The gonadal folds and protrusions of E2-treated embryos ex- amined with the SEM were observed with a light microscope in plastic-embedded samples. They showed diverse tissues depend- ing on the section level. In some sections, they looked like epi- thelial protrusions resembling tiny ovaries with a thick cortex formed by several layers of epithelial cells, and the medullary cords were fragmented (Fig. 2B). In other sections, the multilayer cortex covered a very limited area of the surface epithelium, and small medullary cords remained (Fig. 3A). Although at stage 26 the morphological aspect of sectioned E2 samples resembled under- developed ovaries, immunofluorescence revealed intense staining of Sox9 in cells of the medullary cords underlying protrusions of the surface epithelium (Figs. 2E and 3B).