In vitro induction of germinal vesicle breakdown by natural steroids in zebrafish oocytes. %GVBD was calculated by determining the percentage of oocytes that had undergone GVBD in a group composed of more than 20 oocytes cultured in parallel for three hours. Each value in the mean (± S.D.) of three separate experiments using ovaries of three separate females. ** indicate significant differences between the %GVBD induced by same concentration of 17,20β-DHP at the P < 0.01 level.