In albino mice of the street strain ovaries from the 10th day of fetal life until the 5th day after birth were studied histologically and cytologically by the squash technique. The gonad is in the indifferent stage until the 13th day of intrauterine life, when differentiation into a male and a female gonad is recognizable. Germ cells stop oogonial division along with this differentiation and enter into meiosis on the 14th day. This happens in the ovary simultaneously for all oocytes, i.e. in the course of one or two days. Thus leptotene and zygotene figures are seen together with the last oogonial divisions on the 14th and 15th day of fetal life; on the 16th day a great number of pachytene nuclei appear and on the 17th and 18th day practically all oocytes are in the pachytene stage. Pachytene nuclei seem either to degenerate or to go into diplotene stage. From the 18th or 19th day diplotene figures increase in number, until by the 4th or 5th day of life all oocytes are in late diplotene, the so-called resting stage of the meiotic prophase. In this stage oocytes are arrested until they in a later period grow in size and enter maturation division. From the 16th day of intrauterine life and until the 3rd or 4th day after birth a massive degeneration of oocytes in the pachytene stage takes place. Different strains of mice (Bagg, dlb, hairless and naked) follow the same pattern of oogenesis.