yolk sac
Quick Summary
The yolk sac is a membranous sac found in most vertebrates that contains the yolk. It attaches to the embryo's intestinal cavity via a yolk stalk and is supplied with blood vessels that transport nutrients from the yolk to the developing embryo.1
In human development, cells from the blastocyst form a primitive yolk sac. While in other vertebrates like birds and amphibians, this sac is large and stores significant yolk, in humans and other mammals, it contains very little yolk. The yolk stalk connects the elongating gut to the yolk sac, and though it detaches early in the second month of development, the shrunken sac often remains and can be found in the afterbirth.2
In animals with large yolky eggs, the yolk sac is a key adaptation. It remains connected to the embryo by a stalk and is surrounded by the embryo's cellular layers. Blood vessels in the yolk sac's walls transport yolk products to the embryo as it is broken down and utilized, causing the sac to shrink and eventually be drawn into the embryo's body.3
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