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 
Example Sentences
Recent Examples on the Web 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
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
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