202
J. MILAIRE
derm interposed between these two precartilages, there is no evidence
that it takes part in the metabolic changes involved in the process of
chondrification.
In the third phase taking place at the end of the paddle stage,
chondrification involves the different autopodial elements in a characteristic manner: while a cell condensation in which no precartilage can
be distinguished takes place in the whole proximal part of the autopodium, the three median metapodial elements condense in the broad
part of the footplate (Fig. 5(a, b)). The synthesis of glycogen starts
in the proximal zone of each of the three metapodial precartilages,
whence it spreads distally. However, the carpal (or tarsal) and the
metacarpal (or metatarsal) zones of each precartilaginous rod are quite
distinct soon after the beginning of this process; a small group of cells
is interposed between them which never form glycogen (Fig. 5(a)). At
about the same time several round precartilaginous masses become
visible in the more proximal zone of the footplate which became compact at an earlier stage, but as yet no glycogen is present. Of these proximal precartilages of the autopodium, the radiale and ulnare first show the
metabolic signs of chondrification (increase of glycogen). The more distal
intermedium and three central elements are involved at a later stage, at
the same time as the metapodial precartilages I and V. The different
phalanges then chondrify in proximo-distal sequence in each growing
digital bud (Fig. 6(a,b)).
The different elements in the limb chondrify in the same order in
which their presumptive mesodermal areas came under the inductive
influence of the a.e.r. Experiments on chick and Xenopus limb buds
(see Section Ila) have shown that the most proximal cells of the early
mesodermal bud possess the intrinsic ability to form the girdle cartilage.
In mammalian embryos, these skeletal elements begin to chondrify at
about the same time as the stylopodial precartilage. This suggests that
the stylopodial presumptive mesoderm is elaborated at a very early
stage of limb development, a result of the first interactions between the
compact ventral mesoderm and the overlying thickened ventral ectoderm. The assumption that the whole ventral part of the ectoderm plays
a morphogenetic role at this early stage of limb development is strongly
supported by the presence in this ectodermal area of all substances which
are found at later stages in the a.e.r. Nevertheless, the possibility remains
that even at this early stage, a functional division has already taken
place in the thickened ectoderm, its proximal zone being mostly concerned in stimulating the proliferation of mesoderm and its marginal
zone in inducing the regional properties in the newly formed mesodermal area. The latter role is certainly assumed by the marginal zone
of the ectoderm during the following inductive phase, when it leads to the
J. MILAIRE
derm interposed between these two precartilages, there is no evidence
that it takes part in the metabolic changes involved in the process of
chondrification.
In the third phase taking place at the end of the paddle stage,
chondrification involves the different autopodial elements in a characteristic manner: while a cell condensation in which no precartilage can
be distinguished takes place in the whole proximal part of the autopodium, the three median metapodial elements condense in the broad
part of the footplate (Fig. 5(a, b)). The synthesis of glycogen starts
in the proximal zone of each of the three metapodial precartilages,
whence it spreads distally. However, the carpal (or tarsal) and the
metacarpal (or metatarsal) zones of each precartilaginous rod are quite
distinct soon after the beginning of this process; a small group of cells
is interposed between them which never form glycogen (Fig. 5(a)). At
about the same time several round precartilaginous masses become
visible in the more proximal zone of the footplate which became compact at an earlier stage, but as yet no glycogen is present. Of these proximal precartilages of the autopodium, the radiale and ulnare first show the
metabolic signs of chondrification (increase of glycogen). The more distal
intermedium and three central elements are involved at a later stage, at
the same time as the metapodial precartilages I and V. The different
phalanges then chondrify in proximo-distal sequence in each growing
digital bud (Fig. 6(a,b)).
The different elements in the limb chondrify in the same order in
which their presumptive mesodermal areas came under the inductive
influence of the a.e.r. Experiments on chick and Xenopus limb buds
(see Section Ila) have shown that the most proximal cells of the early
mesodermal bud possess the intrinsic ability to form the girdle cartilage.
In mammalian embryos, these skeletal elements begin to chondrify at
about the same time as the stylopodial precartilage. This suggests that
the stylopodial presumptive mesoderm is elaborated at a very early
stage of limb development, a result of the first interactions between the
compact ventral mesoderm and the overlying thickened ventral ectoderm. The assumption that the whole ventral part of the ectoderm plays
a morphogenetic role at this early stage of limb development is strongly
supported by the presence in this ectodermal area of all substances which
are found at later stages in the a.e.r. Nevertheless, the possibility remains
that even at this early stage, a functional division has already taken
place in the thickened ectoderm, its proximal zone being mostly concerned in stimulating the proliferation of mesoderm and its marginal
zone in inducing the regional properties in the newly formed mesodermal area. The latter role is certainly assumed by the marginal zone
of the ectoderm during the following inductive phase, when it leads to the