POSTNATAL GROWTH IN THE ALBINO RAT 9 



smaller than in mine but the females are heavier. At six weeks 

 and ten weeks both sexes average smaller in Donaldson's series, 

 but at five months and one year they are decidedly larger. 



In general, therefore, it is evident that Donaldson's rats re- 

 mained smaller than mine up to the age of six weeks, but sur- 

 passed them in the later stages. The probable errors were not 

 calculated for the gross body weights (except at six weeks, smaller 

 series) ; but as will be seen later the variation for the gross weight 

 is not materially different from that of the net body weight, the 

 probable errors of which, in the smaller series, are given in table 

 2. The differences between Donaldson's results and mine are 

 greater than the probable errors and therefore appear significant. 

 The explanation of these differences will be considered later under 

 variability. 1 



The relative growth of the sexes of the albino rat was noted 

 by Donaldson ('06), who found that, beginning about the end of 

 the first week, the female grows more vigorously, overtaking and 

 usually exceeding the male in body weight from the fifteenth to 

 the fifty-fifth day of age. My data (table 1) show that in 16 

 newborn litters containing both sexes the males averaged greater 

 in every case; at seven days, the males exceed in 11 litters, the 

 females in 5 ; at fourteen days, the males exceed in 3, the females 

 in 1; at twenty days, the males exceed in 10, the females in 3; 

 at thirty days; the males exceed in 2, females in none; at six 

 weeks, the males exceed in 10, the females in 2; at ten weeks, 

 the males exceed in 6, the females in 1 ; at five months the males 

 exceed in 4, females in 1; at one year, the males exceed in 2, 

 the females in 1. According to litters, therefore, the excess of 

 average weight was invariably in favor of the male at birth, and 

 also in the majority of cases at all succeeding ages. 



The ratio of the average gross body weight in the male to 

 that in the female at corresponding ages in my data is given in 

 table B. 



1 It may be pointed out in this connection that whereas my rats represent for 

 the most part a 'random sample' of the general population at each age, those of 

 Dr. Donaldson were taken while young, and used throughout the period of obser- 

 vation. 



