Regenerative failure of double half limbs in Notophthalmus viridescens

Abstract

IT HAS been proposed that positional information in epimorphic fields is specified in terms of polar coordinates¹, where one component of positional information is a value corresponding to position on a circle, and the other to position on a radius. In the newt limb, the circular sequence of positional values lies around the circumference and the radial sequence lies in the proximal–distal axis of the limb. French et al.¹ have proposed that various different regulative phenomena in such epimorphic fields as insect legs, insect imaginal disks, and amphibian limbs can be accounted for by two principles of cellular behaviour. The first of these is shortest intercalation, whereby any discontinuities in positional information are resolved by intercalation of missing positional values during growth. The second principle is that distal transformation can only take place from a complete set of positional values in the circular sequence. Unless a complete circle is either present at a site of amputation, or can be generated by intercalation from an incomplete circle, no regeneration (distal transformation) will occur. These principles have been used to account for the number, location, handedness and orientation of supernumerary regenerates produced after grafting experiments in amphibians² and insects³. The results reported here directly support the idea that a complete circle of positional values in the circular sequence is necessary for limb regeneration in adult newts.