Studies of digit regeneration and their implications for theories of development and evolution of vertebrate limbs.

Simple amputation of various combinations of hindlimb digits of Notophthalmus viridescens resulted in a variety of abnormal regenerates. The abnormalities included the formation of adventitious digits and branched digits, fusion between digits, and increases and decreases in the normal number of phalanges. The type of abnormality was correlated with the proximal-distal level of amputation and with the identity of the digits removed. These results are discussed in the context of a model for normal digit formation. An important feature of this model is that the normal regeneration of separate digits is dependent upon precise and orderly interactions between different parts of the limb circumference. In the experiments we report, the disruption of these orderly interactions leads to a variety of abnormalities. In addition, the model for digit formation presented here is discussed in light of its implications for evolutionary trends in the tetrapod limb.