L I M B M O R P H O G E N E S I S I N V E R T E B R A T E S
207
strongly supported by experiments with genetically abnormal chick
limb buds in which the mesodermal factor may be lacking (wingless,
syndrome, Zwilling, 1956c) or abnormally distributed in the preaxial
part of the bud (polydactyly, Zwilling, 1956d). Indirect supporting
evidence may also be found in our descriptive data: with the exception
of the dogfish, in all species studied the differentiation of the a.e.r. first
takes place in the postaxial part of the marginal ectoderm; in the mole
embryo, alkaline phosphatase is present in the contiguous postaxial
mesoderm while this enzyme is absent from the distal zone of the preaxial mesoderm. Recent observations gathered in mouse embryo limb
buds have revealed that most, if not all, mesodermal structures of the
autopodium differentiate gradually from the postaxial border of the
footplate to more preaxial levels, this morphological gradient running
parallel to the functional properties histochemically demonstrated in
the a.e.r. It thus seems that, at least in mammals, the morphogenetic
properties asymmetrically distributed in the postaxial mesoderm are
mainly concerned in inducing the formation of the footplate. However,
while in this group of Vertebrates the whole ventral mesoderm seems
to play an active role in the determination of the two proximal skeletal
segments, the a.e.r. is only necessary to promote the development of the
more distal autopodium. This inductive structure becomes fully differentiated and acquires its most specific histochemical properties immediately before the growing stage of the limb paddle. The a.e.r. then
remains the only differentiated part of the limb ectoderm and shows
typical histochemical features during the whole period of the footplate
outgrowth. The acceleration of the process of chondrification in the
carpal (or tarsal) and metapodial precartilages is another convincing
evidence of the strong inductive influence exerted at this stage by the
a.e.r. By its metabolic supply, the marginal venous sinus, a temporary
vessel induced by the a.e.r., seems to take an important part in the
active outgrowth of the footplate mesoderm.
Finally, even when degeneration of the a.e.r. has begun and some of
its histochemical characteristics are lost, this structure seems to preserve some weakened morphogenetic properties in five restricted areas
opposite each metapodial precartilage. This last ectodermal influence,
probably resulting from the persistence of the maintenance factor in
the former precartilages, leads to the formation of digits. This active
aspect of digit formation is associated with degenerative phenomena
taking place in the interdigital mesoderm, which is a consequence of the
complete loss of all morphogenetic properties in the underlying marginal
ectoderm.
One may thus be justified in believing that in mammals, and probably
also in reptiles, the morphogenetic role played by the ectoderm in limb
207
strongly supported by experiments with genetically abnormal chick
limb buds in which the mesodermal factor may be lacking (wingless,
syndrome, Zwilling, 1956c) or abnormally distributed in the preaxial
part of the bud (polydactyly, Zwilling, 1956d). Indirect supporting
evidence may also be found in our descriptive data: with the exception
of the dogfish, in all species studied the differentiation of the a.e.r. first
takes place in the postaxial part of the marginal ectoderm; in the mole
embryo, alkaline phosphatase is present in the contiguous postaxial
mesoderm while this enzyme is absent from the distal zone of the preaxial mesoderm. Recent observations gathered in mouse embryo limb
buds have revealed that most, if not all, mesodermal structures of the
autopodium differentiate gradually from the postaxial border of the
footplate to more preaxial levels, this morphological gradient running
parallel to the functional properties histochemically demonstrated in
the a.e.r. It thus seems that, at least in mammals, the morphogenetic
properties asymmetrically distributed in the postaxial mesoderm are
mainly concerned in inducing the formation of the footplate. However,
while in this group of Vertebrates the whole ventral mesoderm seems
to play an active role in the determination of the two proximal skeletal
segments, the a.e.r. is only necessary to promote the development of the
more distal autopodium. This inductive structure becomes fully differentiated and acquires its most specific histochemical properties immediately before the growing stage of the limb paddle. The a.e.r. then
remains the only differentiated part of the limb ectoderm and shows
typical histochemical features during the whole period of the footplate
outgrowth. The acceleration of the process of chondrification in the
carpal (or tarsal) and metapodial precartilages is another convincing
evidence of the strong inductive influence exerted at this stage by the
a.e.r. By its metabolic supply, the marginal venous sinus, a temporary
vessel induced by the a.e.r., seems to take an important part in the
active outgrowth of the footplate mesoderm.
Finally, even when degeneration of the a.e.r. has begun and some of
its histochemical characteristics are lost, this structure seems to preserve some weakened morphogenetic properties in five restricted areas
opposite each metapodial precartilage. This last ectodermal influence,
probably resulting from the persistence of the maintenance factor in
the former precartilages, leads to the formation of digits. This active
aspect of digit formation is associated with degenerative phenomena
taking place in the interdigital mesoderm, which is a consequence of the
complete loss of all morphogenetic properties in the underlying marginal
ectoderm.
One may thus be justified in believing that in mammals, and probably
also in reptiles, the morphogenetic role played by the ectoderm in limb