Cancer lit Course and Its Cavmt 7 



Experimentally it hat been poetible to prodaoe at will 

 cancer in rats, mice and rabbits througb the application 

 of Eoentgen rays, by infecting them with certain para- 

 sitic worms, or through the application of irritating sab- 

 stances like tar to their skiu. In all of these oases the 

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

 and generally the appearance of fully developed cancor 

 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 wo find in a wound heal- 

 ing during a short i)eriod 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. 



