334 Tumors. 



ranee of the origin of these growths and to bring into the prob- 

 lem new difficulties. 



Among the more prominently urged hypotheses may be men- 

 tioned the idea that an infections cause underlies the cellular 

 proliferation, a view which has had numerous adherents in re- 

 cent years. At first bacteria were sought as causative factors, 

 some of which as the causes of productive inflammation had mani- 

 fested functions suggesting analogies with tumor formation ; 

 but nothing has been shown beyond the mere secondary presence in 

 tumors of various saprophytes, pyogenic and other bacteria ; and 

 no microphytes have been encountered which can be regarded 

 as responsible for the proliferation of cells into tumors. In time 

 the line of investigation was directed to the protozoa. The in- 

 clusions, often shown by a variety of methods of staining in many 

 tumor cells, and apparently of foreign character (that is, intracellu- 

 lar and nuclear structures not conforming to normal parts of cells), 

 led a number of microscopists to regard them as of some type 

 of protozoan character and as the parasitic causes of cancer and 

 malignant tumors generally. However subsequent investigations 

 directed to the final proof of this discovery have shown that these 

 supposedly foreign elements are nothing more than altered blood 

 corpuscles, lymphocytes, nuclei of the tumor cells, degeneration 

 products or artefacts, produced by stains and reagents in the 

 tissues. In the same way the discovery of yeast fungi having 

 pathogenic properties capable of causing inflammatory granu- 

 lomatous formations by inoculations in small experiment ani- 

 mals, resulted in no definite or generally applicable conclusions ; 

 because these blastomycetes were not to be found in true tumors. 

 or if occasionally found occurred only on the surface of the 

 growth and failed to give rise to characteristic tumors. With 

 the demonstration of parasites as causative agents left in this unsat- 

 isfactory condition, consideration should be given to the so-called 

 invisible microorganisms ; or we should remember that perhaps 

 with our present technique the infectious agents in question 

 cannot be rendered visible. 



A number of investigators insist upon the possibility that an 

 infectious factor underlies some types of tumors (Sticker), 

 because as a matter of fact there have been noted a number of 

 peculiar circumstances which point to the existence of a general 

 extraneous (not individual) cause; as the endemic occurrence of 

 cancer, its occurrence in connection with dermal excoriations, or 



