70 EXPERIMENTAL RESEARCHES 



however, the limit of their growth is reached 

 and there now follows gradual resorption, which 

 is concluded after another week or two. If the 

 tumour at its maximum development is trans- 

 ferred to another rat, it does not gain any 

 foothold there, whilst if it is re-inoculated into 

 a mouse it again proliferates luxuriantly. It is 

 possible to keep on carrying out this zig zag 

 inoculation from mouse to rat and from rat to 

 mouse, and again from mouse to rat, etc., for 

 any length of time without the slightest check 

 to the energy of proliferation being encountered. 

 The question now arises, how it is possible to 

 explain, on the basis of these facts, the immunity 

 of the rat towards mouse-tumour cells. 



We can at once exclude the existence of a 

 natural immunity by antibodies in the rat, in 

 view of the marked initial proliferation of the 

 tumour. On the other hand it would be possible 

 to imagine that the resorption of the tumour 

 might be the result of an active immunisation of 

 the rat. That such immunisation ultimately 

 occurs may be proved by the fact that the result 

 of re-inoculating a rat, in which a first tumour 

 has been absorbed, with a second tumour, is 

 always negative. 



I do not, however, consider it permissible to 



