TUMOR GROWTH 439 



the theory of organismal differentials, based on the comparative studies of 

 transplantation of normal tissues and of tumors, and the concepts of Little 

 and Strong. These authors, did not determine differences which existed be- 

 tween the individuality differentials of tumors and their hosts, but they dealt 

 instead with certain factors which they believed were needed in a specific 

 manner for the growth of transplanted tumors. 



As we have shown in the foregoing pages, there is good reason for assuming 

 that the problem of the transplantability of tumors is complicated by a number 

 of variable factors, including changes in growth energy of the tumor, adapta- 

 tion of the tumor cells to the hosts, different degrees of sensitiveness of differ- 

 ent tissues or cells to injurious conditions, and lastly, processes of immunity, 

 which again depend upon complex conditions, such as the ability of the tumor 

 to give off antigens and to absorb and neutralize antibodies. While genetic 

 factors enter also into these conditions in conformity with the fact that the 

 organismal differentials may act as antigens and that the range of reactivity 

 of an organism to environmental conditions is limited by constitutional fac- 

 tors, still, the fact that also external factors are involved in these processes 

 makes it impossible to account for the transplantability of tumors entirely on 

 the basis of Mendelian heredity, and to refer modifications of transplantability 

 entirely to genetic mutations, either in the host or in the tumor cells them- 

 selves. These difficulties have been discussed in the preceding chapters. 



As to the relations between the origin of spontaneous tumors and the fate 

 of transplanted tumors, it is certain that tumor cells even more than regenerat- 

 ing cells have properties which differ from those of normal cells. As the 

 result of acquired characteristics, tumor cells may be more readily accessible 

 to certain injuries than some types of normal cells. Various physical and 

 chemical agencies affect the former somewhat differently from the latter, but 

 the changes which have taken place during the cancerous transformation are 

 in all probability not specific, in the sense that they depend upon alterations in 

 the constitution of the organismal differentials of the affected tissues, as the 

 result of which it would be possible for antibodies to develop against the in- 

 dividuality differentials of the tumor cells in the host in which they originate. 



It is especially the recent investigations of Blumenthal, in which the effects 

 of transplanted normal tissues and transplanted tumors on the distribution 

 of the leucocytes in the circulating blood were compared, which again con- 

 firmed in a very convincing manner the essential similarity in the principles 

 underlying the transplantation of both normal and tumor tissues, and which 

 again demonstrated the fact that in both instances it is the nature of the 

 organismal differentials in the host and transplant which primarily determines 

 the outcome of these transplantations. However, in addition to the organismal 

 differentials which normal tissues and tumors have in common, there occur 

 other substances which are also the same in both and which likewise may 

 function as antigens, and lastly there are at least indications that various 

 types of tumors may possess specific antigenic substances which distinguish 

 them from normal tissues. 



Qrganismal differentials and in particular individuality differentials are 



