21.5 The external genitalia

• Indifferent stage
• Differentiated stage of the male genitalia
• Differentiated stage of the female genitalia

Indifferent stage

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.

Fig. 54
In front, the cloacal membrane is delimited by a swelling, the future genital tubercle, that at the rear continues in the two cloacal folds.

During the 7th week the urorectal septum 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 19, so that the urogenital sinus communicates freely with the amniotic cavity.

Fig. 55
During the 7th week the urorectal septum (8) subdivides the cloacal membrane, so it is separated into urogenital and anal membranes.

Fig. 56
From the cloacal folds around the urogenital orifice arise the urethral folds and, from the one around the anal orifice, the anal folds. Outside the urethral folds a further prominence arises on both sides, the genital swelling.