THE OVUM. 45 



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 pecu- 

 liar lymphatic tissue, which has 110 honiologue 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 Arnphioxus 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 vitelliue 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. 



