Frog embryos in the first tailbud stage (i.e., shortly after the closure of the medullary tube) were joined in parabiosis. The normal controls, in which sex-differentiation took place during the 3rd week, gave 100 [female][female] and 96 [male][male]. Among the 56 pairs of twins, 29 were 1-sexed (either both animals [female][female] or both cf d") and 27 were 2-sexed. In the latter group the [female][female] showed underdevelopment of the ovaries and partial sex-reversal. The [male]-differentiating factor acts, therefore, first as an inhibitor and later as a suppressor of the [female] one (antagonistic character of the two sex-differentiators). The sex-differentiators do not act like hormones, but show a distinct localization, comparable to Spemann''s "Organisatoren.".