Definition

1. Noun:
   • A membranous sac enclosing the yolk in the eggs of birds, reptiles, marsupials, and some fish: This structure surrounds the nutrient-rich yolk and circulates nutrients to the developing embryo.
   • A membranous structure in mammalian embryos: This structure functions as an early circulatory system, providing nutrients and aiding in blood cell formation until the fetal heart becomes functional.

Usage Examples

• Noun:
  • The yolk sac is a vital source of nutrition for the chick before it hatches.
  • In human development, the yolk sac is one of the first structures to form within the gestational sac.

Advanced Usage

• "Yolk sac tumor": A type of germ cell tumor that histologically resembles the embryonic yolk sac.
  • The pathology report confirmed the presence of a yolk sac tumor.

Variants and Related Words

• Yolk (n): The yellow, nutrient-rich inner part of an egg.
  • Separate the egg yolk from the white.
• Vitelline (adj): Pertaining to the yolk of an egg or the yolk sac.
  • The vitelline duct connects the yolk sac to the midgut of the embryo.

Synonyms

• Vitelline sac: A technical synonym for the yolk sac.

Related Terms (Note: Not phrasal verbs or idioms, as this is a scientific term)

• Allantois: An extra-embryonic membrane involved in gas exchange and waste removal in bird and reptile eggs; in mammals, it contributes to bladder formation and the umbilical cord.
• Amnion: The innermost membrane that encloses the embryo of a mammal, bird, or reptile, containing the amniotic fluid.

Noun

1. membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo
2. membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional