a part inside the body of a person, an animal or a plant, that is like a bag in shape, has thin skin around it, and contains liquid or air （人、动植物体内的）囊，液囊，气囊 Oxford Collocations DictionarySac is used after these nouns:- egg
Word Originmid 18th cent. (as a term in biology): from French sac or Latin saccus ‘sack, bag’.