A membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles, and it is important in the formation of the umbilical cord and placenta in mammals[VHOG].
Comment:
[development-note] "a diverticulum of the hindgut endoderm [ISBN:0073040584 (Vertebrates, Kardong)]"; [homology-note] "Structures homologous to the four extraembryonic membranes of reptiles and birds appear in mammals: amnion, chorion, yolk sac, and allantois.[well established][VHOG]" xsd:string {date_retrieved="2012-09-17", external_class="VHOG:0000738", ontology="VHOG", source="ISBN:978-0072528305 Kardong KV, Vertebrates: Comparative Anatomy, Function, Evolution (2006) p.187", source="http://bgee.unil.ch/"}