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During the third month, after the herniated midgut derivatives return to the abdominal cavity from the umbilical cord, the dorsal mesentery fuses with the dorsal body wall in three areas. The three areas are as follows:

Most of the dorsal mesogastrium between its midline attachment and the spleen together with the mesoduodenum containing the pancreas swing to the left and come to lie adjacent to the left suprarenal gland and kidney. Since the suprarenal glands and kidneys are covered with peritoneum on only their ventral surface, they are said to be retroperitoneal in position. The segment above of the dorsal mesogastrium and mesoduodenum fuses with the ventral surface of the inferior vena cava, aorta, left suprarenal gland and left kidney. As a result most of the duodenum and pancreas become retroperitoneal. The short mesentery segment between the left kidney and spleen does not fuse thereby forming the lienorenal ligament. The tail of the pancreas and splenic blood vessels pass through this ligament to reach the hilum of the spleen. The segment of the dorsal mesogastrium between the stomach and the spleen is called the gastro-lienal ligament and contains gastric branches of the splenic artery. The lienorenal and gastrolienal ligaments form the left boundary of the lesser sac.

The mesentery of the midgut extends from the duodenal-jejunal junction to the distal segment of the transverse colon. It can be divided into a proximal, intermediate and distal portion. Only the small intermediate portion fuses with the dorsal body wall. The proximal portion suspends the jejunum and ileum. It becomes the mesentery proper that attaches obliquely to the dorsal body wall from upper left to lower right and contains most of the superior mesenteric vessels. As the cecum and ascending colon move into their definitive position at the far right, their mesentery and blood vessels fuse with the right side of the dorsal wall covering over the right ureter and part of the duodenum and right kidney. The distal portion suspends the transverse colon and persists as the transverse mesocolon. It attaches to the dorsal wall in a horizontal manner at the level of the pancreas and fuses with the dorsal aspect of the greater omentum.

The mesentery of the hindgut extends from the distal segment of the transverse colon to the rectum. It suspends all of the hindgut derivatives (distal segment of the transverse colon, descending colon and sigmoid) and contains the inferior mesenteric vessels. The hindgut derivatives are pushed to the far left side by the returning midgut. The mesentery of the descending colon and its contained vessels fuse to the left side of the dorsal wall covering over the left ureter and caudal part of the left kidney. The mesentery attached to the sigmoid persists as the sigmoid mesocolon.

Source: Atlas of Human Embryos.