VII.
LIMB
MORPHOGENESIS
319
outgrowth, the other with the individuation of the limb parts.
Amprino's ideas are deficient in that they offer no mechanism for limb
outgrowth. However, once the tissue has been laid down individuation
of limb elements may result from the sort of spatio-temporal set of
'influences' from proximal areas which he suggests. We must recognize
that we are not yet describing actual mechanisms; we are merely
deducing possible interactions and are, therefore, still vague about
many aspects of interpretation.
Saunders and Gasseling (1959) have studied host-graft reactions in
post-hatching cases of limb duplication produced by operative means.
The evidence to date indicates that the homograft reactions involved
the duplicate hands but did not always extend to more proximal
elements which, according to carbon marks, should have been formed
from graft tissue. The extent to which more proximal host tissues may
migrate because of accessory distal structures is not known. That this
sort ofthing may occur is indicated by a variety of observations (studies
on duplicate mutants, Landauer, 1956; luxate mice, Hovelacque and
Noel, 1923; Carter, 1954). Saunders' experiments, in conjunction with
others which he now has in progress, may provide interesting support
for these ideas.
B. Mechanisms
Several observations have been mentioned as suggesting some sort of
mechanism for one or another aspect of limb development. A number of
investigators have been impressed with the close association between
the marginal blood vessel which is in the mesoderm closely adjacent to
the ectodermal ridge and feel that this vascularization may somehow be
related to distal growth of the limb. Carter (1954) has observed variations in the size and pattern of the marginal sinus and its branches
which were related to anomalies of luxate mouse embryo limbs. Descriptive studies have impressed observers (O'Rahilly, Gardner and Gray,
1956; Milaire, 1955, 1956, etc.) with the close association between
the blood vessels and the ectodermal ridge (the blood vessel is largest
where the ridge is most prominent) and has led them to suspect some
causal dependence between the two. In Tschumi's (1957) experiments
with Xenopus, typical marginal vascularization was lost in ectodermfree grafts in which distal development failed and was duplicated in
association with accessory apical ridges when they formed. Tschumi
believes that the ectodermal ridge determines the development and
course of the marginal vein. The latter, in turn, by supplying needed
metabolites by means of its blood flow promotes apical proliferation.
Recently Gasseling and Saunders (1959 and personal communication)
LIMB
MORPHOGENESIS
319
outgrowth, the other with the individuation of the limb parts.
Amprino's ideas are deficient in that they offer no mechanism for limb
outgrowth. However, once the tissue has been laid down individuation
of limb elements may result from the sort of spatio-temporal set of
'influences' from proximal areas which he suggests. We must recognize
that we are not yet describing actual mechanisms; we are merely
deducing possible interactions and are, therefore, still vague about
many aspects of interpretation.
Saunders and Gasseling (1959) have studied host-graft reactions in
post-hatching cases of limb duplication produced by operative means.
The evidence to date indicates that the homograft reactions involved
the duplicate hands but did not always extend to more proximal
elements which, according to carbon marks, should have been formed
from graft tissue. The extent to which more proximal host tissues may
migrate because of accessory distal structures is not known. That this
sort ofthing may occur is indicated by a variety of observations (studies
on duplicate mutants, Landauer, 1956; luxate mice, Hovelacque and
Noel, 1923; Carter, 1954). Saunders' experiments, in conjunction with
others which he now has in progress, may provide interesting support
for these ideas.
B. Mechanisms
Several observations have been mentioned as suggesting some sort of
mechanism for one or another aspect of limb development. A number of
investigators have been impressed with the close association between
the marginal blood vessel which is in the mesoderm closely adjacent to
the ectodermal ridge and feel that this vascularization may somehow be
related to distal growth of the limb. Carter (1954) has observed variations in the size and pattern of the marginal sinus and its branches
which were related to anomalies of luxate mouse embryo limbs. Descriptive studies have impressed observers (O'Rahilly, Gardner and Gray,
1956; Milaire, 1955, 1956, etc.) with the close association between
the blood vessels and the ectodermal ridge (the blood vessel is largest
where the ridge is most prominent) and has led them to suspect some
causal dependence between the two. In Tschumi's (1957) experiments
with Xenopus, typical marginal vascularization was lost in ectodermfree grafts in which distal development failed and was duplicated in
association with accessory apical ridges when they formed. Tschumi
believes that the ectodermal ridge determines the development and
course of the marginal vein. The latter, in turn, by supplying needed
metabolites by means of its blood flow promotes apical proliferation.
Recently Gasseling and Saunders (1959 and personal communication)