364 THE ORGANS 



egg cell is in condilion to be fertilized, consists in changes in the 

 chromatic elements of the nucleus, which result in the extrusion of 

 the polar bodies, and apparently have as their main object the reduc- 

 tion in number of chromosomes to one-half the number characteristic 

 of the species. This process has been described (page 59). In 

 many of the lower animals maturation of the ovum is completed 

 outside the ovary. In man and the higher animals the entire process 

 takes place within the ovary, the second polar body being extruded 

 just before the escape of the ovum from its follicle. 



~J{i^;::s — — T ^ ;■■?.' ■ !: ■ ■ ■'■ e 





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Fig. 256. — Section through Graafian FolUcle of Ape's Ovary. X90. (Szymono- 

 wicz.) Later stage of development than Fig. 255. a, Germ hill; &, ovum with clear 

 zona pellucida, germinal vesicle, and germinal spot; d, follicular epithelium (membrana 

 granulosa); e, foUicular cavity;/, t heca folliculi ; g, blood-vessel. 



The youngest of the Graafian follicles are found just under the 

 tunica albuginea near the germinal epithelium, from which they orig- 

 inate (Fig. 252, 5). As the follicle matures it passes deeper into the 

 cortex. With complete maturity the follicle usually assumes macro- 

 scopic proportions — 8 to 12 mm. — and often occupies the entire 

 thickness of the cortex, its theca at one point touching the tunica 

 albuginea. Thinning of the follicular wall nearest the surface of the 

 ovary next takes place (Fig. 259), while at the same time an increase 

 in the liquor folliculi determines increased intrafollicular pressure. 

 This results in rupture of the Graafian follicle and the discharge of 

 its ovum, together with the liquor folliculi and some of the follicular 

 cells. 



