TUMORS AND NORMAL TISSUES 347 



However, according to this investigator the serum of rats immunized against 

 a rat tumor does not affect autogenous macrophages of the immunized animal 

 or those of another immunized rat, although it may injure macrophages of 

 the rat spleen, which are homoiogenous in nature. 



The analysis of the individuality differentials will be continued in the next 

 chapters, where we shall further discuss immunity against tumors and 

 hereditary factors as they apply to tumor transplantation. 



Transplantation of Heterogenous Tumors and the 

 Species Differential 



In heterotransplantation of normal tissues we observed not only survival 

 of the transplants, but also growth phenomena in some of the tissues, but 

 both of these processes had a very limited duration, and growth, if it took 

 place, was much weaker than after homoiotransplantation of the correspond- 

 ing tissues ; furthermore, the proliferative processes ceased sometime previous 

 to the death of the grafts. We also noted that different kinds of tissues showed 

 different degrees of resistance to the injurious action of the primary, pre- 

 formed heterotoxins. While more sensitive tissues, such as thyroid, kidney, 

 and also skin, were destroyed so rapidly that a pronounced cellular (lympho- 

 cytic) reaction on the part of the host tissue against the transplant could not 

 develop, or was much diminished in intensity, heterotransplanted cartilage 

 proved more resistant, and it lived long enough to allow a very marked 

 connective-tissue reaction as well as an accumulation of polymorphonuclear 

 leucocytes and lymphocytes around the graft. 



If instead of using normal tissues, we carry out heterotransplantation of 

 tumors, the results are in principle the same, although there exist some 

 quantitative differences, which are due at least partly to the greater prolifera- 

 tive momentum inherent in tumors. In addition, the possibility must be con- 

 sidered that tumors manifest a greater power of adaptation to certain injurious 

 conditions than normal tissues, and, as we have seen, they may be able 

 to neutralize, in some way, substances which tend to inhibit their growth. 



Under these circumstances, it is to be expected that the range of condi- 

 tions under which tumors can grow should be somewhat wider than that of 

 normal tissues after homoiotransplantation as well as after heterotransplan- 

 tation, though as a rule, tumors are about as sensitive to heterotoxins as are 

 normal tissues. After heterotransplantation of tumors there may be a pre- 

 liminary period during which the growth may be quite active; but soon it 

 ceases, degenerative processes set in, and the tumors are destroyed. The 

 degree of growth and the duration of this preliminary period depend upon 

 the inherent proliferative momentum of the tumor, the sensitiveness of the 

 tissues of which it is composed, and the degree of difference between the 

 species differentials of host and of transplant. Only if the species of donor 

 and host are very nearly related may the growth be more intense and the 

 growth period of greater duration. Thus heterotransplantations between rats 

 and mice may succeed relatively well, temporarily ; but the results are much 

 more unfavorable if less nearly related species are used. Also, in the case 



