316
EDGAR
ZWILLING
plate mesoderm from the flank and both intact and dissociatedreaggregated somites from the posterior region of two and three day
embryos. Such assemblies of non-limb mesoderm with limb-bud ectoderm were grown either as flank or C-A grafts. In every case the
ectodermal ridge degenerated during the course of the two days following the operation. In a number of operations a small bit of limb-bud
mesoderm was placed in contact with one portion of the inner surface of
the ridge along with a considerable amount of non-limb mesoderm. The
ridge persisted only where it was in contact with the small piece of limb
mesoderm (which gave rise to a long filamentous outgrowth) while the
rest of the ridge degenerated and flattened. Presumptive thigh tissues
from older limb buds (stages 26 and 27) behave like non-limb mesoderm in this respect. When such tissues were placed in contact with the
ridge of younger buds (Saunders, Gasseling and Cairns, 1959) the latter
flattened and was not apparent after 48 hours. Similar tissue from the
prospective thigh region of younger limb buds not only maintained the
ridge, but, under its influence, formed distal structures.
As limb development proceeds definite asymmetries become evident;
the limb outgrowth is not uniform but has definite bulges and irregularities which are characteristic for a given limb type. Careful observation
reveals that thickened ectodermal ridge is associated with these irregularities; it is always present over a region which is involved in an
outpushing. Experiments were designed to test the two major possibilities ; does the ridge form independently and then induce mesodermal
outgrowth or do the thickened regions of the ectoderm form in relation
to some pattern present in the mesoderm? If the thickest regions from
two or three ectodermal ridges are placed in tandem along the distal
edge of one limb mesoblast do they all remain thick and active as outgrowth inducers or do they become modified under the influence of the
mesoderm? This experiment has been done (Zwilling, 1956a) and in
every case the composite ectodermal ridge fused to form a single ridge
which, over a period of time, acquired a typically normal configuration.
The thickened regions at either end flattened out and the ridge could
not be distinguished from a normal one. Normal limbs developed.
Essentially the same results were obtained when limb-bud ectoderm
was rotated 180° in relation to the mesoderm and when wing-bud
ectoderm was placed on leg-bud mesoderm and vice versa. The ectodermal ridge configuration conformed to the pattern normally expected
of the mesoderm (Zwilling, 1956b, 1955). Precisely the same results were
obtained when genetically normal ectodermal ridges were combined
with mesoderm from polydactylous limb buds (Zwilling and Hansborough, 1956); pre-axial thickening of the ridge persisted in regions
where flattening normally occurs and accessory distal structures formed
EDGAR
ZWILLING
plate mesoderm from the flank and both intact and dissociatedreaggregated somites from the posterior region of two and three day
embryos. Such assemblies of non-limb mesoderm with limb-bud ectoderm were grown either as flank or C-A grafts. In every case the
ectodermal ridge degenerated during the course of the two days following the operation. In a number of operations a small bit of limb-bud
mesoderm was placed in contact with one portion of the inner surface of
the ridge along with a considerable amount of non-limb mesoderm. The
ridge persisted only where it was in contact with the small piece of limb
mesoderm (which gave rise to a long filamentous outgrowth) while the
rest of the ridge degenerated and flattened. Presumptive thigh tissues
from older limb buds (stages 26 and 27) behave like non-limb mesoderm in this respect. When such tissues were placed in contact with the
ridge of younger buds (Saunders, Gasseling and Cairns, 1959) the latter
flattened and was not apparent after 48 hours. Similar tissue from the
prospective thigh region of younger limb buds not only maintained the
ridge, but, under its influence, formed distal structures.
As limb development proceeds definite asymmetries become evident;
the limb outgrowth is not uniform but has definite bulges and irregularities which are characteristic for a given limb type. Careful observation
reveals that thickened ectodermal ridge is associated with these irregularities; it is always present over a region which is involved in an
outpushing. Experiments were designed to test the two major possibilities ; does the ridge form independently and then induce mesodermal
outgrowth or do the thickened regions of the ectoderm form in relation
to some pattern present in the mesoderm? If the thickest regions from
two or three ectodermal ridges are placed in tandem along the distal
edge of one limb mesoblast do they all remain thick and active as outgrowth inducers or do they become modified under the influence of the
mesoderm? This experiment has been done (Zwilling, 1956a) and in
every case the composite ectodermal ridge fused to form a single ridge
which, over a period of time, acquired a typically normal configuration.
The thickened regions at either end flattened out and the ridge could
not be distinguished from a normal one. Normal limbs developed.
Essentially the same results were obtained when limb-bud ectoderm
was rotated 180° in relation to the mesoderm and when wing-bud
ectoderm was placed on leg-bud mesoderm and vice versa. The ectodermal ridge configuration conformed to the pattern normally expected
of the mesoderm (Zwilling, 1956b, 1955). Precisely the same results were
obtained when genetically normal ectodermal ridges were combined
with mesoderm from polydactylous limb buds (Zwilling and Hansborough, 1956); pre-axial thickening of the ridge persisted in regions
where flattening normally occurs and accessory distal structures formed