308
EDGAR
ZWILLING
removed; the earlier the bud the greater the distal deficiency. The
proximal parts which did develop were quite normal in size and form.
Shortly afterward Zwilling (1949) described the development of a
wingless mutation of chickens which provided support for Saunders'
experimental findings. In the homozygous wingless embryos normal
wing buds are present early on the third day, but fail to elaborate very
much beyond this stage. Histological evidence revealed that the apical
ectodermal ridge regressed during the third day and was missing by the
fourth. The extent of proximal development was related to the duration
of the ridge. That is, the ridge was not eliminated uniformly and in
cases in which it persisted for a somewhat longer time more of the
proximal limb elements were formed. The wingless mutant, then,
represents a genetic confirmation of Saunders' experiments.
Later, several investigators obtained evidence which led them to
believe that the ectodermal ridge exerts an inductive influence* on the
subjacent mesoderm (Outgrowth inductor'—Zwilling, 1955, 'inductorlike influence' or 'inductive action'—Saunders, Cairns, and Gasseling,
1957, 'influence determinante'—Hampé, 1956a). Zwilling's evidence
was based on a procedure by which the ectoderm and mesoderm components of chick embryo limb buds may be precisely separated and
then recombined; the altered limb buds may then be raised as grafts.
Denuded limb-bud mesoderm (over which flank ectoderm of the host
subsequently grows) or similar mesoderm which has been covered by
non-limb ectoderm failed to form distal limb elements. Later it was
found that similar results were obtained when lateral limb-bud ectoderm
was placed over the distal end of denuded limb-bud mesoderm. Distal
limb structures developed only when limb bud ectoderm, complete with
an ectodermal ridge, was placed over the limb mesoderm. Saunders'
evidence came from experiments in which blocks of presumptive thigh
mesoderm from leg buds were grafted to various levels (Saunders,
Gasseling and Cairns, 1955; Saunders, Cairns and Gasseling, 1957) on a
wing bud. When such blocks of proximal leg mesoderm were placed in
proximal levels of a wing bud and developed in the shoulder region they
retained proximal characteristics and induced typical thigh feathers
from the wing's shoulder ectoderm. However, when the presumptive
thigh mesoderm was grafted to the distal tip and was in contact with
the ectodermal ridge of a wing bud, it formed typical distal elements
which retained their leg characteristics and influenced the wing ectoderm
* The use of the term 'inductive influence' for the action of the ectodermal ridge
requires some comment. The term is used in a most general sense, does not have any
connotation about mechanism but merely indicates that the ridge is active in promoting
limb outgrowth. The term 'inductor' is not synonymous with 'organizer', nor has it been
used to imply any 'organizer' activity. Secondary inductors are not endowed with
properties usually associated with an 'organizer' (Holtfreter, 1951; Grobstein, 1956).
EDGAR
ZWILLING
removed; the earlier the bud the greater the distal deficiency. The
proximal parts which did develop were quite normal in size and form.
Shortly afterward Zwilling (1949) described the development of a
wingless mutation of chickens which provided support for Saunders'
experimental findings. In the homozygous wingless embryos normal
wing buds are present early on the third day, but fail to elaborate very
much beyond this stage. Histological evidence revealed that the apical
ectodermal ridge regressed during the third day and was missing by the
fourth. The extent of proximal development was related to the duration
of the ridge. That is, the ridge was not eliminated uniformly and in
cases in which it persisted for a somewhat longer time more of the
proximal limb elements were formed. The wingless mutant, then,
represents a genetic confirmation of Saunders' experiments.
Later, several investigators obtained evidence which led them to
believe that the ectodermal ridge exerts an inductive influence* on the
subjacent mesoderm (Outgrowth inductor'—Zwilling, 1955, 'inductorlike influence' or 'inductive action'—Saunders, Cairns, and Gasseling,
1957, 'influence determinante'—Hampé, 1956a). Zwilling's evidence
was based on a procedure by which the ectoderm and mesoderm components of chick embryo limb buds may be precisely separated and
then recombined; the altered limb buds may then be raised as grafts.
Denuded limb-bud mesoderm (over which flank ectoderm of the host
subsequently grows) or similar mesoderm which has been covered by
non-limb ectoderm failed to form distal limb elements. Later it was
found that similar results were obtained when lateral limb-bud ectoderm
was placed over the distal end of denuded limb-bud mesoderm. Distal
limb structures developed only when limb bud ectoderm, complete with
an ectodermal ridge, was placed over the limb mesoderm. Saunders'
evidence came from experiments in which blocks of presumptive thigh
mesoderm from leg buds were grafted to various levels (Saunders,
Gasseling and Cairns, 1955; Saunders, Cairns and Gasseling, 1957) on a
wing bud. When such blocks of proximal leg mesoderm were placed in
proximal levels of a wing bud and developed in the shoulder region they
retained proximal characteristics and induced typical thigh feathers
from the wing's shoulder ectoderm. However, when the presumptive
thigh mesoderm was grafted to the distal tip and was in contact with
the ectodermal ridge of a wing bud, it formed typical distal elements
which retained their leg characteristics and influenced the wing ectoderm
* The use of the term 'inductive influence' for the action of the ectodermal ridge
requires some comment. The term is used in a most general sense, does not have any
connotation about mechanism but merely indicates that the ridge is active in promoting
limb outgrowth. The term 'inductor' is not synonymous with 'organizer', nor has it been
used to imply any 'organizer' activity. Secondary inductors are not endowed with
properties usually associated with an 'organizer' (Holtfreter, 1951; Grobstein, 1956).