VII. LIMB MORPHOGENESIS
309
to form foot armament. Thus, instead of thigh structures, typical toes,
complete with claws and scales, developed. Hind-limb mesodermal
tissue, then, retained its limb type characteristics even when grafted to
a wing, but digits formed from presumptive thigh tissue only when it
developed in contact with the ectodermal ridge.
Hampé (1956a) followed leg buds from which the distal tips (including
the presumptive tarso-metatarsus and digits) had been removed. Only
proximal limb elements formed unless the ectoderm from the tip
(complete with ridge) was grafted to the cut surface of the mesoblast
from which the distal tip had been removed.
Tschumi (1955, 1956, 1957) has shown that the formation of distal
structures in the Xenopus limb also depends on the presence of an
apical ectodermal ridge. In this animal an ectodermal ridge, albeit not
so pronounced as in the chick embryo, is present. There are several
important differences between the ectodermal ridge of this amphibian
(probably typical of others) and that of the chick embryo. The ridge of
Xenopus may regenerate after it is damaged or removed and some
undifferentiated non-limb ectoderm may, when in contact with the
apical region of limb-bud mesoderm, form a ridge.
Tschumi found that distal structures did not develop from ectodermfree cores of limb-bud mesoderm which were grafted into the host's
abdominal wall so that they did not make contact with the abdominal
skin. As in Saunders' ridge exicision experiments, the extent of development was greater in older limb-bud mesoblasts.
3. Is the Ectoderm Necessary ?
I shall return to an account of additional evidence for a positive role
of the ectodermal ridge in limb development. At this point, however,
I should like to present the ideas of two investigators who do not
subscribe to this viewpoint. Amprino leads one group which denies that
the ectodermal ridge is an active partner in limb development and Bell
claims that the ectoderm is 'not necessary' for limb morphogenesis.
Amprino and Camosso (1955a, 1959) have confirmed Saunders' observations of distal deficiencies following removal of the ectodermal ridge
alone. They find, in contrast, that removal of subjacent mesoderm along
with the ectodermal ridge leads to marked regulation and the formation
of a nearly normal limb even though the missing ectodermal ridge does
not regenerate. Amprino agrees that a covering of limb ectoderm may be
required for proper limb morphogenesis, but denies that the ridge itself
is essential or that it has any sort of inductive role. Distal structures,
according to him, form from the incompletely determined distal mesoderm as a consequence of some 'influences' (1958a and b) which spread
from the proximal, more determined, areas. Amprino believes that the
309
to form foot armament. Thus, instead of thigh structures, typical toes,
complete with claws and scales, developed. Hind-limb mesodermal
tissue, then, retained its limb type characteristics even when grafted to
a wing, but digits formed from presumptive thigh tissue only when it
developed in contact with the ectodermal ridge.
Hampé (1956a) followed leg buds from which the distal tips (including
the presumptive tarso-metatarsus and digits) had been removed. Only
proximal limb elements formed unless the ectoderm from the tip
(complete with ridge) was grafted to the cut surface of the mesoblast
from which the distal tip had been removed.
Tschumi (1955, 1956, 1957) has shown that the formation of distal
structures in the Xenopus limb also depends on the presence of an
apical ectodermal ridge. In this animal an ectodermal ridge, albeit not
so pronounced as in the chick embryo, is present. There are several
important differences between the ectodermal ridge of this amphibian
(probably typical of others) and that of the chick embryo. The ridge of
Xenopus may regenerate after it is damaged or removed and some
undifferentiated non-limb ectoderm may, when in contact with the
apical region of limb-bud mesoderm, form a ridge.
Tschumi found that distal structures did not develop from ectodermfree cores of limb-bud mesoderm which were grafted into the host's
abdominal wall so that they did not make contact with the abdominal
skin. As in Saunders' ridge exicision experiments, the extent of development was greater in older limb-bud mesoblasts.
3. Is the Ectoderm Necessary ?
I shall return to an account of additional evidence for a positive role
of the ectodermal ridge in limb development. At this point, however,
I should like to present the ideas of two investigators who do not
subscribe to this viewpoint. Amprino leads one group which denies that
the ectodermal ridge is an active partner in limb development and Bell
claims that the ectoderm is 'not necessary' for limb morphogenesis.
Amprino and Camosso (1955a, 1959) have confirmed Saunders' observations of distal deficiencies following removal of the ectodermal ridge
alone. They find, in contrast, that removal of subjacent mesoderm along
with the ectodermal ridge leads to marked regulation and the formation
of a nearly normal limb even though the missing ectodermal ridge does
not regenerate. Amprino agrees that a covering of limb ectoderm may be
required for proper limb morphogenesis, but denies that the ridge itself
is essential or that it has any sort of inductive role. Distal structures,
according to him, form from the incompletely determined distal mesoderm as a consequence of some 'influences' (1958a and b) which spread
from the proximal, more determined, areas. Amprino believes that the