(deutoplasm) n. a substance, rich in protein and fat, that is laid down within the egg cell as nourishment for the embryo. It is absent (or nearly so) from the eggs of mammals (including humans) whose embryos absorb nutrients from their mother.
The membranous sac, otherwise known as the vitelline sac, that lies against, and is attached to, the front of the embryo during the early stages of its existence. During development, the sac decreases proportionately in size to the body, reducing finally to a narrow duct that passes through the umbilicus. The yolk sac is believed to assist in the transportation of nutrients from the mother to the early embryo.... yolk sac
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