A long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus (don't confuse with allantois or urachus). Also known as the vitellointestinal duct, the yolk stalk, the omphaloenteric duct, or the omphalomesenteric duct. It appears at the end of the fourth week, when the yolk sac presents the appearance of a small pear-shaped vesicle (the umbilical vesicle).