SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 99 



plantable tumors would show a different reaction for the two 

 sexes. But this does not explain the results outlined above that 

 there was a significant difference between the sexes when all age 

 classes are taken into consideration, whereas no such difference 

 is encountered with adult mice alone. 



2. Female mice mature earlier than male mice from the same 

 litter. If this is taken into account, the sex difference behavior 

 of the dBrB tumor may be partly explained as a result of the age 

 factor alone. Female mice maturing faster than males would 

 be more able to counteract the tumor tissue (susceptibility de- 

 creases with age up to maturity). In other words, the males 

 would show a larger percentage of positive reactions, thus giving 

 a significant difference between the sexes, although the result is 

 more probably due to the factor of age. The age factor is also 

 able to explain the divergence of the two curves in figure 13. 

 Male mice grow old faster than females. The adult females are 

 able to resist the tumor tissue until by the end of the sixth observa- 

 tion no positive palpable reaction can be observed, whereas the 

 males growing old faster (first stages of senescences) are unable to 

 withstand the tumor reaction as readily. These two facts are of 

 primary genetic importance. 



k. Sex (dBrA). Contrary to the findings for the dBrB tumor, 

 there is no significant difference between the sexes for the normal 

 dBrA tumor reactions when all age groups are massed. 



Tabulation 



Females 388 negative : 10 reactions ±2.10 or 2.51% ± 0.53 

 Males 282 negative : 5 reactions ±1.49 or 1.74% ± 0.52 



Difference 0.77% ± 0.74 



The difference is thus 1.04 times the probable error 



If individuals from age group no. 4 only are compared we 

 get the following results: 



Males 154 negative : 2 reactions ±0.94 or 1.28% ± 0.70 

 Females 281 negative : 1 reaction ±0.67 or 0.35% ± 0.24 



Difference 0.93% ± 0.74 



The difference is thus 1.26 times the probable error 



