16 HELEN DEAN KING AND HENRY H. DONALDSON 



Although the males in the first generation of captive Grays 

 were heavier than the females at birth, the latter grew more 

 rapidly during early postnatal life. At thirteen days of age 

 the average weight of the females exceeded that of the males 

 by 0.9 gram, and was 3.6 grams greater at thirty days. The 

 weight relation of the sexes was reversed, however, when the 

 animals were sixty days old, and at all subsequent ages males 

 were much heavier than females. 



In his first study of body growth in the albino rat, Donald- 

 son ( '06) stated that males were heavier than females at birth 

 and up to seven days of postnatal life, when a period of more 

 active growth on the part of the females began. Growth 

 graphs, constructed from data for the two sexes taken at 

 intervals of one or two days, cross at the fourteen-day period, 

 and that for the females runs above the one for the males 

 until the sixty-day period. At this point the graphs recross, 

 and that for the males runs at a higher level until its end. 

 There is, therefore, a very striking similarity between the 

 growth relations of the sexes in the strain of Albinos studied 

 by Donaldson and those of individuals belonging in the first 

 generation of captive Grays. Like relations were not found, 

 however, in several subsequent studies on the growth of the 

 albino rat (Jackson, '13; King, '15 a; Donaldson, '24), nor 

 do they appear in the findings for the later generations of 

 gray rats. In all of these cases the males have a heavier 

 body weight than the females throughout the entire life span. 



It seems probable that the differences in the weight rela- 

 tions of the sexes in these various series of Norway rats were 

 due to environmental and nutritive conditions, not to factors 

 inherent in the strains. At the time that Donaldson began 

 his studies the rat was not as commonly found in biological 

 laboratories as it is to-day, and little was known regarding 

 the conditions necessary to maintain the animals in health 

 and to enable them to grow and reproduce at a normal rate. 

 Donaldson's rats were fed chiefly on milk-soaked bread and 

 on corn, with the occasional addition of a little meat. Nu- 

 trition studies of later years (Osborne and Mendel, '16, '16 a, 



