II. I^OSTHODS. 



All the inbred rata used in thif=i investigation were 

 bred at the Wistar Institute by Dr. Helen D. King. Two strains 

 were used, referred to in this paper' as strains A and B, The 

 original parents of each strain v/ere picked at random, a male 

 and female from each of two unrelated litters. The A male v/as 

 mated to his sister, A female, and the B male to his sister. 

 Their respective litters constituted, generation lA and IB. 

 From this point inbreeding v/as carried on by selecting from 

 the litter the healthiest appearing rats and mating brother 

 to sister v;ithin the same litter, this constituting the clos- 

 est possible inbreeding. At about thirty days of age the young 

 rats were taken from their mother and those to be used by the 

 writer were shipped to the Johns Plopkins University. There 

 were no fatalities en route, and all arrived apparently in 

 good condition. The system of numbering individual rats as 

 a.ppears in the tables is as follows: the first nunberi refers 

 to the generation of inbreeding, the letter to the strHin (A 

 or E) and the last number identifies the individual. 7or ex- 

 ample 7A909 is analyzed as follows: 7th generation inbred, 

 A strain, individual 90, female. 



It seemed advisable to secure normal control muting 

 strains from different laboratories in order, to avoid any pos- 

 sibility of inbreeding. Control strain H is from the Johns 

 Hopkins Laboratory, W from the stock rats of the V.'iatar Insti- 

 tute, C from Columbia University, B a litter bought from a 

 Baltimore dealer, and E a lart,e male given to me by Ir. Her- 

 bert L'. Fvans of the Johns Hopkins I.'edioal School. Care was 

 taken in mating the control series to avoid any approach to 



