REPRODUCTION 619 



the ovaries to produce ova, and, as they become functionally 

 active, the secondary sexual characters develop (p. 606). 



A. Ovary. — The ovaries are oval structures lying in a fold of 

 peritoneum — the broad ligament. The cells covering the genital 

 ridge grow downwards as the oogonia. These become disposed 

 (1) as a covering layer of columnar cells ; (2) as interstitial cells 

 (p. 608) ; (3) as clumps of cells forming the Graafian follicles. 

 The central cell of each of these undergoes further growth, 

 becomes laro-er than the surrounding cells and forms the 

 primary oocyte. This cell throws out the first polar body 

 and becomes a secondary oocyte. A second polar body is 



Spermatogonium. Oogonium. 



Growth period. 



MataratioQ 

 period. 



Fig. 235. — Scheme of Spermatogenesis and Oogenesis. 



then thrown out, and half the chromosomes are thus elimi- 

 nated and the mature ovum is formed (fig. 235). This becomes 

 surrounded by a capsule, the zona j)ellucida. The surround- 

 ing cells forming the zona granulosa multiply, and a fluid, 

 the liquor folliculi, appears among them, dividing them 

 into a set attached to the capsule of the follicle and a set 

 surrounding the ovum. When the follicle is ripe, it projects on 

 the surface of the ovary, and finally bursts, setting free the 

 ovum, which escapes into the peritoneal cavity and passes into 

 the trumpet-shaped fimbriated upper end of the Fallojnan tube 

 through which it reaches the uterus. The ruptured Graafian 

 follicle generally becomes filled with blood, and later with the 



