318
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ZWILLING
distal structures. Fully 'determined' proximal tissues are not so effective
in influencing the distal regions as are those of an intermediate, less
stabilized, zone. The exact location of these regions varies with limb buds
of different ages.
Saunders et al., however, interpret the same sort of result to be
confirmatory evidence for the existence of an apical ectoderm maintenance factor and additional evidence for its asymmetric distribution.
According to them these results are best explained by the assumption
that the original anterior end of the inverted distal tip is deficient in
maintenance factor and normally would not support a thickened ridge
(it would form the preaxial surface of the forearm and hand and
possibily digit II of the wing). When, as a consequence of the inversion,
it is in contact with the region of high maintenance factor concentration
of the posterior part of the proximal stump the maintenance factor
can influence the persistence of the originally anterior ridge. Since the
concentration is maintained in the original posterior region of the tip
this means that there are two concentrations of maintenance factor and
two regions of persistence of the thickened ridge with, consequently,
two major outgrowths which give rise to duplicate hands.
Both groups agree that the basal regions of older limb buds are
incapable of causing duplications in younger distal tips and that distal
tips from older buds become more restricted in their ability to respond
to younger proximal regions. The extensive series of operations of
Amprino and Camosso (1959) can, actually, be used to bolster the
'maintenance factor' interpretation of limb duplication, since the sites
and combinations which produce duplications in grafted distal tips fit
readily into Saunders' scheme. There are few discrepancies in results in
these experiments, only differences in interpretation. Amprino rejects
the possibility of the existence of a maintenance factor, first because of
his rejection of the importance of the ectodermal ridge and secondly
because he cannot visualize how a supposedly diffusable (I prefer
'transmissable') factor can maintain the asymmetrical concentrations
required for the interaction hypothesis. He recognizes that some sort of
polarized transmission may be involved (as it must be for any sort of
'influences' from proximal tissues, whether acting directly or indirectly
via the ridge). As long as we are in the realm of hypothesis this should be
no difficulty, since there are many situations in which there is a graded
distribution of morphogenetic potencies with relation to a centre, presumably of production. We are still at too primitive a stage to go into
the geometry of such a situation at the present.
In my opinion these seemingly opposed ideas are not entirely mutually contradictory. There are, evidently, at least two major (related)
processes involved in limb development. One is concerned with distal
EDGAR
ZWILLING
distal structures. Fully 'determined' proximal tissues are not so effective
in influencing the distal regions as are those of an intermediate, less
stabilized, zone. The exact location of these regions varies with limb buds
of different ages.
Saunders et al., however, interpret the same sort of result to be
confirmatory evidence for the existence of an apical ectoderm maintenance factor and additional evidence for its asymmetric distribution.
According to them these results are best explained by the assumption
that the original anterior end of the inverted distal tip is deficient in
maintenance factor and normally would not support a thickened ridge
(it would form the preaxial surface of the forearm and hand and
possibily digit II of the wing). When, as a consequence of the inversion,
it is in contact with the region of high maintenance factor concentration
of the posterior part of the proximal stump the maintenance factor
can influence the persistence of the originally anterior ridge. Since the
concentration is maintained in the original posterior region of the tip
this means that there are two concentrations of maintenance factor and
two regions of persistence of the thickened ridge with, consequently,
two major outgrowths which give rise to duplicate hands.
Both groups agree that the basal regions of older limb buds are
incapable of causing duplications in younger distal tips and that distal
tips from older buds become more restricted in their ability to respond
to younger proximal regions. The extensive series of operations of
Amprino and Camosso (1959) can, actually, be used to bolster the
'maintenance factor' interpretation of limb duplication, since the sites
and combinations which produce duplications in grafted distal tips fit
readily into Saunders' scheme. There are few discrepancies in results in
these experiments, only differences in interpretation. Amprino rejects
the possibility of the existence of a maintenance factor, first because of
his rejection of the importance of the ectodermal ridge and secondly
because he cannot visualize how a supposedly diffusable (I prefer
'transmissable') factor can maintain the asymmetrical concentrations
required for the interaction hypothesis. He recognizes that some sort of
polarized transmission may be involved (as it must be for any sort of
'influences' from proximal tissues, whether acting directly or indirectly
via the ridge). As long as we are in the realm of hypothesis this should be
no difficulty, since there are many situations in which there is a graded
distribution of morphogenetic potencies with relation to a centre, presumably of production. We are still at too primitive a stage to go into
the geometry of such a situation at the present.
In my opinion these seemingly opposed ideas are not entirely mutually contradictory. There are, evidently, at least two major (related)
processes involved in limb development. One is concerned with distal