Hindgut formation

(A) A wedge of mesoderm divides the hindgut. Recall that the allantois diverticulum is trapped in the connecting stalk.
(B, C) A migrating urorectal septum of mesoderm pinches the base of the diverticulum off of the hindgut. This results in 2 portals to the outside world, 1 of which is still connected to the gut tube (anal) and 1 of which is a blind pouch (urogenital).