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One of the most salient characteristics of the Amniotes is the complete loss of the gills. The Protamniote itself must have entirely abandoned water-breathing. But we do not find in the embryos of the Amniotes any trace of gill-leaves, or of real respiratory organs on the gill-arches. It is very probable that the urinary bladder of the Dipneusts is the first structure of the allantois.
The branchial arches also, which separate the clefts, develop into very different parts. In the fishes they remain gill-arches, supporting the respiratory gill-leaves. Median section of the head of a Petromyzon-larva.
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