AMPHIBIAN LIMB REGENERATION
243
may seem to be. Inverted limbs (Butler, 1951, 1955) regenerate distal
structures, not proximal ones. As Faber (1965) has pointed out, even
isolated proximal halves of blastemata form distal parts. It is as though
the blastema first developed distalmost structures and then more proximal ones until the presence of proximal structures in the stump prevented
duplication of similar proximal parts in the regenerate. Perhaps ideas
expressed by Rose (1964, for review) may be of interest here. He has
shown that in hydroids, distal structures inhibit regeneration of similar
structures when they are in direct axial alignment. In the absence of
distal parts, stem sections of Tubularia will regenerate the distalmost
organs. Similar experimental evidence of distal inhibition has been
lacking for limb regeneration in Amphibia. Recently, however, Purdy
(1967) has found that supernumerary limbs can be evoked in the axolotl
as a result of the mechanical separation of the shaft of the humeral or
femoral cartilage from the epiphysis. Accessory limb formation will only
occur when the parts of the limb distal to the cartilage separation swing
back out of line with the skeletal break so that a direct-line axial continuity of distal and proximal limb parts is prevented. When cartilage
refusion occurs at the skeletal break so that distal and proximal limb
segments become aligned again, supernumerary limbs never appear.
Purdy interprets these results to mean that distal limb structures, in
appropriate axial alignment, will inhibit the regenerationlike activity
which initiates the formation of supernumerary limbs. Rose (1962) cites
other examples of vertebrate limb regeneration and supernumerary limb
evocation which are amenable to similar explanations. It is tempting to
incorporate the amphibian limb regeneration system into a general
theory which includes invertebrate regeneration systems as well. The
concept of a "distal organization center," as postulated by Faber, may
be a first step in this direction. Faber (1965) regards the center as composed of blastemal cells which from the moment of their origin possess
intrinsic distal differentiation tendencies. He further believes that the
apical epidermal cap is somehow important in facilitating the appearance
of the center of proliferation and organization (see also Hearson, 1966)
in the blastemal cells beneath it. As in hydroid regeneration, a hierarchy
of determinative steps are postulated proceeding proximalward until the
regional determination of the blastema corresponds with that of the
amputation level of the stump. Rose (1962a, 1964) adds to this concept
the postulate that the nerves of an amphibian limb convey, perhaps by
setting up a potential difference along the limb stump, information from
the stump to the developing regenerate. Since the distal tip of the newt
243
may seem to be. Inverted limbs (Butler, 1951, 1955) regenerate distal
structures, not proximal ones. As Faber (1965) has pointed out, even
isolated proximal halves of blastemata form distal parts. It is as though
the blastema first developed distalmost structures and then more proximal ones until the presence of proximal structures in the stump prevented
duplication of similar proximal parts in the regenerate. Perhaps ideas
expressed by Rose (1964, for review) may be of interest here. He has
shown that in hydroids, distal structures inhibit regeneration of similar
structures when they are in direct axial alignment. In the absence of
distal parts, stem sections of Tubularia will regenerate the distalmost
organs. Similar experimental evidence of distal inhibition has been
lacking for limb regeneration in Amphibia. Recently, however, Purdy
(1967) has found that supernumerary limbs can be evoked in the axolotl
as a result of the mechanical separation of the shaft of the humeral or
femoral cartilage from the epiphysis. Accessory limb formation will only
occur when the parts of the limb distal to the cartilage separation swing
back out of line with the skeletal break so that a direct-line axial continuity of distal and proximal limb parts is prevented. When cartilage
refusion occurs at the skeletal break so that distal and proximal limb
segments become aligned again, supernumerary limbs never appear.
Purdy interprets these results to mean that distal limb structures, in
appropriate axial alignment, will inhibit the regenerationlike activity
which initiates the formation of supernumerary limbs. Rose (1962) cites
other examples of vertebrate limb regeneration and supernumerary limb
evocation which are amenable to similar explanations. It is tempting to
incorporate the amphibian limb regeneration system into a general
theory which includes invertebrate regeneration systems as well. The
concept of a "distal organization center," as postulated by Faber, may
be a first step in this direction. Faber (1965) regards the center as composed of blastemal cells which from the moment of their origin possess
intrinsic distal differentiation tendencies. He further believes that the
apical epidermal cap is somehow important in facilitating the appearance
of the center of proliferation and organization (see also Hearson, 1966)
in the blastemal cells beneath it. As in hydroid regeneration, a hierarchy
of determinative steps are postulated proceeding proximalward until the
regional determination of the blastema corresponds with that of the
amputation level of the stump. Rose (1962a, 1964) adds to this concept
the postulate that the nerves of an amphibian limb convey, perhaps by
setting up a potential difference along the limb stump, information from
the stump to the developing regenerate. Since the distal tip of the newt