No. 581] GERM CELLS AND SOMATIC CELLS 301 



if we inject daily from the second to the fifth day after in- 

 oculation mice with either of these two substances, tumor 

 growth is not noticeably retarded through the injections. 

 Tumors, in an early stage of development, are resistant 

 to this inhibiting effect. If we now give four intravenous 

 injections, one on each successive day, from the second to 

 the fifth day, and later again to the same mice four injec- 

 tions from the ninth to the thirteenth day, the latter series 

 of injections which had been effective in other animals not 

 previously injected from the second to the fifth day, have 

 now almost completely lost their efficacy. The mice have 

 through the first set of injections become immune against 

 the action of colloidal copper and hirudin as far as the 

 effect of these substances on tumor growth is concerned. 



We next inquired into the mechanism of this immunity, 

 and especially were we concerned with the place where 

 this immunity is produced. It was conceivable that this 

 took place either in some organ of the injected animal or 

 in the tumor cells themselves. The following experiments 

 showed that both possibilities were realized : If we inject 

 the mice on the four days preceding inoculation with 

 tumor, immunity is produced, at least in the case of col- 

 loidal copper. This proves that some organ in the host 

 animal contributes to the immunity, inasmuch as in this 

 case the preliminary injections exerted their influence 

 without having had a chance to act on the tumor cells. 

 But the tumor cells themselves also become actively im- 

 mune, as shown in the following manner: We inject ani- 

 mals with either colloidal copper or hirudin from the 

 second to fifth day, and again from the ninth to the 

 thirteenth day after inoculation. Two days after the last 

 injection we used the tumors of some of the injected ani- 

 mals for reinoculation into a new set of mice. Nine to 

 thirteen days after this second inoculation the mice be- 

 longing to the second set are injected with colloidal copper 

 and hirudin, respectively. Now we find that these tumors 



also are almost entirely resistant against the effect of 

 colloidal copper and hirudin, although in this case no pre- 

 liminary injections had been given to the animals which 



