L I M B M O R P H O G E N E S I S I N V E R T E B R A T E S
189
chemical peculiarity, the presence of several chemical substances can be
demonstrated at this stage in the entire thickened ventral ectoderm.
An increase of RNA in that part of the ectoderm has been demonstrated
in the rat, the mouse (Fig. 1(c)), the mole, the chick and four reptilian
embryos. It is largely concentrated in the cytoplasm of the inner cell
layer, close to the basal membrane. Synthesis of both alkaline and acid
phosphatases occurs simultaneously with that of RNA in the whole
thickened ectoderm; the enzyme acting in alkaline conditions was detected there in the rat, the mouse and the mole (Fig. 1 (a)) embryos, while
acid phosphatase was demonstrated in the same region of the mouse
embryo (Fig. 1(b)), the only species in which this stage has been studied.
It must be mentioned here that in the mouse embryo, the acid phosphatase activity of the ventral ectoderm is higher in the marginal zone
which will later differentiate into the a.e.r. (Fig. 1(b)). However, in
reptiles and also in the chick embryo, the a.e.r. is both morphologically
and histochemically differentiated at a very early stage of limb development, though in these species, the entire ventral ectoderm of the limb
bud is always histochemically different from the dorsal: in the chick
embryo limb bud, the activity of alkaline phosphatase is strong in the
early a.e.r., it is weak in the thickened ventral ectoderm and nearly
non-existent in the dorsal ectoderm. Both mueopolysaccharides and
glycogen are present in the a.e.r. of the two chelonian reptiles studied
but only glycogen is observed in the ventral ectoderm of the same limb
buds (Milaire, 1957, Fig. 4, Plate XVI).
These facts make the first interactions between the mesodermal bud
and its ectodermal covering evident. The thickening of the ventral
ectoderm and the new metabolic processes simultaneously involved are
clearly the result of some influence exerted by the condensed and histochemically differentiated underlying mesoderm. However, although this
applies to the whole ventral ectoderm in all forms studied, the marginal
ectodermal zone of mammalian limb buds and the early a.e.r. of avian
and reptilian limb buds show specific histochemical properties from the
beginning of limb development.
3. Asymmetrical Growth of the Limb Buds
Soon after these early morphological changes have occurred in the
distal part of the limb bud, the latter enters a period of active growth
during which, according to experimental data, new mesodermal areas
are successively induced distally. As they grow, the limb buds progressively bend toward the tail of the embryo; this is the first
external indication of the presence of asymmetrical conditions in limb
morphogenesis. At this stage, several morphological as well as
G2
A . M . 2
189
chemical peculiarity, the presence of several chemical substances can be
demonstrated at this stage in the entire thickened ventral ectoderm.
An increase of RNA in that part of the ectoderm has been demonstrated
in the rat, the mouse (Fig. 1(c)), the mole, the chick and four reptilian
embryos. It is largely concentrated in the cytoplasm of the inner cell
layer, close to the basal membrane. Synthesis of both alkaline and acid
phosphatases occurs simultaneously with that of RNA in the whole
thickened ectoderm; the enzyme acting in alkaline conditions was detected there in the rat, the mouse and the mole (Fig. 1 (a)) embryos, while
acid phosphatase was demonstrated in the same region of the mouse
embryo (Fig. 1(b)), the only species in which this stage has been studied.
It must be mentioned here that in the mouse embryo, the acid phosphatase activity of the ventral ectoderm is higher in the marginal zone
which will later differentiate into the a.e.r. (Fig. 1(b)). However, in
reptiles and also in the chick embryo, the a.e.r. is both morphologically
and histochemically differentiated at a very early stage of limb development, though in these species, the entire ventral ectoderm of the limb
bud is always histochemically different from the dorsal: in the chick
embryo limb bud, the activity of alkaline phosphatase is strong in the
early a.e.r., it is weak in the thickened ventral ectoderm and nearly
non-existent in the dorsal ectoderm. Both mueopolysaccharides and
glycogen are present in the a.e.r. of the two chelonian reptiles studied
but only glycogen is observed in the ventral ectoderm of the same limb
buds (Milaire, 1957, Fig. 4, Plate XVI).
These facts make the first interactions between the mesodermal bud
and its ectodermal covering evident. The thickening of the ventral
ectoderm and the new metabolic processes simultaneously involved are
clearly the result of some influence exerted by the condensed and histochemically differentiated underlying mesoderm. However, although this
applies to the whole ventral ectoderm in all forms studied, the marginal
ectodermal zone of mammalian limb buds and the early a.e.r. of avian
and reptilian limb buds show specific histochemical properties from the
beginning of limb development.
3. Asymmetrical Growth of the Limb Buds
Soon after these early morphological changes have occurred in the
distal part of the limb bud, the latter enters a period of active growth
during which, according to experimental data, new mesodermal areas
are successively induced distally. As they grow, the limb buds progressively bend toward the tail of the embryo; this is the first
external indication of the presence of asymmetrical conditions in limb
morphogenesis. At this stage, several morphological as well as
G2
A . M . 2