Intercalary regeneration in the amphibian limb and the rule of distal transformation.

The applicability to the amphibian limb of the rule of distal transformation, which states that tissue from any level can only become more distal, has been tested during intercalary regeneration following various types of shift-level transplantation. Following the grafting of distal blastemas to proximal levels, such that part of the presumptive pattern is missing, complete limbs nevertheless formed (Series I). That the intercalated tissue arose entirely from the stump was shown by exchanging blastemas between black and white animals. When the proximal stump was irradiated and its contribution eliminated, intercalary deletions were produced (Series II). This was not due to the inability of irradiated and normal tissue to communicate since irradiated distal blastemas grafted onto proximal stumps still stimulated intercalary regeneration (Series III). When proximal blastemas were grafted to distal levels intercalary regenerates were obtained in about 20% of the cases (Series IV) and under these circumstances the grafted blastema was the sole source of intercalated tissue. The precise structure of these intercalated elements was impossible to ascertain, but it is suggested that they might be of reversed polarity as found in insects. These results are dicussed in relation to similar experiments on the insect limb.