The external
genitalia development:
Indifferent stage
Ø During the third week the cloacal
membrane is formed and lies below the umbilical cord.
Ø 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.
Ø It is now delimited in front by a
prominent mesenchymatous eminence with an epithelial covering, the anlage of
the future genital tubercle.
Ø 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.
Ø During the 7th week the urorectal septum
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.
Ø As we have seen, the urogenital membrane
dissolves during the 7th week, so that the urogenital sinus communicates freely
with the amniotic cavity.