14 THE EGGS OF MAMMALS 



minal epithelium contain mature nuclei (Plate II, Figs. 23- 

 24) in which the modification presaged in the younger 

 animals attains culmination. These definitive ova show then 

 a modified type of meiosis which involves essentially the dis- 

 appearance of leptotene and synizesis, and the formation 

 of an atypical pachynema and diplonema. Evans and 

 Swezy (1931) obtained confirmation of these findings in the 

 guinea pig, cat, dog, monkey and man. They point out 

 that instead of being long-lived, the egg cells of mammals 

 are subject to heavy mortality and exhibit a very short 

 life cycle, correlated apparently with the length of the normal 

 ovarian rhythm. In those animals in which the oestrus and 

 ovarian cycles coincide {e.g., rat, mouse, guinea pig) the 

 length of the oestrus cycle is a measure of the lifetime of the 

 ovum in the ovary. 



These rather straightforward histological findings seem 

 to indicate, on the whole, that the definitive ova originate 

 from the germinal epithelium. All our recent knowledge of 

 the rhythmic activity of the ovary with its periodic produc- 

 tion of large numbers of young ova (Allen, Kountz and 

 Francis, 1925) militates against the assumption of a single 

 large initial store of ova gradually being exhausted through- 

 out sexual maturity. 



The Experimental Investigation of the Growth of 



Egg Cells 



Any attempt to analyze the experimental data pertinent 

 to the problem of the origin of the definitive ova encounters 

 two difficulties. First of all many of the experiments are 

 concerned with the simple Weismannian problem and ignore 

 certain now obvious endocrinological implications. And 

 secondly, the difficulty of experimental treatment of mam- 

 malian embryos makes for a hiatus in our knowledge that 

 can only be bridged by indirect deduction. 



The information that we do have at hand is derived from 

 experiments concerned with the effects resulting from (1) bi- 

 lateral ovariectomy, (2) partial ovariectomy, (3) ovarian 



