The midgut rapidly increases in length and soon forms a U-shaped loop called primary intestinal loop (also called midgut loop). The yolk stalk is attached at the apex of this loop. The mesentery of the midgut necessarily elongates with the elongation of the midgut. The superior mesenteric artery
(formed by fusion of the vitelline arteries) runs in center of the midgut mesentery and supplies branches to all parts of the midgut.
The segments of the primary intestinal loop proximal and distal to the attachment of the yolk stalk are respectively designated as cranial and caudal limbs of the loop. The cranial limb develops into distal part of duodenum, jejunum and proximal part of ileum.From caudal limb of the midgut loop are derived distal part of ileum, cecum and vermiform appendix, ascending colon and right two thirds of the transverse colon. The yolk stalk disintegrates and disappears during.the 6th week of development. At about the same time a small swelling, called ce-cal bud, appears on the caudal limb of the midgut loop.
The main events in the development of the intestines from the mid-gut can be summarized as follows: (1) elongation (2) herniation into the umbilical cord, (3) rotation, (4) re-entry into the abdominal cavity, and (5) fixation