One of the mysteries of the English language finally explained.
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.
- ‘Inside the egg are a series of fluid-filled membranes which permit the embryo to survive: the amnion, allantois, yolk sac, and chorion.’
- ‘The urachus is a remnant of allantoic origin, connecting the embryonal cloaca to the allantois during early intrauterine life.’
- ‘The allantois is a structure that collects the waste that the embryo produces before it hatches.’
- ‘Initially, the bladder is continuous with the allantois.’
- ‘In the fetus, the urachus connects the bladder with the allantois.’
Mid 17th century: modern Latin, based on Greek allantoeidēs ‘sausage-shaped’.
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