DIFFERENTIALS IN CLOSELY INBRED MICE 105 



age of 13 months, and two the age of 12 months at the time the experiment 

 was started. The serial transplantations were continued for periods that varied 

 between 5^2 months and 34^ months. The youngest thyroid recovered after 

 completion of the serial transplantation was 11 months and 10 days old and 

 the oldest one was 41^ months old; the age of the others was intermediate 

 between these ages. The age of 41^2 months exceeds considerably the average 

 age of A mice and exceeds, probably, also the oldest age which mice belonging 

 to this strain reach. The number of serial transplantations made in these vari- 

 ous experiments ranged between three and seven, and the time during which a 

 transplant remained in a single host varied between 2j^ months and 6 months ; 

 but in several instances the transplant was left only 2^2 weeks in the last host. 

 On microscopic examination it was found that the thyroid was preserved, but 

 in four experiments it showed either slight or partial sclerosis, while in six 

 experiments it showed complete sclerosis. In the latter case, all the acini were 

 surrounded by rings of hyaline-fibrous tissue, which separated the acini from 

 one another. In the center, a number of acini had been reduced to thin cell 

 strands or had been lost entirely through pressure of the stroma, which injured 

 the epithelium, and, in addition, interfered with its nourishment. Several 

 capillaries were seen, however, in the stroma. These thyroid transplants, there- 

 fore, resembled closely the non-transplanted thyroids of older mice belonging 

 to strain C57. In four cases a limited number of acini were surrounded by 

 thin hyaline rings ; but over larger areas the acini were lying close together. 

 Lymphocytic infiltration was either lacking entirely, or it was slight. Probably 

 two factors are involved in the development of this sclerotic condition: (1) 

 The age of the donor. An age of the donor above 13 months favors complete 

 sclerosis. There was no sclerosis when the transplant was less than 12 months 

 old. (2) The length of time during which the transplant remained in the 

 strange hosts. Complete sclerosis was found in cases in which the donor was 

 only 6 or 12 months old ; here there is little doubt that the thyroid at the time 

 of transplantation was not yet sclerotic, but that it acquired the sclerosis in 

 the course of the serial transplantations. Therefore, if the transplanted thyroid 

 remained long enough in different hosts, or if it attained a certain age, it 

 tended to become completely sclerotic. This condition was especially marked 

 if the donor of the thyroid had reached the age of 14 months, in which event 

 the transplant either was already somewhat sclerotic or had a greater tendency 

 to become so. This interpretation agrees with our findings in the preceding 

 experiments, where there were likewise indications that both the factors men- 

 tioned here may play a role in producing thyroid sclerosis. 



It follows from these experiments that by means of serial transplantations 

 in the same inbred strains, it will in all probability be possible to keep whole 

 organs alive up to an age which much exceeds that usually attained under 

 normal conditions, and that the reactions of the host against such transplants 

 may be lacking entirely. It may perhaps be possible even to keep such trans- 

 plants alive indefinitely by serial transplantation. However, it may be suggested 

 that in future experiments of this. kind, the transplant be allowed to remain for 

 longer periods of time in the same host and that, correspondingly, the number 



