358 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



endothelial system. This is perhaps indicated by the fact that the lymphocytes 

 and leucocytes of the blood may react as early as within the first few days 

 against a homoio- or heterotransplant of normal tissue or tumor. 



But there are several observations which indicate that the application of 

 X-rays, injections of tar, or perhaps even of trypan blue, may have results of 

 a quite different kind ; they may promote the induction of autogenous cancers 

 elsewhere in mice under the influence of tar or other carcinogenic substances 

 (Mayngord and Parsons, Maisin and Masse, Andervont). A related phenom- 

 enon is probably the effect of X-rays or tar on the development of rabbit 

 fibroma following the injection of virus mentioned above. In addition, Rous 

 and his collaborators produced carcinomata in rabbits through intravenous 

 injection of rabbit papilloma virus, in places where the skin had been irritated 

 through previous applications of tar. These effects cannot be due to an inhibi- 

 tion of immune processes developing against strange tissues or tumors and 

 their organismal differentials, but mechanisms of a different kind must be 

 active in these experiment. 



Taken altogether, these experiments add further data in support of the 

 conclusion that in principle the host reacts in a similar way against normal 

 tissues and against tumors, but that secondary factors may be added in the 

 case of tumors, which may induce certain modifications in the types of reaction 

 which occur; and furthermore, that it is the organismal differentials which 

 normal tissues and tumors have in common. 



Transplantation of Benign Tumors 



We have, so far, analyzed some of the principal factors which determine 

 the growth of transplanted malignant tumors, with particular regard to the 

 significance of organismal differentials of host and transplant. It will be of 

 interest, now, to compare with the growth of cancerous tissue, that of benign 

 tumors. In experiments beginning in 1901, and continuing at various periods 

 during the course of the following thirteen years, we transplanted, at various 

 times, altogether four mammary fibroadenomata and two mammary fibromata 

 of the rat, and, with S. Leopold, a mixed mammary tumor, a chondromyx- 

 adenoma of a dog. Similar experiments were subsequently reported by Rib- 

 bert, Borrel and Petit, and more recently, by Mann, Robinson and Grauer, 

 Heiman, Heiman and Krehbiehl, Umehara, Picco, Oberling, and the Guerins, 

 Emge and Wulff, as well as by Wolfe, Burack and Wright. 



From our investigations the following conclusions may be drawn : Benign 

 tumors show a reaction in certain respects intermediate between that of 

 normal tissues, which after serial transplantation manifest at most only a 

 very limited and transitory regenerative growth, and that of malignant tumors. 

 If they grow at all, they usually do so only very slowly and after a relatively 

 long preceding latent period, during which, however, mitotic proliferation may 

 occur. In the majority of cases we had to deal with tumors of the mammary 

 gland, which were composed of adenomatous as well as of fibrous constit- 

 uents, and both could take part in the subsequent proliferation; the fibrous 

 portion evidently did not represent merely the stroma of the epithelial struc- 



