4U HELEN DEAN KING AND HENRY H. DONALDSON 



rats with the heavier weights at birth tend to maintain their 

 supremacy at all age periods. While variations in the length 

 of the gestation period may account for the unlike birth 

 weights of the members of these litters, they cannot explain 

 the differences in the growth rate and in the ultimate body 

 weights attained which must be due to constitutional causes. 

 There is, moreover, a constant, though small, sex difference 

 in the birth weights of rats, males tending to be heavier and 

 slightly more variable than females. This sex difference 

 persists throughout life and becomes greater as the indi- 

 viduals grow older, even when environmental and nutritive 

 conditions are the same for all animals. Seemingly, body- 

 weight variability at birth and that found at all age periods 

 during postnatal life in individuals of the same sex, as w r ell 

 as in those of different sex, are indicative of differences in 

 growth capacity which depend, in large measure, upon con- 

 stitutional causes. Sailer's ('27) statement regarding the 

 cause of variability in the body weights of the mouse is doubt- 

 less applicable to many, if not to all, animals: 



In der Anlage der Tiere, also genotypisch begriindete Unterschiede 

 im Gewichtsverlauf und damit auch fiir die von ihr erreichte End- 

 grosse bestehen. 



There are, however, many factors that are not genetic that 

 unquestionably influence body weight in the rat and so tend 

 to increase its variability. The age and physical condition 

 of the mother, the size of the litter, and the length of the 

 gestation period, all affect the fetal young and so contribute 

 to the variability found in the birth weights of the newborn 

 (King, '15). In some cases, also, all of the ova liberated at 

 a given period of ovulation are not fertilized at the same 

 time, as is shown most convincingly when superfecundation 

 occurs (King, '13). The advantage thus given to some of 

 the embryos likewise tends to cause variations in the weights 

 at birth. 



During postnatal life the growth rate and also the varia- 

 bility in body weight at different age periods are influenced 

 considerably by temperature, nutrition, housing conditions, 



