TUMORS AND NORMAL TISSUES 349 



tiveness to these substances, or a greater power to absorb and neutralize them 

 was responsible for the increased resistance. That these tar cancer cells, how- 

 ever, still possessed the species differential of the mouse is indicated by the 

 fact that it was possible to elicit an active immunity against them by a previous 

 immunization of a rat with mouse liver serving as an antigen. But the best 

 known experiments of this kind are those of Putnoky, who has been able 

 to propagate an Ehrlich mouse carcinoma continuously in rats since 1929 

 by re-transplanting it every ten days. This mouse tumor grew very rapidly in 

 rats for from ten to fourteen days, during which time many rats were killed 

 by its growth. Following this period, necrosis and death of the tumor set in in 

 animals which survived. In other series of heterotransplantations, various 

 investigators have succeeded in keeping mouse tumors alive in rats and in 

 inducing temporary growth, but invariably the growth energy has decreased 

 after a number of successive rat to rat transplantations and then the tumors 

 died out, although after re-transplantation to the mouse, if the tumor had not 

 been too seriously injured, it could be propagated indefinitely. 



Why is it that in these few instances it has been possible to propagate 

 indefinitely, under the conditions mentioned, the mouse tumor in rats? Pre- 

 sumably the various factors enumerated above were responsible for its 

 successful development in heterogenous hosts. The tumor chosen by Putnoky, 

 being a very rapidly growing one, the growth momentum of the cells could 

 overcome injurious conditions up to a certain limit. This marked growth 

 intensity found a morphological expression in the small amount of stroma 

 present in this carcinoma. It was to be expected that such a rapidly growing 

 tumor would be able to absorb and neutralize a larger quantity of either 

 natural or immune heterotoxins than would a slowly growing tumor. Addi- 

 tional factors to be considered in these heterogenous tumor growths are the 

 greater power of resistance of the tumor cells and, furthermore, the strain of 

 rats into which the transplantations are made. Putnoky found that the constant 

 propagation of the Ehrlich mouse carcinoma in rats succeeded only if a Hun- 

 garian strain of rats was used ; in English rats the tumor regressed spontane- 

 ously after about ten or fourteen days. This explains why several other investi- 

 gators, who used other than Hungarian strains of rats, were unable to obtain 

 equally favorable results. However, more recently de Baloghi succeeded in 

 repeating Putnoky's experiments with another strain of Ehrlich mouse carci- 

 noma. This tumor behaved biologically and structurally in a similar way to 

 Putnoky's tumor; but de Baloghi obtained long-continued heterogenous 

 growth in diverse races of white and gray rats. The favorable results noted 

 in these experiments were therefore not primarily due to racial factors in the 

 host, but apparently to the selection of the most vigorous tumors possessing 

 a great growth momentum. 



There remains the question as to whether the differences in the growth 

 of different mouse tumors in the rat are due to a change in the genetic con- 

 stitution of these tumors, implying a difference in their organismal (species) 

 differentials. There is no definite reason for assuming such a change. These 

 rat tumors can readily be re-transplanted into mice, where they grow in the 



