Before the urorectal septum has an opportunity to fuse with the cloacal membrane, the membrane ruptures, exposing the urogenital sinus and dorsal anorectal canal to the exterior. Later on, an ectodermal plug, the anal membrane, forms to create the lower third of the rectum.
Before the urorectal septum has a chance to fuse with the cloacal membrane, the membrane bursts, revealing the urogenital sinus and dorsal anorectal canal to the exterior. Later on, an ectodermal plug, the anal membrane, forms to create the lower third of the rectum.