The progress zone model for specifying positional information.

The Progress Zone Model proposes that positional information in a growing system can be specified by the time the cells spend in a zone where growth occurs. In the vertebrate limb, the Progress Zone is specified by the Apical Ectodermal Ridge. The best evidence for the model is that killing cells in the zone at an early stage leads to loss of proximal structure as cells remain much longer in the zone as it becomes repopulated.