The relationship of the zone of polarizing activity to supernumerary limb formation (twinning) in the chick wing bud

The role of the polarizing zone mesoderm in development of supernumerary distal wing parts after 180° rotation of the wing tip was investigated. Postaxial mesoderm with and without polarizing tissue was repositioned preaxially in the wing bud and duplications occurred only when polarizing zone tissue was included. When the polarizing zone was removed and the distal tip of the wing reoriented, no duplication resulted. Similarly when the polarizing zone was removed, the distal tip reoriented and postaxial, nonpolarizing mesoderm introduced to restore the tissue mass of the stump, no twinning occurred. However, with the distal aspect reoriented on a stump from which postaxial, nonpolarizing mesoderm was removed, twinning occurred in 92.9% of the cases. Further, when the polarizing zone was removed, the distal aspect reoriented and a small piece of polarizing tissue returned, twinning resulted in 93.5% of the cases. These results indicate that polarizing zone tissue is required for the twinning that results after 180° rotation of the wing tip.