The gonadal ridge
For both sexes the gonads arise in the gonadal ridges (inset in figure 5). These are bilateral, ridge-like protrusions that appear ventromedially to the nephrogenic cord. They are generated in the 5th week through the proliferation of the coelomic epithelium and the thickening of the underlying mesenchyma.
At this point, the gonadal ridge represents the primitive gonadal primordium. In order for this to develop into the definitive and gender-specific gonads, the immigration of the primordial germ cells is necessary.
Indifferent gonadal primordium
In the 6th week, the primordial germ cells infiltrate into both gonadal ridges. The primordial germ cells become surrounded by the coelomic epithelial cells that have proliferated and advanced into the depths of the mesenchyma. These germinal cords are still connected with the surface of the coelomic epithelium. At this point, the male and female gonadal primordia cannot be distinguished and, for this reason, this condition is referred to as the indifferent gonadal primordium.