The Nobmal Devexjopment O F The: Mammary Glands O F Virgin Female: Mice O F Ten Strains Varying I N Susceptibility to Spontaneous Neoplasms

Certain straiiis of mice are particularly susceptible to the development of spontaneous mammary gland adenocarcinoma. Other straiiis show a marked resistance to such atypical growths. The predisposition to tumor development thus appears to be determined at least part ially by hereditary fact or s. That the predisposition to tumor incidence might be determined or indicated by structural or developmental factors was pointed out by Gibson (1930). Using a small iiumber of animals from two strains of albino mice, the individuals of one strain being susceptible to tumor development arid representatives of another strain more or less resistant, Gibson reported definite embryonic and developmental anomalies in the mammary glands of the tumor-susceptible strain. These anomalies largely involved the nipples. In addition, the mammary glands of the susceptible animals developed more slowly than did those of mice of the immune strain. More significant strain differences were observed in the regressed mammary glands of old mice. Cystic or metaplastic areas were present in the glands of old animals of the tumor-susceptible strain. Recently the iiormal development of the mammary glands of the mouse has been exteiisively discussed by Turner and Gomez (1933) and by Cole (1933). The endocrine factors stimulating mammary hyperplasia in other laboratory animals (Turner, 1932) also induced the growth of the mammary glands of the mouse (Turner et al., 1932; Rradbury, 1932 ; Turner and Gomez, 1934). Theelin stimulated the growth of the duct system of the glands of male mice arid of female mice ovariectomizecl before puberty. The resulting glands resembled those of virgin females. The additional and simultaiieous stimulating effect of the hormone of the corpus luteum was necessary for the proliferation of the finer mammary elements. Gardner, Diddle, Allen, and Strong ( 1934) observed that extended theeliii treatment alone was accompanied by the formation of localized lobular areas in the mammary glands of the males of at least one strain of mice. I n view of the greater understanding of the factors inducing mammary hyperplasia and the strain deviations observed by Gibson, the iiormal development of the mammary glands of a number of strains was followed through the earlier period of life. The observations were