194
J. MILAIRE
group the histochemical pattern of the thickened ectoderm shows that
metabolic activity is restricted to the axillary (or inguinal) ectoderm
and the a.e.r., while the intermediate portion of the ectoderm is histochemically unreactive. Both the axillary ectoderm and a.e.r. are provided with acid phosphatase in embryos of the rat, mouse (Fig. 3(c)) and
chick, and also with glycogen and alkaline phosphatase in the mole
embryo (Fig. 3(b)), but at a somewhat later stage.
The main morphological and histochemical changes occurring in the
growing limb buds may be summarized and interpreted as follows.
In all species (except the dogfish), the compact and basophilic ventral
mesoderm of the limb buds becomes a proliferating centre, the area of
maximum activity being in the proximo-postaxial part. This proliferation occurs close to the underlying axillary thickened ectoderm which is
histochemically differentiated in all forms studied, and which may also
show specific histochemical properties in some species. Although there is
at present no experimental evidence supporting this idea, these facts
lead us to believe that the thickened axillary (or inguinal) ectoderm
could exert an inductive influence on the contiguous proximal mesoderm. The new mesodermal cells originating in the proximal part of the
limb bud, still provided with RNA, seem to reach the limb axis and
perhaps also the extreme distal zone of the limb bud by migrating under
the ventral thickened ectoderm. The predominance of these activities
in the postaxial part of the limb bud provides a satisfactory explanation
of the asymmetrical growth of the appendage.
Other morphological and histochemical events occur simultaneously
in the postaxial half of the limb bud and are more closely connected
with new morphogenetic activities taking place in the extreme distal
part of the bud. The unequal distribution of alkaline phosphatase in the
mesoderm of the mole embryo limb buds is the only observation of a
descriptive kind supporting the existence of specific properties in the
postaxial mesoderm. However, two metabolic processes which at first
occurred in the entire ventral ectoderm and then only in the differentiating a.e.r. have been demonstrated in some other ectodermal structures subject to an inductive influence: the synthesis of both RNA and
alkaline phosphatase takes place in the otic and olfactory placodes and
also in the early dental and prehypophyseal rudiments as soon as these
ectodermal organs begin to differentiate (Milaire, 1959). The enzymatic
activity disappears in these structures at the end of their first inductive
phase, except in some restricted areas, such as the presumptive zones
for the endolymphatic duct and the horizontal semi-circular canal,
which are probably subjected to secondary inductive influences. Thus,
without giving formal evidence of inductive interaction between the
mesoderm and the ectoderm of the limb bud, these histochemical data
J. MILAIRE
group the histochemical pattern of the thickened ectoderm shows that
metabolic activity is restricted to the axillary (or inguinal) ectoderm
and the a.e.r., while the intermediate portion of the ectoderm is histochemically unreactive. Both the axillary ectoderm and a.e.r. are provided with acid phosphatase in embryos of the rat, mouse (Fig. 3(c)) and
chick, and also with glycogen and alkaline phosphatase in the mole
embryo (Fig. 3(b)), but at a somewhat later stage.
The main morphological and histochemical changes occurring in the
growing limb buds may be summarized and interpreted as follows.
In all species (except the dogfish), the compact and basophilic ventral
mesoderm of the limb buds becomes a proliferating centre, the area of
maximum activity being in the proximo-postaxial part. This proliferation occurs close to the underlying axillary thickened ectoderm which is
histochemically differentiated in all forms studied, and which may also
show specific histochemical properties in some species. Although there is
at present no experimental evidence supporting this idea, these facts
lead us to believe that the thickened axillary (or inguinal) ectoderm
could exert an inductive influence on the contiguous proximal mesoderm. The new mesodermal cells originating in the proximal part of the
limb bud, still provided with RNA, seem to reach the limb axis and
perhaps also the extreme distal zone of the limb bud by migrating under
the ventral thickened ectoderm. The predominance of these activities
in the postaxial part of the limb bud provides a satisfactory explanation
of the asymmetrical growth of the appendage.
Other morphological and histochemical events occur simultaneously
in the postaxial half of the limb bud and are more closely connected
with new morphogenetic activities taking place in the extreme distal
part of the bud. The unequal distribution of alkaline phosphatase in the
mesoderm of the mole embryo limb buds is the only observation of a
descriptive kind supporting the existence of specific properties in the
postaxial mesoderm. However, two metabolic processes which at first
occurred in the entire ventral ectoderm and then only in the differentiating a.e.r. have been demonstrated in some other ectodermal structures subject to an inductive influence: the synthesis of both RNA and
alkaline phosphatase takes place in the otic and olfactory placodes and
also in the early dental and prehypophyseal rudiments as soon as these
ectodermal organs begin to differentiate (Milaire, 1959). The enzymatic
activity disappears in these structures at the end of their first inductive
phase, except in some restricted areas, such as the presumptive zones
for the endolymphatic duct and the horizontal semi-circular canal,
which are probably subjected to secondary inductive influences. Thus,
without giving formal evidence of inductive interaction between the
mesoderm and the ectoderm of the limb bud, these histochemical data