Studies by the author [Gibbs, 1938, 1941] have shown a correlation between the growth phases of the developing skin and hair follicles and those of the developing mouse ovary. The decrease in thickness of the epidermis, which takes place from the fourth to the thirteenth day, is followed by a phase of increase in thickness. This coincides with the first appearance of the follicular fluid in the ovary at the twelfth or thirteenth day which is followed by a period of rapid ovarian growth [Engle, 1931]. This fact suggested the possibility of a persistence of the relationship between variations in the thickness of the skin and the normal ovarian cycle. Such changes in the thickness are probably due to a variation in the mitotic activity of the epidermis, and it was considered possible that they might also be correlated with the cyclic mitotic activity of the ovary of the mouse