INTRODUCTION 7 



genesio-, homoio- and heterotransplantation. The transplantation of various 

 kinds of tissues and organ pieces into the same animal from which they were 

 taken and to which, therefore, they belonged, is called autotrans plantation. 

 Here we find that lymphocytes are practically lacking around the graft; con- 

 nective tissue cells are attracted in only a moderate number and instead of 

 producing dense fibrous tissue, which is characteristic of their reaction against 

 a strange individuality differential, they form only a loose embryonal stroma 

 around the transplanted cells. The blood vessel supply is rich and in the course 

 of a relatively short time the transplant assumes about the condition of the 

 normal tissue or organ in the host. All tissues from the same organism behave 

 in this respect, in principle, in the same way, except that some tissues can 

 withstand the injury connected with the process of transplantation much 

 better than others. We may then conclude that it is not the organ or tissue 

 differentials which determine these injurious reactions of the host cells 

 towards the grafts, but the individuality differentials. The chemical consti- 

 tution of liver and of kidney is very different, but this difference has no effect 

 on the host cells — they react in about the same way towards liver and kidney, 

 provided these tissues possess the same individuality differential ; however, a 

 slight difference in the chemical constitution of the individuality differential 

 sets unfavorable reactions in motion ; and it makes little difference whether the 

 strange individuality differential is attached to organ differentials of kidney, 

 liver, skin, cartilage, uterus or thyroid. The various organ differentials all 

 behave in about the same way. 



This, then, is the first important fact: the host cells recognize in a very 

 subtle way differences in individuality differentials. But they can do more 

 than this. As stated above, they are able to recognize the degree of difference 

 and to react accordingly. Thus, when a piece of tissue from brother to brother 

 is transplanted — a method designated as syngenesiotrans plantation — the cells 

 of the one which functions as host are not as much stimulated or excited by 

 the presence of a tissue so closely related to his own as by the tissues from a 

 non-related individual {hornoiotrans plantation) , the individuality differentials 

 being more similar in the first case. This observation holds good especially if 

 the parents belong to closely inbred strains ; otherwise brothers and sisters may 

 be genetically similar to each other to very different degrees and therefore, in 

 some instances, the reaction against a tissue of a brother may be about the 

 same as against that of a stranger, and if the strangeness exceeds a certain 

 limit, it is no longer the lymphocytes which are active, but the connective 

 tissue cells and the injurious substances of the bodyfluids. On the contrary, 

 in certain inbred strains the individuality differentials of all the animals 

 belonging to such a strain may have become so similar that no or only very 

 slight differences can be established between brothers and not directly related 

 individuals within the same strain. 



On the other hand, if a piece of tissue is transplanted from one animal to 

 another which is genetically still further removed than in cases of homoio- 

 transplantation, as when animals from different species serve as host and 

 graft, the reactions are more severe. This procedure is called heterotrans- 



