32 S. ZUCKERMAN 



throughout reproductive life is very insecurely based. Since 

 then, three sets of papers have appeared, stating the opposite 

 view (see p. 36), and it is necessary to see how the work done 

 in my own laboratory stands in the light of these newer 

 studies. 



The more important of our own observations, all consistent 

 with the view that the female mammal usually begins her 

 reproductive life with ovaries that are already furnished with 

 a finite stock of oocytes, are the following : 



1. The total number of oocytes declines sharply with age 

 (rats: Mandl and Zuckerman, 1951a; monkeys: Green and 

 Zuckerman, 1951, 1954). 



On this point our observations merely confirm the careful 

 studies of the rat carried out by Arai (1920a), as well as the 

 impressions obtained by a number of other workers in some- 

 what sketchy studies of other species. 



2. Oocytes without any cellular cover, or surrounded by a 

 single layer of cells, make up at least 90 per cent of those 

 present in the monkey and rat (Green, Mandl and Zuckerman, 

 1951). Given that proper care is taken to enumerate all 

 oocytes, and that account is also taken of age and litter- 

 relationships, no evidence can be found that the total number 

 of oocytes fluctuates with the phases of the oestrous or 

 menstrual cycle (rat: Mandl and Zuckerman, 1950; monkey: 

 Green and Zuckerman, 1951, 1954). 



The contrary view, which has been taken to mean that there 

 is a cyclical generation of oocytes in the mature ovary, has 

 been mainly based on impressions gained from the variations 

 that occur during the course of the menstrual and oestrous 

 cycle, not in the total population of oocytes, but in the num- 

 bers of, and in the incidence of atresia in, large and medium- 

 sized follicles. We have confirmed by numerical study that 

 the latter changes do occur, but have also shown that no value 

 can be attached to views about the neoformation of oocytes 

 in the adult ovary, or about cyclical variations in the intensity 

 of oogenesis, that are based on counts from which the primor- 

 dial germ cells are excluded. The same point is illustrated by 



