184
J. MILAIRE
morphogenesis until 1951, when I began this investigation under the
guidance of Dalcq (1956). By the use of standard histochemical techniques it was possible to detect the presence of a number of well-known
basic substances in the limb buds of three rodents (mouse, rat, guineapig), one insectivore (mole), four reptiles (Emys, Pseudemys, Chamelaeo,
Mabuia), one bird (chick), and in the fin buds of one elasmobranch fish
(Scylliorhinus canicula). We are now in possession of a great deal of data
which enable us to understand the main morphogenetic processes involved in the limb development of vertebrates in general. Although
some of these investigations are not yet complete, I think that it would
be useful to attempt a preliminary collation of the data. This will include information on many vertebrates which have not been studied
experimentally.
Before proceeding with the results of the histochemical experiments,
I should like, with the aid of Zwilling's (1961) recent survey, to recapitulate briefly the main features of experimental work on limb morphogenesis.
II. Experimental Data
In all vertebrates, the earliest limb buds that can be seen in sections
are simple thickenings of the somatopleure covered by ectoderm. The
following description concerns developmental stages after the first
growth activities of this part of the mesoderm. It is well known that at
earlier stages, the dorsal part of the somatopleure already possesses
some limb properties. The pioneering experiments of Harrison (1918,
1925) established that axial relationships were determined at a very
early stage in the presumptive area of the limbs of the amphibian
embryo; more recently, Chaube (1959) has shown that this is also true
of the chick embryo. Various workers, studying the limb buds themselves,
independently suggested, at about the same time, that the apical ectoderm might play an active part in limb development (Steiner, 1928;
Filatow, 1928; Balinsky, 1931; Rotmann, 1933). In 1948, Saunders
gave a convincing demonstration of the role of the apical ectodermal
ridge (a.e.r.) in the development of the chick embryo limb bud; his
work provided a new stimulus to the problem of limb morphogenesis
and it was followed by a great deal of experimental work on the subject.
The results can be summarized as follows:
(a) Attempts to determine the fate of each part of the early mesoderm led investigators to draw maps of the presumptive areas of the
limb bud (Saunders, 1948; Hampe, 1957; Tschumi, 1957). These maps
show that the potentialities of the initial mesoderm are limited to the
more proximal part of the limb (scapular or pelvic girdle). The mesodermal components destined to form the more distal segments of the
J. MILAIRE
morphogenesis until 1951, when I began this investigation under the
guidance of Dalcq (1956). By the use of standard histochemical techniques it was possible to detect the presence of a number of well-known
basic substances in the limb buds of three rodents (mouse, rat, guineapig), one insectivore (mole), four reptiles (Emys, Pseudemys, Chamelaeo,
Mabuia), one bird (chick), and in the fin buds of one elasmobranch fish
(Scylliorhinus canicula). We are now in possession of a great deal of data
which enable us to understand the main morphogenetic processes involved in the limb development of vertebrates in general. Although
some of these investigations are not yet complete, I think that it would
be useful to attempt a preliminary collation of the data. This will include information on many vertebrates which have not been studied
experimentally.
Before proceeding with the results of the histochemical experiments,
I should like, with the aid of Zwilling's (1961) recent survey, to recapitulate briefly the main features of experimental work on limb morphogenesis.
II. Experimental Data
In all vertebrates, the earliest limb buds that can be seen in sections
are simple thickenings of the somatopleure covered by ectoderm. The
following description concerns developmental stages after the first
growth activities of this part of the mesoderm. It is well known that at
earlier stages, the dorsal part of the somatopleure already possesses
some limb properties. The pioneering experiments of Harrison (1918,
1925) established that axial relationships were determined at a very
early stage in the presumptive area of the limbs of the amphibian
embryo; more recently, Chaube (1959) has shown that this is also true
of the chick embryo. Various workers, studying the limb buds themselves,
independently suggested, at about the same time, that the apical ectoderm might play an active part in limb development (Steiner, 1928;
Filatow, 1928; Balinsky, 1931; Rotmann, 1933). In 1948, Saunders
gave a convincing demonstration of the role of the apical ectodermal
ridge (a.e.r.) in the development of the chick embryo limb bud; his
work provided a new stimulus to the problem of limb morphogenesis
and it was followed by a great deal of experimental work on the subject.
The results can be summarized as follows:
(a) Attempts to determine the fate of each part of the early mesoderm led investigators to draw maps of the presumptive areas of the
limb bud (Saunders, 1948; Hampe, 1957; Tschumi, 1957). These maps
show that the potentialities of the initial mesoderm are limited to the
more proximal part of the limb (scapular or pelvic girdle). The mesodermal components destined to form the more distal segments of the