No. 60S] MULTIPLE FACTOliS IX MICK AM) BATS 477 



On the other hand, rate and extent of growth is undoubt- 

 edly chiefly dependent on the nature of the tissue involved, 

 and as we have seen there is every reason to believe that 

 this depends on the interactions of a complex of genetic 

 factors which are independent of each other in inheritance. 



Other Experiments with Ixix rT.ATED Tumors ix Mice 

 Loeb and Fleisher (1912) have icjiortrd a series of 

 investigations on the hereditary factors underlying the 

 susceptibility of mice to a transjjlantable carcinoma. As 

 parent stocks they used three races of mice, one American 

 race, and two European. By breeding tests including 

 several generations the percentage of American mice to 

 show continued growth of the inoculated tumor was found 

 to be eighty-four, while those of the European races I and 

 II were twenty-three and three per cent., respectively. 

 The Fi generation between American and European I 

 gave sixty-eight per cent, susceptible, from this same 

 cross gave thirty per cent, susceptible. When American 

 mice were crossed with European II an Fi generation was 

 obtained in which one hundred per cent, of the animals 

 inoculated were susceptible. Only fourteen animals were 

 tested and the number is too small to establish this as an 

 accurate percentage for this generation. The Fo genera- 

 tion of this cross gave twenty-six per cent, susceptible. 



It is interesting to note that in the Fi generation, where 

 sufficient numbers were obtained to afford critical evi- 

 dence, the percentage of susceptibility was intermediate 

 between those of the parent races. There is a marked 

 decrease in degree of susceptibility in the Fo generation. 

 The fact that some of the animals of the American parent 

 race failed to grow the tumors shows that this race is in all 

 probability not homogeneous, and the same is true of the 

 European races since animals within a single race fail to 

 react similarly to pieces of biologically similar tumor. If 

 such is the case, we should expect an intermediate result 

 in the F^ generation, just as we do when we cross two 

 races differing in size. The F2 generation should also be 



