Is the Progress Zone Model a Victim of Progress?

The prevailing view has long been that mesenchymal cells at the apex of the vertebrate limb bud proliferate in a progress zone (PZ) under the influence of the apical ectodermal ridge (AER). According to the PZ model, proximodistal cell fates are specified according to the time spent in the PZ. New results, considered along with some earlier findings, fail to support this model.