Is it ensure, insure, or assure?
yolk sac n.
A membranous sac attached to an embryo, enclosing food yolk. Also called umbilical vesicle.
|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.