16 HELEN DEAN KING 



rats reared as controls for the inbred series (King, '15 a) the ma- 

 jority of the males exceeded the females in body weight at each 

 weighing period. As no records were taken when the animals 

 were just six weeks old, the relative size of the two sexes at this 

 age was not determined. 



At thirteen days of age, as tables 4 to 7 show, the average 

 body weight of the inbred males was only one gram more than 

 that of the females. Such a slight difference as this would be 

 negligible, considering the possible error in the weighings due to 

 the varying amount of food in the alimentary tract, save for 

 the fact that it is found in every generation group. In some 

 litters the females, were, on the average, heavier than the males 

 at thirteen and also at thirty days of age, and in a few instances 

 the weight of the largest female surpassed that of the smallest 

 male even when the animals were sixty days old. Although 

 the albino female, whether stock or inbred, has a relatively 

 smaller birth weight than the male, she soon comes to have a 

 body weight that is very nearly equal to that of the males in all 

 cases, and often exceeds it. Even though the absolute body 

 weights are less at any given age, therefore, the female grows 

 more vigorously than the male during the first few weeks of 

 postnatal life. An early acceleration in the growth of the fe- 

 male also occurs in the guinea-pig (Minot, '91 ; Castle, '16), and it 

 finds a parallel in man, as Donaldson ('12) has pointed out, 

 since during a certain phase of development, when children are 

 from thirteen to sixteen years old, the average weight of girls is 

 greater than that of boys; while at all other ages boys, as a rule, 

 are the heavier. 



The relative growth in body weight of males and of females in 

 corresponding generations of the two inbred series, or in suc- 

 ceeding generations of the same series, can be determined by 

 referring to the data- in tables 4 to 7. For a comparative study 

 of the body growth of the individuals in the two series it seemed 

 advisable to combine the data for three succeeding generations. 

 Data for the A series of inbreds are given in table 8. 



The growth graphs shown in figure 2 were constructed from 

 the data for the males of the A series, as given in table 1 and 



