8 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



plantation. In this instance the bodyfluids of the host are so different from 

 those to which the tissues of the transplant are adapted that they exert a 

 strongly injurious effect and kill the graft in a relatively short time; the 

 length of time in which this can be accomplished depends, among other factors, 

 upon the degree of resistance of the particular tissue. The reaction of the 

 connective tissue of the host is very strong in heterotransplantation ; besides, 

 it is the polymorphonuclear leucocytes which are attracted first, rather than 

 the lymphocytes, indicating the presence of a substance which acts as a 

 stronger poison, a heterotoxin. The reaction of the lymphocytes is the test for 

 the presence of a milder toxin, namely, homoio- or syngenesiotoxin. However, 

 in places where the toxin action is weaker or at a later period when the 

 acutely-acting toxins have been largely absorbed, lymphocytes may also be 

 attracted and collect in large masses around tissues derived from a strange 

 species. We see, then, that the host cells not only recognize a strange organis- 

 mal differential, but they also distinguish between different degrees of rela- 

 tionship or strangeness. But there is a limit to this power of discrimination. If 

 a certain threshold of strangeness has been reached, the reaction is maximal 

 and cannot be much increased if the tissues from individuals belonging 

 to still further removed classes are used. In this case serological tests are 

 better able to grade differences. The cellular reactions with which we have 

 to deal in transplantation are comparable to a very sensitive balance which 

 indicates small fractions of a milligram and which cannot be used for the 

 detection of differences which are measured by pounds. On the other hand, 

 serological tests are only under very restricted conditions serviceable in the 

 detection of finer differences. Thus, the experiments of Todd (to which we 

 shall refer later) show that under certain circumstances serological tests also 

 may indicate the presence of strange individuality differentials ; but only with 

 one particular kind of structure, the erythrocytes, has this test been used, and 

 even then it did not as a rule reveal the degree of relationship or strangeness 

 between the individuals which were compared. 



Certain experiments show that the similarity or difference between two 

 individuality differentials corresponds to the similarity or difference in the 

 composition of the gene sets in the host and donor, and that the host cells 

 respond, so to speak, to genes which are strange to them. In reality, however, 

 it is not the genes as such to which the host cells react, but the organismal, 

 and in particular the individuality, differentials which develop in accordance 

 with the gene sets. 



That it is the similarity or difference in the gene sets in two individuals 

 which primarily determines the kind of reaction which takes place between 

 host and transplant is also indicated by the fact that if, through close inbreed- 

 ing, we render their gene composition more similar, the individuality differen- 

 tials correspondingly become more and more similar in successive generations 

 and the severity of the reaction of the host against the graft is correspondingly 

 diminished. But it has been found very difficult to produce complete identity of 

 the individuality differentials even under these conditions. It seems, moreover, 

 that in different species closely inbred animals differ in respect to the readiness 



