Experiments on developing limb buds of the axolotl Ambystoma mexicanum.

Various experiments were performed on the limb buds of axolotls to compare the behaviour of amphibian limbs with that previously reported for chick limbs. Following removal of the tip or whole limb bud, extensive powers of regulation were observed since complete limbs always formed. Similarly after distal to proximal grafts intercalary regulation occurred to produce perfect limbs and after proximal to distal grafts serial repetitions resulted. Transplantation and rotation of limb buds to reverse either the dorso-ventral, antero-posterior or both axes resulted in the induction of supernumerary limbs in a large proportion of cases. Such regulatory behaviour of axolotl limb buds is in contrast to the mosaic nature of chick limbs and as a result, theories such as the progress-zone theory which have been formulated on the basis of data from chick limbs are not relevant to general principles of vertebrate limb development. Possible reasons for the diverse behaviour between the two systems are discussed.