Medical Term:

allantois

Pronunciation: ă-lan′tō-is

Definition: A fetal membrane developing from the hindgut (or yolk sac, in humans). In humans it becomes a fibrous cord, the urachus; externally, in other mammals, it contributes to the formation of the umbilical cord and placenta; in birds and reptiles, it lies close beneath the porous shell and serves as an organ of respiration.

Synonym(s): allantoid membrane

[allanto- + G. eidos, appearance]

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