III.
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131
arms results in the production of double radii and ulnae, but single
carpáis and digits (Fig. la). Sometimes this unification involves some
digits but not others. When upper arms are united in parallel and symmetrical orientation, the regenerated humeri remain separate, but
the ulnae and sometimes the radii become unified, as do the two hands
(Fig. lb, c). In all of these cases, however, fusion occurred between
identical regions of the grafted limb regenerates. When limbs are
asymmetrically united, coalescence of the resulting regenerates is not
so simple.
Heterologous grafts of limbs can be of two kinds: opposite extremities
can be grafted so that different levels are joined together in an otherwise
symmetrical relationship; or, limbs can be united at comparable levels
but with their dorsoventral and anterior-posterior axes disoriented.
Under these conditions it becomes difficult or impossible for similar
parts of the duplicate regenerates to become sufficiently close together
to coalesce. When the lower segment of one arm is grafted in ventral-toventral relationship to the opposite upper arm, subsequent amputation
results in a compound stump containing a humerus in one side and a
radius and ulna in the other. Fusion of the subsequent double regenerate
does not occur until equivalent parts of the two developing limbs
become juxtaposed. To achieve this, a developmental compromise
occurs such that the stump component cut through the humerus
regenerates a foreshortened limb, and the opposite side gives rise to an
elongate lower arm. Thus, in the region of the carpáis the two sides are
in register and similar regions combine to produce a unified hand.
In cases where the lower segments of opposite arms are grafted
together in ventral-to-dorsal orientation, the resulting compound limb
regenerate is confronted with a difficult problem if it is to bring about
the apposition of similar parts in preparation for their fusion. Instead,
it gives rise to separate but partially overlapping hands typically
containing seven digits arranged in a cruciform pattern. The second or
third digits in the middle of each hand become apposed and therefore
tend to coalesce. However, the remaining anterior digits of one hand lie
adjacent to the posterior digits of the other, and vice versa, with the
result that they all develop independently. Hence, fusion of parts in
regenerating double limbs tends to take place best between identical
structures situated in close proximity. When this occurs, both participating structures suffer the equal elimination of similar regions, thus
giving rise to a bilaterally symmetrical entity.
B. Regeneration of Half Limbs
A morphogenetic field is defined, in part, as a system which develops
normally despite an increase or a decrease in the material substrate
REGENERATION OF VERTEBRATE APPENDAGES
131
arms results in the production of double radii and ulnae, but single
carpáis and digits (Fig. la). Sometimes this unification involves some
digits but not others. When upper arms are united in parallel and symmetrical orientation, the regenerated humeri remain separate, but
the ulnae and sometimes the radii become unified, as do the two hands
(Fig. lb, c). In all of these cases, however, fusion occurred between
identical regions of the grafted limb regenerates. When limbs are
asymmetrically united, coalescence of the resulting regenerates is not
so simple.
Heterologous grafts of limbs can be of two kinds: opposite extremities
can be grafted so that different levels are joined together in an otherwise
symmetrical relationship; or, limbs can be united at comparable levels
but with their dorsoventral and anterior-posterior axes disoriented.
Under these conditions it becomes difficult or impossible for similar
parts of the duplicate regenerates to become sufficiently close together
to coalesce. When the lower segment of one arm is grafted in ventral-toventral relationship to the opposite upper arm, subsequent amputation
results in a compound stump containing a humerus in one side and a
radius and ulna in the other. Fusion of the subsequent double regenerate
does not occur until equivalent parts of the two developing limbs
become juxtaposed. To achieve this, a developmental compromise
occurs such that the stump component cut through the humerus
regenerates a foreshortened limb, and the opposite side gives rise to an
elongate lower arm. Thus, in the region of the carpáis the two sides are
in register and similar regions combine to produce a unified hand.
In cases where the lower segments of opposite arms are grafted
together in ventral-to-dorsal orientation, the resulting compound limb
regenerate is confronted with a difficult problem if it is to bring about
the apposition of similar parts in preparation for their fusion. Instead,
it gives rise to separate but partially overlapping hands typically
containing seven digits arranged in a cruciform pattern. The second or
third digits in the middle of each hand become apposed and therefore
tend to coalesce. However, the remaining anterior digits of one hand lie
adjacent to the posterior digits of the other, and vice versa, with the
result that they all develop independently. Hence, fusion of parts in
regenerating double limbs tends to take place best between identical
structures situated in close proximity. When this occurs, both participating structures suffer the equal elimination of similar regions, thus
giving rise to a bilaterally symmetrical entity.
B. Regeneration of Half Limbs
A morphogenetic field is defined, in part, as a system which develops
normally despite an increase or a decrease in the material substrate