1. LIFE PROCESSES ')') 



tion takes place when females are relatively old, and that the 

 ova liberated are capable of being fertilized, but that senes- 

 cence has produced changes in the uterus that render it im- 

 possible for the embryos to undergo normal development. 

 These changes evidently occur at various ages in different 

 individuals, which accounts for the curtailing or the lengthen- 

 ing of the reproductive period, but what they are and how 

 they interfere with embryonic development remains to be 

 determined. 



Data given in this section show that sterility in captive 

 gray females had decreased from 70.00 per cent to less than 

 10 per cent at the end of nine generations. The cause of this 

 decrease can probably be attributed largely to a lessening of 

 nervous tension as the animals became more adapted to 

 changed conditions of environment and to the adequate food 

 that they received. Infertility in females of the later gener- 

 ations was due chiefly to diseases that affected the repro- 

 ductive organs or to decreased vitality resulting from lung 

 infection. 



THE SEX RATIO 



The proportion of the sexes at birth in wild Norway rats 

 has not been determined as yet. Data obtained by Miller 

 ('11) showed that there were twenty-three males and twenty- 

 eight females, or a sex ratio of 82.1 males to 100 females, in 

 five litters cast by caged females of unknown ages. Among 

 trapped animals, according to Lantz ('09) and Donaldson 

 ('12), males are in excess, as a rule. This finding does not 

 indicate the relative proportion of the sexes among adult 

 animals generally, however, since the females, if suckling 

 young, probably remain in or close to the nest most of the 

 time and therefore are not as readily trapped as are tin- 

 males. Sex ratios for gray Norway rats, given in previous 

 publications (King, '24, '27) were based on data obtained 

 from the series of these animals that form the material for 

 the present study. 



