A comparative morphological study of the mammary glands with reference to the known factors influencing the development of mammary carcinoma in mice.

Early in the study of mammary carcinoma in mice attention was given to the correlation of mammary gland structure and the occurrence of malignant growth. Haaland (31), in a rather extensive study, emphasized the frequency with which small adenomatous areas, previously described by Apolant (6), were encountered in the glands of mice with mammary carcinoma as compared with those of mice dying without cancer. That these localized areas of alveolar hyperplasia, occurring in otherwise atrophic glands, did not represent developmental malformations was indicated by their absence in mammae of young animals, and their precancerous nature was established by demonstrating transitional stages between them and frank carcinoma. It was pointed out further that the hyperplasia was not dependent upon pregnancy, as it occurred in virgin mice; but it was felt that the lesions were related to inflammatory changes caused, perhaps, by the presence of nematodes within the glands. The development of highly inbred strains of mice represented a definite refinement in the study of mouse mammary carcinoma and made possible the study of glandular alterations preceding cancer formation, for now it could be predicted with relative certainty that a given mouse would or would not develop mammary cancer later in life. Gibson (28) compared the life history of the mammae of two strains of albino mice, one a low tumor the other a high tumor strain. She observed a *endency toward anomalous nipple formation and a generally slower glandular development in the mice of the high-tumor strain. Also, carcinomas were