CANCER RESEARCH 37 



organisms might be responsible for the cancerous cell proliferation. 

 We must of course remember that our present knowledge of the factors 

 underlying cancer was only gradually acquired ; furthermore that certain 

 experiences concerning animal cancer which we shall discuss later, sug- 

 gested microorganisms as the direct stimulating agencies while the other 

 factors which we analyzed so far would represent merely indirect causes, 

 making the infection with microorganisms more easy. We know indeed 

 that to a certain extent microorganisms can call forth cell proliferation. 

 Thus the tubercle bacillus may cause a limited growth of connective 

 tissue and as especially Borrel has pointed out the organisms causing 

 smallpox and vaccinia may even produce a slight proliferation of the 

 infected epithelium. But in all those cases the proliferation soon ceases 

 and toxic substances produced by bacteria lead usually soon to the death 

 of a great part of the newly-formed cells. 



Based on such considerations many attempts have been made to 

 prove the constant presence of certain parasitic microorganisms in 

 human cancer. Under the microscope it is possible to recognize in 

 cancer cells certain inclusions which are usually absent in normal tissues 

 and a number of investigators claimed such included bodies definitely as 

 protozoa (among others, Thoma, Sjoebring, Leyden, Gaylordand Eisen). 

 Even the life cycle of these protozoa was apparently determined by some 

 of these authors. It could, however, be shown that similar cell inclu- 

 sions may originate otherwise and did in all probability therefore not 

 represent protozoa. Also bacteria (Doyon), mucor (Schmidt), chytri- 

 diaceae (Behla) and other microorganisms were held responsible. Others 

 (Sanfelice, Leopold) believed yeast-like organisms to be frequently 

 demonstrable in human cancer; they cultivated some yeasts in culture 

 media and by injecting the organisms into animals believed to have 

 reproduced the disease. Careful studies by many investigators, how- 

 ever, could not confirm these interpretations. In the case of yeasts it 

 has for instance been shown that although they occasionally occur in 

 cancers, they are on the whole rare and do not reproduce the disease if 

 injected into animals (Busse, Nichols, Loeb, Moore and Fleisher). 

 They act in the body in a similar manner as inert foreign bodies, and 

 we found a yeast which we isolated from a sarcoma to lead, after intra- 

 venous injections, to the death of the animal through occlusion of the 

 kidney tubules, without ever producing cancer. 



These persistent claims of the discovery of a microorganism as the 

 cause of cancer which could in no case be substantiated led in the case of 

 many pathologists, especially of those mainly interested in the careful 

 study of the structure of pathological tissues, to a reaction which in- 

 duced them to deny the possible importance of microorganisms in the 

 causation of cancer ; they were inclined to hold on the whole the factors 

 already established as sufficient to explain the origin of cancer. On the 



