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J. MILAIRE
migrate towards the distal part of the limb bud, where they enter the
zone of influence of the marginal ectoderm. In mammalian embryos,
this region of the ectoderm shows specific histochemical peculiarities
from the earliest stages of the limb development, even before its morphological differentiation into a.e.r. and even in the preaxial part of the
limb bud, where the formation of this structure is always delayed. In the
chick and reptilian embryos, the a.e.r. differentiates morphologically at
a very early stage of limb development and shows the same metabolic
changes as the marginal ectoderm of young mammalian limb buds.
The above interpretation is not entirely new. The idea that undifferentiated mesoderm is formed in the proximal part of the limb bud
and from there migrates towards the distal zone to be induced by the
a.e.r., is strongly supported by the recent experiments of Hampe (1960)
on the chick embryo. He cut the hind-limb buds of different stages at
the presumptive knee level and grafted an ectodermal jacket, including
the a.e.r. at the stage of its maximal activity, on the preserved limb
bud stump containing in each case the presumptive material for the
stylopodium. He found that the older the host mesodermal stump, the
fewer the distal structures that would form in the resulting chimaeric
limbs. It thus seems that during limb development, the proximal part
of the limb bud is gradually deprived of undifferentiated mesoderm able
to form distal structures when subjected to the influence of the a.e.r.
However, it remains highly desirable that the morphogenetic properties
supposedly possessed by the ventral and axillary zones of the limb bud
ectoderm should be investigated further by experiment.
After these first interactions between the mesoderm and the ectoderm in the whole ventral part of the early limb bud, new morphological and histochemical changes take place in its postaxial part. They
indicate the intervention of a second group of asymmetrical processes in
limb morphogenesis. The most obvious change is a significant increase
in cell proliferation in the proximal part of the postaxial mesoderm.
From this genuine proliferating centre, the new mesodermal cells seem
to move in two directions: some group together in the axial zone of the
stump where they soon form the compact stylopodium precartilage,
others migrate distally and are later used in the formation of the
zeugopodium and of the footplate. Simultaneously, the same postaxial
mesoderm acquires specific morphogenetic properties which have been
ingeniously demonstrated in experiments with the chick embryo limb
bud (see Section He). Histochemical peculiarities in the same part of the
mesoderm in the mole embryo also conform to this pattern. These mesodermal properties are supposed to induce the differentiation of the a.e.r.
in the postaxial part of the marginal ectoderm and to maintain this
structure in good inductively active conditions. This assumption is
J. MILAIRE
migrate towards the distal part of the limb bud, where they enter the
zone of influence of the marginal ectoderm. In mammalian embryos,
this region of the ectoderm shows specific histochemical peculiarities
from the earliest stages of the limb development, even before its morphological differentiation into a.e.r. and even in the preaxial part of the
limb bud, where the formation of this structure is always delayed. In the
chick and reptilian embryos, the a.e.r. differentiates morphologically at
a very early stage of limb development and shows the same metabolic
changes as the marginal ectoderm of young mammalian limb buds.
The above interpretation is not entirely new. The idea that undifferentiated mesoderm is formed in the proximal part of the limb bud
and from there migrates towards the distal zone to be induced by the
a.e.r., is strongly supported by the recent experiments of Hampe (1960)
on the chick embryo. He cut the hind-limb buds of different stages at
the presumptive knee level and grafted an ectodermal jacket, including
the a.e.r. at the stage of its maximal activity, on the preserved limb
bud stump containing in each case the presumptive material for the
stylopodium. He found that the older the host mesodermal stump, the
fewer the distal structures that would form in the resulting chimaeric
limbs. It thus seems that during limb development, the proximal part
of the limb bud is gradually deprived of undifferentiated mesoderm able
to form distal structures when subjected to the influence of the a.e.r.
However, it remains highly desirable that the morphogenetic properties
supposedly possessed by the ventral and axillary zones of the limb bud
ectoderm should be investigated further by experiment.
After these first interactions between the mesoderm and the ectoderm in the whole ventral part of the early limb bud, new morphological and histochemical changes take place in its postaxial part. They
indicate the intervention of a second group of asymmetrical processes in
limb morphogenesis. The most obvious change is a significant increase
in cell proliferation in the proximal part of the postaxial mesoderm.
From this genuine proliferating centre, the new mesodermal cells seem
to move in two directions: some group together in the axial zone of the
stump where they soon form the compact stylopodium precartilage,
others migrate distally and are later used in the formation of the
zeugopodium and of the footplate. Simultaneously, the same postaxial
mesoderm acquires specific morphogenetic properties which have been
ingeniously demonstrated in experiments with the chick embryo limb
bud (see Section He). Histochemical peculiarities in the same part of the
mesoderm in the mole embryo also conform to this pattern. These mesodermal properties are supposed to induce the differentiation of the a.e.r.
in the postaxial part of the marginal ectoderm and to maintain this
structure in good inductively active conditions. This assumption is