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Whereas in the well-organized seminiferous cords the germ cells entered the mitotic resting stage (Fig. 3A), in the ovarian regions they entered the prophase of the first meiotic division as evidenced by the synaptonemal complexes clearly detected with the electron microscope (Fig. 3B). At 18.5 dpc, B6.Ytir gonads that differentiated as ovo-testes or as ovaries were distinguishable by their location within the abdominal cavity and the presence or absence of Sox9-positive Sertoli cells. Although ovotestes showed intra-abdominal descent and were located 2-3 mm below the kidneys, ovaries devoid of Sox9-positive cells remained in the upper position attached to the kidneys. Although several B6.Ytir embryos had either bilateral ovotestes or ovaries, some embryos had an ovotestis and a contralateral ovary (Fig. 4, A and B). The extension of well-organized seminiferous cords containing Sox9-positive Sertoli cells varies in individual gonads. In some gonads Sox9-positive cells forming seminiferous cords occupied almost the whole gonad (Fig. 4C). In others, however, the seminiferous cords were located mainly in the middle region, whereas at the cranial and caudal gonadal regions the Sox9-positive cells appeared scattered and failed to form seminiferous cords (Fig. 4, A and D). FIG. 3. Two electron micrographs of the same areas shown as rectangles A and B in Figure 2. Serial thin sections (10 nm) adjacent to the semi-thin section (1.0 ^m) shown in Fig. 2 were taken. A) In the seminiferous cords formed in the medullary region of the gonad, two germ cells (gc) apparently entering in mitotic resting stage and three nuclei of Sertoli cells (s) are seen. B) In the caudal region of the gonad, synaptonemal complexes (arrowheads) in germ cells (gc) indicate that they have entered the prophase of the first meiotic division. Epithelial cells (e) are also shown. FIG. 4. This figure shows the distribution of Sox9-positive cells in B6.Ytir gonads at 18.5 dpc. A) Cross section of an ovotestis showing Sox9-positive Sertoli cells forming the seminiferous cords (sc) at the dorsal side. Scattered Sox9-positive cells are also seen along the ventral ovarian side (o). B) A representative image of a serially sectioned B6.Ytir ovary (o) in which Sox9-positive cells were not found. C) Part of a B6.Ytir ovotestis showing abundant seminiferous cords with green fluorescent Sox9-posi-tive cells (sc). In this gonad the male tissue (seminiferous cords and interstitial Leydig cells) predominates over the ovarian tissue (sex cords) that remains as a thin cortical layer toward the gonadal surface. D) Although the cranial region shows numerous Sox9-positive cells in the seminiferous cords (sc), they are scant and randomly distributed in the ovarian cords (oc). Bar = 100 ^m.