SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 69 



growth activity or fluctuations are observed. (See the work 

 on the transplantable neoplasms of the Japanese waltzing mice 

 discussed below.) 



The first work that showed the importance of race was that 

 of Jensen. He discovered the interesting fact that if tumor 

 tissue derived from a BerUn stock was inoculated into Berlin 

 mice and into Hamburg mice there was a decided difference in 

 the results. The percentage of susceptible individuals among 

 the original stock was distinctly greater. He did not attempt 

 any analysis of this difference. This work, however, attracted 

 the attention of several students of genetics. 



Genetic research is primarily concerned in the recognition 

 of races and variations within a species. The modern geneticist 

 analyzes his variations by the method of hybridization. He must, 

 however, be sure that the races used are homogeneous for all 

 the factors that determine the differences encountered in the 

 character under investigation. This fixation of characteristics 

 is accomplished by inbreeding. 



b. Mendelian segregation. The next step was taken by Doc- 

 tors Tyzzer and Little, working on two transplantable tumors 

 that arose in a closely inbred strain of Japanese waltzing mice. 

 The first (J. W. A.) was a carcinoma of the mammary gland 

 that grew in 100 per cent of all mice (of that particular Japanese 

 waltzing strain) inoculated. The first hybrid generation (Fi, 

 produced by the hybridization of the two parent stocks) pro- 

 duced sixty-two individuals, sixty-one of which grew the trans- 

 planted tissue, even faster than animals of the original Japanese 

 waltzing strain. No explanation of this apparent increased 

 activity on the part of the tumor cell was offered. In the general 

 discussion of this paper we shall consider this phenomenon and 

 attempt a genetic explanation. In the second filial generation 

 (r2, produced by mating the Fi individuals inter se) only three 

 out of the 183 mice obtained were found to be susceptible to 

 the inoculated tissue. Tyzzer and Little applied the multiple- 

 factor hypothesis to explain these results. This hj^Dothesis postu- 

 lates that in the production of certain characteristics several 

 independent (or linked) mendelian factors may be necessary. 



