Most samples of E2-treated embryos showed ovotestis-like gonads in which the ovarian domain displayed fragmented medullary cords formed by Sox9 epithelial cells and a thick cortex with elongated columnar cells. In the testis domain, the remnants of medullary cords contained Sox9þ cells on the surface epithe lium made by one layer of cuboidal cells (Fig. 5B and C). While at FPT Sox9 was undetectable in medullary cords of differentiated ovaries from stage 26 onwards (Fig. 2F), current results prove that down-regulation of SOX9 was considerably delayed in E2-treated embryos.