Phenotypically wingless (ws) chick embryos have wing buds characterized by the spreading of mesenchymal cell death in an anterior-to-posterior direction beginning at stage 19. It has been argued that this may reflect the absence of a functional polarizing zone (ZPA). When tested by preaxial grafting into normal wing buds (stages 20–21), wingless ZPAs (stage 18–19) had duplicating properties identical with those of normal ZPAs. Equally, normal chick or quail ZPA (stages 20–22) grafted into the posterior margin of wingless wing buds (stages 18–20) failed to inhibit the pattern of cell death or to evoke any improvement in their developmental performance. The wingless (ws) condition is not, therefore, due to a ZPA deficiency. Possible explanations are the prior programming for cell death of the wingless mesenchyme, or somitic deficiency, but it appears more likely that the mutant limb mesenchyme fails to transmit or respond to factor(s) produced by the ZPA.

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