Hyperplasia of the breasts is regularly noted in pregnancy. This change in the breasts, for a time passively accepted as a fact, has in late years been made the subject of intensive study, which aimed to discover the primary cause of the tissue growth, and to analyze the nature of the stimulus which gave rise to it. The subject is not only of great interest to biologists, but to investigators in all branches of medicine. The sweeping analogies, drawn from some of the theories which have been based on experimental work, are so far-reaching that confirmation is imperatively needed; otherwise there is danger of basing future research, some in allied, others in entirely separate fields, on an insecure or even false foundation.
The breast hyperplasia of pregnancy can best be studied in animals, and as a matter of laboratory convenience, small animals are preferred. Most workers have employed