A) Embryology: In pentadactyl amniotes like mammals and crocodilians, the primary axis of cartilage formation (red line) always develops into digit 4. Embryological condensations in this figure are labeled C1–C5 based on their spatial relation to the primary axis (C4). B) Gene expression (Late phase): In the mouse, expression of HoxD10-D12 is absent only in digit 1. In the chicken, expression is absent in the anterior digit, but it is one position closer to the primary axis, at the embryological position of C2. C) Comparative morphology: The wing digits are morphologically 1, 2, and 3. The hypothesis of a homeotic frame shift proposes that digits 1,2 and 3 have all shifted one embryological position towards posterior in the evolution of the bird line, such that digits D1, D2 and D3 (color coded: cream, blue and green) in the wing develop from the embryological positions C2, C3 and C4. If the frame shift hypothesis is correct, we expect to find that HoxD gene expression in crocodilians will be absent only at embryological position C1.