TRANSPLANTATION AND INDIVIDUALITY. 145 



differential as far as in addition it differentiates species, the one 

 differential being superimposed upon the other. That such a 

 common factor truly representing the individuality does exist can 

 be shown through a comparative study of transplantation of one 

 part of an organism into the same and other individuals, related 

 and not related, of the same species, and into individuals of 

 various different species. Such transplantations reveal the pres- 

 ence of an individuality differential, either directly through the 

 interaction of tissues and of tissues and body fluids, or in certain 

 cases indirectly through the immunity which follows such trans- 

 plantation of tissues or parfs of organisms. 



A systematic use of transplantation for the analysis of indi- 

 viduality is of rather recent origin. Carried on, however, in a 

 more or less haphazard way the study of transplantation dates 

 back a considerable number of years. My own interest in trans- 

 plantation as a means of analyzing the biochemical difference in 

 the constitution of individuals was first aroused about nineteen 

 years ago, and again a few years later, when I compared the re- 

 sult of transplantation of tumors in the same individual and in 

 another individual of the same species. The transplanted pieces 

 behaved quite differently in both of these cases. 



AUTO AND HOMOIOTRANSPLANTATION. 



It is, however, only recently that the difference in the result of 

 auto and homoiotransplantation has been more generally recog- 

 nized. On the other hand, that heterotransplantation does usu- 

 ally not succeed has been established considerably earlier. 



THE MECHANISM WHICH DETERMINES THE INTERACTION OF 



TISSUES. 



For a number of years my associates and myself have carried 

 out experiments tending to analyze the factors which connect 

 individuality and transplantation, and here I wish to describe 

 very briefly a few of our experiments and to draw some more 

 general conclusions, the latter merely in a tentative manner. Our 

 experimental analysis is not yet concluded and at the present 

 time there are under way certain investigations which we hope 

 will help to clear up some of the doubtful points. For our pur- 



