- an extraembryonic membrane that encloses the yolk of eggs in birds, reptiles, and marsupials and that circulates nourishment from the yolk to the developing embryo.
- a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.
- the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
- the corresponding part in the embryo of mammals, which contains no yolk
