The endochondral bones of the limb are patterned in the precartilage condensations in limb buds. Within the limb primordium that forms of the flank of the embryonic trunk, skeletal elements arise in a proximodistal seqence 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 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. Development appears von Baerian, to recapitulate the evolutionary sequence. The developmental and molecular basis of limb patterning. Before the cartilage condendations arise in the limb primordium, key patterning events take place. The AER runs anterior to posterior, subdividing the bud into dorsal and ventral halves.