Chordate Defining Traits
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A chordate embryo develops five traits at some stage: a notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail, and endostyle (or its thyroid derivative).
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A human embryo develops all five traits, though the notochord, tail, and endostyle are later lost or transformed.
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Losing a trait later does not remove an organism from the phylum — the criterion applies at some point.