74 LEONELL C. STRONG 



tions of reactions of positive growths is determined not by any 

 characteristics of the tumor cell alone, but by its reaction with 

 the genetic constitution of the strain of mice under investigation. 

 Again, the rate of growth of the transplanted mass is constant 

 provided one is dealing with a constant homogeneous race of 

 individuals. 



A genetic interpretation of the observed results (that by 

 progressive inoculations into a foreign strain one can increase 

 the percentage of positive indications) would be somewhat as 

 follows: Since all races of 'market mice' have had a common 

 origin, they have therefore some genetic factors that relate to 

 cancer in common. Within a single family individuals are more 

 closely related to one another than to individuals from another 

 family. When an investigator found that an individual of the 

 foreign strain grew the transplanted tumor, he would naturally 

 pick out individuals within the family of susceptible mice and 

 eliminate those from other apparently non-susceptible families. 

 There is thus an unconscious tendency to select, from the stock, 

 individuals more susceptible to the transplantable tissue — more 

 susceptible because they approach more nearly the genetic 

 constitution of the mouse that grew the original tumor spon- 

 taneously. An investigator not realizing the full significance of 

 the race factor would believe that he was dealing with only one 

 variable — that of the behavior of the tumor cell itself.^ This 

 explanation is offered not as conclusive proof of what necessarily 

 was involved, but as what may possibly have occurred in the 

 experiments that gave the conflicting results previously obtained 

 by various workers. 



c. Transitional conditions. Certain investigators believe that 

 a carcinoma may be transformed directly into a sarcoma and 

 vice versa. 



Several investigators have already foreseen the difficulty 

 encountered in such a transformation from epithelial to con- 

 nective tissue. Among these may be mentioned Ehrlich, Apolant, 

 Ribbert, and, more recently, Woglom. It has been suggested 



2 Of course, healthy and vigorous tumor tissue alone must be used, or varia- 

 tions due to infection will enter in. 



