amnion vs sac

amnion and sac both are nouns.

Word	Noun	Adjective	Verb	Adverb
amnion	Yes	No	No	No
sac	Yes	No	No	No

As nouns, sac is a hypernym of amnion; that is, sac is a word with a broader meaning than amnion:

amnion: thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)
sac: a structure resembling a bag in an animal

amnion (noun)	sac (noun)
thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)	a structure resembling a bag in an animal
	a case or sheath especially a pollen sac or moss capsule
	an enclosed space