amniotic sac vs sac

amniotic sac and sac both are nouns.

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

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

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

amniotic sac (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