THEORIES AND PROBLEMS OF CANCER 231 



of his hypothesis of " Atrepsia." In his opinion the immunity 

 is connected with the nutrition available for the tumour cells. 

 He found that when transferred for a short time to a rat and 

 then back to a mouse, the tumour cells continued to multiply 

 again with their original vigour; a continuance of this zigzag 

 method of transplantation did not render the tumour less trans- 

 missible in the mouse, though it would die out if left too long in 

 the rat. He assumed two kinds of atrepic immunity, both 

 dependent upon what he calls " X-stoff," which supplies the 

 tumour cells with nutriment, either directly or indirectly. In 

 the one, in cases in which the tumour is in an animal similar to 

 that in which it originated, the " X-stoff" facilitates absorption ; 

 in the other, in cases in which the tumour is transferred to an 

 animal of another species, the part of the " X-stoff" which is itself 

 carried over with the graft forms the nutriment of the tumour 

 cells and is soon consumed. The " X-stoff," on this assumption, 

 must obviously be produced continuously when the tumour 

 cells, transferred to a similar animal, continue to grow in- 

 definitely but is used up gradually in cases of immunity. I have 

 kept a strain of tumour, given to me in 1906 by Prof. Ehrlich, 

 growing in mice and have produced several hundreds of pounds 

 weight of it, without any changes taking place excepting such as 

 can be accounted for as the result of experimental treatment. 



It seems quite in accord with other facts that mouse tumour 

 should die out when inoculated into rats, as many normal 

 mouse tissues have been shown to behave in the same way and 

 the same thing happens if normal tissue of one kind of animal be 

 introduced into another kind. The striking fact connected with 

 this is, that the cells from one species of animal will sometimes 

 multiply for a certain time in the bodies of another species 

 before they are destroyed but this appears to happen only when 

 the species are fairly nearly related. Jobling 1 has shown that 

 transplanted pieces of a malignant growth from a human subject 

 continued to grow in monkeys during a maximum period of six- 

 teen days but failed entirely to grow in rats and mice. The new 

 environment evidently supports the transferred cells during a 

 time proportionate to its similarity to the natural environment. 

 If the new environment be so nearly alike to the original that the 

 most resistant cells survive, the action of selection may produce 



1 Monographs of the Rockefeller Inst, for Medical Research, No. I, June 1910, 

 p. 120. V 



