196
J. MILAIRE
of the limb bud ectoderm and shows quite specific histochemical
characteristics. The elective presence of alkaline phosphatase has been
demonstrated in the a.e.r. of all species studied (rat, mouse, guinea-pig,
mole, chicken and dogfish); these findings are illustrated in two of our
previous papers (Milaire, 1956 and 1961). The presence of alkaline
phosphatase in the a.e.r. may be regarded as one of the most constant
histochemical features of developing vertebrate embryos. Acid phosphatase also characterizes the a.e.r. in the limb buds of rat (Fig. 3(d)),
mouse, guinea-pig and chick embryos (see also Milaire, 1961, Figs. 2,
3 and 4). This enzyme seems to be closely associated with the special
role of this structure, for it was observed in quite high concentration
during very early stages of the mouse embryo. In all species studied,
there was more RNA in the a.e.r. than in any other part of the limb bud
ectoderm. In the rat and the mole embryos, glycogen is present in the
early differentiating a.e.r. but is no longer present when the appendage
reaches the paddle stage (the rat embryo has mucopolysaccharides as
well).
During the growing period of the footplate, the mesoderm, adjacent to
the a.e.r. and under its inductive influence, has several histochemical
peculiarities. It shows an increase of RNA in all species studied, and less
alkaline phosphatase than is present at younger developmental stages
(e.g. the mole embryo). In the rat embryo, the same outer mesodermal
cells also contain glycogen, which is synthetised by the cells surrounding
the marginal venous sinus (Milaire, 1956, Fig. 13, Plate IX). The distribution of the polysaccharide suggests the diffusion of blood glucose from
that embryonic vein to the mesoderm and is an example of the trophic
role of embryonic vessels in organogenesis.
The specific histochemical features demonstrated at this stage in the
distal part of the limb bud are thus in complete agreement with the
morphogenetic processes involved in the formation of the footplate.
After the mesodermal cells originating proximally have reached the
distal part of the limb bud, the a.e.r. exerts on the distal mesoderm a
brief but strong influence stimulating its growth and its regional differentiation. Specific processes have been demonstrated both in the
inducing ectodermal structure and in the reacting outer mesoderm. In
the latter, some basic substances required by these metabolic changes
are provided by a large venous vessel which may itself be regarded as a
mesodermal structure induced by the a.e.r.
b. Formation of limb cartilages {the first two proximal segments)
The description of the morphological features of limb development in
its later stages, such as the formation of muscles or the establishment of
the nervous and circulatory patterns, will not be dealt with in the present
J. MILAIRE
of the limb bud ectoderm and shows quite specific histochemical
characteristics. The elective presence of alkaline phosphatase has been
demonstrated in the a.e.r. of all species studied (rat, mouse, guinea-pig,
mole, chicken and dogfish); these findings are illustrated in two of our
previous papers (Milaire, 1956 and 1961). The presence of alkaline
phosphatase in the a.e.r. may be regarded as one of the most constant
histochemical features of developing vertebrate embryos. Acid phosphatase also characterizes the a.e.r. in the limb buds of rat (Fig. 3(d)),
mouse, guinea-pig and chick embryos (see also Milaire, 1961, Figs. 2,
3 and 4). This enzyme seems to be closely associated with the special
role of this structure, for it was observed in quite high concentration
during very early stages of the mouse embryo. In all species studied,
there was more RNA in the a.e.r. than in any other part of the limb bud
ectoderm. In the rat and the mole embryos, glycogen is present in the
early differentiating a.e.r. but is no longer present when the appendage
reaches the paddle stage (the rat embryo has mucopolysaccharides as
well).
During the growing period of the footplate, the mesoderm, adjacent to
the a.e.r. and under its inductive influence, has several histochemical
peculiarities. It shows an increase of RNA in all species studied, and less
alkaline phosphatase than is present at younger developmental stages
(e.g. the mole embryo). In the rat embryo, the same outer mesodermal
cells also contain glycogen, which is synthetised by the cells surrounding
the marginal venous sinus (Milaire, 1956, Fig. 13, Plate IX). The distribution of the polysaccharide suggests the diffusion of blood glucose from
that embryonic vein to the mesoderm and is an example of the trophic
role of embryonic vessels in organogenesis.
The specific histochemical features demonstrated at this stage in the
distal part of the limb bud are thus in complete agreement with the
morphogenetic processes involved in the formation of the footplate.
After the mesodermal cells originating proximally have reached the
distal part of the limb bud, the a.e.r. exerts on the distal mesoderm a
brief but strong influence stimulating its growth and its regional differentiation. Specific processes have been demonstrated both in the
inducing ectodermal structure and in the reacting outer mesoderm. In
the latter, some basic substances required by these metabolic changes
are provided by a large venous vessel which may itself be regarded as a
mesodermal structure induced by the a.e.r.
b. Formation of limb cartilages {the first two proximal segments)
The description of the morphological features of limb development in
its later stages, such as the formation of muscles or the establishment of
the nervous and circulatory patterns, will not be dealt with in the present