yolk sac

noun

, Embryology.

1. 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.
2. a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.

yolk sac

noun

1. the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
2. the corresponding part in the embryo of mammals, which contains no yolk

yolk sac

1. A sac that is attached to the gut of an embryo and encloses the yolk in bony fish, sharks, reptiles, mammals, and birds. In most mammals, the yolk sac functions as part of the embryo's circulatory system before the placenta develops.

Word History and Origins

Origin of yolk sac¹

First recorded in 1860–65

Example Sentences

The study led by at the University of Exeter Living Systems Institute has revealed how early embryo cells decide between contributing to the foetus or to the supporting yolk sac.

From Science Daily

What is important is that gastruloids recapitulate some features of early development even without the external cues from the placenta or yolk sac that typically direct the organization of an early embryo.

From Scientific American

There is a yolk sac, which has some of the roles of the liver and kidneys, and a bilaminar embryonic disc - one of the key hallmarks of this stage of embryo development.

From BBC

And in some tissues of adult rodents, the immune cells known as macrophages are descended from yolk sac cells, not the bone marrow cells that give rise to most macrophages.

From Science Magazine

That model contains embryonic tissues and tissues that can go on to produce structures surrounding the embryo such as the placenta and yolk sac.

From Washington Times