42 S. ZUCKERMAN 



course, conceivable that the nuclear changes described by 

 Aron et al. in the ovary of the mature guinea pig constitute 

 no more than the meiotic changes which usually occur pre- 

 pubertally in most other mammals. For example, de Wini- 

 warter (1920) has noted that the reduction division of oocytes 

 may occur in the ovaries of cats which have just passed 

 puberty. To the best of my knowledge, however, active 

 meiotic changes are not usually seen, except in degenerating 

 follicles or follicles about to ovulate, in the ovaries of adult 

 rats, mice, rabbits, ferrets and monkeys. 



None of these reports, indicating or suggesting that meiosis, 

 and in Gerard and Herlant's case mitosis as well, may occur 

 in the germinal cells of the ovaries of mature females of 

 certain species, is furnished with a control account of the 

 histology of the foetal ovary, at those times when oogonia are 

 unquestionably multiplying. This deficiency was recognized 

 by Rao, but is not referred to by the other workers concerned. 

 Bujard (1947) describes the ovary of the immature guinea pig, 

 but nothing he records allows one to determine the extent to 

 which the presumed oogenetic phenomena in the adult ovary, 

 as outlined by Aron et al., and as confirmed by ourselves, 

 correspond with the histological manifestations of actual 

 oogenesis, which presumably is still in progress at birth. 

 Oogenesis in the mature animal must by definition mean the 

 continued formation of several oocytes from some precursor 

 cell, in the same sense that a single spermatogonium is in the 

 end responsible for the production of a very large number of 

 spermatozoa. An essential criterion of oogenesis would, there- 

 fore, have to be the multiplication of oogonia by mitosis, in 

 the same sense that the occurrence of meiosis represents the 

 end, not an intermediate phase, in the process of formation of 

 the ovum. 



The studies reported by Burkl (1954, 1955) and Burkl and 

 Kellner (1954a, b, 1955) are in general of a more experimental 

 kind than those of Aron et al. In his first paper, Burkl (1954) 

 reports that oestrogen (hexoestrol implantation) increases the 

 rate of neoformation of oocytes in rats, as judged by the 



