292 



HISTOLOGY. 



Many follicles degenerate without discharging their egg cells. Cells 

 from the stratum granulosum and leucocytes are said to invade them and 

 after absorbing the egg protoplasm they disintegrate. The zona pellucida, 

 a clear layer around the egg cell, becomes conspicuously folded and per- 

 sists for some time (Fig. 331). The basement membrane of the stratum 

 granulosum has been said also to thicken and become convoluted. These 

 degenerating or atretic follicles are finally reduced to inconspicuous scars 

 or they disappear. After the menopause the degeneration of the egg 

 cells becomes general. 



zp 



FIG. 335. THE OVUM AS DISCHARGED FROM A VESICULAR FOLLICLE OF AN EXCISED OVARY OP A 



WOMAN THIRTY YEARS OF AGE. Examined fresh in liquor folliculi. (Nagel.) 

 c. r., Corona radiata; n., nucleus; p., granular protoplasm; p. s., perivitelline space; y., yolk; 

 s. p., zona pellucida. (From McMurrich's " Embryology.") 



Oogenesis. The maturation of the ovum is comparable with that 

 of the spermatozoon. Just as an indefinite number of generations of 

 spermatogonia produced by ordinary mitosis, terminates in primary sper- 

 matocytes, so the oogonia terminate in primary oocytes. Both the primary 

 spermatocyte and oocyte give rise by two reduction divisions, in which 

 one half the somatic nuniber of chromosomes is involved, to four mature 

 sexual cells. In case of the ovum, however, only one of the four is 

 capable of fertilization. 



The sexual cells in the germinal epithelium and in the islands of the 



