336 HELEN DEAN KING 



The inbred strain of rats was composed of two series, A and B, 

 both derived from the same ancestral stock. In every generation 

 of each series the females that were used for breeding were paired 

 twice with a brother from the same litter, thus producing the 

 strictly 'inbred' litters that alone furnished the breeding stock in 

 the following generation. These same females were then paired 

 twice with an unrelated Albino male taken from the general 

 stock colony. For convenience, litters with the latter parentage 

 are here designated as ' half-inbred' litters. 



The early generations of these inbred animals suffered severely 

 from malnutrition, due to improper feeding. Nutritive condi- 

 tions were improved after the fourth generation, and the animals 

 quickly regained their normal size and fertility. iVt no stage of 

 the investigation was any attempt made to influence the produc- 

 tiveness of the animals, other than by keeping them under 

 environmental and nutritive conditions that were as uniform and 

 as favorable as it was possible to make them. 



A. Litter size 



The normal fertility of any race can properly be estimated only 

 from the total number of offspring produced by many females 

 during the entire period of their reproductive activity. The 

 fertility in the inbred strain of rats cannot be measured by this 

 standard, unfortunately, since the plan of the experiment called 

 for only four litters from each breeding female, and after this 

 number was obtained the females were usually discarded. Ac- 

 cording to Cram.pe ('84), the Albino female has, on the average, 

 only three or four litters. On this basis the litter data obtained 

 for the inbred series shows the total productiveness of the greater 

 proportion of the females that were used for breeding. Crampe's 

 estimate for litter production is, I believe, too low, since the 

 breeding history of a considerable number of stock Albinos, 

 recently obtained, shows that the females had an average of 5.3 

 litters each. Records for the inbred series undoubtedly cover the 

 most productive period in the life of the females, and if the fer- 

 tility of the strain was impaired to any extent by inbreeding it is 

 probable that all of the litters cast would have been smaller than 

 normal. 



