Yolk sac

thin membrane which surrounds the yolk of an egg and supplies nutrients to the embryo (in mammals, the yolk sac ceases to function when the placenta forms)

The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation.It is a critical landmark, identifying a true gestation sac.It is quite echogenic (light) to ultrasound, and reliably seen early.
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Noun
1. membranous structure that functions as the circulatory system in mammal embryos until the heart becomes functional
(synonym) vitelline sac, umbilical vesicle, vesicula umbilicus
(hypernym) sac
2. membranous structure enclosing the yolk of eggs in birds reptiles marsupials and some fishes; circulates nutrients to the developing embryo
(hypernym) sac

Not all yolk has to do with birds’ eggs. Human embyros have a yolk sac, a membane outside the embryo but connected by a tube (the yolk stalk) though the umbilical opening to the embryo’s midgut. The yolk sac serves as an early site for the formation of blood and in time is incorporated into the primitive gut of the embryo.

SACCO VITELLINO. LECITOCELE