10 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



Ovulation and Menstruation.-At birth, or shortly after, ova cease 

 forming The number at this time in both ovaries has been placed be- 

 "0 ooo and 800,000. Cellular degeneration reduces this supply 



FIG. 5.-Uterine tube and ovary with mature Graafian follicle (Ribemont-Dessaignes). 



until, at 18 years, the total is from 35,000 to 70,000. Of these, relatively 

 few reach maturity, only about 200 ripe ova developing in each ovary 

 during the thirty years of sexual activity. Several years after the meno- 



Ovum 



Follicle cells 





Zona pellucida 

 FIG. 6. Immature follicle containing six ova. From the ovary of a young monkey. X 43. 



pause no more ova are to be found. When an ovum is ripe, the Graafian 

 follicle is large and contains fluid, probably under vascular and muscular 

 pressure. The ripe follicles form bud-like protuberances at the surface of 



