III.
REGENERATION OF VERTEBRATE
APPENDAGES
143
The most compelling factor in determining what is to be regenerated
appears to be the nature of the stump at the level of amputation. Thus,
when an appendage is transected in the region of an abnormality,
regeneration tends to recapitulate the defective mode of development
responsible for the original malformation. This does not necessarily
reflect a genetic aberration; rather, it is more plausibly attributed to the
morphological defects in the stump. Amputation proximal to structural
aberrations present in extremities, however, is seldom followed by
repetition of the first anomaly. Under these conditions, there are formed
normal regenerates commensurate with the unaltered anatomy of the
stump at the level of amputation. This proves that abnormalities
resulting from environmental injuries, even those operating during
embryonic life, do not affect the genetic constitution of normal appearing
tissues. Indeed, the nature of the experimental interventions thus far
applied to such developing systems are not of the kind one would
expect to alter genetic expression (any more than acquired characteristics
are inherited). In this regard, it would be particularly interesting to
investigate the effects on regenerating appendages of agents which do
act upon genetic materials. Thus, treatment with chemical or physical
agents known to alter or interfere with nucleic acid synthesis, or infection with viruses, are among the factors deserving priority in future
investigations of morphogenesis and regeneration.
VIII. Polarity and Regeneration
Students of regeneration have focused particular attention on the
interesting phenomenon of regeneration from reversed stumps in
attempts to learn more about the factors which determine the polarity of
regenerates. In most vertebrate appendages (except tail fins of amphibians, for example), regeneration only occurs along the longitudinal
axis, i.e., at right angles to transverse amputations. If the amputation
plane is oblique, the regenerate may at first grow perpendicular to the
cut but eventually reorientates itself parallel to the original longitudinal
axis of the stump. If one side of a limb is removed, regeneration does not
occur laterally to complete the integrity of the extremity. Rather, if
regeneration under these circumstances does take place, it gives rise to
structures growing out to one side but nevertheless exhibiting proximodistal polarity (Weiss, 1923b, 1925b; Sciacchitano et al., 1930).
Regardless of the relative axial orientations of stumps and amputation
plane, regenerates tend to be organized along the original longitudinal
axis of the appendage. It is noteworthy, however, that a regenerate
always develops into a distal structure despite the orientation of the
stump from which it grows.
This conclusion follows from the results of numerous investigations
REGENERATION OF VERTEBRATE
APPENDAGES
143
The most compelling factor in determining what is to be regenerated
appears to be the nature of the stump at the level of amputation. Thus,
when an appendage is transected in the region of an abnormality,
regeneration tends to recapitulate the defective mode of development
responsible for the original malformation. This does not necessarily
reflect a genetic aberration; rather, it is more plausibly attributed to the
morphological defects in the stump. Amputation proximal to structural
aberrations present in extremities, however, is seldom followed by
repetition of the first anomaly. Under these conditions, there are formed
normal regenerates commensurate with the unaltered anatomy of the
stump at the level of amputation. This proves that abnormalities
resulting from environmental injuries, even those operating during
embryonic life, do not affect the genetic constitution of normal appearing
tissues. Indeed, the nature of the experimental interventions thus far
applied to such developing systems are not of the kind one would
expect to alter genetic expression (any more than acquired characteristics
are inherited). In this regard, it would be particularly interesting to
investigate the effects on regenerating appendages of agents which do
act upon genetic materials. Thus, treatment with chemical or physical
agents known to alter or interfere with nucleic acid synthesis, or infection with viruses, are among the factors deserving priority in future
investigations of morphogenesis and regeneration.
VIII. Polarity and Regeneration
Students of regeneration have focused particular attention on the
interesting phenomenon of regeneration from reversed stumps in
attempts to learn more about the factors which determine the polarity of
regenerates. In most vertebrate appendages (except tail fins of amphibians, for example), regeneration only occurs along the longitudinal
axis, i.e., at right angles to transverse amputations. If the amputation
plane is oblique, the regenerate may at first grow perpendicular to the
cut but eventually reorientates itself parallel to the original longitudinal
axis of the stump. If one side of a limb is removed, regeneration does not
occur laterally to complete the integrity of the extremity. Rather, if
regeneration under these circumstances does take place, it gives rise to
structures growing out to one side but nevertheless exhibiting proximodistal polarity (Weiss, 1923b, 1925b; Sciacchitano et al., 1930).
Regardless of the relative axial orientations of stumps and amputation
plane, regenerates tend to be organized along the original longitudinal
axis of the appendage. It is noteworthy, however, that a regenerate
always develops into a distal structure despite the orientation of the
stump from which it grows.
This conclusion follows from the results of numerous investigations