Xo. G30] 



l\ I MIL r RELATION SHIP 



49 



than the homoio-tissues, the lymphocytic reaction is on 

 the contrary decidedly weaker in the case of tissues of 

 other species. Lymphocytes do not usually invade and 

 destroy the tissue of a foreign species, although they may 

 collect in a considerable number in the neighborhood of 

 the graft and may occasionally invade it here and there 

 in small areas and destroy these. They may appear in 

 large quantities after the graft has been destroyed. This 

 relative inactivity of the lymphocytes is surprising, if we 

 consider the much greater strangeness between graft and 

 host which results from heterotransplantation. But it 

 may be just on account of the great strength of the hetero- 

 toxins that the reaction is weakened. "We have seen above 

 that the development of the specifically attracting sub- 

 stances depends upon the active metabolism of the graft 

 as evidenced by the failure of the erythrocytes to at- 

 tract the lymphocytes. In the case of heterotransplan- 

 tation the action of the toxins may be so strong that it 

 interferes with these specific metabolic activities of the 

 transplanted cells, and thus the reaction would naturally 

 become weaker. It may also be that the scarcity of ves- 

 sels around and in the graft may contribute to this result, 

 these vessels acting as the channels through which the 

 lymphocytes reach the graft. But that this factor can 

 not be the only reason is clear from a study of certain 

 areas of the transplant occasionally found in which the 

 blood vessel supply may be somewhat better, but in 

 which, nevertheless, the lymphocytic infiltration of the 

 graft is absent or slight. 



There is then a graded reaction of various tissues of 

 the host towards transplants. At one end of the series 

 we find the autotransplants ; these are followed by the 

 various kinds of syngenesio-, by homoio- and hetero- 

 transplants. Autotransplants call forth a marked vas- 

 cular reaction. This reaction decreases in the direction 

 towards heterotoxins; it is already very weak in most 

 cases of homoiotoxins. The fibroblasts remain relatively 

 well preserved, form least fibrillar material and tend to 

 the production of myxoid tissue under the influence of 



