INTRODUCTION 9 



with which the stage of identity of the individuality differentials is approached 

 or becomes manifest, and the transplantation method can be applied in order 

 to test to what degree the gene composition in the individuals belonging to 

 a closely inbred family or strain has become similar, or, expressed differently, 

 the degree of homozygosity which has been reached in such a strain. 



That it is the strange genes in the graft on which the reaction of the host 

 against the transplant depends is confirmed, also, by experiments in which two 

 inbred strains were hybridized and the reactions of individuals belonging to 

 the parent strains against tissues or organs of the hybrids were compared with 

 the reactions of the hybrids against transplants from the parent strains. In 

 the former case the reactions were more severe than in the latter case; this 

 corresponds to the fact that only one-half of the hybrid genes is represented 

 in the inbred parent strains, while the genes of parent strains are all present 

 in the gene sets of the hybrids. 



In the course of phylogenetic evolution, gene sets which are characteristic 

 of the more highly differentiated species have gradually evolved from the 

 gene sets of other more primitive ancestor species, and the organismal 

 differentials have undergone a corresponding development. On the other hand, 

 in the fertilized ovum the chromosomes and gene sets are the same as in the 

 cells of the adult organism. Yet there are indications that in the fertilized tgg 

 the individuality differential is not yet fully formed, but that it develops from 

 a precursor substance in the course of embryonal life; it is certain that at least 

 the mechanism which makes the differences in the individuality differentials 

 of host and transplant manifest undergoes such an evolution. Even in very 

 young guinea pigs, before the age of sexual maturity, these mechanisms of 

 defense against a strange individuality differential are not yet fully developed, 

 .as is indicated by transplantation experiments of tissues into hosts of various 

 ages. The connective tissue reaction is diminished in intensity and the lympho- 

 cytes may have therefore a better chance to become active in these young 

 animals. 



As to the number of genes which determine the nature of the individuality 

 differentials, no definite statement can be made. However, considering the 

 difficulty in eradicating reactions against other than autotransplants, even in 

 individuals belonging to strains closely inbred through a large number of 

 generations, and considering the improbability of ever obtaining an autoge- 

 nous reaction after homoiotransplantations in non-inbred strains, also in view 

 of the fact that the reactions are so very finely graded and that a homoio- or 

 syngenesio-reaction after transplantation of a piece of tissue belonging to 

 another individual may appear as late as several months following transplanta- 

 tion, it is very likely that the number of genes entering into the composition 

 of the individuality differential is great and that perhaps all the genes partici- 

 pate, although different ones possibly to a different degree. Both organismal 

 differentials and organ and tissue differentials depend entirely, or to a large 

 extent, on the constitutions of chromosomes and genes ; but the genes and 

 combinations of genes which preponderate as determiners of these two types 

 of differentials are evidently not the same and there are indications that it is 



