THE OVARIES. 



247 



favorable conditions, to the production of mature germ-cells. Of the thousands of 

 primary ova contained within the ovaries just before puberty, only comparatively few 

 attain perfection, between 300-400 probably being the maximum number liberated 

 during the usual period of sexual activity. When enclosing an ovum destined for 

 complete development, the primary follicle enters upon a period of active growth, the 

 flat mantle cells of the egg-sac changing into a single layer of cuboid epithelium. 



The growing follicles are distinguished by the rapid proliferation of their cuboid 

 epithelium that results in the production of a stratified follicular epithelium surrounding 

 the ovum. Outside this polygonal epithelium, the stroma condenses into a connective 



Germinal epithelium 



Primordial ovum 



Ova 



FIG. 296. Section of developing ovary from human embryo, showing intergrowth between derivatives 

 from germinal epithelium and stroma-tissue from Wolffian body. X 560. 



tissue envelope, the theca, which subsequently differentiates into an outer and an inner 

 tunic, the former being composed of concentrically disposed fibrous tissue and the 

 latter of round or spindle cells and numerous capillaries. After the formation of the 

 follicular epithelium, the ovum itself begins to grow, the expansion proceeding uni- 

 formly and affecting all parts of the cell, including the nucleus and nucleolus. It 

 attains its maximum diameter long before the follicle reaches full growth. Through 

 the activity of the follicular epithelium the egg becomes invested with a protecting 

 envelope, the zona pellucida or radiata, after which little or no further increase in 

 the size of the egg occurs. At first solid, the growing follicle is converted into a 

 vesicle containing fluid by, at first, the progressive vacuolation and breaking down of 

 the cells of the middle layers of the follicular epithelium and, later, by the transudation 

 from the surrounding blood-vessels. This fluid, the liquor folliculi, increases to sucli 

 an extent that it soon occupies the greater part of the expanding egg-sac, now entering 

 upon its final stage of growth. 



The maturing follicles, also known as vesicular, occupy the deeper parts of the 

 cortex and reach to the medulla. With their continued expansion they appropriate 

 more and more of the cortex, until the entire thickness of the latter and, sometimes, 

 part of the medulla are occupied by the ripe follicle, which just before its rupture 

 attains a diameter of from 1-2 cm. and models the free surface of the ovary as a tense 

 rounded elevation. After rupture and liberation of the ovum, the follicle is converted 

 into a corpus luteum. 



