Determination of temperature sensitive stages for sexual differentiation of the gonads in embryos of the turtle, Emys orbicularis

In order to determine the temperature sensitive stages for sexual differentiation of the gonads in Emys orbicularis , eggs of this turtle were shifted at different stages of embryonic development from the male‐producing temperature of 25°C to the female‐producing temperature of 30°C and reciprocally. Based on the series of developmental stages described by Yntema (′68) for Chelydra serpentina , temperature begins to influence sexual differentiation of Emys orbicularis at stage 16, a stage in which the gonads are still histologically undifferentiated. Its action lasts over the first steps of histological differentiation of the gonads. The minimal exposure at 25°C required for male differentiation of all individuals extends from stage 16 to somewhat before stage 21. For 100% female differentiation, incubation at 30°C must be longer, from stage 16 to somewhat before stage 22. Shorter exposures at 25°C or 30°C during these periods result in different percentages of males, females, and intersexes. Our results show that there is a critical stage (stage 16) which is the same for both male and female differentiation of the gonads. The thermosensensitive periods are rather long, corresponding to 11–12 days at 25°C and 30°C.