The yolk sac is one of the embryonic appendices of amniotes. The sac develops from the endoblast and the mesoblast during embryogenesis. It surrounds the embryo's nutrient reserves (egg vitellus, or yolk) and supplies it with food during its development.
The yolk sac in fish
The yolk sac in fish refers to what remains of the vitiellus in the young alevin, feeding it before it is able to sustain itself.
Salmon alevins are still dependent on their yolk sac. © Earl Steele, CC by-nc-nd 2.0