TUMORS AND NORMAL TISSUES 341 



originated in a white American mouse, and which we used in many of our 

 experiments, could be transplanted into the large majority of American 

 white mice, but into a much smaller percentage of German or English mice. 

 We find, therefore, all kinds of transitions between transplantable and non- 

 transplantable tumors. The larger the number of animals which are tested 

 for their suitability as hosts, the greater becomes the chance that in the end 

 we shall find an animal in a mixed strain in which the tumor will take ; which 

 means that the cells remaining alive after transplantation will continue to 

 multiply. And between this condition of relative non-transplantability and a 

 perfect transplantability in 100 per cent of all animals of the same species, 

 we find all intermediate grades. However, the more closely inbred a strain 

 is, in which a tumor originated, the larger becomes the number of animals 

 belonging to this strain into which, as a rule, the tumor can be successfully 

 transplanted, whereas, the tumor may not grow after transplantation into 

 other strains. 



There are, in a general way, two factors which determine the degree of 

 transplantability of a tumor, as expressed by the average number of takes, 

 namely, (1) the relation of the individuality differential of the host to that 

 of the transplant, and (2) certain factors which differentiate normal tissues 

 from tumor tissues, and which may vary quantitatively in the case of dif- 

 ferent tumors ; among these are variations in growth energy and processes 

 of adaptation, which may take place between tumor and host. In all cases 

 the individuality differentials in host and transplant seem to assert them- 

 selves, even in those tumors in which the transplantability is 100 per cent, 

 for here, also, a transplanted tumor differs in its relation to the host from 

 a spontaneous tumor developing in the same animal. We had already noted, 

 in our earlier transplantations, this difference between spontaneous autog- 

 enous tumors and transplanted homoiogenous tumors. While spontaneous 

 tumors have a tendency to recur after extirpation, transplanted tumors are, 

 as a rule, more sharply separated from the host tissue and can much more 

 readily be completely removed ; they behave like strange organisms implanted 

 in the host, from which they draw their nourishment but from which they 

 often remain separated by a capsule; their vascularization is less adequate, 

 and not rarely they grow even more rapidly in the host than do spontaneous 

 autogenous tumors. Notwithstanding such a rapid growth of the homoiog- 

 enous tumors, it is, after all, a precarious existence which they lead in the 

 strange host, as shown by the fact that they are usually more readily dam- 

 aged by the injection of certain unsuitable substances into the host than are 

 autogenous spontaneous tumors. These various differences in the behavior 

 of spontaneous and homoiogenous transplanted tumors are, perhaps, partly 

 due to the process of transplantation as such, but they are largely caused by 

 the difference in individuality differentials of host and transplant; an in- 

 jurious reaction against the transplant takes place in the strange host, and 

 such an injurious effect is the more evident the greater the dissimilarity 

 between organismal differentials of host and transplant. In a general way, 

 it may be stated that these primary reactions are similar to those which are 



