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
195
lend support to the assumption that the differentiation of the a.e.r. in a
restricted zone of the ventral thickened ectoderm is the result of a
localized mesodermal influence distributed only in the postaxial part of
the limb bud.
The widest part of the marginal venous system differentiates in the
postaxial half of the distal mesoderm in close association with the a.e.r.
This topographic relationship between these temporary limb structures
was previously reported in the human embryo (O'Rahilly, Gardner
and Gray, 1956) and in the rat embryo (Milaire, 1956). In the latter
report, it was suggested that there might be a causal relationship
between them, the vessel being probably induced by the ectodermal
ridge. Tschumi's (1956) more recent experiments with Xenopus have
shown that the venous marginal system becomes reduced in ectodermfree grafted limb buds.
4. Formation of the Limb Paddle
As soon as the a.e.r. is fully differentiated, the distal zone of the limb
buds enters an active growing period, which lasts for about 12 hours in
the mouse and the rat embryos. This leads to the formation of the limb
paddle or footplate, that broad and flattened part of the limb bud which
will form the autopodium. At the beginning of this growing period,
active proliferation is observed in the proximo-ventral zone of the mesoderm, but it gradually decreases as if the new mesodermal cells originating in the stump of the limb bud had slowly migrated towards more
distal regions and been used in the formation of the footplate.
Two different descriptive aspects of limb morphogenesis will be apparent in the following account: (a) The morphogenetic processes taking
place in the distal part of the limb bud and leading to the differentiation
and growth of new mesodermal areas, (b) The histochemical aspects of
the developing cartilaginous skeleton of the limb bud.
a. Genesis of the limb paddle
Of all the histological and histochemical aspects of limb morphogenesis, those observed during the formation of the limb paddle provide
the most convincing data supporting the role of the a.e.r. in limb
development. Previously limited to the postaxial border of the limb
bud, this ectodermal structure as well as the underlying marginal venous
sinus have now extended along the entire cephalo-caudal edge of the
footplate. Though the axillary (or inguinal) zone of the ectoderm is still
thick and histochemically differentiated at the beginning of this stage,
the normal thickness of the ectoderm is gradually restored in the whole
proximal part of the limb bud and on the dorsal and ventral surfaces
of its distal part. The a.e.r. soon remains the only differentiated zone
195
lend support to the assumption that the differentiation of the a.e.r. in a
restricted zone of the ventral thickened ectoderm is the result of a
localized mesodermal influence distributed only in the postaxial part of
the limb bud.
The widest part of the marginal venous system differentiates in the
postaxial half of the distal mesoderm in close association with the a.e.r.
This topographic relationship between these temporary limb structures
was previously reported in the human embryo (O'Rahilly, Gardner
and Gray, 1956) and in the rat embryo (Milaire, 1956). In the latter
report, it was suggested that there might be a causal relationship
between them, the vessel being probably induced by the ectodermal
ridge. Tschumi's (1956) more recent experiments with Xenopus have
shown that the venous marginal system becomes reduced in ectodermfree grafted limb buds.
4. Formation of the Limb Paddle
As soon as the a.e.r. is fully differentiated, the distal zone of the limb
buds enters an active growing period, which lasts for about 12 hours in
the mouse and the rat embryos. This leads to the formation of the limb
paddle or footplate, that broad and flattened part of the limb bud which
will form the autopodium. At the beginning of this growing period,
active proliferation is observed in the proximo-ventral zone of the mesoderm, but it gradually decreases as if the new mesodermal cells originating in the stump of the limb bud had slowly migrated towards more
distal regions and been used in the formation of the footplate.
Two different descriptive aspects of limb morphogenesis will be apparent in the following account: (a) The morphogenetic processes taking
place in the distal part of the limb bud and leading to the differentiation
and growth of new mesodermal areas, (b) The histochemical aspects of
the developing cartilaginous skeleton of the limb bud.
a. Genesis of the limb paddle
Of all the histological and histochemical aspects of limb morphogenesis, those observed during the formation of the limb paddle provide
the most convincing data supporting the role of the a.e.r. in limb
development. Previously limited to the postaxial border of the limb
bud, this ectodermal structure as well as the underlying marginal venous
sinus have now extended along the entire cephalo-caudal edge of the
footplate. Though the axillary (or inguinal) zone of the ectoderm is still
thick and histochemically differentiated at the beginning of this stage,
the normal thickness of the ectoderm is gradually restored in the whole
proximal part of the limb bud and on the dorsal and ventral surfaces
of its distal part. The a.e.r. soon remains the only differentiated zone