180 



THE ESSENTIALS OF HISTOLOGY 



has a proper wall of basement-membrane, strengthened by a layer 

 derived from the strorna, and contains an ovum, surrounded by a 

 number of epithelium-cells. 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. 



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 occurs at about the time of menstrua- 

 tion. 1 



ff* 



FIG. 217. SEMI-DIAGRAMMATIC REPRESENTATION OF A MAMMALIAN OVUM. 



(Highly magnified.) 



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



The ova are large spheroidal cells (fig. 217). When mature, as in 

 the largest Graafian follicles, each ovum is surrounded by a thick 

 transparent striated membrane (zona pellucida]. Within this is the 

 protoplasm of the cell (vitellus), filled with fatty and albuminous 

 granules. Lying in the vitellus, generally eccentrically, is the large 

 clear round nucleus (germinal vesicle], which contains an intranuclear 

 network, and usually one well-marked nucleolus (germinal spot). Both 

 the ova and the epithelium of the Graafian follicles are developed 



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

 stage of maturity, undergo a process of retrograde metamorphosis and eventually 

 disappear. 



