Cells outside the embryo that, although derived from the zygote, are not part of the embryo proper and contribute to the fetal membranes (yolk sac, allantois, amnion, and chorion).
Source: PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 753 (https://doi.org/10.1515/pac-2015-1202)