The formation of auricles in human embryos was evaluated between Carnegie stage (CS)19 and CS23, and the findings were correlated across the stages. The auricle was categorized into 11 steps according to Streeter's criteria with modifications. Mesenchyme cell condensation was observed at Step 7, and two layers of cartilage consisting of the auricle were recognized at Step11. The representative steps at each CS shifted from Step 3 to Step11 during CS16 and CS23, although several steps overlapped between adjacent CSs. These results indicate that observations of the auricle between CS19 and CS23 may be utilized for determining embryo staging as convincing supportive evidence of external features reflecting the internal histological structure, although other findings should also be taken into account.
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