TRANSPLANTATION OF TUMORS 391 



Thus Bashford and Murray found that the Jensen mouse carcinoma, which 

 grew readily in Danish mice, but only with difficulty in English mice, began 

 to grow at last also in the latter in the course of continued transplantations. 

 Similarly, the first Rous chicken sarcoma which, according to Rous and 

 Murphy, at first took only in blood relatives of the animal in which it had 

 originated, after further transplantation grew well also in non-related 

 chickens of the same variety, and after still further propagation it became 

 adapted even to growth in different varieties of fowl. On the other hand, the 

 second Rous sarcoma, an osteochondroma, grew from the start in all varieties 

 of fowl, in conformity perhaps with the relatively low degree of sensitiveness 

 of cartilage to differences in individuality differentials. Rous and Murphy 

 observed also a selective process, which led to the opposite effect ; by 

 selecting weakly growing tumors for further transplantation, a line of 

 tumors was propagated which tended to undergo retrogression. In this case 

 evidently the tumor cells had been injured through unfavorable organismal 

 differentials of the host, or through certain secondary factors — an injury 

 similar to that obtained by heating — and after successive transplantations 

 these injuries accumulated. We have already referred to the experiments of 

 Duran-Reynals, in which marked adaptive changes were observed in Rous 

 chicken sarcoma cells after transplantation into ducks ; these changes affected 

 primarily the agent situated in the cells, but secondarily, the cells themselves 

 seemed to undergo corresponding adaptive changes, presumably under the 

 influence of the agent they contained. 



Similar in certain respects were the adaptations which Roffo noted in a 

 transplantable rat tumor. Through continuous selective tranplantation he 

 succeeded in adapting this tumor to growth in different varieties of rats. At 

 last it could be successfully transplanted in 70 per cent of wild rats, in which 

 it had not been able to grow at all in the beginning. It was of interest that 

 also in these experiments there was a parallelism between the increase in 

 transplantability and growth energy of the tumors, indicating that we may 

 not have had to deal solely with an increase in transplantability due to special 

 adaptive processes, but also to an increase in growth energy. Similar adaptive 

 changes were apparently observed by Gheorgiu when he transplanted mouse 

 tumors into very young rats. With successive passages the process of complete 

 retrogression in the heterogenous animals became more and more delayed, 

 until at last growth extended as long as to the twenty-seventh day following 

 transplantation. After several passages in newly-born rats, in which presum- 

 ably the mechanisms of reaction against strange organismal differentials are 

 not yet fully developed, the tumors could be transplanted also into older suck- 

 lings, but here the tumor did not live as long as in the very young animals. The 

 retrogression and absorption in these young animals seemed to follow without 

 the aid of leucocytes (lymphocytes). After reinoculation into mice the tumors 

 grew with increased intensity. There are still additional experiences which 

 point to adaptations taking place in tumors in the course of serial trans- 

 plantations and causing an increase in their transplantability. Furth and others 

 observed that also leukemic cells after continued serial transplantations be- 



