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J. MILAIRE
strated in the whole postaxial mesoderm. These structural details and
histochemical differentiations in RNA content and phosphatase activity
form the morphological basis of morphogenetic properties experimentally shown in these two cellular fields.
b. Ectodermal asymmetries
While the dorsal ectoderm of the limb bud remains thin and has little
of histochemical interest, two morphological changes occur in the ventral
ectoderm of mammal embryos. The first one, which concerns only the
preaxial half of the limb bud, is a spreading of the thickening process of
the ventral ectoderm to the lateral wall of the embryo, i.e. across the
boundary between the limb bud and the embryo itself. This newly
thickened ectodermal area and the contiguous limb bud ectoderm form
the 'axillary (or inguinal) zone', overlying the compact and proliferating
axillary (or inguinal) mesoderm (Fig. 2(a)). The formation of the a.e.r.
is the second morphological change to be considered at this stage in the
ectoderm. In mammals, reptiles and birds, it takes place only in the
postaxial part of the limb bud, ventral to its free edge where it underlies
the largest portion of the distal venous system (Fig. 2(b)). The evolution
of this ectodermal zone in different vertebrates runs parallel to the evolution of the animals themselves. In the dogfish embryo, an ordinary
ectodermal fold is present along the whole free edge of the paired fin
buds, as if their ectodermal jackets were growing faster than their
mesodermal contents (Milaire, 1961, Fig. 2). This aspect of a folded
ectoderm is still visible in the a.e.r. of reptiles (Milaire, 1957, Fig. 14,
Plate XVI) and, to a lesser extent, in the chick embryo (Milaire, 1961,
Fig. 6). However, in these forms, the previously two-layered folded
ectoderm has become a multi-layered ectodermal ridge. A transverse
section through a mammalian limb bud shows a triangular a.e.r. with a
straight base opposite the distal mesoderm and a round top which becomes sharper during later stages.
If the various histochemical observations made at this stage in the
thickened limb bud ectoderm of all species are taken into consideration,
they fall into three groups. In the first group, the substances found are
distributed in the entire thickened ectoderm, in its axillary and ventral
parts as well as in the a.e.r. This is so for alkaline phosphatase in the
mouse (Fig. 3(a)) and the mole embryos, and for glycogen in the mole. In
the second group these substances are found in the marginal zone only,
which may or may not be differentiated into a.e.r. (i.e. in the pre- and
postaxial parts of the marginal ectoderm in mammalian limb buds).
This is so for RNA in embryos of the mouse (Fig. 2(b)), rat and mole and
also for alkaline phosphatase in the rat embryo (Milaire, 1956, Fig. X,
p. 335) and in the dogfish embryo (Milaire, 1961, Fig. 2). In the third
J. MILAIRE
strated in the whole postaxial mesoderm. These structural details and
histochemical differentiations in RNA content and phosphatase activity
form the morphological basis of morphogenetic properties experimentally shown in these two cellular fields.
b. Ectodermal asymmetries
While the dorsal ectoderm of the limb bud remains thin and has little
of histochemical interest, two morphological changes occur in the ventral
ectoderm of mammal embryos. The first one, which concerns only the
preaxial half of the limb bud, is a spreading of the thickening process of
the ventral ectoderm to the lateral wall of the embryo, i.e. across the
boundary between the limb bud and the embryo itself. This newly
thickened ectodermal area and the contiguous limb bud ectoderm form
the 'axillary (or inguinal) zone', overlying the compact and proliferating
axillary (or inguinal) mesoderm (Fig. 2(a)). The formation of the a.e.r.
is the second morphological change to be considered at this stage in the
ectoderm. In mammals, reptiles and birds, it takes place only in the
postaxial part of the limb bud, ventral to its free edge where it underlies
the largest portion of the distal venous system (Fig. 2(b)). The evolution
of this ectodermal zone in different vertebrates runs parallel to the evolution of the animals themselves. In the dogfish embryo, an ordinary
ectodermal fold is present along the whole free edge of the paired fin
buds, as if their ectodermal jackets were growing faster than their
mesodermal contents (Milaire, 1961, Fig. 2). This aspect of a folded
ectoderm is still visible in the a.e.r. of reptiles (Milaire, 1957, Fig. 14,
Plate XVI) and, to a lesser extent, in the chick embryo (Milaire, 1961,
Fig. 6). However, in these forms, the previously two-layered folded
ectoderm has become a multi-layered ectodermal ridge. A transverse
section through a mammalian limb bud shows a triangular a.e.r. with a
straight base opposite the distal mesoderm and a round top which becomes sharper during later stages.
If the various histochemical observations made at this stage in the
thickened limb bud ectoderm of all species are taken into consideration,
they fall into three groups. In the first group, the substances found are
distributed in the entire thickened ectoderm, in its axillary and ventral
parts as well as in the a.e.r. This is so for alkaline phosphatase in the
mouse (Fig. 3(a)) and the mole embryos, and for glycogen in the mole. In
the second group these substances are found in the marginal zone only,
which may or may not be differentiated into a.e.r. (i.e. in the pre- and
postaxial parts of the marginal ectoderm in mammalian limb buds).
This is so for RNA in embryos of the mouse (Fig. 2(b)), rat and mole and
also for alkaline phosphatase in the rat embryo (Milaire, 1956, Fig. X,
p. 335) and in the dogfish embryo (Milaire, 1961, Fig. 2). In the third