||This article needs additional citations for verification. (August 2010)|
|Human embryo of 2.6 mm.|
|Human embryo from thirty-one to thirty-four days|
|Latin||vesicula umbilicalis; saccus vitellinus|
|Gray's||subject #11 54|
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system of the human embryo, before internal circulation begins.
In humans 
It 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 reliably seen early in human pregnancy using ultrasound.
Blood is conveyed to the wall of the sac by the primitive aorta, 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 it nutritive material is absorbed from the yolk sac and conveyed to the embryo.
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.
As a rule the duct undergoes complete obliteration during the seventh week, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum, which is situated about 60 cm proximal to the ileocecal valve, 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.
In fish 
All bony fishes, some sharks and rays have yolk sacs at some stage of development, with all oviparous fished retaining the sac after hatching. Lamniform sharks are ovoviviparous, in that their eggs hatch in utero, in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-cannibalism: stronger pups consume their weaker womb-mates.
The yolk sac starts forming itself during the second week of the embryonic development, at the same time of the shaping of the amniotic sac. The hypoblast starts proliferating laterally and descending.
In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.
- Primary yolk sac/primitive yolk sac: it is the vesicle constituted in the second week, its floor is represented by the Heuser membrane and its ceiling by the epiblast. It is also known as the exocoelomic cavity.
- Secondary yolk sac: this first transformation is determined by the modification of its cover, in the connection zone between the epiblast and the Heuser membrane. We can observe a structure. The two parts detach and the inferior one, which is smaller, forms a cyst destined to be eliminated. The upper one is now covered only by the epiblast.
- The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas. A little portion of the sac, in the upper part, constitutes the intestinal tube. On the other side, the distal part forms a little vesicle that is what remains of the yolk sac.
Additional images 
See also 
- Meisner, A & Burns, J: Viviparity in the Halfbeak Genera Dermogenys and Nomorhamphus (Teleostei: Hemiramphidae). Journal of Morphology 234, pp. 295–317, 1997
- Peter Scott: Livebearing Fishes, p. 13. Tetra Press 1997. ISBN 1-56465-193-2
- Leonard J. V. Compagno (1984). Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations. ISBN 92-5-104543-7. OCLC 156157504.
- "Text for first three lectures". Retrieved 2007-10-13.