Internal sex organs: Both genetically male (XY) and genetically female (XX) fetuses start out with two sets of internal primordial structures, the Wolffian and the Mullerian.

  • In the presence of testicular hormones, the Wolffian ducts develop (e.g., prostate and vas) and the Mullerian ducts regress.
  • In the absence of testicular hormones the reverse happens. The Wolffian ducts regress and the Mullerian ducts develop (uterus and fallopian tubes).

External sex organs: Both male and female fetuses also start out with a single set of external primordial structures:

  •  Testosterone stimulates these structures to differentiate into penis and scrota, becoming recognizably male by about week 9-10 of gestation.
  •  In the absence of testosterone, these same structures become clitoris and labia, regardless of the levels of estrogen or progesterone. So, no hormonal influence from the female gonads (ovaries) appears to be needed for differentiation of female external genitalia. 

(Hines, Melissa, PhD. Brain Gender. p 21-43. NY:Oxford University Press, 2004.)

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