The hindgut contributes following derivatives to the digestive tube: left one third of transverse colon, descending colon, sigmoid colon, rectum, and major part of the anal canal. However, the hindgut also gives rise to some derivatives which are not part of the digestive system but belong to the uro-genital system; these derivatives are: urinary bladder and urethra. The digestive tube derivatives of the hindgut are supplied by the inferior mesenteric artery.
The hindgut comes into existence during the 4th week of the development when the posterosuperior part of the yolk sac comes to lie within the tail fold of the embryo. The initial part of the hindgut is tubular, but at its termination the hindgut dilates into cloaca. The cloaca ends at the cloacal membrane. This membrane is composed of fused endoderm and ectoderm, and separates the cloaca from the amniotic cavity. The cloaca receives the allan-tois ventrally and mesonephric ducts laterally.
As already mentioned, the descending colon loses its mesentery and becomes fixed to the posterior abdominal wall. The sigmoid colon is formed by elongation of the terminal part of the colon. It retains a short mesentery.
Partitioning Of Cloaca at the distal end of the anorectal canal.
During the 5th and 6th weeks of The zone of fusion of the anorectal
development the cloaca becomes di- septum with the cloacal membrane is
vided into two parts by the formation of represented in the adult by the perineal
a septum called urorectal septum body.
The anterior sybdiyision of the cloaca The Posterior subdivision of the
is known as primitive urogenital si- cloaca' ie- the anoreqtal canal, gives
nus,.whereas the posterior subdivision nse to rectum and upper two thirds of
is called anorectal canal. the anal canal-