OOGENESIS IN THE WHITE MOUSE 263 



was selected for this work as it seemed to offer a number of 

 advantages. The sexual cycle is completed in about sixty 

 days (Kirkham, '16) of which the period of gestation occupies 

 twenty or so. For this reason it is possible to secure a complete 

 series of ovaries from birth to sexual maturity, as well as during 

 embryonic life, without involving an excessive amount of material. 

 The small size of the ovaries of even adult mice reduces the 

 manipulation, a consideration not to be despised when making 

 a large number of preparations. 



.Von Winiwarter and Sainmont state — reserving a full de- 

 scription for a later chapter which, as yet, has not appeared — 

 that the definitive ova are formed after birth in the young 

 kitten about three and a half or four months old, shortly before 

 sexual maturity. This new formation of egg-cells takes place 

 by a differentiation of cells from the germinal epithelium of the 

 ovary. If this post-natal formation is of more than an exceed- 

 ingly limited and special significance, it will be found to occur 

 in other forms. Accordingly, in a form like the mouse which 

 has a short sexual cycle, making it fairly easy to secure a com- 

 plete series of ovaries between birth and sexual maturity, it 

 should be possible to confirm or disprove such a new formation 

 of egg-cells. 



In this study, the emphasis is laid on oogenesis, which is used 

 to mean the development of the definitive germ cells or ova 

 from undifferentiated cells into primary oocytes in mature or 

 nearly mature Graafian follicles. Further development, that 

 is, maturation, will not be discussed, being beyond the limits 

 planned for this study. My results agree with those of von 

 Winiwarter and Sainmont in that, in the mouse, this process of 

 oogenesis or differentiation of the definitive ova takes place 

 after birth in the period before sexual maturity. Furthermore, 

 in the mouse, as they found in the cat, the germ cells formed 

 before birth during embryonic and foetal life all degenerate and 

 have nothing to do with the development of the definitive ova. 

 The development of these cells, however, and the morphogenesis 

 of the ovary will be briefly described in order that the process of 

 oogenesis proper may receive the setting necessary for its apprecia- 

 tion and understanding. 



