Figure 6.9   Hox genes and the development of the tetrapod limb. (a) The sequence of growth of a tetrapod limb bud, reading from top to bottom, showing how the stylopod (humerus/femur), zeugopod (forearm/calf) and autopod (hand/foot) differentiate. The pattern is determined by turning on (filled squares) and off (open squares) of Hox genes D-9 to D-13. (b, c) Interpretation of the forelimbs of the osteolepiform fish Eusthenopteron (b) and the tetrapod Acanthostega (c) in terms of development. The developmental axis (solid line) branches radial elements (dashed lines) in a pre-axial (anterior) direction in both forms, and the digits of tetrapods condense in a post-axial direction. (a, based on Shubin et al. 1997; b, c, courtesy of Mike Coates.)

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