190
J. MILAIRE
histochemical differences can be demonstrated between the preaxial (or
cephalic) and the postaxial (or caudal) parts of each limb bud. They are
as follows:
a. Mesodermal asymmetries
On the morphological side, the clearest mesodermal asymmetry is
observed in the venous vascular pattern of the limb buds of all species.
The fore-limb buds receive arterial blood through an axial artery proceeding from the dorsal aorta and the hind-limb bud from an axial
artery proceeding from the umbilical artery. When the axial arteries
reach the centre of the mesoderm, they divide into radiated terminal
capillaries which are collected by a marginal venous system underlying
the epidermis. Most of the blood is collected in a large sinus in the postaxial part of the limb bud. This sinus then enters the preaxial part of the
limb bud and divides into numerous narrower and intertwined vessels
which join the anterior cardinal vein (or the umbilical vein) when they
reach the body wall of the embryo. Several differences may also be found
between the preaxial and the postaxial parts of the mesoderm itself. In
mammalian and reptilian embryos, the mesoderm is compact and contains RNA in its ventral outer zone, but the extent differs in the two
parts of the limb bud. In the preaxial part, the compact, RNA-containing mesoderm is seen only in three to ten layers of cells, depending on
the species. These two features are also characteristic of the outer
mesoderm in the axillary zone from where they continue for some distance into the body of the embryo (Fig. 2(a); also Milaire, 1957, Fig. V(a),
p. 453). In the postaxial part, however, these characteristics are much
more widespread, especially in the proximal region (Fig. 2(b); also Milaire,
1957, Fig. V(b, c); p. 453). In a transverse section through the postaxial
part of the fore-limb bud of a 10-day mouse embryo, these features may
be seen as far as the middle of the dorso-ventral length of the bud (Fig.
2(b)). The rate of mitotic activity in the ventral compact mesoderm was
calculated by counting the dividing cells in the fore-limb buds of nine
10-day mouse embryos. In each case, a great number of mitoses was
found in the proximal part of this cellular field, mostly in the postaxial
half of the limb bud, where 10 to 15 dividing cells could be observed in a
single section. Hence in the absence of any contradictory reports, we
may consider the proximo-ventral mesoderm of the limb bud as a cell
proliferation zone which seems to be more active in the postaxial part
of the bud.
The distribution of alkaline phosphatase in the mole embryo limb
buds shows another clear-cut but purely histochemical difference between the pre- and postaxial parts of the mesoderm. Present in the
proximal part only of the preaxial mesoderm, this enzyme was demon-
J. MILAIRE
histochemical differences can be demonstrated between the preaxial (or
cephalic) and the postaxial (or caudal) parts of each limb bud. They are
as follows:
a. Mesodermal asymmetries
On the morphological side, the clearest mesodermal asymmetry is
observed in the venous vascular pattern of the limb buds of all species.
The fore-limb buds receive arterial blood through an axial artery proceeding from the dorsal aorta and the hind-limb bud from an axial
artery proceeding from the umbilical artery. When the axial arteries
reach the centre of the mesoderm, they divide into radiated terminal
capillaries which are collected by a marginal venous system underlying
the epidermis. Most of the blood is collected in a large sinus in the postaxial part of the limb bud. This sinus then enters the preaxial part of the
limb bud and divides into numerous narrower and intertwined vessels
which join the anterior cardinal vein (or the umbilical vein) when they
reach the body wall of the embryo. Several differences may also be found
between the preaxial and the postaxial parts of the mesoderm itself. In
mammalian and reptilian embryos, the mesoderm is compact and contains RNA in its ventral outer zone, but the extent differs in the two
parts of the limb bud. In the preaxial part, the compact, RNA-containing mesoderm is seen only in three to ten layers of cells, depending on
the species. These two features are also characteristic of the outer
mesoderm in the axillary zone from where they continue for some distance into the body of the embryo (Fig. 2(a); also Milaire, 1957, Fig. V(a),
p. 453). In the postaxial part, however, these characteristics are much
more widespread, especially in the proximal region (Fig. 2(b); also Milaire,
1957, Fig. V(b, c); p. 453). In a transverse section through the postaxial
part of the fore-limb bud of a 10-day mouse embryo, these features may
be seen as far as the middle of the dorso-ventral length of the bud (Fig.
2(b)). The rate of mitotic activity in the ventral compact mesoderm was
calculated by counting the dividing cells in the fore-limb buds of nine
10-day mouse embryos. In each case, a great number of mitoses was
found in the proximal part of this cellular field, mostly in the postaxial
half of the limb bud, where 10 to 15 dividing cells could be observed in a
single section. Hence in the absence of any contradictory reports, we
may consider the proximo-ventral mesoderm of the limb bud as a cell
proliferation zone which seems to be more active in the postaxial part
of the bud.
The distribution of alkaline phosphatase in the mole embryo limb
buds shows another clear-cut but purely histochemical difference between the pre- and postaxial parts of the mesoderm. Present in the
proximal part only of the preaxial mesoderm, this enzyme was demon-