amnion http://upload.wikimedia.org/wikipedia/commons/e/e7/Gray14.png EV:0100122 UBERON:0000305 MESH:A16.254.403.277 amniotic uberon CALOHA:TS-0033 amnios EMAPA:16109 Structures homologous to the four extraembryonic membranes of reptiles and birds appear in mammals: amnion, chorion, yolk sac, and allantois.[well established][VHOG] VHOG:0000721 EHDAA:136 BTO:0000065 EHDAA2:0000116 NCIT:C12365 FMA:80223 GAID:1298 A thin but tough extraembryonic membrane of reptiles, birds and mammals that lines the chorion and contains the fEtus and the amniotic fluid around it. [Biology_Online][VHOG] UMLS:C0002630 The thin innermost layer of the extraembryonic membranes that contains the amniotic fluid; the membrane forms a closed sac in which the embryo and later, the fetus, is suspended and protected. mesoderm-derived structure extraembryonic membrane