A fetal membrane, part of the yolk sac, that appears during some placentation processes. [ https://orcid.org/0000-0003-3308-6245 http://www.ncbi.nlm.nih.gov/pubmed/29416852 http://www.ncbi.nlm.nih.gov/pubmed/10761048 https://github.com/obophenotype/uberon/issues/1426 ]