All humans begin with both Wolffian (male) and Mullerian ducts (female) and the gonads (male and female) development of male and female embryos is indistinguishable.
The reproductive system is derived from the intermediate mesoderm of the peritoneum.
The thick plate of epithelium extends deeply, pushing before it the mesoderm and forming a distinct projection.
This is the gonadal ridge.
The gonadal ridge (or urogenital ridge[1]) is the precursor to the gonads.
The gonadal ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells).
The gonadal ridge appears at approximately five weeks, and gives rise to the sex cords.
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At first, the mesonephros and gonadal ridge are continuous, but as the embryo grows the gonadal ridge gradually becomes pinched off from the mesonephros.
Some cells of mesonephric origin join the gonadal ridge.
Furthermore, the gonadal ridge still remains connected to the remnant of that body by a fold of peritoneum, namely the mesorchium or mesovarium.
About the seventh week the distinction of sex in the gonadal ridge begins to be perceptible.
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