The yolk sac is situated on the ventral aspect of the embryo. It is filled with fluid, the
vitelline fluid, which possibly may be utilized for the nourishment of the embryo during
the earlier stages of its existence. Blood is conveyed to the wall of the sac by the
primitive aortae , and after circulating through a wide-meshed capillary plexus, is
returned by the vitelline veins to the tubular heart of the embryo. This constitutes the
vitelline circulation, and by means of its nutritive material is absorbed from the yolk-sac
and conveyed to the embryo. After the end of the fourth week the yolk-sac presents the
appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive
tube by a long narrow tube, the vitelline duct.
The yolk-sac is also responsible for the initial circulation and is in charge of delivering
nutrients, via a primitive aorta, to the developing embryo. The yolk-sac provides
nutrition to the developing embryo until the placenta takes over.
How does the yolk-sac disappear?
As the pregnancy advances, the yolk-sac progressively increases from the 5th to end of
the 10th gestational week, following which the yolk sac gradually disappears and is often
sonographically undetectable after 14-20 weeks.
Readers, did you know about yolk-sac earlier?
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Information courtesy Google
Day 21. Uterus Day 22. Vertebral Column