Influence of Pregnancies on Cancer. 39 



undertook investigations into the influence of pregnancy in various 

 strains of mice. 



Accordingly we kept a number of mice from various strains 

 after they were weaned separated from males throughout their 

 life. Other mice from the same strains were allowed to breed in 

 the usual manner. A few months after we had begun our experi- 

 ments we received a recent communication by Bashford, who 

 stated that he did not notice any influence of pregnancies on the 

 incidence of cancer in mice. 



Our results are as follows : 



I. English Strain. — Never bred mice. 



Group A (24 non-breeding mice) : 46 per cent, have tumors. 

 The corresponding percentage in bred English mice varies in 

 different groups between 62 per cent, and 75 per cent. 



91 per cent, of the tumors in never-bred mice occurred in mice 

 older than 12 months, 36 per cent, in mice above 17 months of 

 age, while in breeding English mice the corresponding per- 

 centages were 32 per cent, and 4.6 per cent respectively.. 



Group B (58 non-breeding mice) : Experiment not yet finished. 

 17 per cent, of the mice have had tumors so far, almost all of 

 them between 10 and 12 months of age. All others, 22 of which 

 are already more than 12 months old, have not yet had tumors 

 In control mice 68 per cent, had tumors at or under 12 months of 

 age. We find therefore that even in English mice that are pre- 

 vented from breeding a considerable number of tumors occur, 

 but that the number of mice affected by cancer is distinctly de- 

 creased and that the age at which the tumors develop is higher in 

 non-breeders than in breeders. 



II. No. 8 (136 non-breeding mice). In 3.6 per cent, of the 

 non-breeders the development of tumors was observed, in 100 

 per cent, of these above the age of 12 months, in 80 per cent, above 

 the age of 20 months. 



In breeding mice of the same strain 30 per cent, had tumors, 

 72 per cent, above 12 months of age and 36 per cent above 17 

 months. We find here a great reduction in the incidence of cancer 

 and an increase in the age at which the tumors develop in non- 

 breeding mice. 



III. No. %}/2 (48 non-breeding mice). In 10 per cent, tumors 



