The baglike structure that contains a clear fluid in which the developing embryo floats is called the Amnion.
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What is an Amnion?</h3>
The  membrane that  covers the embryos of  reptiles, birds, and the mammals when first formed is called Amnion. 
 It contains amniotic fluid, which serves as an amniotic sac that provides a protective shield for the developing embryo. Together with the amnion, contains the
-  chorion 
- the yolk sac
- allantois
with different functions to  protect the embryo.
The amniotic sac in humans is enclosed in the uterus while for birds, reptiles ,it is enclosed in a shell.
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