The fetal membrane lying below the chorion in many vertebrates, formed as an outgrowth of the embryo's gut. In birds and reptiles it grows to surround the embryo; in eutherian mammals it forms part of the placenta.
- ‘The urachus is a remnant of allantoic origin, connecting the embryonal cloaca to the allantois during early intrauterine life.’
- ‘In the fetus, the urachus connects the bladder with the allantois.’
- ‘Initially, the bladder is continuous with the allantois.’
- ‘Inside the egg are a series of fluid-filled membranes which permit the embryo to survive: the amnion, allantois, yolk sac, and chorion.’
- ‘The allantois is a structure that collects the waste that the embryo produces before it hatches.’
Mid 17th century modern Latin, based on Greek allantoeidēs ‘sausage-shaped’.