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THE AMERICAN NATURALIST [Vol. LIV 



chemical reaction than any reaction known so far and 

 more delicate than the immune reactions. 



These substances which function as individuality-dif- 

 ferentials are produced in the active metabolism of cells ; 

 they can be demonstrated through their reaction after 

 transplantation of metabolically active tissue, but not 

 after transplantation of erythrocytes contained in a 

 blood clot. The syngenesiotoxins are least toxic. Usually 

 they call forth a lymphocytic reaction only slowly; but 

 ultimately the lymphocytes appear even here and invade 

 and destroy the strange tissue. At a time when the 

 vascular and fibroblastic reactions should occur, the 

 toxins may as yet be so weak that these two kinds of 

 tissues may behave normally. In other cases, however, 

 even the syngenesiotoxins may be strong enough to call 

 forth an altered reaction of vessels and fibroblasts ; the 

 vessels may fail to be present in a large number by the 

 graft and the fibroblasts may produce more fibrous 

 tissue. This occurs typically in response to homoio- 

 toxins; furthermore, in the presence of homoiotoxins 

 lymphocytes appear early and in large masses. They, as 

 well as the fibroblasts, may invade and destroy the 

 strange tissue. In the case of the heterotoxins the direct 

 injurious effect of the body fluids is much more pro- 

 nounced, and in a general way this effect is the greater, 

 the more distant the relationship between host and graft, 

 but in this case also the cellular reactions of the host may 

 secondarily contribute to the destruction of the strange 

 tissues. The fibroblasts of the host have a tendency to 

 form dense fibrous tissue around the graft and the blood 

 and lymph vessels are usually extremely sparse in the 

 direct neighborhood and in the interior of a heterograf t ; 

 apparently the strange cells do not exert an attracting or 

 stimulating influence on the growth of capillaries. While 

 thus in these two respects the heterotoxins produce an 

 effect which might have been foreseen from a study of 

 homoiotoxins, the behavior of the lymphocytes seems to 

 be different from what might have been expected. In- 

 stead of being attacked by lymphocytes even more actively 



