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Updated: May 3, 2025
Parts of the discarded gill-arches are gradually transformed into the three bones of the mammal's internal ear; just as other parts are converted into mouth cartilages, and as it is believed one of the gill clefts is converted into the Eustachian tube. In the Monotreme and Marsupial the ear-hole begins to be covered with a shell of cartilage; we have the beginning of the external ear.
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