758 



EMBRYOLOGY 



their appearance. The prevailing idea is, indeed, that the great major- 

 ity of these never arrive at maturity and that they undergo atrophy at 

 various stages of their development. In the adult, according to Wal- 

 deyer, the smallest Graafian follicles measure g^- to r J T of an inch 

 (30 to 40 JJL\ and the smallest ova, a little more than T ^ T of an inch 

 (26 p). The primordial ova have the form of rounded cells, each with a 

 large clear nucleus and a nucleolus. Other structures are developed in 

 and surrounding these cells as the ova arrive at their full development. 



Fig. 198. Ovum of the cat, within the ovary, directly reproduced from a photograph of a 

 preparation by Dahlgren, x 235 (Wilson). 



The ovum lies in the discus proligerus within the Graafian follicle. 



The most important stage in the development of the ova and 

 Graafian follicles is observed at about the beginning of puberty. At 

 this time a number of follicles (twelve, twenty, thirty or even more) 

 enlarge, so that all sizes are observed, between the smallest primordial 

 follicles, -%^-Q of an inch (30 /u), and the largest, nearly -|- an inch 

 (12 millimeters) in diameter. In follicles that have attained any consid- 

 erable size, there are the fully-developed ova, one in each follicle 

 except in rare instances, when there are two; and these ova have a 

 diameter of about T J^ of an inch (200 /JL). In the process which cul- 

 minates in the discharge of the ovum into the fimbriated extremity of 



