Fluid-filled space that surrounds the developing embryo of a mammal, bird, or reptile, bounded by the amniotic sac.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 720 (https://doi.org/10.1515/pac-2015-1202)