Ovaries of fetal mice obtained during the twelfth to eighteenth day of gestation and from the first to third postnatal day have been studied by electron microscopy. The interrelationships of follicle cells to oögonia and to connective tissue cells have received special attention. On day 12, when the female gonad can first be distinguished, relatively few large oögonia are visible. These are loosely aggregated and are accompanied by only a few follicle cells.

On days 13 and 14 loose groups of oögonia become better demonstrated, and now are always surrounded peripherally by follicle cells. Beginning on day 14 very thin cytoplasmic processes of the follicle cells are interposed between adjacent oögonia. Subsequently, by following this process the follicle cells are drawn into the groups of oögonia or oöcytes. Increasing numbers of connective tissue cells and capillaries are growing into the cortex from the medulla and hilar regions. By day 18 and postnatal day 1 this ingrowth has separated the groups of oöcytes and follicle cells further. By postnatal days 2 and 3 most oöcytes have become enclosed by a follicular epithelium of either flattened, cuboidal or columnar cells. Many stages of zona pellucida formation can be recognized. It is postulated that the manner of fetal and early postnatal differentiation of the ovary explains a number of follicular configurations which deviate from the normal. At no time have cords of cells been observed to originate from germinal epithelium. By contrast the underlying tunica albuginea is derived by ingrowth of cells from the medulla.