310 THE MAMMALIAN EMBRYO. [CHAP. 



in the thickened mass a cavity which gradually becomes 

 more and more distended and filled with an albuminous 

 fluid. 



As the cavity enlarges, the OYum, around which are 

 several layers of cells, forms a prominence projecting 

 into it. The follicle cells are known as the membrana 

 granulosa, and the projection in which the ovum lies as 

 the discus or cumulus proligerus. The whole structure 

 with its tunic is known as the Graafian follicle. 



If the ovary of a mature female during the breeding 

 season be examined, certain of the protuberances on its 

 surface maybe seen to be considerably larger 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 ovum is surrounded by a 

 radiately striated membrane, the zona radiata, internal 

 to which in the nearly ripe egg a delicate membrane 

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

 of the discus are supported upon an irregular 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 renders it probable 

 that it is the remnant of the first formed membrane 

 in the young ovum and is therefore the vitelline mem- 

 brane. 



Maturation and impregnation of the ovum. As 

 the ovum placed in the Graafian follicle approaches 

 maturity the germinal vesicle assumes an excentric 



