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Sox9-positive cells were located in the medial region of the gonad where they organize the incipient seminiferous cords together with the germ cells (Fig. 1C). In the B6.Ytir genital ridges showing Sox9-positive cells at 13.5 dpc, their number and the area of distribution along the medial region showed individual variations. Some gonads had scant randomly distributed Sox9-positive cells, whereas others revealed only a few at the mesonephric-gonad boundary (Fig. 1, D and E). In contrast to normal male gonads of B6 where seminiferous cords formed by Sox9-positive cells segregated from the coelo-mic epithelium at 12.5 dpc, in B6.Ytir gonads such segregation was delayed approximately for 1 day and was evident at 13.5 dpc. Furthermore, although well-formed seminiferous cords were evident in the middle region of the B6.Ytir gonads, at the cranial and caudal regions Sox9-pos-itive cells were scant or absent (Fig. 1F). At 15.5-17.5 dpc, seminiferous cords formed by Sox9-positive cells are organized in the middle region of all B6.Ytir gonads that form ovotestis. They expand toward the cranial and caudal regions where ovarian tissues are undergoing regression as suggested by the presence of abundant pyknotic cells (Fig. 2). In addition to the immunolo-calization of Sox9 in Sertoli cells, the presence of testicular and ovarian tissues in B6.Ytir ovotestis is supported by the different meiotic behavior of germ cells in each of them (Fig. 2). FIG. 2. Light microscopy of a semithin section of a plastic-embedded B6.Ytir ovo-testis at 15.5 dpc. The medullary region displays well-organized seminiferous cords (sc) formed by Sertoli (s) and germ cells (gc). In the cortical region, the transitional ovarian sex cords formed by epithelial cells (arrows), germ cells (gc), and dark picnotic cells (arrowheads) are evident. Rectangles A and B correspond to the two insets of the electron-micrographs shown in Figure 3. Bar = 50 ^m.