30 HELEN DEAN KING 



The graphs in figure 10 show in a very striking manner the 

 great difference between the size of the rats in the first six in- 

 bred generations and those in the later generations. At the 

 300 day period the space between graph A and graph C repre- 

 sents a difference of 87 grams in the average body weights of the 

 two groups of males. Females of the earlier generations (graph 

 D) were likewise far inferior in body size to the females of sub- 

 sequent generations (graph B), and at 300 days of age the space 

 between graph B and graph D indicates a difference of 64 grams 

 in favor of the females in the later generation group. 



The earliest data on the growth in body weight of the albino 

 rat are those of Donaldson ('06) who studied the growth changes 

 in a series of animals reared at The University of Chicago. Other 

 investigators, Jackson, Slonaker, Ferry and Hoskins have pub- 

 lished records for the growth in body weight of various series of 

 albino rats reared under different environmental conditions. 

 All of the latter data agree, in the main, with those of Donald- 

 son, although as the rat is very responsive to external conditions 

 some series of records show more rapid and vigorous growth 

 than others. 



Donaldson's growth graphs for albino rats may be taken as 

 representing the average run of stock animals. His graph for 

 the males is reproduced as graph A in figure 11, and that for 

 the females is shown as graph A in figure 12. 



As controls for the present series of inbred albino rats thir- 

 teen litters of stock albinos, comprising fifty males and fifty 

 females, were reared in The Wistar Institute animal colony 

 under the same environmental and nutritive conditions as the 

 later generations of the inbred series. In selecting litters for 

 controls care was taken to pick out only those in which the young 

 were large and vigorous at birth. The animals chosen, there- 

 fore, represent the best, not the average, stock in The Wistar 

 colony. The average body weights at various ages of the males 

 and females in this selected group of stock albinos, reproduced 

 from table 3 of a previous publication (King, '15 a), are given 

 in table 14. 



