The zone of polarizing activity: evidence for a role in normal chick limb morphogenesis

When an impermeable barrier is placed so as to divide the early chick limb-bud into anterior and posterior parts then development continues only on one side of the barrier. The detailed results are inconsistent with mosaic development. They can readily be explained by supposing that pattern is specified by the concentration of a diffusible morphogen controlled by the zone of polarizing activity. A simulation of appropriate concentration profiles is presented and its relevance to similar experiments published elswhere is discussed. It seems probable that the zone of polarizing activity is active during normal development.