Early development of external genitalia is similar in males and females. From the 4th to the 7th week, the external genitalia are sexually un-differentiated. Distinguishing sexual characteristics begin to appear during the 9th week. However, the external genitaiia are not fully differentiated until the 12th week of development.
In the 4th week of development, a pair of swellings, called cloacal folds, develop on either side of the doacal membrane. Just cranial to the cloacal membrane these folds unite to form a midline swelling called genital tubercle.
During the 7th week the urorectal septum fuses with the cloacal membrane and divides the cloacal membrane into an anterior part called urogenital membrane, and a posterior part called anal membrane. The cloacal folds also become divided into two parts. That part of the cloacal fold flanking the urogenital membrane is now known as urethra! fold (also called urogenital fold), and the part flanking the anal membrane is now called anal fold. Very soon afterwards, a new pair of swellings develop on each side of the urethral folds, these swellings are called genital swellings (also called labioscrotal swellings).
By the end of 7th week the urogenital membrane disintegrates and disappears. As_a result, the phallic part of the urogenital sinus loses its floor and now opens into the amniotic cavity. The opening of the urogenital sinus is in the form of a cleft-like groove called urethral groove. This groove is lined by endoderm and bounded on each side by a urethral fold.