476 THE AMEBIC AX XATURALIST [Vol. LI 



all of them within a single gamete formed by the Fj ani- 

 mals. Since susceptibility in the Fo generation will de- 

 pend upon the presence of all of the factors ordinarily 

 found in the Japanese race, it follows that the greater 

 the number of factors involved, the rarer will be the ap- 

 pearance of a susceptible animal in the F2 generation. It 

 further follows that the susceptible animals of the Fg 

 generation probably will not possess the factors in a 

 homozygous condition, as did the Japanese grandparents, 

 and therefore they will not, in most cases, breed true, as 

 did the Japanese grandparents, to the character of sus- 

 ceptibility. 



For a more detailed discussion of these results from a 

 genetic point of view, the reader is referred to Tyzzer 

 and the writer 's earlier paper. It will suffice at the pres- 

 ent time to emphasize the fact that it is the inherent nature 

 of the tissue of the host animal that is being studied. The 

 tumor itself is as near a biologic constant as one can ob- 

 tain. Variation in its growth therefore means variation 

 in the attitude towards it, taken by the host tissue. This 

 attitude appears to be dependent upon a complex of dis- 

 tinct factors. If a change or substitution is made in any 

 one of the.members of this complex, a different reaction is 

 obtained. The behavior of the different factors in any 

 such complex is distinctly that of independent units in in- 

 heritance. The fact that the reactions of susceptibility 

 and non-susceptibility are dependent on multiple factors 

 seems established. If the tissue of the adult mouse may 

 be analyzed in this way, the conclusion is far reaching. 

 If the reaction of the tissue depends on its substance, and 

 its substance depends in turn on a certain hereditary com- 

 plex of factoi->. it is logical to suppose that the rate and 

 exti iit of <]e\-.']())nnent of the tissue as well as other proc- 

 esses ..r vimiilicinice to the organism depend, in so far as 

 tlicy niv lio'.'.liiai-y. on similar complexes of genetic fac- 

 tors. Kn\ir..niiic]it inidouhtodly influences certain char- 

 nct.T- in tlinr .l.-vclnptneiit far more than others, and in 

 this r('S])c<-t >izt' ai)pi'ai> to bo one of the most susceptible. 



