ABSTRACT
In order to investigate the replacement of skeletal elements in the Amphibia, bones were removed from the extremities of anuran tadpoles, and larval and adult urodeles. Following the extirpation of tibiae from the hind limbs of R. pipiens tadpoles, skeletal regeneration occurred in only a few of the smaller legs. Ulnae removed from the forelimbs of adult T. viridescens were not replaced even when the bone had been previously fractured or when it was replaced by blastema tissue from amputated arms. Cartilaginous skeletal regeneration did occur, however, after extirpation of ulnae from the forelimbs of larval A. maculatura. Likewise, ulnae removed from young forelimb regenerates of adult Triturus were regularly reconstituted, but as such regenerates grew older the percentage of ulna replacement progressively diminished.
In view of these results, it is concluded that in order for skeletal regeneration to occur the extirpated structure must be cartilaginous, and it must be removed from a limb otherwise capable of regeneration after amputation. This, together with the evidence that skeletal regeneration, like total limb regeneration, is dependent upon adequate innervation, substantiates the contention that these two phenomena are different manifestations of the same process. It is suggested that the absence of skeletal regeneration is dependent upon the developmental state of surrounding tissues, such that in larval or recently regenerated urodele limbs sufficient potential chondroblasts persist after skeletal extirpation to ensure replacement, while in adult limbs chondrogenic cells, being restricted to the periosteum, are nearly all removed with the bone.