Normal anterior pattern formation after barrier placement in the chick leg: further evidence on the action of polarizing zone.

Impermeable barriers were inserted into the stage-20 to -21 leg bud to test whether or not such an interruption of diffusion of the proposed morphogen from the polarizing zone would result in failure of leg elements to develop anterior to the barrier. Tantalum foil was placed at somite levels 30/31 or mid-31 through the dorsoventral extent of the leg bud separating anterior from posterior mesoderm and ectodermal ridge. In the resulting legs, structures developed anterior to the level of the barrier. For example, legs with foil at the 30-/31-somite level developed digits 1 and 2. We conclude that either the barrier is not an effective block of diffusion of polarizing zone morphogen or that the influence of the polarizing zone is not required for determination of leg structures at these stages.