III.
REGENERATION OF VERTEBRATE
APPENDAGES
139
antlers. Therefore, there exist a number of interesting correlations
between the dimensions and configurations of antlers, and age of animal,
size of pedicle, date of growth initiation and duration of development.
These and other factors (e.g. hormones, diet) unquestionably play significant roles in directing the morphogenesis of developing antlers.
A. Regeneration of Abnormal Extremities
It has been noted above how abnormal regenerates can be induced to
develop from originally normal stumps by means of various kinds of
experimental interventions. Indeed, limbs rendered experimentally
abnormal can occasionally give rise to normal regenerates by virtue of
their regulatory capacities. Is it possible, however, for originally
abnormal appendages to regenerate normal parts, or must they always
develop true to form? To answer this question it is first necessary to
obtain appendages which are atypical as a result of abnormalities
originating during their ontogenetic development. Inasmuch as such
malformations are generally restricted to some regions of an appendage
but not others, the type of structure regenerated might be expected to
vary according to whether amputation was performed through a more
proximal normal area or through the abnormality itself. In the latter
case, the morphogenesis of the regenerate might well be adversely
affected by the abnormal anatomical relationships of the stump tissues,
depending on the degree and nature of the defect. If, however, the
abnormal appendage is amputated through a more basal normal region,
the regenerate would be expected to be normal unless the basic directing
factors responsible for morphogenesis had been altered. Thus, if the
original malformation had occurred as a result of genetic mutations,
regeneration from any level would be expected to recapitulate the
original developmental patterns. Therefore, the character of a regenerate
produced by an atypical limb amputated proximal to the abnormality,
can be used as a criterion for assessing whether the cause of the original
malformation was the result of primary defects in the sources of morphogenetic instructions or secondary aberrations in the mechanisms by
which these prospects are fulfilled. If the first abnormality is repeated
in the regenerate, despite the existence of a normal stump, an imperfection in the morphogenetic blueprint is indicated. Correction by the
regenerate of the original mistake in the appendage signifies that
extraneous factors interfered with the proper execution of morphogenetic information during ontogeny. This type of analysis constitutes
an effective way of judging the effects of various kinds of experimental
interventions on morphogenetic mechanisms, thereby revealing something of their nature.
The kind of structure which regenerates from an abnormal extremity
REGENERATION OF VERTEBRATE
APPENDAGES
139
antlers. Therefore, there exist a number of interesting correlations
between the dimensions and configurations of antlers, and age of animal,
size of pedicle, date of growth initiation and duration of development.
These and other factors (e.g. hormones, diet) unquestionably play significant roles in directing the morphogenesis of developing antlers.
A. Regeneration of Abnormal Extremities
It has been noted above how abnormal regenerates can be induced to
develop from originally normal stumps by means of various kinds of
experimental interventions. Indeed, limbs rendered experimentally
abnormal can occasionally give rise to normal regenerates by virtue of
their regulatory capacities. Is it possible, however, for originally
abnormal appendages to regenerate normal parts, or must they always
develop true to form? To answer this question it is first necessary to
obtain appendages which are atypical as a result of abnormalities
originating during their ontogenetic development. Inasmuch as such
malformations are generally restricted to some regions of an appendage
but not others, the type of structure regenerated might be expected to
vary according to whether amputation was performed through a more
proximal normal area or through the abnormality itself. In the latter
case, the morphogenesis of the regenerate might well be adversely
affected by the abnormal anatomical relationships of the stump tissues,
depending on the degree and nature of the defect. If, however, the
abnormal appendage is amputated through a more basal normal region,
the regenerate would be expected to be normal unless the basic directing
factors responsible for morphogenesis had been altered. Thus, if the
original malformation had occurred as a result of genetic mutations,
regeneration from any level would be expected to recapitulate the
original developmental patterns. Therefore, the character of a regenerate
produced by an atypical limb amputated proximal to the abnormality,
can be used as a criterion for assessing whether the cause of the original
malformation was the result of primary defects in the sources of morphogenetic instructions or secondary aberrations in the mechanisms by
which these prospects are fulfilled. If the first abnormality is repeated
in the regenerate, despite the existence of a normal stump, an imperfection in the morphogenetic blueprint is indicated. Correction by the
regenerate of the original mistake in the appendage signifies that
extraneous factors interfered with the proper execution of morphogenetic information during ontogeny. This type of analysis constitutes
an effective way of judging the effects of various kinds of experimental
interventions on morphogenetic mechanisms, thereby revealing something of their nature.
The kind of structure which regenerates from an abnormal extremity