THE GENETICS OF TUMOR TRANSPLANTATION 299 



Age, sex and other biological factors. — In 1920 Little (45) showed that 

 temporary growth of tumors destined to eventual regression and disappear- 

 ance was more readily obtained in very young animals than in young adults. 

 Strong later showed that the same is true of very old animals as compared 

 with those in the prime of physiological activity. 



A difference in the rate at which the sexes acquired the ability to elimi- 

 nate transplants of tumors was also demonstrated by Little (Table 10). 



Table 10 



DlFFKRKNCE IN THE RaTE AT WhICH THE SeXES ACQUIRE THE ABILITY TO 



Eliminate Transplanted Tumors 



Males 

 Females 



Per Cent 



Showing 



Mass 



14-51 ± I-5I 

 15-38 ± 1-55 

 12.12 + I . 76 

 25.74 ± 2.07 



Diff. ± P.E. 



13. 16 + 2.71 



Diff. 



p1^ 



0.87 + 2. i6| 0.4 



S-o 



In this case the mice used were those of a hybrid generation in which 

 some of the animals would presumably show progressive growth of the 

 implants and others (the majority) would show regression and eventual dis- 

 appearance. The female mice in the older age group gave a significantly 

 higher percentage of "takes" than did the males. This was in all prob- 

 ability due to the earlier assumption by some of these animals of the 

 biological make-up which reflects the presence of genetic factors for sus- 

 ceptibility. Female mice mature distinctly more rapidly than do males. 

 They would, therefore, begin earlier to express their characteristic genetic 

 constitution. This actually is the case. 



As a contrast to the hybrid mice among which are to be found a number 

 of animals with a genetic constitution favoring susceptibility, may be cited 

 the results of inoculating females of a completely non-susceptible strain. 

 These results are shown in Table 11. 



Here it will be noted that the non-susceptible genetic constitution is 

 expressing itself rapidly and definitely in a significant decrease in positive 

 observations. 



