312 BIOLOGY OF THE LABORATORY MOUSE 



and subsequent growth of the mammary rudiments into extensive duct 

 systems. 



It has been found that lymphosarcoma in one Hne of mice was nearly 

 twice as frequent in females as in males (6i). In another study, daughters 

 from reciprocal matings showed the same difference in incidence of leukemias 

 as the sons (59). 



A brown degeneration occurring in the adrenal glands of both sexes of 

 mice has been described. Efforts to correlate this degeneration with varia- 

 tion of estrogenic hormones and the incidence of mammary cancer have been 

 made (16, 19). 



The reproductive physiology of strains of mice with various percentages 

 of mammary tumors has been investigated (57, 43, 44, 37, 10, 68, 7, 80). 

 Special characteristics of the estrous cycle such as unusual duration of 

 phases of the cycle have not been consistently correlated with tumor 

 incidence. 



It has been observed that the frequency of breeding had in some cases 

 marked influence on the incidence of mammary tumors in mice (3, 4, 53, 25). 

 Whether the result was related to the rapidity of the pregnancies in them- 

 selves or to the irritation of stagnating products in the mammary ducts is 

 not certain. 



In an extensive study of mice painted with tar it has been found that the 

 males showed a distinctly delayed tar tumor reaction as compared with the 

 females (40). 



With unusual addition of hormones. — Experimental studies have shown 

 that sex hormones can awaken malignant changes at least on a substratum 

 that is usually thought of as hereditarily susceptible to cancer. Thus 

 Murray (71) found that mammary tumors appeared in fifteen male mice out 

 of 210 castrated at 3 to 4 months of age when ovaries from sisters were 

 implanted. An inbred Jackson Laboratory tumor strain of mice was used. 

 Feminization of the male mouse, in which the mammary rudiments undergo 

 little if any development throughout life (30) , induced growth of mammary 

 glands and also the development of mammary tumors. This was confirmed 

 by dejongh (21). 



Following the injection of estrogen,* males from high tumor strains of 

 mice developed mammar\' tumors as frequently as multiparous females 

 (41, 42, 27, 32). Males from low tumor strains developed tumors with 



* Estrogen: a generic name for female sex hormone. The term as here used is 

 intended to include synthetic as well as naturally occurring hormones. 



