Digestive system

Formation of the intestines

Following transverse rotation of the developing heart and with the head curving round (cephalic fold), an endodermal tube is formed: the foregut. It is located between the ventrally-lying cardiac prominence and the septum transversum on one side, and the dorsal notochord and the neural tube on the other side. Its rostral end is sealed by the oropharyngeal membrane which separates it from the oral depression (stomatodeum). The oropharyngeal membrane can be seen from S10 onwards and is directly rostral to the prechordal plate. Endoderm and ectoderm are in direct contact at this point.
Inflection at the caudal end of the body (caudal fold) leads to the formation of a caudal endodermal diverticulum: the hindgut. Caudal folding causes the allantois to move ventrally to become incorporated into the body of the embryo. The caudal end of the posterior intestine widens into a cloaca into which the allantois also leads. The cloaca remains closed from the anal depression (proctodeum) by a cloacal membrane until S19. Ectoderm and endoderm are again in direct contact with each other.