The thickened germinal epithelium gives rise (in the case of the female) to the ova and the follicular epithelium. Whether the genital ridge is provided with a core of stroma or no, the germinal epithelium is always in contact on one side with the stroma, from which it is at first separated by a well-marked boundary line; but after a certain time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epithelium, and break it up into a sponge-like structure formed of trabeculæ of germinal epithelium interpenetrated by vascular strands of stroma. The trabeculæ of the germinal epithelium form the egg-tubes of Pflüger.
With reference to the distribution of the stroma in the germinal epithelium, it may be said in a general way that there is a special layer close to the surface of the ovary, which, after the formation of fresh ova has nearly ceased, completely isolates a superficial layer of the germinal epithelium from the deeper and major part of it. The superficial layer is frequently (but erroneously) regarded as constituting the whole of the germinal epithelium. The layer of stroma below the superficial epithelium forms in the mammalian ovary the tunica albuginea. As the follicles are formed. in the trabeculæ of germinal epithelium the stroma grows in around them, and forms for each one of them a special tunic.
The adult ovaries differ in a corresponding manner to the embryonic genital ridges as to the presence of a core of stroma. The ovaries which are without such a core in the embryo, are also without it in the adult, and are formed of a double layer of tissue entirely derived from the germinal epithelium with its ingrowths of stroma, and composed, for the most part, of ova in all stages of development. In the case of the other ovaries there is a hilus of stroma—the zona vasculosa—internal to the egg-bearing region.
In Mammalia, proportionately to the ovary, the zona vasculosa is at a maximum, and in Birds and Reptiles it is relatively far less developed. In these forms the germinal epithelium covers the whole surface of the ovary. In Elasmobranchii the structure of the ovary is somewhat different, owing to the presence in the ovarian ridge of a large quantity of a peculiar lymphatic tissue, which has no homologue in the other ovaries; and still more to the fact that the true germinal epithelium is in most forms entirely confined to the outer surface of the ovary, on which it forms a layer of thickened epithelium in the embryo ([fig. 17]), and of ovigerous tissue in the adult.
In the ovary of Mammalia and Reptilia and possibly other forms there are present in the zona vasculosa during embryonic life cords of epithelial tissue derived from the Malpighian bodies; these cords have no function in the female, but in the male assist in forming the seminiferous tubules.
In considering the development of the ova it is again convenient to distinguish between Amphioxus and the Craniata.
In Amphioxus the germinal cells destined to become ova are first distinguished by the larger size of their germinal vesicles and by the presence of certain refracting granules in their protoplasm. They subsequently rapidly enlarge and form protuberances on the surface of the ovary, which are enveloped for three-quarters of their circumference by the flattened epithelioid cells of the peritoneal membrane, which thus form a kind of follicle. As the ova become ripe yolk granules are deposited in their protoplasm, first in the superficial layer and subsequently throughout. The germinal vesicle also passes from the centre to the surface. A vitelline membrane is formed when the ova are mature.
In the Craniata the ova are developed from the cells of the germinal epithelium. In the types with larger ova (Teleostei, Elasmobranchii, Amphibia, Reptilia, Aves), at a very early period, sometimes (Elasmobranchii) even before the formation of the genital ridge, certain of the cells which are destined to form ova become distinguished by their greater size, and by the possession of an abundant clear protoplasm and a large spherical granular nucleus. ([Fig. 18], po.) Such special cells form primitive germinal cells, and are common to both sexes.
For a considerable period after their first formation these cells remain stationary in their development; but their number increases, partly, it appears, by an addition of fresh ones, and partly by division. Owing to the latter process the germinal cells come to form small masses or nests. The following description of the further changes of these cells in the female refers in the first instance to Elasmobranchii, but holds good in most respects for other types as well.
It is convenient to distinguish two modes in which the primitive germinal cells may become converted into permanent ova, though the morphological difference between the two modes is of no great importance.