The sac is surrounded by a network of vitelline arteries. The vitelline arteries are the arterial counterpart to the vitelline veins. The mucopolysaccharides attract water to raise the vitelline membrane. The actual egg is covered by a vitelline membrane. The yolk sac remains connected to the gut tube via the vitelline duct. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane. These contents digest the jelly coat and eventually the vitelline membrane. From here, a short oviduct passes to the vitelline duct. Vitelline glands are scattered around the caeca. In the fetus the ileum is connected to the navel by the vitelline duct. In non-mammals it is called the vitelline membrane or vitelline envelope. The vitelline duct narrows progressively and disappears between the 5th and 8th weeks gestation. It receives blood from the vitelline vein, umbilical vein and common cardinal vein. This constitutes the vitelline circulation, which in humans serves as a location of haematopoiesis. The vitelline arteries form the celiac, superior and inferior mesenteric arteries of the gastrointestinal tract.