III.
REGENERATION OF VERTEBRATE
APPENDAGES
105
analyses of morphogenesis. By observing changes in the outcome of the
regenerate ensuing inflicted alterations in the stump, something of the
nature of the morphogenetic influences resident in the appendage can
be revealed.
II. Phylogenetic Aspects of Regeneration
Though the majority of morphogenetic studies on regeneration have
dealt with the amphibian limb, it is important to remember that few
valid generalizations can be made which do not apply as well to other
regenerating systems. In this respect, the comparative approach to
problems of morphogenesis and regeneration is certain to be most
rewarding. Among the many different kinds of appendages in the lower
vertebrates that are capable of regeneration, a variety of tissue combinations is represented. Indeed, tissues present in one kind of extremity
may be entirely lacking in another. Such observations lead to the
possibility that tissues not universally present in all structures capable
of regeneration probably play roles of relatively minor importance in
the regeneration process. Conversely, the comparative inspection of
many different regenerating structures among the vertebrates can reveal
certain features shared by all such systems. These common attributes
promise to include factors that are generally of greater importance in
the process of regeneration.
A further reason for not neglecting regenerative structures in nonamphibian vertebrates is that it is thereby possible to perform comparable experiments on relatively different types of appendages, thus
either certifying the validity of, or casting doubt upon, the results
obtained in one form. For example, extirpation of bones from urodele
limbs does not interfere with regeneration following amputation through
the boneless regions. In fact, such regenerates are replete with normal
skeletal parts, despite the absence of bones in the adjacent stump. Yet
it would be incorrect to conclude from this that bones in general are
unnecessary for the induction of new skeletal regenerates, for removal
of the bony fin rays from the fins of teleost fishes, followed by amputation through the rayless portion of the fin, results in the production of a
new fin regenerate without bony rays. In this instance, therefore, the
older ray stumps are necessary to the development of new rays in the
regenerate. In still other structures, namely, the taste barbels of the
catfish, the regeneration of the entire barbel fails to occur after amputation unless the skeletal component, in this case a central rod of cartilage,
is present. The taste barbel, therefore, depends upon its skeleton not
only for certain morphogenetic influences (as in the case of the teleost
fin), but also for the very cells which are to give rise to the regenerate.
REGENERATION OF VERTEBRATE
APPENDAGES
105
analyses of morphogenesis. By observing changes in the outcome of the
regenerate ensuing inflicted alterations in the stump, something of the
nature of the morphogenetic influences resident in the appendage can
be revealed.
II. Phylogenetic Aspects of Regeneration
Though the majority of morphogenetic studies on regeneration have
dealt with the amphibian limb, it is important to remember that few
valid generalizations can be made which do not apply as well to other
regenerating systems. In this respect, the comparative approach to
problems of morphogenesis and regeneration is certain to be most
rewarding. Among the many different kinds of appendages in the lower
vertebrates that are capable of regeneration, a variety of tissue combinations is represented. Indeed, tissues present in one kind of extremity
may be entirely lacking in another. Such observations lead to the
possibility that tissues not universally present in all structures capable
of regeneration probably play roles of relatively minor importance in
the regeneration process. Conversely, the comparative inspection of
many different regenerating structures among the vertebrates can reveal
certain features shared by all such systems. These common attributes
promise to include factors that are generally of greater importance in
the process of regeneration.
A further reason for not neglecting regenerative structures in nonamphibian vertebrates is that it is thereby possible to perform comparable experiments on relatively different types of appendages, thus
either certifying the validity of, or casting doubt upon, the results
obtained in one form. For example, extirpation of bones from urodele
limbs does not interfere with regeneration following amputation through
the boneless regions. In fact, such regenerates are replete with normal
skeletal parts, despite the absence of bones in the adjacent stump. Yet
it would be incorrect to conclude from this that bones in general are
unnecessary for the induction of new skeletal regenerates, for removal
of the bony fin rays from the fins of teleost fishes, followed by amputation through the rayless portion of the fin, results in the production of a
new fin regenerate without bony rays. In this instance, therefore, the
older ray stumps are necessary to the development of new rays in the
regenerate. In still other structures, namely, the taste barbels of the
catfish, the regeneration of the entire barbel fails to occur after amputation unless the skeletal component, in this case a central rod of cartilage,
is present. The taste barbel, therefore, depends upon its skeleton not
only for certain morphogenetic influences (as in the case of the teleost
fin), but also for the very cells which are to give rise to the regenerate.