ON SPECIFIC THERAPEUTICS. 71 



identify these two manifestations of immunity. 

 For in the first place it is highly improbable 

 that, during the period of rapid proliferation of 

 the tumour in the rat's organism, any consider- 

 able absorption of tissue elements should occur. 

 Further, previous to that period, a production of 

 antibodies has not been proved. Lastly, since 

 the tumour, when re-inoculated into the mouse, 

 never shows ever so slight a reduction of viru- 

 lence, one would have to suppose with von 

 Dungern that the anti-substance formed in the 

 rat's organism was activated only by the com- 

 plements of rats but not of mice. This, again, 

 has up to the present not been proved. 



A far simpler and more natural explanation 

 of all these phenomena is afforded by my hypo- 

 thesis of atrepsy. According to this, the mouse- 

 tumour cells require for their growth not only 

 the ordinary nutritive substances which the rat 

 can also supply to them in ample quantity, but, 

 besides that, some well-defined substance which 

 is present only in the mouse's organism. 



In the rat, then, these cells would only be 

 able to go on growing so long as there still 

 remained with them some of the specific growth- 

 stimulating substance that was introduced with 

 them at the time of inoculation. When it has 



