The development of the sexually indifferent gonads and ovaries of the mouse was studied in embryos, fetuses, newborns and prepuberal animals from day 10 post-coitum through post-natal day 32. The process of gonadal morphogenesis consists of three main phases. During colonization, the genital ridge area is seeded by primordial germinal cells and by epithelial cells contributed by the mesonephric tubules. At the end of sexual indifference, the newly differentiated ovary appears as a voluminous

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... an consisting of a solid mass of closely associated germinal and somatic (mesonephric) cells ; at its dorsal margin, numerous mesonephric tubules continue to contribute epithelial cells to the somatic cell population of the gonad. During the phase of organization, the mass becomes invaded from its dorsal side by septa of connective tissue and by blood vessels, and cleaved into elongated formations of germinal and somatic (mesonephric) cells, the ovigerous cords. Due to the anatomical continuity of the ovarian cell mass and mesonephric tubules, and the direction of the invasive process, the ovigerous cords become the intraovarian prolongations of the mesonephric tubules developing gradually in a dorso-ventral, center-to-periphery direction. The development of the earliest follicles also occurrs during this phase : following early meiotic prophase, the germinal cells in the innermost (most dorsal) segments of the ovigerous cords enlarge, and the surrounding mesonephric cells become distributed coronally around them and become follicle cells. Adjacent follicles remain connected by narrow segments of cords not containing germinal cells. During the phase of compartmentalization, the mesonephric tubule-ovigerous cord continuum becomes subdivided into independent structures. This is due to maturation of the primitive stroma surrounding the various components of the continuum and its progressive transformation into a thick, fibrous connective tissue. The mesonephric tubules become separated from the ovigerous cords and condensed into an irregular epithelioid cell mass which remains confined at the hilus of the ovary. The ensuing compartmentalization of the various segments of the ovigerous cords results in the individualization of the follicles and the transformation of the somatic-cell-only portions of the cords into islets of interstitial (steroidogenic) cells. The above findings demonstrate that the follicle and interstitial (steroidogenic) cells of the mouse ovary are of mesonephric origin. Introduction.