DIFFERENTIALS IN CLOSELY INBRED MICE 103 



transplantations between 2 months old mice, the results approached those 

 found in autogenous reactions. These experiments confirm, then, our previous 

 findings, which indicated great variations in the transplantations in this strain 

 and the occurrence of autogenous as well as of homoiogenous reactions. We 

 noted furthermore that transplants into hosts as old as 18 months may remain 

 very well preserved, but in general the results were better in experiments in 

 which host and donor were young than in those in which they were older. 



In strain C57, almost all mice older than 12 months are affected to a variable 

 extent with sclerosis of the thyroid gland. If the various organ pieces, includ- 

 ing thyroid gland, of mice ranging in age between \S l / 2 and 18 months, were 

 transplanted into young, 2 to 3 months old mice, the preservation of the grafts 

 was, on the whole, very good ; but in one-half of the transplanted thyroids 

 sclerosis was found, and in five out of six of these pieces there was much or 

 moderate lymphocytic infiltration. Therefore, the sclerosis of the thyroid, 

 which develops in older mice of this strain, may remain unchanged in young 

 hosts. 



In strain C3H, tissues were transplanted from 13 to 13^ months old mice 

 to 35^2 to 4 months old animals. The results approached those of autogenous 

 transplantations and only rarely a slight lymphocytic infiltration was found. 

 As controls, transplantations were made from strain C3H to strain C57. The 

 donors ranged in age from 15 months and 13 days to 13 months and 20 days; 

 the hosts' age ranged from almost 3 to 4 months. The average grade was 1.37, 

 which corresponds to a marked homoiogenous reaction. In one instance the 

 thyroid was partly sclerosed. The relatively old age of the donor did not 

 modify noticeably the strength of the reaction of the host against the trans- 

 plants, and the marked difference between the severity of the reaction after 

 transplantation of tissues into different strains and into the same strain is 

 quite evident. 



In one experiment, four thyroid glands were transplanted from two \]/2 

 months old CBA mice to a 19 months old CBA mouse. Two months later, at 

 autopsy, two thyroids were found. There was no lymphocytic infiltration and 

 the thyroid tissue was well preserved, but a small amount of hyaline tissue had 

 developed around certain acini. Also, in the parathyroid hyaline septa were 

 noted. This observation agrees with that made in some other instances in 

 which thyroid glands were transplanted from young into old mice. We are 

 inclined to interpret this condition as a partial sclerosis of the thyroid gland. 

 It seems, then, that after transplantation of the thyroid gland from young into 

 old mice of the same strain there may be, in some cases, relatively much de- 

 velopment of dense fibrous-hyaline connective tissue around certain acini. 

 The conditions usually present in the old hosts would favor sclerotic changes 

 in the thyroid tissue, and if this tendency is sufficiently strong, it may lead to 

 sclerosis even in the non-transplanted thyroid gland of the host; otherwise, 

 there was no indication of a specific reaction of the host against the individu- 

 ality differential of the transplant, and in some transplantations into old mice 

 the results may approach those found in autogenous grafts. 



These experiments confirm, therefore, our former ones, in which it has been 



