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THE AMERICAN NATURALIST [Vol. XLIX 



without doubt regard tumor cells as a kind of somatic 

 cells. 



One of the most characteristic properties of cancer cells 

 is their ability to grow after transplantation into other 

 animals of the same species. This applies not to all, but to a 

 certain number of spontaneous cancers; the majority of 

 spontaneous tumors are not transplantable into other in- 

 dividuals of the same species. They grow, however, 

 usually after transplantation into the same individual in 

 which they originated. There does not exist as far as 

 their origin is concerned any essential difference between 

 these two kinds of cancers— those transplantable and not 

 transplantable into other individuals. The cancers used 

 in experimental tumor investigation take their origin 

 from somatic cells ; but it appears some are less sensitive 

 to the difference in the chemical composition of the body 

 fluids which exists between different individuals of the 

 same species than others, and those less sensitive can be 

 transplanted, while others can not. 



In those tumors which are transplantable, relatively 

 few tumor cells give after inoculation into other animals 

 origin to the new tumors, and the tumor cells after the 

 first transplantation not rarely multiply with greater 

 vigor than they did in the original animal, an effect caused, 

 as I could show, through the stimulating influence of the 

 cutting and otherwise manipulating the tumor cells. In 

 each animal therefore there are produced many successive 

 generations of tumor cells, and after transplantation into 

 another individual each surviving cancer cell produces 

 again new generations. Consecutive transplantations into 

 many individuals have been carried out with the same 

 tumor. The potential proliferative power of the cancer 

 cells is therefore enormous. It is, however, not so much 

 the intensity of the proliferative power of the tumor 

 cells which we wish to consider as the potential duration 

 of their life. It has been shown that epithelial, as well as 

 connective tissue tumors can be transplanted through 

 many generations and can survive for a long time the 

 animal in which the tumor originated. Thus I was able 



