Cancer Its Course and Its Causes 7 



Experimentally it has been possible to produce at will 

 cancer in rats, niice and rabbits through the application 

 of Roentgen rays, by infecting thera with certain para- 

 sitic worms, or through the application of irritating sub- 

 stances like tar to their skin. In all of these cases the 

 stimulus has to act over a relatively long period of time, 

 and generally the appearance of fully developed cancer 

 is preceded by increased growth activity of the stimu- 

 lated areas which leads to the production of what is often 

 called a precancerous condition. Such a stage often pre- 

 cedes cancer in man. Through the long continued stim- 

 ulation the cells are kept in a continuous state of in- 

 creased multiplication, such as we find in a wound heal- 

 ing during a short period of time; if this excessive mul- 

 tiplication continues sufficiently long, the cells change 

 their character, they keep on multiplying in excess and 

 invade neighboring tissues even after the direct stim- 

 ulus has been removed. This effect of stimulation is 

 very much promoted through the hereditary predisposi- 

 tion to a certain kind of cancer, but it seems that if the 

 external stimulus is very strong and long continued, a 

 deficiency in hereditary predisposition becomes less im- 

 portant; but an analysis of the quantitative relation be- 

 tween the strength of the predisposing hereditary fac- 

 tors and the external or internal stimulating factors has 

 so far been attempted in only a few cases. 



There are some cancers where at present we have no 

 knowledge of stimulating factors, where, apparently 

 merely on the basis of some error during the development 

 of the embryo, cancer develops, or where ova undergo 

 parthenogenetic development and lead to embryo-like 

 tumors in the ovary or perhaps elsewhere. From these 

 abnormal embryo-like structures cancer may develop. 

 These kinds of cancer frequently appear relatively early 

 in life, while usually cancer is more frequent in older in- 

 dividuals. 



