chorion

https://doi.org/10.1351/goldbook.10516
  1. In human embryology, the cellular, outermost extra-embryonicmembrane, composed of trophoblast lined with mesoderm.
    Note: The chorion develops villi, becomes vascularized by allantoic vessels, and forms the fetal part of the placenta.
  2. In mammalian embryology, the cellular, outer extra embryonic membrane, not necessarily developing villi.
  3. In biology, the noncellular membrane covering eggs of various animals, e.g., fish and insects.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 735 (https://doi.org/10.1515/pac-2015-1202)