miscellaneous: stimuli underlying tumor-formation 571 



which have no leucocytic apparatus, are easier to work with 

 than animals and undoubtedly their tumors w411 continue to 

 yield to careful study many facts which must throw inter-' 

 esting side-lights on animal tumors, and the time is not far 

 removed when they will be studied in many laboratories for this 

 purpose. 



Neither does it seem to me a valid objection to the above 

 argument that cancers of rats and mice in which no parasite 

 has been found are easily inoculable, grow rapidly, and soon 

 destroy the host animal. Here the case is quite different from 

 that of a naked bacterium. The cancer graft carries into the 

 body of the mouse or rat a compact mass of cells, hundreds of 

 thousands of them, functioning as a unit. If the graft heals on, 

 the cancer cannot fail to continue its malignant growth. If, 

 however, it were possible to divide this mass of cells into its 

 component elements we should then have a suspension of can- 

 cer-cells comparable to a bacterial culture. The cancerous mass 

 would then have lost not only its unity but its strength, and 

 undoubtedly the body would then react to it very differently, 

 that is to say, just as it often does to a few cancer cells that 

 have drifted away from the main body of the tumor and are 

 surrounded by leucocytes and destroyed in a thrombus, or just as 

 it does to an injected bacterial suspension of a feeble para- 

 site, a cancer parasite let us assume, each separate foreign 

 cell being surrounded and overwhelmed by the leucocytes. 

 If we could devise a satisfactory method of overcoming the 

 resistance of the normal body in our experimental animals, 

 a resistance due to leucocytes and anti-bodies, we should then 

 have the pre-cancerous stage, about which we have heard so 

 much in recent years, and in reality know so little, and it would 

 probably be easy to produce tumors in some of them, even with 

 crown-gall bacteria. 



How the oncologist approaches the problem of the etiology of 

 tumors will depend very largely upon his training. In looking 

 toward the solution of the cancer problem, not much is to be 

 hoped from such pathologists as have only a descriptive knowl- 

 edge of tumors but much from some of the younger biologists, 

 especially those who are well trained in bio-physics and bio- 



