200 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



strength of resistance which different transplanted tissues oppose to the in- 

 jurious actions of the bodyfluids and cells of a host possessing different in- 

 dividuality differentials : (3) by the production of certain degenerative changes 

 in the tissue, which may increase very much the strength of the lymphocytic 

 reaction of the host; such an effect we have observed especially with trans- 

 plants of ovarian and adrenal-cortical tissue ; in addition, certain specific reac- 

 tions may take place between two adjoining autogenous tissues within the 

 same organism, if the equilibrium between these tissues is disturbed. Such 

 reactions may, for instance, take place between autogenous grafts of pig- 

 mented skin into defects of white skin in the guinea pig, and similar reactions 

 may occur between the squamous epithelium of the cervix and the cylindrical 

 epithelium of the uterus, particularly under the influence of estrogenic stimu- 

 lation. 



Other factors of a secondary nature exist, which may affect the strength 

 and character of the reactions of the host against organismal differentials, such 

 as age of host or donor and stimulation leading to an increase in the growth 

 momentum of the transplanted tissue. This increase enables the graft to over- 

 come various obstacles present in the host and especially also an unfavorable 

 constitution of the individuality differentials. In particular, hormones which 

 stimulate the transplanted cells to grow may enable them better to withstand 

 the attacks of the host. There may be associated with this increase in growth 

 momentum a diminished differentiation of the transplanted tissue, which may 

 also indirectly contribute to the decrease in the severity of the reaction of the 

 host against the transplant. We have seen furthermore that there are loca- 

 tions in the host where the transplants are, to some extent, protected against 

 the host reactions, and different mechanisms of these various kinds may be 

 active in different places. 



As to the character of these organismal differentials, they are essentially 

 genetically determined, but they are not identical with the genes; they are 

 gene derivatives which lead to the production of the chemical substances 

 characteristic of these differentials. While the genes in the donor of the graft 

 strange to the host are mainly responsible for the intensity of the host reac- 

 tion, there are some indications that also genes in the host which are not 

 present in the transplant, may play a certain role in this reaction. However, 

 it seems that in addition the ability of the host to react against strange indi- 

 viduality differentials may vary. The mechanism underlying this difference 

 between different hosts has not been determined, and it cannot be excluded at 

 present that the function of certain organs or tissues, such as the reticulo- 

 endothelial system, is involved. Besides, different species to which the hosts 

 belong differ quite distinctly in the mode of their reactions against a strange 

 individuality differential. 



The character of the individuality and species differentials directly or in- 

 directly determines the mutual compatibility or incompatibility of two differ- 

 ent organisms; but there does not exist a simple reciprocal relation between 

 the reactions of individual or species A to individual or species B or vice 

 versa, but it follows from what has been stated, that A may differ from B in 



