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ZWILLING
in contact with the ectodermal ridge continued their outgrowth. An
extensive portion of the original distal surface, which ordinarily would
have been active in outgrowth, failed to continue, even though covered
by lateral limb ectoderm. These are clear cases of ectodermal ridge
influence.
Suggestive supporting evidence may be noted in descriptive studies
which link limb excesses (polydactyly) with an enlarged apical ridge and
limb deficiencies (ectromelia) with a reduced ridge. Forsthoefel's (1959)
study of the luxoid mouse mutant is a case in point. Additional evidence
for the positive role of the ectodermal ridge comes from combinations
of 'wingless' limb-bud mesoblasts with ectoderm from genetically normal
buds (Zwilling, 1956c). Just as in the mutant limb buds, the genetically
normal ectodermal ridges regressed, but before they disappeared completely the mutant mesoderm was induced to produce considerably more
outgrowth than would have occurred and than did occur in the controls.
Again, this represents a positive performance of the ectoderm.
Even more impelling evidence comes from some of my experiments
which have not yet been published in extenso but which have been
mentioned briefly elsewhere (Zwilling, 1956d). Small fragments (-1--2
mm) of limb-bud mesoderm do not form recognizable limb structures
when grown on chorio-allantoic membranes either singly or in clusters
of 7-10 fragments. Nodules of cartilage, without any particular form,
develop. When 7-12 similar fragments are picked at random and placed
in an ectodermal jacket from a limb bud so that at least several of them
are in contact with the ridge and are then grown as flank grafts or on a
C-A membrane they consistently form recognizable limb structures,
frequently complete with digits. Limb type properties are retained, i.e.
wing-bud fragments form wing elements when in contact with leg
ectoderm, duck mesoderm fragments placed in chick limb-bud ectoderm
form webbed digits, etc. Limb-bud mesoderm has been completely
dissociated into a cell suspension, the cells have been compacted by
centrifugation and then grown as C-A grafts either without ectoderm, enclosed in a jacket of non-limb (flank) ectoderm or stuffed into
limb-bud ectoderm. The ectoderm-free mesoderm formed cartilaginous
nodules, the mesoderm which was in flank ectoderm formed sheets or
rods of cartilage which, however, had no joints or resemblance to limb
elements, while the mesoderm which developed in association with limb
ectoderm, contrary to some earlier experiments (Zwilling, 1956d), formed
jointed limb elements and readily identified digits. This certainly indicates the positive inductive action of the limb ectoderm.
Finally there is the positive contribution made by the ectoderm in
chimaeric limbs which develop from the combination of duck ectoderm
with chick mesoderm. A thin but definite webbing was regularly found
EDGAR
ZWILLING
in contact with the ectodermal ridge continued their outgrowth. An
extensive portion of the original distal surface, which ordinarily would
have been active in outgrowth, failed to continue, even though covered
by lateral limb ectoderm. These are clear cases of ectodermal ridge
influence.
Suggestive supporting evidence may be noted in descriptive studies
which link limb excesses (polydactyly) with an enlarged apical ridge and
limb deficiencies (ectromelia) with a reduced ridge. Forsthoefel's (1959)
study of the luxoid mouse mutant is a case in point. Additional evidence
for the positive role of the ectodermal ridge comes from combinations
of 'wingless' limb-bud mesoblasts with ectoderm from genetically normal
buds (Zwilling, 1956c). Just as in the mutant limb buds, the genetically
normal ectodermal ridges regressed, but before they disappeared completely the mutant mesoderm was induced to produce considerably more
outgrowth than would have occurred and than did occur in the controls.
Again, this represents a positive performance of the ectoderm.
Even more impelling evidence comes from some of my experiments
which have not yet been published in extenso but which have been
mentioned briefly elsewhere (Zwilling, 1956d). Small fragments (-1--2
mm) of limb-bud mesoderm do not form recognizable limb structures
when grown on chorio-allantoic membranes either singly or in clusters
of 7-10 fragments. Nodules of cartilage, without any particular form,
develop. When 7-12 similar fragments are picked at random and placed
in an ectodermal jacket from a limb bud so that at least several of them
are in contact with the ridge and are then grown as flank grafts or on a
C-A membrane they consistently form recognizable limb structures,
frequently complete with digits. Limb type properties are retained, i.e.
wing-bud fragments form wing elements when in contact with leg
ectoderm, duck mesoderm fragments placed in chick limb-bud ectoderm
form webbed digits, etc. Limb-bud mesoderm has been completely
dissociated into a cell suspension, the cells have been compacted by
centrifugation and then grown as C-A grafts either without ectoderm, enclosed in a jacket of non-limb (flank) ectoderm or stuffed into
limb-bud ectoderm. The ectoderm-free mesoderm formed cartilaginous
nodules, the mesoderm which was in flank ectoderm formed sheets or
rods of cartilage which, however, had no joints or resemblance to limb
elements, while the mesoderm which developed in association with limb
ectoderm, contrary to some earlier experiments (Zwilling, 1956d), formed
jointed limb elements and readily identified digits. This certainly indicates the positive inductive action of the limb ectoderm.
Finally there is the positive contribution made by the ectoderm in
chimaeric limbs which develop from the combination of duck ectoderm
with chick mesoderm. A thin but definite webbing was regularly found