Rule of Distal Transformation of Blastema
An intriguing phenomenon characteristic of limb regeneration is, that just only the part of the limb removed distal to the level of amputation is regenerated. For instance, suppose a fore limb is cut by the middle of upper arm, removing the distal half of the upper arm, wrist and hand leaving behind a stump consisting of just only the proximal half of the upper arm. In this case the blastema made at the cut end of the stump generates a regenerate consisting of the distal part of upper arm, lower arm, wrist and hand in this type of order; the proximal part of the upper arm is not duplicated in the regenerate. So a complete fore limb is regenerated with no any duplication. The blastema all the time forms structures or parts distal to its own level of origin, along the proximo-distal axis of the limb. In addition, regardless of the level of amputation the blastema formed all the time regenerates only the distal structures, even if the polarity of stump is reversed as illustrated in an elegant experiment on axolotl larva. This is termed as the Rule of Distal Transformation of Blastema and applies equally to limb regeneration in urodeles and anurans amphibians also to leg regeneration in insects.