WordReference Random House Unabridged Dictionary of American English © 2025
yolk′ sac′, [Embryol.]
  1. Developmental Biologyan 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. Developmental Biologya similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.
  • 1860–65

Collins Concise English Dictionary © HarperCollins Publishers::
yolk sac
  1. the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
'yolk sac' also found in these entries (note: many are not synonyms or translations):
extraembryonic membrane - yolk stalk

Forum discussions with the word(s) "yolk sac" in the title:


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