The midgut herniates out through the umbilicus between the 6th and 10th weeks. The superior mesenteric artery also herniates out and this artery acts as an axis around which the midgut rotates, first 90 degrees and then a further 180 degrees as the bowel is retracted back inside the abdominal cavity. The midgut therefore rotates a total of 270 degrees counter-clockwise.
This physiological herniation occurs because of both the rapid growth of the liver forcing abdominal contents outwards and the rapid growth of the midgut. As a result of this growth, the small intestine is much longer than the large intestine.
The cloaca is the terminal dilated part of the hindgut. It comes into direct contact with the ectoderm at the cloacal membrane (there is no mesoderm between these two layers so the membrane is said to be bilaminar). It is the cloaca that is divided by the urogenital septum to form the rectum and the urogenital sinus.
The inferior recess of the lesser sac forms the greater omentum.
The yolk sac closes off from the gut in the 5th week.
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