During the third week the cloacal membrane is formed and lies below the umbilical cord.
In the fourth week, as the lower abdominal wall is being formed, the cloacal membrane is shifted caudally by immigrating mesenchyma coming from various sources.
It is now delimited in front by a prominent mesenchymatous eminence with an epithelial covering, the anlage of the future genital tubercle.
At the end of the 5th week several cloacal folds form on both sides of the cloacal membrane. At their anterior end they join, delimited by the genital tubercle.
During the 7th week the urorectal septum (stage 16) divides the cloacal membrane into a urogenital membrane (ventrally) and an anal membrane (dorsally). From this moment on the ventral section of the cloacal swellings around the urogenital sinus are called urethral folds and in the dorsal section around the anus, anal folds. Lateral to the urethral folds a further pair of tori form, the genital swelling.
As we have seen, the urogenital membrane dissolves during the 7th week (stage 19), so that the urogenital sinus communicates freely with the amniotic cavity.
It is important to remember that the morphology of the external genitalia in both sexes is very similar up to the 9th week.