36 



Scientific Proceedings (55). 



per cent.; hybrids between European and No. 10, 82 per cent.; 

 cancer incidence. 



2. The tumor rate in the different strains remains approxi- 

 mately constant in the succeeding generations; a few times we 

 noticed however an apparent increase in the tumor rate after the 

 first two generations. Also different substrains obtained through 

 selection of certain individuals from a large strain which may or 

 may not differ in color or in other peculiarities from other sub- 

 strains, show usually a similar tumor rate as the main strain. 



Thus the substrain "English tan" had a tumor incidence of 

 73 per cent., "English sable" 76 per cent., "English 101" 65 per 

 cent. It is however, in some cases, possible to detach from the 

 main strain a substrain, that differs markedly from the main 

 strain in the tumor rate. Thus the "English silver," a substrain 

 of the "English," has only a tumor incidence of 8% per cent. 

 In this case the factor or factors determining the origin of tumors 

 seem to be linked with the factor or factors of the silver color. 



3. Various strains of mice do not only differ in regard to the 

 incidence of cancer, but also in regard to the age at which the 

 cancers appear. Thus in the English strain as a whole (excluding 

 the silver substrain) tumors appeared in 68 per cent, of the cases 

 in mice at the age of 12 months or below, and in only 4.6 per cent, 

 of the mice the tumors appeared above the age of 17 months, 

 while in the strain No. 8 approximately 72 per cent, of the animals 

 have tumors above the age of 12 months and 36 per cent, above 

 17 months. In the "Carter" strain the tumors appear in 63 per 

 cent, at or below the age of 12 months and in the hybrids between 

 European and No. 10, which have the high tumor rate of 82 per 

 cent, the tumors appear in 69 per cent, above 12 months and in 

 22 per cent, above 17 months of age, therefore considerably later 

 than in the English, who also have a very high tumor rate. In 

 various English substrains the tumors appear therefore much 

 earlier than in all the other strains, although the tumor incidence 

 as a whole is not as high in the English strain as in the hybrids 

 between European and No. 10. In valuating this fact we have 

 however to take into consideration the usually shorter duration of 

 life of the English in contradistinction to some other strains. 



The youngest mice in which we observed tumors were 5^ 

 months old at the time their tumors were first noticed. 



