The action of oestrogens on the development and periodic changes of the female accessory sexual organs of the mammals is well known, but it now appears probable that this secretion also influences the growth of the various parts of the ovary itself. A striking coincidence has recently been demonstrated between the positions of high concentrations of oestrogen in the mouse ovary and local areas of mitotic activity in the germinal epithelium, and it is not unlikely that the three nuclear divisions of oogenesis in this animal are induced by oestrogen [Bullough, 1942b]. Further, it appears that the growth of the follicles and corpora lutea, as well as the mitotic activity of the interstitial and connective tissue cells, are due to the action of this hormone [Bullough, 1942c]. Experiments were therefore planned to test these theories as far as possible. It has already been shown [Bullough, 1942d] that