The endodermal cloaca is divided into a dorsal and a ventral part by means of a partition, the urorectal septum, which grows downward from the ridge separating the allantois from the cloacal opening of the intestine and ultimately fuses with the cloacal membrane and divides it into an anal and a urogenital part. The dorsal part of the cloaca forms the rectum, and the anterior part of the urogenital sinus and bladder. The urorectal septum eventually becomes part of the perineal body.
Cloaca of human embryo from twenty-five to twenty-seven days old.
Malformation of the urorectal septum can lead to several different types of fistulas.