The primitive gut is formed form the endodermal sac which is carved out of the receding yolk sac and the folding embryo. The portion before the yolk sac becomes the foregut guarded by an anterior intestinal portal and the portion behind the yolk sac becomes the midgut. The portion facing the yolk sac directly is the midgut. The esophagus is formed from the elongation of the tube between the pharynx and the stomach. and the foregut ends at the level of the bile duct. The stomach now rotates with its ventral mesogastrium so that its lesser curvature, which is continuous with the ventral mesogastrium now lies to the right and the dorsal mesograstium carried to the left forming the omental bursa behind it.. The duodenum then comes to lie right of the stomach. The spleen is formed within the rotating dorsal mesogastrium. The pancreas is formed by two diverticula for the gut tube. The largest is the dorsal pancreas while the smaller is the ventral pancreas . With the rotation of the stomach, the ventral pancreas is carried behind the stomach to meet the dorsal and the two fuse to form the definitive pancreas.
The ventral diverticulum of the pancreas also forms the gall bladder and common bile duct and later the liver. The liver develops from the junction of the yolk sac and foregut from primordium of cells which form the heptatic rudient and grows into the septum transversum. It is initially surrounded by three veins, umbilical, vitelline and hepatocardiac veins. The hepatocardia veins drain the lier iectly into teh sinus vnosus. But as soon as develop continues the liver allows the obliteration of many veins and finally forms within its substance the ductus venosus, which later becomes the inferior vena cava at birth.
The midgut herniates during the development of the gut and undergoes rotation and then returns back at about 3 months of intrauterine life due to the decrease in size of the kidneys and other intrabdominal organs.
