HEREDITY AND TRANSPLANTATION OF TUMORS 379 



transplantations in the F 2 generation of hybrids between susceptible and 

 non-susceptible strains, such as the different degrees of resistance, the growth 

 momentum, and the mode of inoculation of a tumor. The number of success- 

 ful takes in the F 2 generation depends, therefore, not only on the relations of 

 the organismal differentials of host and transplant, but also on various sec- 

 ondary factors, and the number of takes in F 2 hybrids might be quite different 

 if these secondary conditions were altered. Moreover, if we use the percentage 

 of takes of tumors as the criterion for the presence of genes in the tumor, which 

 are compatible with those of the host, we apply a relatively coarse standard of 

 measurement. There is no intergrade between take and non-take ; the tumor 

 either grows continuously after transplantation, or it does not grow ; it may 

 grow for a while, then retrogress and ultimately disappear. But there exist 

 various finer kinds of distinction between degrees of compatibility or non- 

 compatibility of host and graft, such as variations in growth energy, in the 

 length of the latent period, and in the number of cells or size of a piece of 

 tumor tissue which permits a successful transplantation. If these variables 

 should also be considered, the gradations in the fate of tumor transplants in the 

 F 2 generation would be much greater, and correspondingly, the number of 

 factors which are supposed to determine the compatibility between host and 

 transplant would likewise be found to be much greater. By taking merely into 

 account the proportion of the number of takes to the number of non-takes in 

 the F 2 generation, a threshold value is determined, namely, a slight excess or 

 deficit in the sum of the large number of variables which decide the continuous 

 growth or lack of growth of the transplanted tumor. Such a determination 

 is not identical with the study of the differences between the individuality 

 and species differentials of host and transplant. 



In transplantation of normal tissues, much finer standards of measurement 

 are used in the evaluation of the compatibility between host and transplant than 

 in transplantation of tumors, where the percentage of successful transplanta- 

 tions alone is considered, and consequently the number of genetic factors on 

 which this compatibility depends has been found to be great in the case of the 

 former. 



Although the non-genetic factors which we have discussed play a certain 

 role in the transplantation of cancerous tissues, and, to a less extent, also of 

 normal tissues, still, in both it is essentially the genes which fix the con- 

 stitution of the organismal differentials, and therefore the result of trans- 

 plantation depends largely on the relation of the individuality and species 

 differentials in transplant and host. We have already referred to some of the 

 essential facts which prove this conclusion. Autotransplantation succeeds as a 

 rule, and autotransplantation means transfer under conditions in which the 

 individuality differentials and their precursors, the genes, are the same in 

 host and graft. Conditions might be different if somatic mutations were char- 

 acteristic of tumors. In this case, an incompatibility might arise even between 

 autotransplant and host; but this has not been observed. More or less ap- 

 proaching autotransplantation is transplantation into closely inbred strains ; 

 but it must again be stated very definitely that even the inbred mouse strains 



