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.
Prior to the fusion of the urorectal septum with the cloacal membrane, the membrane ruptures, exposing the urogenital sinus and dorsal anorectal canal. Subsequently, an ectodermal plug, known as the anal membrane, forms to create the lower third of the rectum.