CHAP. XX.] FEMALE GENERATIVE ORGANS. 243 



is covered by a peculiar layer of columnar epithelium-cells, 

 called germinal epithelium. 



The Graafian follicles are cavities dotted about in the stroma 

 in large numbers. The smaller ones lie near the surface. The 

 larger are more deeply imbedded, and only approach the sur- 

 face when they are ready to discharge their contents. The 

 follicles have each their own proper wall or tunic, derived from 

 the connective tissue of the stroma, and each is lined by a layer 

 or layers of granular epithelium-cells, and contains an ovum. 

 The granular layer of cells, closely lining the cavity of the 

 follicle, is termed the membrana granulosa, but at one or other 

 side it is heaped up into a mass of cells which projects into the 

 cavity of the follicle and envelops the ovum. This mass of 

 cells which immediately surrounds the ovum is called the 

 discus proligerus. 



As the follicle matures, fluid collects in the cavity, and, 

 increasing in amount, the follicle gradually becomes larger and 

 more tense. It now approaches the surface and begins to form 

 a protuberance like a small boil upon the outside of the ovary. 

 Finally the wall of the ovary and the wall of the follicle burst 

 at the same point, and the fluid (liquor folliculi) containing the 

 ovum, with the loose, irregular mass of cells, the discus pro- 

 ligerus, clinging to it, is set free. At the moment of rupture, 

 the ovum is received by the Fallopian tube and afterwards con- 

 veyed to the uterus. After the follicle has discharged its con- 

 tents, it has done its work, and it passes through a series of 

 changes which are more or less persistent, but eventually it is 

 obliterated and disappears. Thus in the very same ovary some 

 of the follicles are mature, or approaching maturity ; others are 

 undergoing development; while others are retrograding and 

 disappearing. 



The ova are formed from the germ-epithelium on the surface 

 of the ovary, the cells of which become enlarged and dip down 

 into the stroma in the form of little elongated masses. From 

 these groups of cells the Graafian follicles and the ova are pro- 

 duced. The ovum is a highly developed cell about -j^g- inch 

 in diameter. It has (1) a thick, surrounding envelope or 

 membrane, called the vitelline membrane or zona pellucida ; 

 (2) within the membrane or cell-wall is the protoplasm of 

 the cell, rilled with fatty and albuminous granules, and usually 



