330 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



difficult to exchange skin from areas in A to B, which differ in color, than 

 from areas where the color is the same in host and donor. According to the 

 concept of organismal differentials, on the other hand, the same individuality 

 differential should attach to the black and to the white skin in the same animal, 

 and it should make no difference as far as the reaction of the host against the 

 transplant is concerned, which part of the skin of A is grafted to B. Of course, 

 the tissue differentials of these two parts of the skin might differ and it might 

 thus be easier to transplant pigmented than unpigmented skin, but this dif- 

 ference would apply also to transplantation of white and pigmented skin in 

 the same individual. We must assume that the tissues in the same individual 

 possess the same organismal differentials and that these alone determine the 

 specific reaction of a particular host against a transplant, while tissue and 

 organ differentials would call forth the same reaction in all hosts, irrespective 

 of the character of the organismal differentials in host and transplant. When 

 differences in tissue or organ differentials are superimposed, in a certain 

 individual, upon those in organismal differentials, the former do not call forth 

 reactions specific for an individual in the same sense in which the latter do. 

 This holds good in general, although in some cases the character of the tissues 

 may help to determine the reaction of the host against individuality differentials. 



Both Schoene and Schultz stress the importance of athrepsia in transplan- 

 tation, by which is understood a condition in the graft caused by lack of 

 foodstuffs, mainly of a protein nature, but also of salts which are specifically 

 needed by tissues transplanted into certain hosts ; furthermore, importance is 

 attributed to anaphylactic reactions. Although in a general way both these 

 authors regard the presence of toxic substances as a possible additional factor 

 in determining the fate of heterotransplants, the existence of heterotoxins 

 affecting interspecies transplantation is denied by them, because it can be 

 observed that the margin of a transplant may be better preserved than its 

 central parts. They assume that if a heterotoxin were active in such cases 

 it should first show its injurious effects in the peripheral part of the graft. 

 However, as we have seen, the better oxygen supply in the periphery as com- 

 pared to the center of the graft, may overbalance and obscure the effect of 

 specific heterotoxins. 



There are still other secondary factors which have to be considered and 

 which may explain some difficulties in transplantation : for instance, the un- 

 equal sensitiveness of different tissues to the lack of a sufficient amount of 

 oxygen during the process of grafting, or directly following it, may play a 

 role also in the transplantation of the fertilized ovum; differences in the 

 structure of tissues, such as the density of the cutis, may be of some im- 

 portance in skin grafting; and lastly, the different effects of hormones in 

 different hosts and in the same hosts under varying conditions, may affect the 

 fate of the transplanted sex organ. 



On account of the difficulty in obtaining a sufficient number of suitable 

 animals for certain transplantations, the conclusions of Schultz are based on 

 a very limited number of experiments, but they, as well as our earlier ones, 

 indicate that within certain limits a parallelism exists between transplant- 

 ability and the phylogenetic relationship between heterogenous hosts and 

 transplants, and the experiments of Schultz in addition suggest, with certain 



