The cloaca becomes flattened laterally and extends in the sagittal plane. At S14, as the caudal end of the body unrolls, a septum is formed in the frontal plane. The septum grows caudally, in the angle between the allantois and the hindgut: urorectal septum. It subdivides the cloaca into the rectum (anorectal channel) and the primary urogenital sinus. The urorectal septum enlarges near the cloacal membrane and forms the deep perineum (corpus perinei). Formation of the anal depression (proctodeum) can be seen on the ectodermal side. The anal membrane breaks down at S19, which allows for a link between ectodermal and endodermal anus to be made.
The endodermal part of the hindgut is supplied by the inferior mesenteric artery, the ectodermal part by the middle and inferior rectal arteries from the internal iliac artery.