It is formed by ectoderm and endoderm coming into contact with each other. As the human embryo grows and caudal folding continues, the urorectal septum dividing the cloaca into a ventral urogenital sinus and dorsal anorectal canal. 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 3rd of the rectum.
It has been suggested that developmental errors may be associated with enlarged clitorises.