40 



Scientific Proceedings (55). 



developed, in 100 per cent, of these above 12 months and in 60 

 per cent, above 17 months, while in breeding mice of the same 

 strain, the tumor incidence was 14 per cent., in 69 per cent, of 

 which the tumors occurred above 12, in 8 per cent, above 17 months 

 of age. There is here only a slight reduction in the cancer rate in 

 non-breeding mice and a more marked increase in the age at which 

 cancer occurred in non-breeding mice as compared to breeding 

 mice of this strain. 



IV. Carter (44 non-breeding mice) : 4J/2 per cent, have tumors, 

 100 per cent, above 20 months of age as compared to 35 per cent, 

 tumor incidence in breeding mice, in 63 per cent, of which the 

 tumor developed at 12 months of age or below while in none it 

 appeared in a mouse older than 17 months. Here we find therefore 

 again a very marked difference in the same direction as in the 

 previous strains. 



Three further experiments the details of which do not need to 

 be given here confirm these results. We may therefore conclude: 



1. In breeding mice cancer of the breast occurs in a consider- 

 ably larger number of individuals than in non-breeding mice. 



2. In non-breeding mice the age at which cancer occurs is much 

 higher than in breeding mice, and 



3. The number of mammary cancers which occur in non-breed- 

 ing mice varies in different strains. They are relatively more 

 frequent in non-breeding mice from such strains in which in 

 breeding mice the tumor incidence is greater and is relatively less 

 frequent in non-breeding mice from strains in which also in breed- 

 ing mice the tumor incidence is less. 



4. In our preceding communication we stated that in some 

 strains which are naturally poor breeders (the "creams"), the 

 incidence of cancer is relatively low; while this factor may con- 

 tribute to the low rate of tumors in such strains, it is in itself not 

 sufficient to explain all or even the major part of the hereditary 7 

 difference in the incidence in cancer in the various strains of mice. 



