III. REGENERATION OF VERTEBRATE APPENDAGES
137
above, the kind of information conveyed to the blastema can be changed
by experimental interventions in the stump, thus bringing about
corresponding alterations in the regenerate. Of particular interest to the
student of regeneration and morphogenesis, however, are those instances
where the blastema normally and spontaneously develops into something
different from the original lost part. Such occurrences constitute hypomorphic regeneration.
Some instances of this kind are, in a sense, errors of omission. That is,
a structure may develop quite normally except that one or more parts
present in the stump are either missing or deficient in the regenerate.
Incomplete structures of this sort probably reflect the inability of some
histological components of their stumps to regenerate. This kind of
event is exemplified by the regenerating lower jaws of urodele amphibians, in which mandibular tissues reconstitute themselves normally,
but the branchial skeleton, hyoid apparatus and tongue fail to develop
(Goss and Stagg, 1958a, b). The legs of postmetamorphic Xenopus laevis
normally regenerate malformed structures consisting of tapered, sometimes branched, cartilaginous outgrowths which, at best, are poor
copies of the original appendages (Beetschen, 1952; Skowron and
Komala, 1957). The notochord of the larval urodele tail cannot replace
itself as the tail regenerates. In the same case, however, is encountered
an example of hypomorphic regeneration more in the nature of an error of
commission. The young larval urodele tail is capable of regenerating
segmented cartilaginous vertebrae despite the total absence of corresponding elements in the stump (Holtzer et al., 1955). This occurrence is
less remarkable, however, in view of the eventual spontaneous development of cartilaginous vertebrae in the normal urodele tail. Be novo
chondrification can also occur in regenerating limbs previously deprived
of their skeletal parts. Nevertheless, it is noteworthy that cartilage can
regenerate in the absence of homologous histological precursors.
The regenerating tail of the lizard is an almost classic example of the
replacement of a lost part by an outgrowth of different structure. The
principal departure from the original in this case involves the skeleton.
Instead of the development of segmented vertebrae, as are present in
the tail stump, there is regenerated a tapering tube of cartilage within
which is the ependymal component of the spinal cord. Although the
cartilaginous tubular skeleton of the regenerate is unsegmented, the
surrounding muscular tissue is arranged metamerically. Since new
ganglia are not reconstituted in lizard tail regenerates, and inasmuch as
the spinal cord regenerate is isolated within the enveloping chondrified
sheath, the muscles can only receive their innervation from nerve fibres
which grow directly from the stump tissues. The skin of such tail regenerates differs from the stump skin in the details of its scalation and
137
above, the kind of information conveyed to the blastema can be changed
by experimental interventions in the stump, thus bringing about
corresponding alterations in the regenerate. Of particular interest to the
student of regeneration and morphogenesis, however, are those instances
where the blastema normally and spontaneously develops into something
different from the original lost part. Such occurrences constitute hypomorphic regeneration.
Some instances of this kind are, in a sense, errors of omission. That is,
a structure may develop quite normally except that one or more parts
present in the stump are either missing or deficient in the regenerate.
Incomplete structures of this sort probably reflect the inability of some
histological components of their stumps to regenerate. This kind of
event is exemplified by the regenerating lower jaws of urodele amphibians, in which mandibular tissues reconstitute themselves normally,
but the branchial skeleton, hyoid apparatus and tongue fail to develop
(Goss and Stagg, 1958a, b). The legs of postmetamorphic Xenopus laevis
normally regenerate malformed structures consisting of tapered, sometimes branched, cartilaginous outgrowths which, at best, are poor
copies of the original appendages (Beetschen, 1952; Skowron and
Komala, 1957). The notochord of the larval urodele tail cannot replace
itself as the tail regenerates. In the same case, however, is encountered
an example of hypomorphic regeneration more in the nature of an error of
commission. The young larval urodele tail is capable of regenerating
segmented cartilaginous vertebrae despite the total absence of corresponding elements in the stump (Holtzer et al., 1955). This occurrence is
less remarkable, however, in view of the eventual spontaneous development of cartilaginous vertebrae in the normal urodele tail. Be novo
chondrification can also occur in regenerating limbs previously deprived
of their skeletal parts. Nevertheless, it is noteworthy that cartilage can
regenerate in the absence of homologous histological precursors.
The regenerating tail of the lizard is an almost classic example of the
replacement of a lost part by an outgrowth of different structure. The
principal departure from the original in this case involves the skeleton.
Instead of the development of segmented vertebrae, as are present in
the tail stump, there is regenerated a tapering tube of cartilage within
which is the ependymal component of the spinal cord. Although the
cartilaginous tubular skeleton of the regenerate is unsegmented, the
surrounding muscular tissue is arranged metamerically. Since new
ganglia are not reconstituted in lizard tail regenerates, and inasmuch as
the spinal cord regenerate is isolated within the enveloping chondrified
sheath, the muscles can only receive their innervation from nerve fibres
which grow directly from the stump tissues. The skin of such tail regenerates differs from the stump skin in the details of its scalation and