TRANSPLANTATION AND INDIVIDUALITY. 157 



HETEROTRANSPLANTATION. 



If we compare with these results of auto, syngenesio and 

 homoiotransplantation, heterotransplantation (transplantation 

 into a strange species ) we find some interesting differences. 

 After heterotransplantation tissues generally live only a short 

 time, which varies between a few days or even less and two weeks 

 or slightly more. In a few exceptional cases tissue may even live 

 as long as three or four weeks. For a short period there may be 

 found a slight proliferation of the heterotransplant. But usually 

 the injurious action of the host and particularly of its body- 

 fluids is very marked after heterotransplantation. The quantity 

 of living, well preserved tissue is therefore very much reduced. 

 These differences between the hetero and homoiotransplant are 

 usually quite marked as early as the latter part of the first and 

 the beginning of the second week. There may be added to the 

 direct injurious effect of the host and its body fluids a destructive 

 action of lymphocytes and an invasive action of fibroblasts. But 

 both of these are usually relatively slight ; and especially the 

 lymphocytic reaction is markedly less prominent after hetero- 

 than after homoiotransplantation. Fibroblasts and bloodvessels 

 of the host show little activity around the heterotransplanted 

 parenchyma. The vascularization is therefore poor and the 

 number of fibroblasts growing directly around the heterotissue is 

 usually restricted. The connective tissue that does grow has the 

 tendency to form fibrous tissue. At some distance from the 

 transplanted parenchmya the fibroblasts and bloodvessels behave 

 otherwise as if they had to deal with an inert foreign body. While 

 thus the lymphocyte and connective tissue contribute only slightly 

 to the destruction of the heterotransplant the fibrous tissue ex- 

 erting an injurious pressure on the heterotransplant large masses 

 of lymphocytes may collect in the tissue surrounding the hetero- 

 graft. These lymphocytes, however, are relatively innocuous. 



If we ask, how it comes about that after heterotransplantation, 

 notwithstanding the greater strangeness of host and donor, the 

 destructive action of the lymphocytes is not only not more marked 

 than after homoiotransplantation, but on the contrary much more 

 restricted, we may suggest that after heterotransplantation the 



