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Anatomy, Zoology. the innermost of the embryonic or fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.
Zoology. a similar membrane of insects and other invertebrates.
Historical Examples

amnion cavity: a tube-like insinking from the ventral plate of the embryo, extending cephalad.
Explanation of Terms Used in Entomology John. B. Smith

amnion: the inner of the two membranes enveloping the embryo.
Explanation of Terms Used in Entomology John. B. Smith

Both it and the stalk of the yolk-sack are enveloped by the amnion (am).
The Works of Francis Maitland Balfour, Volume III (of 4) Francis Maitland Balfour

Only in the chancel were they protected by sheep’s amnion stretched on frames.
Pabo, The Priest Sabine Baring-Gould

The amnion in fact begins to pulsate slowly and rhythmically, and by its pulsation the embryo is rocked to and fro in the egg.
The Works of Francis Maitland Balfour, Volume III (of 4) Francis Maitland Balfour

It adheres to the amnion and supplies it with blood-vessels (Bischoff).
The Works of Francis Maitland Balfour, Volume III (of 4) Francis Maitland Balfour

The embryo thus enclosed in the egg finds its protection in the fact that it is encased in a fluid contained in the amnion.
The Meaning of Evolution Samuel Christian Schmucker

Superficial to it is the amnion, and below it is the yolk sac.
Embryology Gerald R. Leighton

An important appendage contained in the Uterus in connection with the developing fetus is that known as “The amnion.”
Private Sex Advice to Women R. B. Armitage

This second fold gives rise to the amnion, and is limited entirely to the somatopleure.
The Works of Francis Maitland Balfour, Volume III (of 4) Francis Maitland Balfour

noun (pl) -nions, -nia (-nɪə)
the innermost of two membranes enclosing an embryonic reptile, bird, or mammal See also chorion
n.

1660s, Modern Latin, from Greek amnion “membrane around a fetus,” said to be originally “bowl in which the blood of victims was caught” [Liddell & Scott], which is variously said to be of unknown origin, from ame “bucket,” or a diminutive of amnos “lamb.”

amnion am·ni·on (ām’nē-ən, -ŏn’)
n. am·ni·ons or am·ni·a (-nē-ə)
The thin, membranous sac filled with a serous fluid in which the embryo or fetus is enclosed and suspended in the uterus. Also called amniotic sac.
am’ni·ot’ic (-ŏt’ĭk) or am’ni·on’ic (-ŏn’ĭk) adj.
amnion
(ām’nē-ən)
A thin, membranous sac filled with a watery fluid (called the amniotic fluid) in which the embryo or fetus of a reptile, bird, or mammal is suspended during prenatal development. Also called amniotic sac.