THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



379 



Each primary ovarian follicle, containing a primitive ovum (egg cell, sex cell) , 

 is composed of a single layer of flat or cuboidal cells, plus a layer of stroma 

 which gives rise to the theca folliculi. As the ovum continues to enlarge, the 

 follicular cells become higher and arranged in a radial manner (Fig. 331, a) . By 

 proliferation, the follicular cells come to form several layers, the innermost 

 layer retaining the radial character and forming the zona radiata. The inner or 

 basal ends of the cells of the zona radiata become clear to form the zona pellucida. 

 In the latter, radial striations appear which have been described as minute 



. c d 



FIG. 331. Four stages in the development of the ovarian (Graafian) follicle 



From photographs of sections of a cat's ovary Hertwig. 



The ovum is not shown in a, b and c. 



channels in the cells, through which nutriment may pass to the ovum. After 

 the follicular epithelium has become several layers thick, a fluid substance 

 known as the liquor folliculi, and probably derived from the cells themselves, 

 comes to lie in little pools among the cells (Fig. 33 1, b and c ) . While the follicle 

 as a whole enlarges, these pools gradually coalesce and form a single large pool 

 which fills the interior of the follicle (Fib. 331, d). Thus the epithelium is 

 crowded out toward the periphery where it forms a layer several cells in thick- 

 ness, known as the stratum granulosum. The ovum itself, with the zona radiata 

 and some other surrounding cells, is also crowded off to the periphery of the 



