354 



HELEN DEAN KING 



in both series, as the inbreeding advanced, there was a marked 

 tendency for relatively more of the females to breed at the earliest 

 possible age. About 30 per cent of the breeding females in the 

 eighteenth to the twenty-fourth generation group of each series 

 threw their first litter at or before the age of ninety days ; only a 

 small proportion of them failed to breed before reaching the age 

 of four months. 



As a whole, the females of the A series of inbreds tended to 

 mature slightly earlier than the females of the B series, but the 



TABLE 9 



Shoiving the approximate age at which breeding females in various generation groups 

 of the two inbred series {A and B) cast their first litters 



difference between the two series was not great, and correspond- 

 ing records were in nearly as close agreement as were those for 

 litter size. 



The youngest breeding female in the inbred strain was a 

 member of the A series of inbreds, and she was eighty days old 

 when she cast her first litter of five young. As the gestation 

 period in the albino rat is about twenty-two days (Donaldson, 

 '15), this female must have conceived when she was two months 

 old. Kirkham and Burr ('15) state that one of their Albino 

 females gave birth to a litter when she was only seventy-seven 



