360 



HELEN DEAN KING 



In the earlier generations of the B series the mortahty in both 

 males and females was considerably greater than that in the 

 animals belonging to the A series: only 5 per cent of the males 

 lived to be fifteen months old, while not a single female reached 

 this age. For the later generation groups the data for the B 

 series were very similar to those for the A series. As a whole, 

 however, the animals in the A series lived longer than did those 

 in the B series. 



The data given in table 10 and in table 11 have been com- 

 bined in table 12. This table shows also mortality data for 377 

 stock albino rats reared in The Wistar Institute animal colony 

 during the past three years. Included in the latter series are the 



TABLE 11 



Showing the mortality at different ages in a group of 151 males and 2S1 females be- 

 longing in the seventh to the twenty-third generations of the B series of inbred rats 



records, elsewhere published (King ,'15), for fifty males and for 

 fifty females of selected stock that were reared as controls for the 

 inbred strain. 



The mortality data for the inbred rats, given in table 12, 

 show that close inbreeding did not tend to shorten, but to lengthen 

 the span of life in both males and females: 50 per cent of the 

 animals belonging to the last group lived to be fifteen months of 

 age, while in none of the other groups did even 30 per cent of the 

 individuals attain this age. It is probable that the relatively 

 high death rate in the animals of the earlier generations was due 

 to the fact that the rats had not regained the vigor that was so 

 greatly impaired in their ancestors by malnutrition. 



