LIFE PROCESSES IN CAPTIVE GRAY RATS 



35 



As indicated in table 10, comparatively few litters were 

 cast when females were not more than 3 months of age. 

 Although the sex organs are capable of functioning at this 

 age, the bodies of the females are not fully grown and so less 

 able to nourish embryos than when they are more mature. 

 The number of litters cast increased with the age of the 

 mothers to the period of greatest productiveness, which came 



TABLE 30 



Litter production and litter size with their coefficients of variation in the second 



to the twenty-sixth generation of captive gray rats. Data arranged 



according to the age of the mothers at the time of parturition 



when females were 7 months old, and then declined as the 

 intervals between litters became longer. Few litters were 

 cast after females reached the age of 20 months. Autopsies 

 made on very old females some times disclosed embryos in the 

 uterus that were being resorbed, probably because senility 

 changes in the uterus so interfered with the nutrition of the 

 young that they died at an early period of development. 



