6 HELEN DEAN KING 



have not reappeared, although the rats have been carried through 

 twenty-eight generations of brother and sister matings. 



In the inbred colony, up to the sixth generation, very little 

 selection of breeding animals was possible; any females that 

 would breed at all were used to continue the series. The change 

 in food was made at the time that -the animals of the fourth 

 inbred generation were reaching maturity. In the course of 

 the two following generations the effects of malnutrition gradually 

 disappeared, and in the sixth generation most of the rats were 

 of* normal size and relatively large litters were being produced. 

 After this time large and vigorous animals were available for 

 breeding, purposes, and it became possible to make a careful 

 selection of the breeding stock. 



From the seventh generation on the selection of the individ- 

 uals which were to serve as progenitors of the succeeding gener- 

 ation was always made among the newborn young, as the sexes 

 can readily be distinguished at this time (Jackson, '12). In 

 the A series of inbreds, which is called the 'male line,' all litters 

 containing an excess of female young were always discarded ; in 

 the B series, the 'female line,' litters with an excess of male young 

 were never reared. Unless the individuals in the litter were of 

 normal size and vigorous at birth they were killed at once. 

 The young which were retained remained with their mother 

 until they were one month old, when they were again carefully 

 examined, and if they did not come up to the norms for stock 

 animals of like age they were discarded. If the young rats ful- 

 filled all requirements as to body weight and vigor they were 

 returned to the cage to be reared as possible breeding stock. 

 This rigid selection left in each generation, as a rule, at least 

 three times the number of animals that were required for breed- 

 ing. When the rats become sexually mature, at about three 

 months of age, they were again inspected, and any that were be- 

 low normal in any way were rejected. Generally only one 

 female of a litter, the first to breed, was taken to continue the 

 line. If, however, the individuals were unusually large and 

 vigorous, two, very rarely three, breeding females were taken 

 from the same litter. 



