20 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



in number during the growth of the ovum until several layers surround it (Fig. 3) . 

 A cavity appearing between these cells becomes filled with fluid and thus forms 

 a sac, the Graafian follicle, within which the ovum is eccentrically located. The 

 cells of the Graafian follicle immediately surrounding the ovum form the corona 

 radiata (Fig. i) when the ovum is set free. 



Ovulation and Menstruation. When the ovum is ripe, the Graafian 

 follicle is large and contains fluid, probably under pressure. The ripe follicles 

 form bud-like projections at the surface of the ovary (Fig. 5) , and at these points 



Ovum 



Follicle cells 



T "& ::--9'-^*^Af9"iL f af-^St^. 

 *) 1 j4tf/#3liSra1lJ9> 



Zona pellncida 

 FIG. 6. Immature follicle containing several ova. From the ovary of a young monkey. X 430. 



the ovarian wall has become very thin. It is probable that normally the bursting 

 of the Graafian follicle and the discharge of the ovum are periodic and associated 

 with the phenomena of menstruation. That ovulation or discharge of the ovum 

 from the ovary may occur independent of the menstrual periods has been proven 

 by the observations of Leopold. Also in young girls ovulation may precede 

 the inception of menstruation and it may occur in women some time after the 

 menopause. 



At birth, or shortly after, all of the ova are formed in the ovary of the female 



