The histology of the reproductive tract of the lactating mouse is described.
During the first 11 days post partum, the largest vesicular follicles are 350μ in diameter, the corpora lutea of pregnancy are larger than the corpora lutea of lactation, and the vagina has a two-layered epithelium. Significant changes occur after 12 days post partum: the size of the follicles in most animals increases to 400–450μ, the corpora lutea of lactation are larger than the old set, and the vagina becomes mucified.
This suggests that lactation in the mouse is divided into two periods: the first in which progesterone and negligible amounts of oestrogen are present lasts until the 11th day. This is followed by a period of increasing levels of oestrogen and a gradual subsidence in that of progesterone.
A hypothesis is proposed to account for the morphological changes that occur during lactation.