III. REGENERATION OF VERTEBRATE
APPENDAGES
147
powers of regenerating limbs have stimulated the postulation of the
limb field hypothesis which, according to Weiss (1939), accounts for the
production of normal limbs despite increases or decreases in the material
substrates from which the regenerates arise. Savchuk (1938), however,
disputed the field concept on the grounds that what regenerates is
determined strictly by the tissues of origin because tissues grafted into
a limb remain unaffected by the field of their new environment. Probably
the truth of the matter is most closely approximated by a view intermediate between these two extremes.
Notwithstanding the demonstrated capacity of regenerating limbs to
compensate for certain types of defects (e.g., missing skeletal parts),
their inability to regulate for more serious kinds of structural alterations
gainsays the unrestricted application of morphogenetic field theory to
regenerating extremities. Thus, half limbs (except when enveloped in
their full complements of skin) form incomplete regenerates, and double
limbs produce outgrowths which, despite secondary unification, are
actually double structures. These and other experimental results invalidate the interpretation of morphogenesis in regenerating limbs in terms
of a single 'field', at least in the broader sense commonly employed in the
physical sciences.
In view of current experimental evidence the field concept is most
reasonably applied to regenerating extremities in a modified, more
conservative form. A morphogenetic field is a territory of developing
tissue which falls within the organizational influences of another region.
The latter, therefore, is capable of governing the differentiation and
morphogenesis of itself as well as of tissues outside its immediate
province. This is well illustrated in the regenerating limb where the
organizational pattern of the blastema is attributable to that of the
subjacent stump. Ordinarily, each histological entity of the stump
induces the differentiation of specific homologous tissues in the contiguous area of the blastema. This is clearly evident from the inductive
effects on a blastema of extra tissues implanted in a limb stump. Yet
experiments have also proved that histodifferentiation of the blastema
can occur in the absence of the corresponding tissue in the stump. This
kind of effect can be ascribed only to other tissues of a different type in
the stump, and constitutes a patent example of the operation of the
morphogenetic field. The details and precise limitations of the regulatory
capacity of regenerating limbs are not fully understood, but experimental
evidence indicates that while residual tissues can compensate for the
absence of various individual parts (e.g., bones, muscles), the removal
of large fractions of limbs (e.g., half limbs) cannot be countervailed by
the remaining portions. Hence, the only logical interpretation of the
morphogenetic properties of a limb is one represented by a series of
F
A.M. I
APPENDAGES
147
powers of regenerating limbs have stimulated the postulation of the
limb field hypothesis which, according to Weiss (1939), accounts for the
production of normal limbs despite increases or decreases in the material
substrates from which the regenerates arise. Savchuk (1938), however,
disputed the field concept on the grounds that what regenerates is
determined strictly by the tissues of origin because tissues grafted into
a limb remain unaffected by the field of their new environment. Probably
the truth of the matter is most closely approximated by a view intermediate between these two extremes.
Notwithstanding the demonstrated capacity of regenerating limbs to
compensate for certain types of defects (e.g., missing skeletal parts),
their inability to regulate for more serious kinds of structural alterations
gainsays the unrestricted application of morphogenetic field theory to
regenerating extremities. Thus, half limbs (except when enveloped in
their full complements of skin) form incomplete regenerates, and double
limbs produce outgrowths which, despite secondary unification, are
actually double structures. These and other experimental results invalidate the interpretation of morphogenesis in regenerating limbs in terms
of a single 'field', at least in the broader sense commonly employed in the
physical sciences.
In view of current experimental evidence the field concept is most
reasonably applied to regenerating extremities in a modified, more
conservative form. A morphogenetic field is a territory of developing
tissue which falls within the organizational influences of another region.
The latter, therefore, is capable of governing the differentiation and
morphogenesis of itself as well as of tissues outside its immediate
province. This is well illustrated in the regenerating limb where the
organizational pattern of the blastema is attributable to that of the
subjacent stump. Ordinarily, each histological entity of the stump
induces the differentiation of specific homologous tissues in the contiguous area of the blastema. This is clearly evident from the inductive
effects on a blastema of extra tissues implanted in a limb stump. Yet
experiments have also proved that histodifferentiation of the blastema
can occur in the absence of the corresponding tissue in the stump. This
kind of effect can be ascribed only to other tissues of a different type in
the stump, and constitutes a patent example of the operation of the
morphogenetic field. The details and precise limitations of the regulatory
capacity of regenerating limbs are not fully understood, but experimental
evidence indicates that while residual tissues can compensate for the
absence of various individual parts (e.g., bones, muscles), the removal
of large fractions of limbs (e.g., half limbs) cannot be countervailed by
the remaining portions. Hence, the only logical interpretation of the
morphogenetic properties of a limb is one represented by a series of
F
A.M. I