The Esr1 (Estrogen receptor alpha) was strongly expressed both during bipotential (stage 23) and differentiated gonad (stage 28) of FPT - and MPT embryos, as well in the mesonephros, adrenal gland and brain (Fig. 9). To investigate whether the powerful effect of E2 treatment on delayed SOX9 inhibition, gonadal growth and re- duced rate of cell proliferation correlates with altered expression of candidate genes involved in ovarian determination, we analyzed aromatase and Fork-head box protein (FoxL2). For bipotential go- nads, one-shift experiments with eggs from L. olivacea indicated that 100% of embryos shifted from FPT to MPT at stage 23 become males, while 100% of those shifted from MPT to FPT developed as females (Merchant-Larios et al., 1997).