Once the cloaca has been partitioned, the development of external genitalia can begin. This begins to occur at approximately four weeks. Early in the fourth week the mesenchyme surrounding the cloacal membrane begins to proliferate. It is critical that this mesenchyme reach a critical cell mass or only rudimentary structures are formed. The proliferation of mesenchyme is known as the genital tubercles. These must fuse at the ventral aspect to form the future glans of penis or clitoris. If the genital tubercles fuse on the dorsal aspect, epispadias may result. Along the sides of the cloacal membrane, outer labio-scrotal and inner urogenital folds also develop.
Soon afterwards, the genital tubercle elongates to form a phallus. Within two weeks, the uro-rectal septum will fuse with the cloacal membrane. This provides a division in the membrane which will form the anus and the urogenital system, dorsally and ventrally respectively. These two membranes are separated by the perineum. Shortly thereafter, the membranes will rupture and there will be an anal orifice or opening and a urogenital orifice. The urogenital membrane is surrounded by the folds described earlier.
