Sequence
A few weeks after fertilization, the initial appearance of the human fetal genitalia is basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance.
In typical fetal development, the presence of the SRY gene causes the fetal gonads to become testes; the absence of it allows the gonads to continue to develop into ovaries. Thereafter, the development of the internal reproductive organs and the external genitalia is determined by hormones produced by fetal gonads (ovaries or testes) and the cells' response to them.
If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings swell, wrap around the penis, and fuse in the midline to form the penile urethra.
If testosterone is not present, normal female development continues, with the development of a perineal urethra and the formation of a uterus, clitoris and vagina.
The Müllerian ducts, which are paired ducts of the embryo which empty into the cloaca, and which develop into the upper vagina, cervix, uterus and oviducts; in the male they disappear except for the vestigial vagina masculina and the appendix testis.
Although female development is usually considered the "default" configuration in the absence of induction, recent evidence has indicated that Wnt4 plays a role in the development of female anatomy.
Read more about this topic: Fetal Genital Development
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