Regeneration of mirror symmetrical limbs in the axolotl.

Mirrow symmetrical, double posterior limbs were produced by two types of operations on tailbud embryos of the axolotl, Ambystoma mexicanum. When the limbs had grown to maturity, they were amputated and a study was made of the structures which regenerated in their place. The majority of the regenerates were also reduplicated, although a significant minority showed other types of symmetry. Reduplicate symmetry was often conserved following several amputations, but the number of skeletal elements present in each regenerate tended to vary: both expansion and contraction were found, the changes always occurring in the center of the pattern. These results are discussed in terms of the possible mechanisms for pattern formation in the anteroposterior axis during regeneration. We suggest that control of normal regeneration is carried out by a monotonic gradient of positional information which is originally set up to control pattern formation during embryonic development.