56 CRANIATA. 



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 con- 

 venient 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 peri- 

 toneal 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. i8,/0.) Such special cells form primitive germi- 

 nal 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 in- 



