8 



HELEN DEAN KING 



rapidly than males. In later generations (tables 2 and 3) the 

 growth rate of females exceeded that of males throughout 

 the suckling period, since at 13 days the average weights of 

 males and females were the same, and at 30 days the weight 

 excess of males was but 2 per cent. Subsequently the growth 

 rate in females decreased while that in males increased. The 



TABLE 2 



Increase in the weight of the body with age for individuals in the sixteenth to the 

 twentieth generation of captive gray rats 



AGE IN 



DAYS 



13 

 30 

 60 

 90 

 120 

 151 

 182 

 212 

 243 

 273 

 304 

 334 

 365 

 395 

 425 

 455 

 486 

 516 

 547 

 578 

 608 



weight excess of males had increased to about 21 per cent by 

 the time the rats were 8 months old. Thereafter the average 

 weight differences between the sexes remained fairly constant. 

 It thus appears that in gray rats, as in albinos (Donaldson, 

 '06), the growth curves are similar to those for man in that, 

 although males tend to be heavier than females at birth and 



