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J. MILAIRE
limb skeleton in the mole were also seen in most other mammalian and
reptilian embryos. This aspect of limb development has not yet been
studied in the chick or dogfish embryos.
At the limb paddle stage, the early morphological as well as histochemical signs of chondrification become visible at about the same time
in the first two precartilages: the girdle and the stylopodium elements.
The girdle blastema appears in the body wall of the embryo, opposite
the proximal part of the limb bud, with its long axis parallel to the
cephalo-caudal axis of the embryo. The stylopodium cartilage develops
perpendicularly to the latter in the axial zone of the limb bud stump.
Both precartilages differentiate in a common core of compact mesoderm containing alkaline phosphatase; this enzyme disappears in the
intermediate zone at a later stage, before joint structures are formed.
5. Final Steps in Limb Morphogenesis
While more distal mesodermal areas are successively involved in the
process of chondrification, the last major interaction between the ectoderm and the mesoderm of the limb bud takes place in the distal part
of the footplate and leads to the formation of digits. In order to make
the following description clearer, these two aspects of limb morphogenesis will still be treated separately, as in the previous section.
a. Final morphogenetic processes
According to Tschumi's (1957) maps of the presumptive mesodermal
areas in the limb bud of Xenopus, the whole mesoderm of the young
limb paddle is used in the proximal segments of the autopodium, i.e.
the carpus and metacarpus, the tarsus and metatarsus. In this species
each presumptive digital area forms in the extreme outer zone of the
footplate mesoderm, as a result of the final inductive influence exerted
by the a.e.r. Authors who have mapped the presumptive mesodermal
areas in the chick embryo (Saunders, 1948; Hampe, 1957) did not
take these late stages of limb development into consideration but they
all agree that the a.e.r. is necessary for the formation of digits. However,
the experimental data concerning the latest inductive activities of this
ectodermal structure are inconclusive and it is not known if these
activities continue in the whole cephalo-caudal part of the ridge
or only in five restricted areas. The histochemical aspects of digit
development suggest such a division of the a.e.r. into five limited functional areas. As soon as the metacarpal or metatarsal precartilages have
condensed in the limb paddle, some signs of a lengthwise growth can be
seen in the distal part of each of them (see Section Vb). During this
lengthening phase, the distal extremity of these precartilaginous rods
gradually brings a mechanical pressure to bear on the contiguous part
J. MILAIRE
limb skeleton in the mole were also seen in most other mammalian and
reptilian embryos. This aspect of limb development has not yet been
studied in the chick or dogfish embryos.
At the limb paddle stage, the early morphological as well as histochemical signs of chondrification become visible at about the same time
in the first two precartilages: the girdle and the stylopodium elements.
The girdle blastema appears in the body wall of the embryo, opposite
the proximal part of the limb bud, with its long axis parallel to the
cephalo-caudal axis of the embryo. The stylopodium cartilage develops
perpendicularly to the latter in the axial zone of the limb bud stump.
Both precartilages differentiate in a common core of compact mesoderm containing alkaline phosphatase; this enzyme disappears in the
intermediate zone at a later stage, before joint structures are formed.
5. Final Steps in Limb Morphogenesis
While more distal mesodermal areas are successively involved in the
process of chondrification, the last major interaction between the ectoderm and the mesoderm of the limb bud takes place in the distal part
of the footplate and leads to the formation of digits. In order to make
the following description clearer, these two aspects of limb morphogenesis will still be treated separately, as in the previous section.
a. Final morphogenetic processes
According to Tschumi's (1957) maps of the presumptive mesodermal
areas in the limb bud of Xenopus, the whole mesoderm of the young
limb paddle is used in the proximal segments of the autopodium, i.e.
the carpus and metacarpus, the tarsus and metatarsus. In this species
each presumptive digital area forms in the extreme outer zone of the
footplate mesoderm, as a result of the final inductive influence exerted
by the a.e.r. Authors who have mapped the presumptive mesodermal
areas in the chick embryo (Saunders, 1948; Hampe, 1957) did not
take these late stages of limb development into consideration but they
all agree that the a.e.r. is necessary for the formation of digits. However,
the experimental data concerning the latest inductive activities of this
ectodermal structure are inconclusive and it is not known if these
activities continue in the whole cephalo-caudal part of the ridge
or only in five restricted areas. The histochemical aspects of digit
development suggest such a division of the a.e.r. into five limited functional areas. As soon as the metacarpal or metatarsal precartilages have
condensed in the limb paddle, some signs of a lengthwise growth can be
seen in the distal part of each of them (see Section Vb). During this
lengthening phase, the distal extremity of these precartilaginous rods
gradually brings a mechanical pressure to bear on the contiguous part