382 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



kemia could not be transferred to any individuals belonging to the non- 

 susceptible parent strain. The results were variable in F 2 hybrids ; leukemia 

 could be transferred in 50% of F 3 hybrids. 



These results could perhaps be explained if we assume that in addition 

 to the gene sets derived from both parents, which determine the organismal 

 differentials of these leukemic cells, there is present in the F x cells an intrinsic 

 stimulus (Gi), derived from the parent which is susceptible to the develop- 

 ment of spontaneous leukemia. The possession of this intrinsic stimulus 

 converts the lymphoid cell into a leukemic cell and makes it possible for this 

 cell to proliferate in an abnormal manner. 



In the inbred strain of mice in which leukemia occurs spontaneously in a 

 high percentage of cases, a factor (Ge) is present which stimulates or 

 otherwise makes it possible for the leukemic cells to multiply and thus to 

 transfer the disease, while in other strains this auxiliary factor is lacking. 

 The activity of both the intrinsic factor within the leukemic cells and the 

 auxiliary factor would enable the cells to overcome the resistance to the 

 growth which is due to the presence of a combination of a set of strange 

 genes and a set of genes identical with those of the host. We have referred 

 to a similar condition already in the preceding chapter, when we discussed 

 the effect of the continued action of Ge (hormones) on transplantation of 

 not yet full cancerous tumors. 



Further complications may be due to the fact that long-continued trans- 

 plantations may modify the immunological characteristics of tumor cells and, 

 according to MacDowell, also of leukemic cells. However, not all leukemic 

 cells arising in F 1 hybrids of two strains, one of which has a high and the 

 other a low incidence of spontaneous leukemia, behave in the manner ob- 

 served by Furth and Barnes. Kirschbaum and L. C. Strong found that the 

 leukemic cells from F 1 hybrids between the CBA and F strains behaved in 

 the same way as typical carcinoma cells originating in such hybrids, while 

 the leukemic cells from other kinds of F x hybrids in which leukemia had 

 been produced experimentally could behave in an entirely different manner. 

 But notwithstanding the existence of such complications which may arise, in 

 general, it may be concluded that it is the organismal differentials in host 

 and transplanted tumor and the genetic factors of which these differentials 

 are the expression which primarily determine the fate of the transplant in a 

 given host. 



The genetic constitution of an individual influences the receptivity or 

 resistance to the inoculation of a tumor by way of the individuality and 

 species differentials of which the genes are the precursor elements. Further- 

 more, there are indications that special growth promoting substances may 

 aid in the growth of transplanted cancer cells in certain cases, and it is 

 probable that these growth promoting substances (Ge) which may be either 

 hormone-like or virus-like, are also ultimately determined genetically. 

 Organismal or individuality differential substances may be fixed in tissues as 

 well as be present in the circulating bodyfluids. In certain respects they 



