In most tetrapods (land vertebrates) the fourth (ring) finger is the first to develop in the embryo. And in birds, the finger on the outside of the hand (posterior, the pinky side) appears first, which suggests that this is the ring finger. However, it can be shown that on the thumb side (anterior) an embryonic finger begins to develop, but quickly disappears—this would have to be digit I. These data argue for an identification of the fully-formed fingers as the index, middle, and ring fingers (II, III, IV).
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