6 HELEN DEAN KING 



In this investigation, as in others continued over a long 

 period, contingencies have arisen at times that had a marked 

 effect on animals then under observation. Unfavorable condi- 

 tions in the colony decreased fertility and the growth rate of 

 individuals in the eleventh to the fifteenth generation. Similar 

 effects followed removal of the colony to temporary quarters 

 in 1932, and its subsequent transfer to a new location the 

 following year. 



Various subjects are discussed here in the same order as 

 in the previous report, and a new section has been added 

 dealing with the mutations that appeared in later generations. 

 Many tables contain a summary of relevant data for the first 

 ten generations in order to show successive steps in the series 

 of changes that occurred in these rats during 14 years of 

 captivity (1919-1933). 



GKOWTH IN BODY WEIGHT 



Changes in the weight of the body with age were determined 

 for 1674 individuals from the fifteen generations covered by 

 this report, approximately the same number of rats being 

 taken from each generation. To facilitate an analysis of the 

 findings, data for five successive generations were combined 

 into the three groups given in reference tables 1 to 3. 



The body weight relations of the sexes during early post- 

 natal life warrant consideration in view of the fact that in 

 the first generation the average body weight of females ex- 

 ceeded that of males at 13 and at 30 days of age (King, '23), 

 but in the second to the tenth generations weights of males 

 exceeded those of females at all age periods (King and 

 Donaldson, '29). A weight excess in young females, although 

 similar to Donaldson's ('06) finding for young albino rats, 

 does not accord with other determinations of the weight rela- 

 tions of the sexes in albinos (Jackson, '13; King, '15; Donald- 

 son, '24). 



When the first report on this strain was written sufficient 

 data on the birth weights of gray rats had not been obtained 

 to furnish the norms that are necessary in order to determine 



