Septum Upper two thirds of the anal ca-This septum is a composite nal are derived form the hin,dggt and, structure formed by two combined hence, are endodermal in origin. The mesodermal septal systems: (1) a su- lower one third of the anal canal is perior mesodermal fold called Torneux ectodermal in origin and develops from fold, and (2) a pair of lateral folds called an ectodermal pit called proctodeum. Rathke folds. The Torneux fold ap- The proctodeum (also termed anal pit) pears as a crescentic wedge of meso- appears when mesenchyme around derm which grows inferiorly in the an- the anal membrane proliferates and gle between the allantois and the hind- raises the ectoderm surrounding this gut. By the growth of this coronal parti- membrane. The anal membrane thus tion the upper part of cloaca becomes comes to lie at the bottom of the ecto-divided into anterior and posterior derm-lined anal pit. During the 8th parts. The Rathke fold appears as a week the anal membrane breaks down pair of mesodermal bars located on and disappears, so that the endo-each side of the caudal part of the clo- dermal part of the anal canal becomes aca. These folds grow toward the mid- continuous with the ectodermal part, line and finally fuse with each other and Due to the disappearance of the anal with the Torneux fold to complete the membrane, the hindgut now opens to urorectal septum. the exterior (ie, the amniotic cavity at The inferior edge of the urorectal this stage of development). The former septum fuses with the cloacal mem- location of the anal membrane is indi-brane by the 7th week and divides this cated in the adult by the pectinate line, septum into two parts: (i) an anterior The dual origin of the anal canal part called urogenital membrane which is the reason why the upper two thirds forms the inferior boundry of the primi- and lower one third of the anal canal tive urogenital sinus, and (ii) a posterior differ in their blood supply, nerve sup-part called anal membrane which lies ply and lymphatic drainage.