i9o8.] LOEB— TUMOR GROWTH AND TISSUE GROWTH. 11 



brought about, we do not yet know ; and this is one of the problems 

 that remain before us. Of one fact we may be reasonably certain ; 

 namely, that the growth-regulating substances to which we referred 

 just now are, in all likelihood, not the primary factors in the produc- 

 tion of tumors. We draw this conclusion because the action of such 

 substances has so far not been shown to be hereditary. They in- 

 fluence the growth as long as they are present. If we liberate tissues 

 or tumors from their influence these substances lose their effect at 

 once or relatively soon. If, however, we are able to transplant cer- 

 tain tumors through forty generations of animals and if the tumors 

 preserve their character as tumors, notwithstanding the individual 

 differences of the different animals into which they are transplanted, 

 then there must be present some factor in or near the tumor cells 

 themselves that constantly stimulates their growth and stirs them 

 restlessly to new activity, until through their activity they destroy 

 their host, and thus prepare their own end. What the character of 

 this local stimulus is, we do not yet know. All the discoveries of 

 organisms that have been announced from time to time were found 

 to be based upon erroneous observations ; but that does not exclude 

 the possibility that, after all, a microorganism in intimate relation 

 with the tumor cell is the local stimulus acting on the tumor cell. 



There are two discoveries that, in themselves of interest, promise 

 to give us a foothold from which to attack successfully this problem : 

 In the first place the endemic occurrence of tumors among animals, 

 to which we alluded above. Here we can determine whether it is 

 caused by hereditary conditions, or whether it is due to microorgan- 

 isms or environmental factors. Secondly, the surprising fact we 

 learned three years ago, that if we inoculate one kind of tumor, an 

 epithelial tumor, a carcinoma, into animals, the carcinoma, in a 

 certain number of cases, causes the surrounding connective tissue 

 to assume, likewise, a cancerous growth. We have here, therefore, 

 actually succeeded in producing a new tumor, a sarcoma. Such a 

 fact was entirely unforeseen. It could be discovered only through 

 the experimental method of investigation. The more unexpected a 

 new fact, the more welcome it is ; the more it promises to change 

 existing conceptions and to open up new roads, where before no way 

 out could be seen. 



