410 



PRACTICAL AN ATOM V. 



is about the size of a pea, consisting of two coats, an 

 outer fibro-vascular and an inner, called the ovi capsule, 

 and which is lined by a layer of cells. These at a certain 

 place accumulate and form a bed, the discus proligerus, in 

 which the ovum is imbedded. The ova are formed by 

 the involution of the germ epithelium from the surface of 

 the ovary. They are gradually inclosed by the stroma 

 and removed from the surface. 



FIG. 177. SECTION OF AN OVAKY. 



e, germ epithelium ; 1, large-sized follicles ; 2, 2, smaller-sized follicles ; o, ovum within 

 a Graafian follicle ; v. v, blood-vessels of the stroma ; g, cells of the membraiia granulosa. 



The ovum is a perfect cell, having a thick cell-wall, 

 nucleus, and nucleolus. It is about T ^ 7 inch in diameter. 

 Its cell-wall is called the vitelline membrane; it is 

 broad, clear, and shining, and appears as a bright ring ; 

 the dark, granular contents are known as the vitellus, or 

 yelk, and within this is imbedded a bright, clear nucleus, 

 about one-fourth the diameter of the ovum. It is called 

 the germinal vesicle, and contains a dark nucleus, the 

 germinal spot, which measures about grW mcn m diam- 

 eter. Before puberty the Graafian vesicles are undevel- 



