392 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



came more virulent, which means that they multiplied more rapidly in the host ; 

 at the same time, certain structural and other changes occured and these cells 

 acquired the ability to propagate in alien strains into which they could not be 

 transplanted in the beginning. However, in such experiments it is difficult to 

 determine how far the increase in transplantability of the leukemic cells is due 

 to the increase in growth momentum and how far it is due to actual adaptive 

 processes to strange individuality differentials. In some cases, on the other 

 hand, the contrary effect, namely, a greater sensitiveness to strange individu- 

 ality differentials, has been observed in the course of continued transfers. 



We have referred above to the experiments of Gheorgiu, in which adaptive 

 processes, arising in mouse tumors, gradually increased the ability of these 

 tumors to grow also in heterogenous, although nearly related species. Similar 

 observations have been made in the Putnoky experiments, in which a mouse 

 carcinoma could be serially transplanted into rats ; but these we have discussed 

 in an earlier chapter. We may, however, add here that in the early passages 

 there was more necrosis than in later ones, in which the tumors were able to 

 maintain themselves also in somewhat older rats. There was, moreover, a 

 diminution in the amount of stroma in the rat-adapted mouse tumors. In these 

 heterotransplantations, as well as in the transplantation of leukemic cells into 

 different strains, certain structural changes took place in the course of con- 

 tinued transplantations; furthermore, rat-adapted tumors, when transplanted 

 back to mice, showed a marked growth energy. 



In a considerable number of experiments it was possible to make tumor or 

 leukemic cells grow in unfavorable strains, if the aggressive power of the hosts 

 had first been depressed by some experimental means. Preliminary treatment 

 of the host animals with X-ray or with trypan blue had this effect. Cancerous 

 or leukemic cells, which had been propagated for some time in such specially 

 prepared hosts, were afterwards able to propagate in otherwise unsuitable 

 strains, even without a preceding experimental depression of the aggressive 

 power of the host animals. Another method, which led to similar results, was 

 used by Margaret R. Lewis, who inoculated mouse sarcoma into mice belong- 

 ing to strains which were genetically unsuitable for the growth of this tumor. 

 The first inoculations of this kind were unsuccessful; but after repeated 

 inoculation of pieces of this sarcoma into the same individual mice, the sar- 

 coma grew in the end, and after the tumors had once succeeded in growing in 

 alien strains they could be further propagated in these strains without much 

 difficulty. Several investigators have found that tumors which did not grow 

 after subcutaneous, intramuscular or intraperitoneal transplantations, grew 

 successfully in the brain or in the anterior chamber of the eye. Such tumors 

 could subsequently be successfully transplanted, also, by subcutaneous or 

 intramuscular inoculation into animals in which originally they would not have 

 grown in these places. But such an increase in the capacity of tumors to grow 

 elsewhere after they had first been transplanted into the anterior chamber of 

 the eye was not noted in some recent experiments which Greene carried out 

 with rabbit tumors. 



As to the mechanism underlying these adaptive changes, it might be assumed 



