extraembryonic mesoderm

https://doi.org/10.1351/goldbook.10701

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)