310 THE MAMMALLVX EMBRYO. [CHAP. 



in the thickened mass a ca'vitv which i^raduallv becomes 

 more and more distended and lilled with an albuminous 

 fluid. 



As the cavity enlarges, the o-v-imi. around which are 

 several layers of cells, forms a prominence projectinsj 

 into it. The folHcle cells are known as the membrana 

 gi-anulosa, and the projection in which the o^um hes as 

 the discus or cumulus proligems. The whole structure 

 with its tunic is known as the Graafian follicle. 



If the ovarv of a mature female durincj the breedinsf 

 season be examined, certain of the protuberances on its 

 surface mav be seen to be considerablv laro-er than others : 

 they are more transparent than their fellows and their 

 outer covering appears more tense : these are Graafian 

 follicles containing nearly or quite ripe ova. Upon pierc- 

 ing one of these follicles with a needle-point the ovum 

 contained therein spirts forth together with a not incon- 

 siderable amount of clear fluid. 



Egg Membranes. The o-^-um is surrounded by a 

 radiately striated membrane, the zona radiata. internal 

 to which in the nearly ripe egg a dehcate membrane 

 has been shown, by Ed. v. Beneden. to exist. The cells 

 of the discus are supported upon an ii'regular granular 

 membrane external to the zona radiata. This mem- 

 brane is more or less distinctly separated from the zona, 

 and the mode of its development rendei"S it probable 

 that it is the remnant of the first formed membrane 

 in the young ovum and is therefore the vitelline mem- 

 brane. 



Matiu'-ation and impregnation of the ovum. As 

 the ovum placed in the Graafian follicle approaches 

 maturitv the germinal vesicle assumes an excentric 



