The gastrointestinal tract develops from the primitive digestive tube that originates from the dorsal part of the yolk sac. Initially the yolk sac is attached to the midgut of the digestive tube, but as early as the fourth week of gestation the gut becomes distinct from the yolk sac. The yolk sac is connected to the digestive tube through the omphalomesenteric (vitteline) duct. The dorsal mesentery separates the digestive tube from the dorsal wall of the embryo, and at this stage there is also a ventral mesentery that separates the anterior part from the ventral embryonic wall. Continuity with the exterior environment is formed only after the rupture of the buccopharyngeal and cloacal membranes. The anatomic formation of the esophagus, stomach, intestine, pancreas and liver is achieved by the fourth week through a series of evaginations, elongations and dilatations. Further development through cell proliferation, growth and morphogenesis then follows.
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