REPRODUCTION 6i 



age of follicles containing an antrum, as compared with the total number of 

 follicles having at least two layers of follicular cells, is ii% at fifteen days. 

 This figure rises to 39% at thirty-seven days, falls to 29% at fifty days, and 

 then ascends sharply until it reaches 50% at sixty-six days when ovulation 

 occurs. Hargitt (62) likewise has noted a decrease in the number of large 

 follicles in rat ovaries two to three weeks before the first ovulation, and finds 

 this to be due to an increased rate of degeneration of such follicles at this 

 time. Ovulation in his animals occurred at about 45 days, and the ovaries 

 at 29-32 days showed a drop in the number of large follicles. In the case 

 of mice, also, a reduction in the number of large follicles in the ovaries of 

 animals 28 days old, as compared with the number at 21 days, has been 

 noted (18). 



According to a recent study with rats (124), ovogenesis between birth 

 and maturity is cyclic, with maxima occurring approximately every ten 

 days. In this investigation, as in others described above, the first maximum 

 wasTound to occur at six or seven days post partum. Other maxima 

 occurred at approximately ten day intervals until the onset of the normal 

 estrous rhythm. Follicular atresia during this period was found also to be 

 cyclic with about ten days between peaks. How this prepuberal rhythm of 

 ovogenesis and atresia is related to the prepuberal fluctuations in the pro- 

 portions of large follicles described by other authors is not yet clear. 



The process of ovogenesis continues, though somewhat more slowly, 

 until fecundity is lost in old age. During maturity it shows fluctuations 

 that coincide with the estrous cycle (see p. 74). The process is less con- 

 spicuous in older mice because the newly formed ova do not attain such large 

 size while still in the germinal epithelium and hence are more easily confused 

 with epithelial cells. Some authors have disputed the continued production 

 of ova by the germinal epithelium during maturity, but recent work quite 

 definitely confirms its occurrence (4, 25). 



Coincident with the occurrence of ovogenesis, continued ovular degenera- 

 tion is likewise going on. As a result there is a more or less steady reduction 

 in the number of ova present in the ovaries. Counts by Arai (8) in the rat 

 show a total of approximately 35,100 ova in both ovaries at birth, 11-10,000 

 at 2 1, days and 63 days, 6,600 at 70 days, 2,000 at 31 months. Except for 

 the period from 23 to 63 days, ovogenesis is not sufficiently rapid to replace 

 the ova lost through ovular degeneration and normal ovulation (For the 

 details of the degenerative changes in atretic ova and follicles see p. 154-) 



In addition to abnormal ova due to degenerative changes, polyovular 

 follicles and polynuclear ova, probably not due to degeneration, have been 



