104 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



shown that there are still disharmonies between the individuality differentials 

 of mice within the same inbred strains, but that the frequency with which the 

 resulting antagonism is found varies in different strains. They show also that, 

 essentially, old hosts react against transplants in a similar manner to young 

 hosts, except that there may possibly be a greater tendency in the old hosts to 

 the development of fibrosis around the transplanted thyroid acini ; but in ex- 

 periments in strain A, similar results were obtained in transplantations in 

 which the age of donors and hosts, brothers and sisters in these cases, was 

 1 or 2 months and in others in which it was 11 months. In addition, these 

 experiments show that if a sclerosed thyroid is transplanted to a young mouse 

 belonging to the same strain, the sclerosis may persist and that such a sclerosed 

 thyroid may also be invaded by lymphocytes. 



Serial transplantation of thyroid gland in inbred strain A. From long- 

 continued transplantations of tumors, we concluded that various mammalian 

 tissues have the potentiality to immortal life, although the organism of which 

 they form a part is mortal. It is not possible to repeat this condition with 

 normal tissues through serial transplantations in the same way as with tumors, 

 because in the case of normal tissues the differences between the individuality 

 differentials of donor and host will, in all probability, be so great that the 

 toxicity of the bodyfluids of the host and the aggressive action of its lympho- 

 cytes and connective tissue will injure and destroy the transplants within a 

 relatively short time. On a former occasion we attempted to overcome this 

 difficulty by using for serial transplantations in the rat a very resistant tissue, 

 namely cartilage, and in this way it could be shown that parts of the cartilage, 

 originally taken from an old rat, after transplantation into younger hosts may 

 remain alive for so long a time that the total age of the transplant exceeds that 

 usually reached by rats. 



Subsequently it occurred to us that this problem might perhaps be attacked 

 successfully in still another manner, namely, by serial transplantation in close- 

 ly inbred strains or families, in which the individuality differentials in all the 

 animals belonging to such a strain or famly had become identical or almost 

 identical. Here, the reactions against the transplants on the part of the hosts 

 should be lacking, or so slight that no serious damage would be inflicted. We 

 have already reported on short serial transplantations of this kind in guinea 

 pigs, but more favorable for this purpose seemed to be the inbred strain A 

 mice, because under natural conditions the life of this mouse is shorter than 

 that of a rat or guinea pig and because strain A was the one in which severe 

 reactions against transplants from donors belonging to strain A would be least 

 likely to occur. We selected for transplantation, the thyroid gland, with or 

 without the parathyroid. In this case, not only isolated tissues, such as fibro- 

 blasts or epidermal cells spreading out diffusely, but a complete organ would 

 be kept alive. 



Ten experiments were carried out, in which one or two thyroid glands were 

 transplanted from a strain A mouse through a number of generations of A 

 mice, the donors in different experiments varying in age between Sy 2 months 

 and 14 months. Three of the donors had reached the age of 14 months, one the 



