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Intercalation and the cellular origin of supernumerary limbs in Xenopus.

The hypothesis that a specialized polarizing zone controls the pattern of the anterior-posterior axis during limb development in Xenopus has been tested by analysing the cellular contribution to supernumerary limbs. Supernumerary limbs were generated by grafting hindlimb buds contralaterally between X. borealis and X. laevis to appose anterior and posterior limb tissues. Cells derived from these two species of Xenopus are readily identified by staining with quinacrine. The analysis of cellular contribution showed that supernumerary limbs consist of approximately half anterior-derived (57%) and half posterior-derived (43%) cells. These data are not consistent with the polarizing zone theory but are consistent with the hypothesis that both supernumerary limbs and normally developing limbs arise from intercalary interactions between limb bud cells with different positional values.