Figure 1.

Limb bud development and adult limb structures are conserved among tetrapods. A scanning electron microscopy image of a 22HH chick forelimb shows limb bud, the structure where all the skeletal elements arise during development. Schematic representations of adult skeletal structures of forelimbs from distinct tetrapods (frog, turtle, bird, and human) are shown. The most proximal skeletal element, the stylopod, is characterized for a single skeletal element (purple); two middle elements represent the zeugopod (orange), and the distal region highly segmented represents the autopod (blue).