586 



ZOOLOGY 



SECT. 



Development. — The ova of Mammals (Fig. 1213), like those 

 of Vertebrates in general, are developed from certain cells of 

 the germinal epithelium, the I'^rimitive ova (jyr. ov.). Each of 



^.ep 



trv 



pr.ov 



these, surrounded by smaller unmodified cells of the epithelium, 

 sinks into the stroma of the ovary, in which it becomes embedded, 

 the small cells forming a Graafian follicle {foil.) which encloses it. 



caps. 



ov 



tnemh 



ov 



yn^mif 



Kic. 1l'14.—T\V(i stages in the doveloiniiont nf the Graafian follicle. ^, with the folliLular 

 fhiid liegiiiiiiiiK to tipijear ; />', after tliu space lias largely iiiereased. en ^m. capsule ; Oixc. 

 iimiuhis iiniligerus ; nwiJi. mcmbraiui granulosa ; of. ovum ; si>. space containing fluid. 

 (After Hertwig.) 



Spaces filled with fluid soon appear among the follicle cells (Fig. 

 121-i, Ai^j.), and these eventually coalesce to form a single cavity. 

 This cavity, which in some Mammals is crossed by strings of 



