44 



HELEN DEAN KING 



It lias been shown that in each of four different strains of 

 rats the sex ratio changed in a similar way as the litter series 

 advanced (King, '24: table 8). This finding seems to indi- 

 cate that the age of the mother has some influence on the sex 

 ratio in her newborn young. As a further test of this assump- 

 tion, litter data for gray rats were arranged according to the 



TABLE 14 



Sex distribution and sex ratios 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 



age of the mothers at the time of parturition, and the sex 

 ratios calculated. The series of ratios thus obtained is given 

 in table 14. 



Males outnumbered females in litters cast when mothers 

 were from 3 to 5 months of age, and also in litters born when 

 the mothers were 11 and 12 months old. At all other age 

 periods of the mothers litters contained an excess of females, 



