amnion

noun

am·​ni·​on ˈam-nē-ˌän 

-ən

plural amnions or amnia ˈam-nē-ə 

1

: a thin membrane forming a closed sac about the embryos or fetuses of reptiles, birds, and mammals and containing the amniotic fluid

2

: a membrane analogous to the amnion and occurring in various invertebrates

amniotic

ˌam-nē-ˈä-tik 

adjective

Examples of amnion in a Sentence

Recent Examples on the Web

The amnion is the innermost layer of the placenta that surrounds the fetus. —Beth Warren, The Tennessean, 2 July 2025

Several important features of real embryos are present in the mimics, the team says, including the predecessors of sperm and eggs and the beginnings of the amnion, the membrane-like structure that encloses the embryo in a watery environment. —Bymitch Leslie, science.org, 17 June 2023

Additional features can be built into these sensors such as amnion fluid infusion and extraction. —IEEE Spectrum, 13 Dec. 2018

Birds, mammals and reptiles called amniotes produce eggs that include an amnion, or inner membrane, that keeps the embryo inside from drying out. —Ashley Strickland, CNN, 17 June 2020

Stem cells, platelet rich plasma (PRP), placenta/amnion and their variants are the latest rage. —Profootballdoc, sandiegouniontribune.com, 1 Sep. 2017

Word History

Etymology

New Latin, from Greek, caul, from amnos lamb — more at yean

First Known Use

1661, in the meaning defined at sense 1

Kids Definition

amnion

noun

am·​ni·​on ˈam-nē-ˌän 

: a thin membrane forming a closed sac about the embryo or fetus of a reptile, bird, or mammal and containing a watery fluid in which the embryo or fetus is immersed

Medical Definition

amnion

noun

am·​ni·​on ˈam-nē-ˌän, -ən 

plural amnions or amnia -nē-ə 

: a thin membrane forming a closed sac about the embryos and fetuses of reptiles, birds, and mammals and containing the amniotic fluid

called also amniotic sac