TUMORS AND NORMAL TISSUES 361 



of these fibroadenomas of the mammary gland under the influence of cer- 

 tain hormones may enable them to overcome the resistance to their growth, 

 caused by unfavorable individuality differentials of hosts, and to grow, there- 

 fore, after homoiotransplantation. There were also some indications that there 

 exist strain differences, a certain tumor growing better in one strain of rats 

 than in another; presumably strains in which the individuality differentials of 

 the rats were similar to that of the tumor were more suitable than strains 

 with more strange individuality differentials. 



While as the result of the growth or of the retrogression of transplanted 

 malignant tumors the bearers of the transplant may become immune against a 

 second transplantation, such an immunity has not been observed in the case of 

 benign tumors; the latter behaved in this respect, as well as in the lack of 

 adaptive processes and in their responsiveness to hormones, similar to normal 

 tissues, while as far as the growth energy and abnormal mode of growth is 

 concerned, they are intermediate between normal tissues and cancers. In 

 cancerous tissue the inner growth factor (Gi) has become so strong and 

 provides so stable a growth momentum to the tumor cells that extrinsic fac- 

 tors (Ge), such as hormones, have no longer any chance to affect the growth 

 to any marked extent. In normal tissues and benign tumors the relation be- 

 tween Gi and Ge differs in favor of Ge, whereas in cancerous growth Gi 

 predominates ; in addition, other changes may have taken place in the tissues 

 during their cancerous transformation, which tend to diminish the effective- 

 ness of regulatory processes. It is then the strength of Gi (endogenous growth 

 factor) which is one of the factors enabling a cancerous tumor to overcome 

 difficulties in transplantation, and especially in serial transplantation, and per- 

 haps the main difficulties in the way of the transplanted cells consist in differ- 

 ences in the individuality and species differentials between host and trans- 

 plant. In tumors in which Gi has not yet reached sufficient strength, there is 

 therefore need for the additional and longer continued action of Ge 

 (exogenous growth factor) and this applies especially to certain benign 

 tumors, where hormones may affect favorably the growth of transplanted 

 fibroadenomas. Also, in certain other tumors which have not yet reached a 

 fully cancerous state, the effect of Ge, acting on the host of the graft, may be 

 required for a successful growth of the tumor in the strange host; thus 

 Gardner noted that estrogen administration in male mice could make possible 

 the development of transplanted tumors of the interstitial gland of the tes- 

 ticle. Although such tumors could occasionally metastasize, on the whole they 

 had reached only a very low degree of growth momentum, or, expressed 

 differently, they had not yet progressed very far in the process of cancela- 

 tion. They therefore could be transplanted successfully only if the host re- 

 ceived, at the same time, estrogen. The same factor, Ge, which helped to 

 cause in the normal tissue the increase in Gi and thus the transformation into 

 cancer, was needed in order to add to the growth momentum of the trans- 

 plant and aid it in surviving and growing under otherwise not quite adequate 

 conditions. There seemed to exist a somewhat similar action of Ge in the 

 transmission of leukemia from hybrids between mice belonging to a leukemic 



