330 



REPRODUCTION 



the germinal spot. Outside the ovum, but not strictly a part 

 of it, is the zona pellucida, a transparent envelope, and out- 

 side the zona pellucida a collection of cells, the corona radi- 

 ata. The perivitelline space is between the ovum proper and 

 the zona pellucida. The zona presents a radial striae, which 

 may facilitate the entrance of the spermatozoon. 



Ova are capable of being impregnated as long as '7-9 days 

 after their discharge from the ovary. Their formation be- 

 gins early in fetal life. The ovum possesses no power of in- 

 dependent motion. It is pass- 

 ive in fecundation ; it is sought 

 by the male element. Its vitel- 

 lus, or yolk (protoplasm), 

 contains nutritive non-living 

 material, deutoplasm, whose 

 function is to furnish food 

 substance to the impregnated 

 ovum until the fetal circulation 

 is established. Deutoplasm in 

 the human ovum is scarcely to 



,-, ,_, be distinguished from the liv- 



FIG. 93. Ovum. (From Yeo . & . 11- 



after Robin.) m g protoplasm, though in the 



ova of birds, e. g.. it is clearly 



a, zona pellucida and yitelhne mem- i 



brane; b, yolk; c, germinal vesicle or marked Off, and Constitutes the 

 nucleus; a, germinal spot or nucleo- r n r .1 



lus; e, interval left by the retraction main bulk of the mature egg, 



of the viteHus from the zona pellucida. c ;_ ,^ developing embryo 



receives no blood from the mother. 



Graafian Follicles.-The Graafian follicles are directly 

 concerned in the development and maturation of ova. These 

 are small vesicles in the cortical ovarian substance sur- 

 rounded by a capsule of thickened ovarian stroma, the 

 tunica vasculosa. Inside the tunica vasculosa, lining the 

 spherical cavity of the vesicle, are several layers of epithelial 

 cells making up the membrana granulosa. The cavity is 

 filled with an albuminous liquid, the liquor folliculi. At one 

 point in its circumference the membrana granulosa is much 



