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EDGAR
ZWILLING
areas of chick embryos. Her experiments, incidentally, also indicate
that antero-posterior and dorso-ventral relationships are established, in
the chick, in the same order as in urodele limbs.
V. Regulatory Phenomena
Embryos or structures therefrom are frequently considered in terms
of their 'mosaic' properties (i.e. the tendency for isolated regions to
develop the parts which they would have formed had they been left
in the intact organism or structure), or 'regulative' properties (i.e. the
ability of embryo (or structure) to form a complete organism or organ
even though considerable amounts of their tissues have been eliminated
during early stages). The more general term 'regulation' is used to
describe many sorts of reparative phenomena and, in the early structure
with which we are concerned, may be distinguished from 'regeneration',
which involves the restoration of parts which have already been
differentiated. The processes responsible for most regulative phenomena
are not known and may involve adjustment in proliferation rates, shifts
of tissues, changes in the fate of tissues and a variety of little-understood
tissue interactions.
Early experiments with amphibian limb discs demonstrated considerable regulative potential of the presumptive limb tissues; half of
a disc could form an entire limb, rotation of the disc could result in
doubling of the limb, etc. In contrast with this the early chorio-allantoic
grafts of chick embryo limb buds seemed to indicate that the buds
represented a mosaic system. In fact Huxley and de Beer (1934) considered the chick limb bud as a 'mosaic stage' in limb development.
This opinion was based on results of experiments of Murray and Huxley
(1925) and Murray (1928) in which various portions of limb buds were
grown on the C-A membrane. Even these experiments revealed a
certain amount of regulative behaviour when it was noted, for example,
that presumptive tibial tissue could form part of a femur (see also
similar results from tissue culture experiments of Fell and Canti, 1934).
Recent analyses have revealed that the chick limb bud, in sharp
contrast to early findings, is highly regulable. This difference is not due
to faults of observation on the part of the early investigators but largely
results from the type of experiment performed and attention paid to the
presence or absence of the ectodermal ridge. It is evident from our
previous discussion of the sequential formation of distal territories that
the early limb bud cannot be a mosaic, since it does not even contain the
presumptive distal anlagen, but consists only of presumptive proximal
elements and a developing system from which distal parts are elaborated.
To a great extent the results of partitioning experiments depend on how
EDGAR
ZWILLING
areas of chick embryos. Her experiments, incidentally, also indicate
that antero-posterior and dorso-ventral relationships are established, in
the chick, in the same order as in urodele limbs.
V. Regulatory Phenomena
Embryos or structures therefrom are frequently considered in terms
of their 'mosaic' properties (i.e. the tendency for isolated regions to
develop the parts which they would have formed had they been left
in the intact organism or structure), or 'regulative' properties (i.e. the
ability of embryo (or structure) to form a complete organism or organ
even though considerable amounts of their tissues have been eliminated
during early stages). The more general term 'regulation' is used to
describe many sorts of reparative phenomena and, in the early structure
with which we are concerned, may be distinguished from 'regeneration',
which involves the restoration of parts which have already been
differentiated. The processes responsible for most regulative phenomena
are not known and may involve adjustment in proliferation rates, shifts
of tissues, changes in the fate of tissues and a variety of little-understood
tissue interactions.
Early experiments with amphibian limb discs demonstrated considerable regulative potential of the presumptive limb tissues; half of
a disc could form an entire limb, rotation of the disc could result in
doubling of the limb, etc. In contrast with this the early chorio-allantoic
grafts of chick embryo limb buds seemed to indicate that the buds
represented a mosaic system. In fact Huxley and de Beer (1934) considered the chick limb bud as a 'mosaic stage' in limb development.
This opinion was based on results of experiments of Murray and Huxley
(1925) and Murray (1928) in which various portions of limb buds were
grown on the C-A membrane. Even these experiments revealed a
certain amount of regulative behaviour when it was noted, for example,
that presumptive tibial tissue could form part of a femur (see also
similar results from tissue culture experiments of Fell and Canti, 1934).
Recent analyses have revealed that the chick limb bud, in sharp
contrast to early findings, is highly regulable. This difference is not due
to faults of observation on the part of the early investigators but largely
results from the type of experiment performed and attention paid to the
presence or absence of the ectodermal ridge. It is evident from our
previous discussion of the sequential formation of distal territories that
the early limb bud cannot be a mosaic, since it does not even contain the
presumptive distal anlagen, but consists only of presumptive proximal
elements and a developing system from which distal parts are elaborated.
To a great extent the results of partitioning experiments depend on how