REACTIONS AGAINST INDIVIDUALITY DIFFERENTIALS 67 



possessing strange individuality differentials and injure them ; but they do so 

 to a very different degree in different cases. The effect may be so slight that it 

 is hardly noticeable; but in other cases the direct injurious action of these 

 substances is quite marked and in different species the relative preponderance 

 of the influence of the host cells and of the bodyfluids varies, the latter being 

 relatively more important in the mouse than in the guinea pig and rat. The 

 fact that an interaction between transplanted tissues and the bodyfluids of the 

 host takes place in every instance makes it difficult to decide whether the host 

 cells are activated by the homoiogenous individuality differential of the trans- 

 plant directly, or only after the latter has combined with the homoiotoxins of 

 the host. 



The growth processes, and in particular the mitotic cell multiplication, which 

 occur in the transplanted tissues are not entirely regenerative in character, 

 but they may be due partly to the continued function of a primary tendency 

 to mitotic proliferation, which is inherent to a very different degree in differ- 

 ent tissues. In tissues in which secondary differentiations have taken place, as 

 for instance, in cartilage and striated muscle tissue, or in epidermal cells at 

 some distance from the source of oxygen supply, the tendency to undergo 

 mitotic proliferation is replaced by amitotic processes. Certain unfavorable 

 environmental factors may likewise prevent mitotic proliferation and instead 

 cause formation of epithelioid and giant cells, and in general favor processes 

 of differentiation instead of mitotic proliferation. 



2. The mechanism which leads to the specific reactions of the lymphocytes 

 of the host against the transplant. We have seen that different homoiogenous 

 tissues may attract the lymphocytes to a different degree, and we shall report 

 on particularly striking instances of such differences between different organs 

 in subsequent chapters when we discuss the transplantation of adrenal gland 

 and anterior hypophysis in mice. In addition, it was possible to demonstrate, 

 in the rat, the attraction which homoiogenous tissues exert on the lymphocytes 

 of a nearby lymph gland of the host, in experiments which Crossen carried 

 out in the guinea pig. He autotransplanted a lymph gland into the subcutaneous 

 tissue and then placed either a piece of autogenous or homoiogenous xiphoid 

 cartilage near the lymph gland, or into the lymph gland itself. While the 

 lymphocytes of the transplanted lymph gland were inactive towards the auto- 

 genous cartilage graft, they were actively attracted by the homoiogenous tissue 

 and they migrated into the homoiogenous transplant. This may be considered 

 as confirmatory evidence for the conclusion that the movement of the lympho- 

 cytes towards the homoiogenous transplants represents a chemotropic reac- 

 tion. 



3. Differences in the intensity of the reaction against strange individuality 

 differentials observed in different families or strains of rats. If tissues are 

 homoiotransplanted from certain families or strains of rats into other families 

 or strains, different average degrees of severity in the reactions may be ob- 

 served. We have analyzed the factors which cause these differences in several 

 series of experiments carried out in rats. For this purpose we compared the 

 reactions in rats from various strains and families, obtained from different 



