Duplicated axolotl regenerates.

Three series of palette stage regenerates were prepared by amputating both arms of juvenile axolotls in the mid-forearm, above the elbow, or close to the shoulder. Within each series, excised regenerates were replaced in their original orientation (as a control) or rotated at 90 degrees intervals about the proximodistal axis, or were transplanted to the contralateral arm with identical rotations. Control grafts occasionally caused the formation of a single extra digit. All experimental rotations provoked duplicate or triplicate structures, ranging from accessory digits to extra forearms. Shoulder level grafts were subject to a pronounced correctional derotation and yielded a variable proportion of duplications. Forearm and elbow-level rotations invariably yielded duplications, which therefore result from an axial discrepancy rather than complete axial opposition between graft and host. These results are incompatible with the clockface model of positional information and demand a modification of other current models. The recorded frequency and orientation of duplications suggest that a limb contains at least two polarized transverse axes which cannot be respecified during regeneration. A substantial discrepancy on either axis reduces the normal regulative interaction between graft and host, allowing either of them to regenerate independently.