Primary sexual characteristics arise when gametes mature and start to produce sex hormones. If the gametes develop into ovaries, the Wollfian ducts are absorbed and the Müllerian ducts develop into Fallopian tubes. If the gametes develop into testes the Müllerian ducts are absorbed and the Wollfian ducts develop into vas deferens and other male genital structures (epidydimis, seminal vesicles, etc.) In a female embryo, the urethral groove deepens to form the vulvar opening and the folds become the labia minora. The labriscrotal swellings form into the labia majora. In a male embryo, the flaps of the groove fold together to form the urethral tube and shaft of the penis, and the labriscrotal swellings form the scrotum
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In some other scientist`s opinions gametes and wolfian ducts do not depend on hormones for development in early stages of pregnency but stablise the ducts so they do not regress at a later stage.