128 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



The reactions were less severe against the transplants from Peromyscus to 

 strain C57. Here the grades were better, especially in early periods after 

 transplantation. At 8 days, the grades were 2— ; at 12 days, they varied be- 

 tween 2— and 1+ ; at 15 days, the grades were 1 and 1 + , and at 20 days, 

 they were 1 in three cases and 1 + in one case. At 8 and 12 days, some pres- 

 ervation of thyroid tissue and also of muscle tissue was found and in the 

 latter there was some nuclear proliferation; also at 15 days a small part of the 

 thyroid was preserved, and, in one case, even at 20 days. The fat tissue was 

 replaced by fibrous tissue or invaded by small vacuolated tissue. At 15 and 

 20 days the muscle tissue was necrotic and more or less organized by connec- 

 tive tissue. At 8 and 12 days, no. lymphocytes but scattered polymorphonuclear 

 leucocytes were seen in certain instances. At 15 days, besides variable num- 

 bers of leucocytes in one transplant, also some lymphocytes were present, 

 whereas at 20 days, only leucocytes, but no lymphocytes, were noted. 



We find, then, a definitely less severe reaction in cases in which C57 mice 

 were hosts and Peromyscus were donors, than in the reciprocal transplanta- 

 tions. Lymphocytic infiltration in general was more common in the former 

 experiments than in the latter. These findings bring, therefore, additional con- 

 firmation of the conclusion, that in addition to the relations of the individu- 

 ality differentials of host and transplant to each other, the strength and mode 

 of the reaction of the host against the transplant is also a factor which has to 

 be taken into account and which may influence the results obtained. The type 

 of reaction which a certain species or strain shows is also, in all probability, 

 due to the inherited genetic constitution. It is furthermore of great interest 

 that in strain C57, the reaction against heterotransplants of Peromyscus may 

 not be stronger than those seen in a type of homoiotransplantations in which 

 the individuality differentials of host and donor are very dissimilar. 



On the basis of these observations, and of others which we cannot describe 

 in detail, we may answer the questions raised in the beginning of this dis- 

 cussion. (1) As to a possible corespondence between the severity of hetero- 

 genous reactions in different combinations of species and the phylogenetic 

 relationship of these species, the data given in tables 1, 2 and 3 indicate a 

 relatively great similarity in all these species as to time of survival 

 and mitotic activity. Both periods were relatively short and, in general, there 

 was no very definite correspondence between phylogenetic relationship of 

 donors and hosts and the fate of the transplants. There was very little differ- 

 ence between the results of experiments in which tissues of rodents were 

 transplanted to other rodents and in those experiments in which tissues were 

 exchanged between rodents and cats; in some instances, transplantations in 

 the latter gave even better results than in the former. 



Even exchange of tissues between mammals and birds, which represent 

 two different classes, could give results not unlike those observed in trans- 

 plantations between species as near as rat and mouse. Only in certain cases 

 was there an indication of a shorter time of survival and mitotic activity 

 after transplantation into different classes. Thus, pigeon skin fared better 

 when grafted into chicken than into a mammalian species. Transplantation of 



