nuclei in a state of degeneration are shewn in [fig. 19]. Thus only a few nuclei out of a nest undergo a complete development. At first the protoplasm of the nest is clear and transparent, but as the nuclei undergo their changes the protoplasm becomes more granular, and a specially large quantity of granular protoplasm is generally present around the most developed nuclei, and these with their protoplasm gradually become constricted off from the nest, and constitute the permanent ova ([fig. 19], do). The relative number of ova which may develop from a single nest is subject to great variation. The object of the whole occurrence of the fusion of primitive ova and the subsequent atrophy of some of them is to ensure the adequate nutrition of a certain number of them.
In the second and rarer mode of development of permanent ova from primitive germinal cells, the nuclei and protoplasm undergo the same changes as in the first mode, but the cells either remain isolated, and never form part of a nest, or form part of a nest in which no fusion of protoplasm takes place, and in which all the cells develop into permanent ova.
The isolated ova and nests are situated, during the whole of the above changes, amongst the general undifferentiated cells of the germinal epithelium, but as soon as a permanent ovum becomes formed the cells adjoining it arrange themselves around it as a special layer, and so give rise to the epithelium of the follicle ([fig. 19], o). The growths of stroma into the germinal epithelium appear shortly after the formation of the earlier follicles.
Mammalia. The development of the ovary in Mammalia differs mainly from that just described in that the formation of primitive germinal cells from the indifferent cells of the germinal epithelium takes place at a relatively much later period.
The stroma grows into the germinal epithelium while it is still formed of rounded indifferent cells, and divides it into trabeculæ as described above. At a later period a number of the cells in the deeper layer of the epithelium, as well as certain cells in the superficial part, become primitive germinal cells, while the remainder of the cells become smaller and are destined to form the follicle cells.
The most conspicuous primitive germinal cells are situated in the superficial layer of epithelium; and the primitive germinal cells in the deeper layers of the germinal epithelium are not nearly so marked as in most Craniata, so that it is difficult in some cases to be sure of their destination till their nucleus commences to undergo its characteristic metamorphosis.