46 Scientific Proceedings (iio). 



With the hope of discovering a strain of mice that would be 

 susceptible to the dBrA tumor and not to the dBrB, our first 

 choice was the wild house mouse, collected in several localities 

 near the laboratory at Cold Spring Harbor, N. Y. 



The first and foremost consideration in an experiment with 

 transplantable tumors is the selection of the strain of mice to be 

 used. Race has been recognized by many investigators in this 

 field as a factor underlying tumor susceptibility and yet it is the 

 one most often ignored. Several investigators have explained 

 their fluctuating results as due to variations in the tumor cell 

 whereas the same results can be explained by assuming fluctua- 

 tions, although slight, in the strain of mice employed. This last 

 explanation, certain to apply in most cases, is the only one accept- 

 able to present-day geneticists without positive histological proof 

 of tumor-cell modifiability. Racial homozygosity can only be 

 obtained by rigid inbreeding, brother to sister matings, for several 

 generations. The time element necessary to produce mammals 

 genetically and biologically uniform is therefore too great for the 

 patience and resources of most investigators. 



As before stated the wild house mouse has been used in this 

 experiment. If collected in the same locality one can be sure of 

 the stock for several reasons (i) mice very seldom migrate, they 

 usually remain in the same building and no doubt sometimes breed 

 in the same nest in which they were born. This is evidenced by 

 the fact that slight variations in coat color tend to be restricted 

 to the same corner of a building, etc. (2) The fact that relatively 

 fewer adult males than females are present, thus necessitating 

 close inbreeding. This inequality of the sexes is brought about 

 possibly by conflicts between individuals of the male sex. Dr. 

 Sewall Wright has pointed out that the fewer the males employed 

 for breeding the quicker homozygosity in the strain is produced. 

 An added precaution was taken in determining whether there is 

 any difference in the two tumors. Both tumors were inoculated 

 into the same mouse, the dBrA tumor into the right axilla, the 

 dBrB tumor into the left. By this method we have eliminated 

 the possibility of even a slight variation in the race employed by 

 comparing the two tumors in the same soil. If there is any 

 difference in the tumor cell we should be able to determine it, 

 since the soil in each case is identical. 



