GENERATIVE ORGANS OF THE FEMALE. 



211 



in the stroma, although, as they increase in size, they may extend 

 towards the surface. 



These vesicles are the Graafian follicles. Each Graafian follicle has 

 proper wall (theca folliculi) formed of a layer derived from the stroma, 

 and contains an ovum and epithelium. In the smallest follicles the 

 ovum is small, and the epithelium of the follicle is formed of a single 

 layer of cells, which are flattened against the ovum. In somewhat 

 larger follicles the epithelium-cells are in two layers, and these are 

 columnar in shape. In still larger ones, each of these two layers is 

 formed of several strata of cells, and fluid has begun to collect between 

 the layers at one part. Of the two layers, the one which lines the 

 cavity of the follicle is termed the membrana granulosa, while the mass 

 of cells which more immediately surrounds the ovum is known as the 

 discus proligerus. 



FIG. 245. SEMI-DIAGRAMMATIC REPKESENTATION OF A MAMMALIAN OVUM. 

 (Highly magnified.) 



zp, zona pellucida ; vi, vitellus ; gv, germinal vesicle : gs, germinal spot. 



In the largest follicles the fluid has much increased in amount, so 

 that the follicle has become gradually larger and more tense. Finally 

 it reaches the surface of the ovary, and projects from that surface, 

 where it eventually bursts, and the liquor folliculi, with its contained 

 ovum, is set free. This event is believed to occur usually at about the 

 time of menstruation. 1 



The ova are large spheroidal cells (fig. 245), about ^ T inch in 

 diameter. When mature, as in the largest Graafian follicles, each ovum 

 is surrounded by a thick transparent striated membrane (zona pellucida). 



1 Some of the Graafian follicles do not burst, but, after attaining a certain 

 stage of maturity, undergo a process of retrograde metamorphosis'aiid eventually 

 disappear. 



