AGE AND INDIVIDUALITY DIFFERENTIALS 185 



which are characteristic of older age in various organs. Carrel has found 

 that the blood serum of older animals is less suitable as a medium in which 

 tissues grow in vitro than is that of younger individuals. However, it is not 

 certain that this factor plays a significant role in the living organism. 



We approached this problem by means of transplantation of the thyroid 

 gland in guinea pigs. Our observations showed that within the first 10 days of 

 extrauterine life of the host the connective-tissue and lymphocytic reactions 

 against homoiogenous thyroid gland are less intense than in adult hosts. 

 Tureen then compared, in our laboratory, transplantations of thyroid glands 

 in which adult guinea pigs were the donors and in which the donor-age was 

 therefore constant, while the age of the host varied. In the group of young 

 hosts the age varied between 4 days and 5 weeks, while in the group of older 

 hosts variations in weight between 500 and 800 grams indicated corresponding 

 variations in age. In the first 4 or 5 days the reaction was about the same in 

 both groups. But from then on a difference developed : in the older guinea pigs 

 there was a more marked formation of fibrous tissue, which destroyed a con- 

 siderable part of the transplant, and, in the majority of cases, destroyed it en- 

 tirely after 20 days or more had elapsed. In the younger animals the formation 

 of the fibrous tissue was considerably less in most of the animals and the 

 thyroid tissue was therefore better preserved. But in a certain number of 

 instances there was a marked fibrous reaction also in the younger guinea pigs. 

 However, because in the majority of the younger group the preservation of 

 the gland was so much better, the homoiogenous individuality differentials 

 were here subsequently given off in larger quantities ; these then attracted the 

 lymphocytes, which, somewhat later, surrounded and invaded the transplant in 

 considerable numbers and, secondarily injured it. In younger animals, as a 

 rule, there is therefore a tendency for the homoiogenous tissue to elicit a syn- 

 genesio rather than a severe homoiogenous reaction. A combination of homoiog- 

 enous individuality differentials, and a relatively older age of the host, led 

 thus to an early increase in the formation of fibrous tissue in or around the 

 transplant. Under the influence of not well compatible individuality differen- 

 tials the stroma developing in the transplants in adult animals is inclined to as- 

 sume prematurely the fibrous character which is so characteristic of the bodily 

 structures in old age. Because of these injurious effects grafts in older hosts 

 were, then, less liable to attract lymphocytes than the better preserved tissues 

 of younger animals. But when in older guinea pigs the preservation of the 

 thyroid tissue happened to be better, in such animals, also, a larger number 

 of lymphocytes were attracted ; hence it is the strong connective-tissue reac- 

 tion in the older animals which in these instances caused the difference in 

 the fate of the graft in the old and in the young guinea pigs. Whether an 

 increased toxicity of the bodyfluids in older hosts contributed to the ac- 

 celerated and intensified injury of the graft is difficult to determine, because 

 the injury by the connective tissue was so marked that it might have obscured 

 a damaging effect of the bodyfluids. We have already remarked that in old 

 mice, transplants of thyroid and cartilage and fat tissue could do as well as 

 in younger animals. 



