A membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles. It is important in the formation of the umbilical cord and placenta in mammals.

• 尿模哺乳动物、鸟类和两栖动物的胚胎中在消化道后部形成的膜质液囊。对哺乳动物脐带和胎盘的形成有重要作用