THE MIDGUT

The midgut forms the intestine, from the entry of the bile duct to the junction of the transverse and descend­ing parts of the colon. Its initial wide connection with the yolk sac is quickly lost.

The early growth of the midgut is very rapid, causing it to hang in a loop from an elongated mesentery in which the midgut (cranial mesenteric) artery runs. The expanding liver claims so large a part of the abdominal cavity that insufficient room remains for the intestine. The long mesentery then permits the midgut to slip out of the abdominal cavity into the umbilical cord, which is a process known as physiological herniation, where growth continues. The cranial limb of the herniated loop becomes the small intestine; the appearance of a diverticulum, the future cecum, indicates the division of the caudal limb into the terminal part of the small intestine and the initial part of the colon. The cranial limb grows more rapidly and soon becomes much coiled. The key event is the rotation of the loop about the arterial axis (Figure 3-64), which is a rotation that carries the originally caudal limb forward on the left, then across the abdomen before it passes caudally on the right side, completing a rotation through approxi­mately 270°. This rotation, clockwise when viewed from above, brings the intestines more or less into their adult disposition when they are returned to the abdomen (Figure 3-65). The return is possible because the rate of liver increase slows and falls behind the general growth of the embryo. The final arrangement may depend on local shortenings of the mesentery and fusions of apposed peritoneum-clad surfaces.