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
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presumptive limb areas and the intermediate part of Wolff's crest, for
the latter persists unchanged during the first growing stages of the limb
buds. The possibility remains that further advances in the knowledge
of metabolic processes of developing embryonic structures will provide a
satisfactory solution to this problem.
As soon as the early mesoderm of the limb bud is formed, the first
morphogenetic event takes place in its whole ventral outer zone, which
becomes compact and acquires some specific histochemical properties.
Some other morphological and histochemical changes then occur in the
ventral ectoderm, as a result of the inductive influence exerted by
the differentiated ventral mesoderm. Workers who have demonstrated
the existence of morphogenetic properties in the mesoderm of the chick
embryo limb bud (see Section lie) all agree that these properties are
asymmetrically distributed in the postaxial part of the mesoderm soon
after the initiation of limb development. However, most experiments
supporting this mesodermal factor have been performed on growing limb
buds during the formation of the a.e.r. or even when this structure is
fully differentiated. At earlier stages of mammalian and reptilian
embryos, the first mesodermal properties appear in the whole ventral
mesoderm, when the limb ectoderm is still completely undifferentiated.
Asymmetrically distributed with respect to the frontal plane of the
limb bud, this mesodermal activity is clearly responsible for the morphological and metabolic changes which occur in the contiguous ventral
ectoderm soon afterwards. Since the same histochemical properties
which later characterize the a.e.r. have been demonstrated in the entire
thickened ventral ectoderm of the early growing limb bud, this large
ectodermal zone may be regarded as an inductive structure, active
during the first stages of limb development. Two different kinds of
activity occur during these early stages in the limb bud mesoderm:
proliferation of cells and differentiation of the stylopodium presumptive
area. Some histochemical results suggest that these two functional
aspects of the ectodermal induction take place in two different zones of
the thickened ectoderm: the ventro-proximal zone and the more restricted marginal zone. The influence of the ventro-proximal ectoderm
seems to be concerned mostly with stimulating cell proliferation in the
proximal mesoderm, and that of the marginal ectoderm with inducing
the regional properties of mesoderm originating from more proximal
levels. The former activity becomes more and more evident at later
stages of limb development when a typical proliferating zone can be
seen in the proximo-ventral mesoderm and when the histochemical properties of the ectoderm become electively distributed in the axillary
and proximal zones. The topographical distribution of new mesodermal
cells originating proximally and containing RNA suggests that they
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presumptive limb areas and the intermediate part of Wolff's crest, for
the latter persists unchanged during the first growing stages of the limb
buds. The possibility remains that further advances in the knowledge
of metabolic processes of developing embryonic structures will provide a
satisfactory solution to this problem.
As soon as the early mesoderm of the limb bud is formed, the first
morphogenetic event takes place in its whole ventral outer zone, which
becomes compact and acquires some specific histochemical properties.
Some other morphological and histochemical changes then occur in the
ventral ectoderm, as a result of the inductive influence exerted by
the differentiated ventral mesoderm. Workers who have demonstrated
the existence of morphogenetic properties in the mesoderm of the chick
embryo limb bud (see Section lie) all agree that these properties are
asymmetrically distributed in the postaxial part of the mesoderm soon
after the initiation of limb development. However, most experiments
supporting this mesodermal factor have been performed on growing limb
buds during the formation of the a.e.r. or even when this structure is
fully differentiated. At earlier stages of mammalian and reptilian
embryos, the first mesodermal properties appear in the whole ventral
mesoderm, when the limb ectoderm is still completely undifferentiated.
Asymmetrically distributed with respect to the frontal plane of the
limb bud, this mesodermal activity is clearly responsible for the morphological and metabolic changes which occur in the contiguous ventral
ectoderm soon afterwards. Since the same histochemical properties
which later characterize the a.e.r. have been demonstrated in the entire
thickened ventral ectoderm of the early growing limb bud, this large
ectodermal zone may be regarded as an inductive structure, active
during the first stages of limb development. Two different kinds of
activity occur during these early stages in the limb bud mesoderm:
proliferation of cells and differentiation of the stylopodium presumptive
area. Some histochemical results suggest that these two functional
aspects of the ectodermal induction take place in two different zones of
the thickened ectoderm: the ventro-proximal zone and the more restricted marginal zone. The influence of the ventro-proximal ectoderm
seems to be concerned mostly with stimulating cell proliferation in the
proximal mesoderm, and that of the marginal ectoderm with inducing
the regional properties of mesoderm originating from more proximal
levels. The former activity becomes more and more evident at later
stages of limb development when a typical proliferating zone can be
seen in the proximo-ventral mesoderm and when the histochemical properties of the ectoderm become electively distributed in the axillary
and proximal zones. The topographical distribution of new mesodermal
cells originating proximally and containing RNA suggests that they