Supernumerary limgs in amphibians: experimental production in Notophthalmus viridescens and a new interpretation of their formation.

The formation of supernumerary limbs was studied in the adult newt, Notophthalmus viridescens. Forelimb blastemas at the stages of medium bud and early digits were either transplanted to the contralateral forelimb with their dorsal-ventral axis opposed to that of the limb stump, or removed, rotated through 180°, and replaced on the same limb stump with both dorsal-ventral and anterior-posterior axes opposed to those of the stump, or as a control, removed, and replaced in normal orientation. Supernumerary limbs were produced in both experimental series, but not in the controls.Following contralateral transplantation, supernumerary limbs arose close to the graft junction at the two positions where dorsal limb tissue was in contact with ventral limb tissue. Both dorsal and ventral supernumerary limbs were of the same handedness as the limb stump and they were mirror-images of the regenerate developing directly from the transplanted blastema. Following 180° rotation, supernumerary limbs arose close to the graft junction at those positions where anterior-ventral and posterior-dorsal limb tissues were in contact. The supernumerary limb which arose in the posterior-dorsal position with respect to the limb stump was a mirror-image of the transplant, and was therefore of opposite handedness to both transplant and stump. The supernumerary limb which arose in the anterior-ventral position was of the same handedness as both transplant and stump. A new model of pattern regulation in epimorphic fields which can account for these results and which has retrospective value in the interpretation of earlier experiments on developing limbs is discussed.