yolk sac

noun

, Embryology.

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.

yolk sac

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

yolk sac

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.

First recorded in 1860–65

The epiblast spreads uniformly over the yolk-sac and not on the one side only as in the former eggs.

In this thickened edge a circular vein arises, which brings back the blood from the yolk-sac to the embryo.

Superficial to it is the amnion, and below it is the yolk sac.

The head and even the region of the pronephros are clearly separate from the yolk-sac; the mouth is coming to be formed.

This point is where the communication exists between the inside of the tube, which is the embryo, and the yolk sac.

April 23, 2024inveterate

[in-vet-er-it ]

Start each day with the Word of the Day in your inbox!

By clicking "Sign Up", you are accepting Dictionary.com Terms & Conditions and Privacy Policies.

Browse