The regulative ability of the limb regeneration blastema ofNotophlhalmus viridescens: Experimentsin situ.

1. The regulative ability of the regeneration blastema of the newt limb (Notophthalmus viridescens) was tested by operationsin situ. Either the anterior, posterior, dorsal, or ventral half of the blastema was removed at various stages during regeneration. 2. All blastemas operated on prior to the stage of four early digits showed a delay in reaching the subsequent stages of regeneration. 3. The blastema is capable of extensive regulation in the anterior-posterior and dorsoventral axes even after many of its cells have begun to differentiate. 4. Early digital stages of regeneration were found to be defective in regulative ability. Additional skeletal elements were present in limbs which had been operated on at the stage of three early digits. Supernumerary digits as well as additional skeletal elements were present in limbs which had been operated on at the stage of four early digits. Removal of the posterior half of the regenerate at one of these late stages resulted in more severe abnormalities than did removal of the anterior half. 5. Either the anterior or the posterior half of a mature limb was removed back to the level of the wrist. In several cases, an almost complete autopodium developed alongside the remaining half autopodium. 6. Removal of half of a regenerate at digital stages gave results similar to those obtained following removal of half of a mature limb. 7. The results are discussed in the context of other experiments on regenerating limbs, and of experiments on other developing systems. It is concluded that amphibian blastemas in common with a number of other systems can develop according to the presumptive fates of their cells, or they can regulate when they are given the opportunity for growth and cell division.