326
EDGAR
ZWILLING
as the ridge was not damaged even the pattern of Polydactyly was
unaltered. Amprino and Camosso attempted to quantitate regulation
in relation to amount of tissue removed and the stage of development when the deletion was made. Their data reveal a definite decline in
regulability in older limb buds. In my opinion part of the over-all
quantitation of Amprino and Camosso is weak because of improper
consideration of the role of the ectodermal ridge. Since removal of the
ridge alone may result in limb reductions which are almost as drastic as
deletion of the ridge and underlying mesoderm, then a comparison of a
medial deletion, in which the ridge is untouched, with a distal deletion,
in which more or less of the ridge is removed, is not, in my opinion, valid.
There is still the question of the possibility of regulation after excision
of distal mesoderm and ectodermal ridge. Amprino and Camosso's
contention that some regulation may occur after this type of operation
is supported by Rudnick's (1946) experiment. In this experiment the
excised piece was grown as a graft while the growth of the donor limb
was followed; this revealed that some regulation occurred since, in some
cases, the total number of digits in graft and donor exceeded the
number normally present in one limb. It is possible that when some of
the ridge is left (as was mentioned above this was quite likely in Amprino
and Camosso's lc operation (1955a)) there is some outgrowth which
may be accompanied by regulation. Certainly Rudnick's experimental
results indicate that such regulation is possible.
Our discussion of the inductive interaction between the ectodermal
ridge and the mesoderm has included a number of examples of regulative
development which depend on the establishment or re-establishment of
a distal outgrowth system. The formation of proximal structures, in
addition to distal elements in my experiments with mesoderm fragments
and reaggregated suspensions placed in contact with limb-bud ectoderm
(vide supra) is unexpected. A priori one might expect that fragmented
or previously disassociated mesoderm from stage 19-20 buds would
form only metatarsals and digits (the presumptive areas not established
at these stages) and not any of the more proximal structures. The fact
that proximal elements may form (Zwilling, unpublished work) in these
experiments indicates some regulatory influence of distal territories over
more proximal tissues. Many other examples which involve regulative
development have been cited above and will not be repeated here. I will
merely summarize by reiterating that the limb system is highly plastic
and regulable; that a good deal of the regulation depends on retention
or re-establishment of the distal growing system and that some types of
partitioning of the distal ridge-mesoderm complex (or ridge alone) in a
limb bud which does not regenerate a ridge will result in a seeming
mosaicism.
EDGAR
ZWILLING
as the ridge was not damaged even the pattern of Polydactyly was
unaltered. Amprino and Camosso attempted to quantitate regulation
in relation to amount of tissue removed and the stage of development when the deletion was made. Their data reveal a definite decline in
regulability in older limb buds. In my opinion part of the over-all
quantitation of Amprino and Camosso is weak because of improper
consideration of the role of the ectodermal ridge. Since removal of the
ridge alone may result in limb reductions which are almost as drastic as
deletion of the ridge and underlying mesoderm, then a comparison of a
medial deletion, in which the ridge is untouched, with a distal deletion,
in which more or less of the ridge is removed, is not, in my opinion, valid.
There is still the question of the possibility of regulation after excision
of distal mesoderm and ectodermal ridge. Amprino and Camosso's
contention that some regulation may occur after this type of operation
is supported by Rudnick's (1946) experiment. In this experiment the
excised piece was grown as a graft while the growth of the donor limb
was followed; this revealed that some regulation occurred since, in some
cases, the total number of digits in graft and donor exceeded the
number normally present in one limb. It is possible that when some of
the ridge is left (as was mentioned above this was quite likely in Amprino
and Camosso's lc operation (1955a)) there is some outgrowth which
may be accompanied by regulation. Certainly Rudnick's experimental
results indicate that such regulation is possible.
Our discussion of the inductive interaction between the ectodermal
ridge and the mesoderm has included a number of examples of regulative
development which depend on the establishment or re-establishment of
a distal outgrowth system. The formation of proximal structures, in
addition to distal elements in my experiments with mesoderm fragments
and reaggregated suspensions placed in contact with limb-bud ectoderm
(vide supra) is unexpected. A priori one might expect that fragmented
or previously disassociated mesoderm from stage 19-20 buds would
form only metatarsals and digits (the presumptive areas not established
at these stages) and not any of the more proximal structures. The fact
that proximal elements may form (Zwilling, unpublished work) in these
experiments indicates some regulatory influence of distal territories over
more proximal tissues. Many other examples which involve regulative
development have been cited above and will not be repeated here. I will
merely summarize by reiterating that the limb system is highly plastic
and regulable; that a good deal of the regulation depends on retention
or re-establishment of the distal growing system and that some types of
partitioning of the distal ridge-mesoderm complex (or ridge alone) in a
limb bud which does not regenerate a ridge will result in a seeming
mosaicism.