DIFFERENTIALS IN CLOSELY INBRED MICE 101 



tissue and lymphocytes, a destruction of a part of the thyroid transplant, and 

 an infiltration of the fat tissue by connective tissue, lymphocytes and small 

 vacuolated phagocytic cells. However, the more severe reactions may have 

 been due to infection, as indicated by the presence of polymorphonuclear leu- 

 cocytes in or around the transplants. 



In strain C57 we find, again, reactions varying between those seen in auto- 

 genous and those in homoiogenous transplantation. Besides the many experi- 

 ments in which the results resembled or approached those obtained in autog- 

 enous transplantations, there were a considerable number of syngenesio- and 

 homoiogenous reactions and, as a rule, a correspondence existed between the 

 behavior of different transplanted tissues. In some instances, the presence 

 of polymorphonuclear leucocytes made the interpretation of the results difficult. 



In New Buffalo strain, in which donors and hosts were from 2 to 6 months 

 old, either almost autogenous reactions or syngenesio-reactions were obtained ; 

 in some instances, a stunting of transplanted thyroids and striated muscle 

 tissue was noted. The average grades were 2.84 in transplantations between 

 litter mates and 2.81 in those between non-litter mates, a difference which is 

 of no significance. In a few older mice belonging to strain Old Buffalo, a few 

 homoiogenous reactions were noted. There was, as a rule, a correspondence 

 in the type of reactions against different tissues in transplantations from the 

 same donor to the same host. On the whole, then, the individuality differen- 

 tials in strain New Buffalo approached identity, but this goal had not yet been 

 quite reached. 



In strain AKA, there was usually a definite homoiogenous reaction against 

 thyroid as well as against cartilage and fat tissue. In some cases the thyroid 

 was destroyed, in others there was an incomplete ring of acini invaded by 

 fibrous tissue and with much lymphocytic infiltration, which helped to destroy 

 the acini. In such cases the fat tissue was usually invaded by fibrous tissue, 

 lymphocytes and phagocytic, vacuolated cells ; the bone marrow was necrotic. 

 In one instance, the thyroid transplant approached an autogenous condition, 

 and, correspondingly, there was very little connective tissue ingrowth or 

 lymphocytic infiltration in the fat tissue. In a few other cases there were 

 syngenesio-reactions ; but for the most part, a marked homoiogenous reaction 

 was found. 



In strain C only a few transplantations were carried out ; the results were 

 good, approaching autogenous or syngenesious conditions. 



Do variations in age of donors or hosts in inbred strains affect the reactions 

 against the individuality differentials? In further experiments we tested the 

 degree to which the individuality differentials, within the various inbred 

 strains, had become similar to one another or identical, by transplanting tissues 

 from relatively old donors to young hosts and vice versa. This procedure gave 

 us also an opportunity to study further the effect of the age of donors and 

 hosts on the reactions against individuality differentials, in experiments which 

 can be more readily carried out in mice than in other species, because mice 

 undergo old age changes and die earlier than do most other mammalian species 

 used in these experiments. 



