380 THE' BIOLOGICAL BASIS OF INDIVIDUALITY 



D, C57 and others are not homozygous; not even the A strain, which ap- 

 proaches homozygosity more than other strains, is completely homozygous. 

 But such inbred strains come near to this state to a degree which enables 

 cancerous tissues to pass the threshold point which makes possible the con- 

 tinued growth of the transplants in other individuals of such a strain; 

 whereas in individuals of different strains this threshold point has not yet 

 been reached. On the other hand, normal tissues transplanted within these 

 inbred strains quite frequently reveal the lack of a perfect identity between 

 the individuality differentials of host and transplant. Analogous conditions 

 are found if we compare transplantations of cancerous and of normal tissues 

 from hybrids between two different inbred strains to parents, and vice versa, 

 in cases in which the donors of the transplants belong to one of the two inbred 

 strains. Tumor transplants, owing to the largely non-genetic characteristics 

 which they have acquired, are able to pass the threshold point separating non- 

 growth from growth in the new host, if parent tissue is grafted to the hybrids ; 

 whereas normal tissues, although they also do not evoke marked signs of in- 

 compatibility in the hybrids, still in many instances, call forth some reactions 

 on the part of the host and may undergo a moderate degree of injury. How- 

 ever, in transplantations from hybrids to parents, tumors as a rule have not yet 

 passed the point of threshold which allows them to grow, while normal tissues 

 are injured in such a host to a higher degree than in the reciprocal transplanta- 

 tions, but because of the use of finer criteria the results are not considered, 

 to the same degree, to be completely unfavorable or negative as they are when 

 tumors are used in this type of transplantation. These differences between 

 normal tissues and tumors are then not caused essentially by differences in 

 the genetic constitution, but either by non-genetic factors, or by the methods 

 applied in the evaluation of the results. If we deal with other strains which 

 have not yet reached so high a degree of homozygosity, we should find various 

 averages in the number of takes; the less the strain has been made homo- 

 zygous by close inbreeding, the greater should be the number of animals 

 which need to be examined in order to arrive at valid figures indicating the 

 connection existing between compatibility of tumor and host and the genetic 

 relationship between donor and host. 



Additional data which prove the significance of the individuality and species 

 differentials for the successful transplantation of tumors are furnished by 

 experiments in immunization against tumor grafts. It is possible, although 

 only to a limited degree, to immunize an animal by a previous transplantation 

 of normal, and especially of embryonal tissue or by injection of red blood 

 cells against a tumor transplant. In order to accomplish such an immunization 

 against the individuality differential of a tumor, the tissue serving as antigen 

 must belong to the same species as the tumor to be inoculated subsequently, but 

 it must differ in the genes which determine individuality from those of the 

 host. Such genes; which occur in the antigen but not in the host, give rise to the 

 immune mechanism, and, in all probability, to the formation of antibodies. 

 If antigen and host are heterogenous in their constitution, then the developing 

 immune mechanisms are directed only against heterogenous, but not against 



