TUMORS AND NORMAL TISSUES 351 



depend upon the genetic constitution, may perhaps have been slightly modified, 

 and that such a modification may have facilitated the growth of the mouse 

 tumor in the rat ; but we do not possess any definite knowledge as to wherein 

 such adaptive processes consist. 



There can be little doubt that even under the most favorable conditions a 

 heterotransplantation of a mouse tumor into rats, which would lead to a 

 permanent growth in the latter, cannot be accomplished. There remains a 

 difference between the growth of these tumors in the rat and in the mouse, 

 and there also remains a difference between the growth of rat-adapted mouse 

 tumors and real rat tumors in the rat. It is therefore not possible to conclude 

 that a heterotransplantation has been fully successful in these experiments. 

 Furthermore, we must not identify mouse-to-rat transplantation with hetero- 

 transplantation in general. There can be no doubt that transplantation of 

 mouse tumors into the subcutaneous tissue or into the peritoneal cavity of 

 farther distant species would have a much more unfavorable outcome and that 

 such tumors would undergo rapid necrosis. 



While the data concerning the degree to which transplantation of tumors 

 from mouse to rat is possible may be considered as well established, and 

 while these data are not in conflict with the conclusion that the concept of 

 organismal differentials applies also to the transplantation of tumors, there 

 have been recorded, from time to time, observations which make it appear 

 that cancers can be successfully transferred also into widely distant species. 

 If this were a fact, it would be contradictory to what is known about the 

 significance of organismal differentials in determining the fate of transplants. 

 Thus it has been stated that human tumors can be transplanted to dog, rabbit 

 or rat; however, should a tumor develop in the new host following such a 

 transplantation, there is the possibility that it may have been a spontaneous 

 growth ; it is very improbable that the growth was derived from the heteorog- 

 enous cells. In the case reported by C. Lewin many years ago, the transplant- 

 able tumor, which formed in the rat following transplantation of pieces of 

 human cancer, was of a very low degree of specificity and was apparently 

 constituted of cells which usually take a prominent part in inflammatory 

 reactions. 



In heterotransplantations of normal tissues we have seen that the toxic 

 action of the bodyfluids of the host is much more evident in the destruction 

 of the transplant than in homoiotransplantations. Although in the latter the 

 toxicity of the bodyfluids does injure the transplant, the cellular and vascular 

 reactions of the host are, here, relatively more important. Likewise, hetero- 

 transplanted tumors are primarily injured by the heterotoxins of the body- 

 fluids of the host, although cellular reactions may secondarily participate in 

 the destruction of the graft. 



Since, after homoiotransplantation of a tumor, even in an animal im- 

 munized against it, it is usually the central part of the graft which dies first, 

 while after heterotransplantation in an immunized animal the peripheral, as 

 well as the central, part shows signs of injury, it has been assumed that im- 

 mune substances exert their injurious action only on heterotransplants, and 



