Forelimb regeneration from different levels of amputation in the newt,Notophthalmus viridescens: Length, rate, and stages.

1. Some aspects of the influence of position on regeneration have been examined by comparing regeneration from two different levels along the newt forelimb. 2. We have defined a series of stages of forelimb regeneration in the newt,Notophthalmus viridescens, in order to facilitate this study. 3. Limbs amputated at either a proximal level (through the humerus) or a distal level (through the radius and ulna) pass through the same stages at the same times after amputation. 4. The histological sequence of events of digit regeneration was compared with that of limb regeneration from a proximal level of amputation and was found to be the same. 5. In limbs amputated at either proximal or distal levels, the rate of elongation of regenerates is the same during the phases of dedifferentiation, blastema accumulation, and blastema growth. 6. During the phase of differentiation and morphogenesis, the rate of elongation of regenerates from the proximal level is significantly greater than that of regenerates from the distal level. 7. The total length of regenerates from proximal and distal levels along the limb is significantly different. 8. The results indicate that positional information does not influence the developmental sequence of events of limb regeneration, but that it does influence the rate of growth of the regenerate and the specification of the structures to be replaced.