748 HANDBOOK OF PHYSIOLOGY. 



columns or tubes. By degrees these tubes become cut off from the surface 

 epithelium, and form cell nests, small, if near the surface, larger if in 

 the depth of the stroma. The nests increase in size from multiplication 

 of their cells, and may even give off new nests laterally by constriction of 

 them in various directions. Certain of the cells of the germinal 

 epithelium enlarge, and form ova ; and the formation of ova also takes 

 place in the nests within the stroma. The ova of a nest may multiply 

 by division. The small cells of a nest surround the ova, and form their 

 membrana granulosa, and the stroma growing up separates the surrounded 

 ova into so many Graafian follicles. The other layers, namely, the mem- 

 brana fibrosa and the membrana vasculosa, are derived from the stroma. 



The smallest follicles are formed at the surface, and make up the cor- 

 tical layer. It is said by some that the superficial follicles as they ripen 

 become more deeply placed in the ovarian stroma; and, again, that as 

 they increase in size, they make their way toward the surface (fig. 443) . 



When mature, they form little prominences on the exterior of the 

 ovary, covered only by a thin layer of condensed fibrous tissue and epithe- 

 lium. Only a few follicles ever reach maturity. 



From the earliest infancy, and through the whole fruitful period of 

 life, there appears to be a constant formation, development, and matura- 



Fig. 446. Germinal epithelium of the swrf ace of the ovary of five days' chick, a, small OTO- 

 blasts; 6, larger ovoblasts. (Cadiat.) 



tion of Graafian vesicles, with their contained ova. Until the period of 

 puberty, however, the process is comparatively inactive; for, previous to 

 this period, the ovaries are small and pale, the Graafian vesicles in them 

 are very minute, and probably never attain full development, but soon 

 shrivel and disappear, instead of bursting, as matured follicles do; 

 the contained ova are also incapable of being impregnated. But, coin- 

 cident with the other changes which occur in the body at the time of 

 puberty, the ovaries enlarge, and become very vascular, the formation 

 of Graafian vesicles is more abundant, the size and degree of development 

 attained by them are greater, and the ova are capable of being fecun- 

 dated. 



The Fallopian Tubes (Oviducts}. The Fallopian tubes are about 

 four inches in length (10 cm.), and extend between the ovaries and the 



