ENDOCRINE SECRETION AND TUMOR FORMATION 313 



greater frequency than multiparous females of the same strains. Mammary 

 gland tumors have not yet been obtained following estrogen injections in 

 male mice from some of the very low tumor strains even though prolonged 

 efforts have been made to produce them (8, 44, 27). It might be assumed 

 that estrogens act in conjunction with some intrinsic factor predisposing 

 to tumor formation. Mammary tumors were produced in males of one low 

 tumor strain following injection of an estrogen only when nursed on high 

 tumor mothers (84). The method of injection is of importance. Using a 

 high tumor strain it was found that 3000 rat units of estrogen over a three 

 day period at two weeks of age was not effective in producing mammary 

 tumors in males but 16 weekly doses of 100 rat units each produced a high 

 incidence of tumors (11). Synthetic estrogens which differ markedly in 

 molecular structure from naturally occurring forms will produce tumors in 

 male mice (48, 75). This makes it difficult to assume that there is car- 

 cinogenic action associated with hormone molecular structure (50). 



Mammary gland carcinomas have developed in females of very low 

 tumor strains following estrogenic treatment, though only after long periods 

 of treatment (9). The incidence of mammary tumors in mixed stocks has 

 been increased as compared to the controls (74). The incidence of mam- 

 mary tumors was increased in female mice of both high and low tumor 

 strains following estrogen injections (81). 



The activity of the corpora lutea may be a contributing factor to mam- 

 mary gland cancer production (58). However, progesterone alone or 

 in combination with estrogen did not alter mammary tumor percentages 

 (50, 28). 



A considerable increase in tumor rate in non-breeding mice of 

 several strains was observed following subcutaneous transplants of three 

 or four anterior lobes of the hypophysis from male and female litter mates 



(58). 



^Modification of the incidence of mammary tumors in mice has been 

 attempted with male hormone preparations. It has been reported that 

 testosterone administered to female mice of a highly susceptible strain will 

 result in a marked fall in the incidence of mammary tumors (51, 72). The 

 mechanism of the inhibition is not understood although evidence has been 

 marshalled indicating that the action is through the pituitary (50). The 

 lowering of incidence when treatment is started with mature animals has 

 not been suitably confirmed (50). 



The appearance of mammary cancer has been prevented by use of the 

 thyrotropic hormone of the pituitar>^ gland (20). In another study the 



