1. amnion

noun. thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals).


  • sac
  • amnios
  • fetal membrane
  • amniote

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Example sentences of the word amnion

1. Noun, singular or mass
At the innermost part of the egg, the embryo is suspended by the amniotic fluid, which is surrounded by a membrane called the amnion.