62 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



body. The keratin, and perhaps also remnants of feathers or other foreign 

 material, can in these cases act as non-specific stimulators for the host fibro- 

 blasts, or even for the transplanted fibroblasts, and this process can some- 

 times lead to the destruction of the transplants ; but in these instances, we 

 have to deal with non-specific reactions of connective tissue cells and not with 

 specific reactions on the part of lymphocytes induced by strange individuality 

 differentials. The great difficulty on the part of the chicken and pigeon skin 

 in forming a typical closed cyst, probably due to the low degree of growth 

 intensity of the avian epidermal cells, is an obstacle to the successful sub- 

 cutaneous transplantation of skin in this class of animals. But we must sharp- 

 ly distinguish between non-specific extraneous factors and specific factors 

 which can be used in the analysis of individuality differentials. In the former, 

 we have to deal with general tissue reactions, and these present important data 

 which can be used in the construction of a physiology of tissues. 



Various difficulties, then, are encountered, which limit a successful analysis 

 of the individuality differentials in birds. They include : ( 1 ) the difficulty of 

 obtaining the organs and tissues most suitable for transplantation in living 

 animals; (2) the presence of complicating, non-specific factors which may 

 cover up the reactions characteristic of the individuality differentials, as, for 

 instance, the connective-tissue reactions which we have mentioned, and (3) 

 the preponderance of lymphocytes in the circulation of birds, with which is 

 associated an excessively strong reaction of lymphocytes against even very 

 slight differences in individuality differentials; the great intensity of this 

 reaction makes the discernment of smaller differences in individuality differ- 

 entials difficult. However, the experiments which we have discussed do prove 

 the presence of very fine differences in individuality differentials in birds, 

 inasmuch as they have shown that such differences exist also between brothers 

 in inbred strains or races of birds. 



However, the great intensity of the lymphocytic reaction in this class of 

 animals makes possible the clearer recognition of the mode of action and of 

 the effects of the infiltration of a tissue with large masses of lymphocytes. In 

 the experiments with chickens it could be seen that these cells are able to 

 cause the disintegration of such structures as tendon-like fibrous tissue, car- 

 tilage, and even bone, resulting either in their complete solution or at least 

 very extensive destruction, and leaving behind a fine network of remnants of 

 these tissues. In mammalian transplantations it has not been possible to ob- 

 serve such marked effects ; this is due presumably to the usually less massive 

 invasion of mammalian tissues by lymphocytes, even in cases in which lympho- 

 cytes form lymph-gland-like accumulations in the tissues, such as we found 

 especially in rat and guinea pig. It appears doubtful whether mammalian and 

 avian lymphocytes otherwise differ markedly in their destructive power on 

 tissue, which is presumably due to the action of enzymes. In the mouse, for 

 instance, one can see in homoiogenous transplantations that lymphocytes as 

 well as connective tissue cells are able to penetrate into hyaline connective 

 tissue with ameboid processes ; they move, in the latter, in the direction of the 

 fibers and lymphocytes may likewise invade pieces of cartilage in the direction 



