The endochondral bones of the limb are patterned in the precartilage condensations in limb buds, after the stages when the regulatory genes described above are expressed. Individual skeletal elements arise in a proximodistal sequence of budding and branching of the condensation. The sequence is shown in PJH Box 10-1, e and f. The ulna branches and buds, and the axis elongates the proximal carpals coming off preaxially. Then, within the proximal autopod, there is a change.
Metacarpals branch off postaxially.
We understand from this study that a turn in the limb axis occurs during limb development that we have supposed occurs during tetrapod limb evolution. Here, development appears to follow von Baer's rules . Three phases of Hox gene regulation pattern the proximodistal axis of the limb bud. Roughly, and importantly in the scenario of pattering proposed by Shubin et al., the timing of the three phases matches the timing of specification of the three limb compartments, the stylopod, zeugopod and autopod. During Phase I, paralogs 9 and 10 are expressed in the distal mesenchyme and this expression is proposed to specify this early mesenchyme to develop as stylopod.