6 INTERNAL SECRETIONS OF THE OVARY 



in supposing that both ovum and fohicular epithehum are de- 

 rived direct from the germinal epithehum. The ovum itself con- 

 sists of an external zona pellucida which encloses a cytoplasmic 

 mass containing the nucleus and certain cytoplasmic inclusions. 



Ovulation, which takes place spontaneously at oestrus in most 

 species, is preceded by certain changes in the follicle. The 

 distension due to the accumulation of liquor folliculi causes the 

 follicle to approach the periphery of the ovary, so that discharge 

 of the ovum becomes possible. The discus proligerus, containing 

 the ovum, breaks away from the membrana granulosa, and at 

 the same time the cells become arranged radially round the 

 ovum, from which they tend to withdraw, forming the corona 

 radiata or cumulus oophorus. Finally, the first polar body is 

 given off, and ovulation takes place. The nature of the final 

 stimulus required for ovulation is not known. If fertilization 

 takes place, the second polar body is given off, and the ovum 

 proceeds down the Fallopian tube. Otherwise, degenerative 

 fragmentation occurs, which has occasionally been mistaken 

 for parthenogenesis. 



Although ovulation is the predestined end of the Graafian 

 follicle, the majority fail to complete the full life history 

 owing to their excessive number, and undergo atresia at some 

 stage of their growth. The degenerative processes usually 

 begin in the granulosa, from which odd cells are shed into 

 the antrum in a state of pycnosis. The degeneration of the 

 ovum, accompanied by spurious maturation divisions, usually 

 takes place later in atresia. Finally, the follicle is either entirely 

 absorbed or metamorphosed into interstitial tissue or a corpus 

 luteum atreticum. Cyclic bursts of atresia seem to take place 

 in the ovary corresponding with the stages of the oestrous 

 cycle. Atresia of small follicles tends to be most common 

 during pregnancy and in the vicinity of large corpora lutea. 

 Atresia of large follicles is very evident in the rabbit (in the 

 absence of the sexual stimulus required for ovulation) and in 

 the guinea-pig where only two or three of each batch of mature 

 fohicles ovulate. For the extensive literature on fohicular 

 atresia the bibliography given by Salazar (541) may be 

 consulted. 



Abnormalities of the ovum and follicle, such as multinucleate 



