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CHARLES S. THORNTON
differentiate into digital elements. The most striking experiments, however, consisted of dividing the paddle stage blastema into distal and
proximal halves and grafting these separately to the back. Surprisingly,
the proximal halves formed only distal structures in about the same
proportion of the cases as did the distal halves. Pietsch (1961a) also
transplanted young limb blastemata (to the tailfin) of
Ambystoma
larvae and found that distal structures were developed primarily. Jordan
(1965), in her blastemal grafts to the brain in Xenopus, found that predominantly distal limb structures were developed from young blastemata.
Similar results were also obtained by Skowron and Walknowska (1963)
in the axolotl.
Of particular interest in all these blastemal graft experiments was the
excellent morphogenesis obtained from young blastemata, isolated as
they were from the limb stumps. Faber (1965, for review) postulates an
"apical organization center" in the blastema to account for these results
and suggests that the final morphology of the limb regenerate is the
product of an interaction between a predominant, and earlier appearing,
"distal field" and a weaker "basal field" provided by the stump tissues.
In addition to understanding the nature of general organ morphogenesis, we must also deal with the problem of particular histogenesis in the
blastema. There is an increasing accumulation of evidence that points to
a stability of both histogenetic and morphogenetic capacities of the
mesenchymatous cells of regeneration. Roguski (1961) implanted the
minced limb tissues of larval Salamandra salamandra (which can regenerate) into limb stumps of adults (which have largely lost regenerative
ability), and obtained good limb regenerates. Mixtures of minced limb
and tail tissues implanted similarly into adult limb stumps regenerated
structures showing both limb and tail characteristics. Lagon (1961) performed similar experiments using X-irradiated axolotl limbs as host
structures and obtained similar results. Both of these investigators concluded that the dissociated cells carried with them into the regenerate
both histospecific and organospecific determinations. Pietsch shares this
view. When limb blastemata of Ambystoma
larvae were grafted in place
of the eye, skeletal muscle of orbital type regenerated around differentiating limb cartilages (Pietsch, 1961b). Limb implants grafted in place of
the eye were amputated in the antibrachial segments (Pietsch, 1962),
and, although cartilage differentiated limb skeletal patterns, muscle
developed orbital muscle patterns within the limb. Since the injured
orbital muscle stumps remained, these undoubtedly provided mesenchymatous cells to the limb blastema. Pietsch concludes that myogenesis
CHARLES S. THORNTON
differentiate into digital elements. The most striking experiments, however, consisted of dividing the paddle stage blastema into distal and
proximal halves and grafting these separately to the back. Surprisingly,
the proximal halves formed only distal structures in about the same
proportion of the cases as did the distal halves. Pietsch (1961a) also
transplanted young limb blastemata (to the tailfin) of
Ambystoma
larvae and found that distal structures were developed primarily. Jordan
(1965), in her blastemal grafts to the brain in Xenopus, found that predominantly distal limb structures were developed from young blastemata.
Similar results were also obtained by Skowron and Walknowska (1963)
in the axolotl.
Of particular interest in all these blastemal graft experiments was the
excellent morphogenesis obtained from young blastemata, isolated as
they were from the limb stumps. Faber (1965, for review) postulates an
"apical organization center" in the blastema to account for these results
and suggests that the final morphology of the limb regenerate is the
product of an interaction between a predominant, and earlier appearing,
"distal field" and a weaker "basal field" provided by the stump tissues.
In addition to understanding the nature of general organ morphogenesis, we must also deal with the problem of particular histogenesis in the
blastema. There is an increasing accumulation of evidence that points to
a stability of both histogenetic and morphogenetic capacities of the
mesenchymatous cells of regeneration. Roguski (1961) implanted the
minced limb tissues of larval Salamandra salamandra (which can regenerate) into limb stumps of adults (which have largely lost regenerative
ability), and obtained good limb regenerates. Mixtures of minced limb
and tail tissues implanted similarly into adult limb stumps regenerated
structures showing both limb and tail characteristics. Lagon (1961) performed similar experiments using X-irradiated axolotl limbs as host
structures and obtained similar results. Both of these investigators concluded that the dissociated cells carried with them into the regenerate
both histospecific and organospecific determinations. Pietsch shares this
view. When limb blastemata of Ambystoma
larvae were grafted in place
of the eye, skeletal muscle of orbital type regenerated around differentiating limb cartilages (Pietsch, 1961b). Limb implants grafted in place of
the eye were amputated in the antibrachial segments (Pietsch, 1962),
and, although cartilage differentiated limb skeletal patterns, muscle
developed orbital muscle patterns within the limb. Since the injured
orbital muscle stumps remained, these undoubtedly provided mesenchymatous cells to the limb blastema. Pietsch concludes that myogenesis