In humans, 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). The vesicle can be seen in the afterbirth as a small, somewhat oval-shaped body whose diameter varies from 1 mm to 5 mm. It is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta. Generally, the duct fully obliterates (narrows and disappears) during the seventh week, but a failure of the duct to close is termed a vitelline fistula. This results in discharge of meconium from the umbilicus. About two per cent of fetuses exhibit a type of vitelline fistula characterized by persistence of the proximal part of the vitelline duct as a diverticulum protruding from the small intestine, Meckel's diverticulum, which is situated about two feet above the ileocecal junction, and may be attached by a fibrous cord to the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct

http://upload.wikimedia.org/wikipedia/commons/8/8c/Gray30.png

yolk-stalk, embryonic yolk stalk, yolk duct, ductus vitellinus, omphalomesenteric duct

uberon

UBERON:0007105

• Meckel's diverticulum