304
EDGAR
ZWILLING
capable of participating with the mesoderm in normal limb morphogenesis. The details of which ectodermal regions are capable of cooperating with limb mesoderm vary with the embryonic stage and the
species used. The epidermis is not without influence on limb development
since it may (in chimaeric combinations, Rotmann, 1933) influence
digital shape.
Most of the early studies of limb development stem from the pioneering experiments of Ross G. Harrison, who provided some of the earliest
and still most rigorous analyses. The data of Harrison and his students,
largely obtained from embryos of Ambystoma punctatum, stress the
importance of the mesoderm for limb development. There were some
dissenting viewpoints based on experiments with urodeles. Filatow
(1928), using Ambystoma mexicanum, grafted limb mesoderm from a
stage in which the limb bud is just barely evident into a pocket of host
epidermis and obtained no limb structures. When he grafted the overlying epithelium, cleaned of all mesoderm, to a non-limb site, limbs
developed in about half of his operated animals. Balinsky (1931)
obtained no limbs from grafts of limb mesoderm (between Triturus
taeniatus and Ambystoma mexicanum) taken from fairly late (stages
29-36) limb buds; the grafted mesoderm was resorbed. The overwhelming weight of consistent and positive evidence, however, came
from the work of the American group and the mesoderm was established
as the chief determiner of limb properties.
It is interesting, though, that the experiments which most strongly
indicate a morphogenetic role for the ectoderm were done with later
stages than those used by the Yale group. Thus Steiner (1928) used limb
buds (not discs) in experiments in which the 'ektodermkappe' of Rana
fusca and Triturus taeniatus buds was injured by cautery. Limb development stopped until the 'ektodermkappe' regenerated. Balinsky's (1931)
ideas about co-operative relations between ectoderm and mesoderm are
based on experiments in which ectoderm of limb buds, following
rotation of 180°, seemed to influence the position of the limbs, while
similar rotation of ectoderm of younger stages had no effect and the
limbs developed in accordance with the position of the mesoderm.
None of the experiments done with chick embryo limb buds (the chief
amniote embryo on which such experiments have been done) during
this early period made a significant contribution to an understanding of
the factors involved in limb morphogenesis. This is not surprising since
the period of the early and late 1920's represents one in which the basic
techniques for manipulating and growing graft tissues were developed.
Early extirpation, chorio-allantoic grafting and culture experiments
(Murray and Huxley, 1925; Murray, 1926; Selby and Murray, 1928;
Fell and Canti, 1934) indicated that young chick embryo limb buds
EDGAR
ZWILLING
capable of participating with the mesoderm in normal limb morphogenesis. The details of which ectodermal regions are capable of cooperating with limb mesoderm vary with the embryonic stage and the
species used. The epidermis is not without influence on limb development
since it may (in chimaeric combinations, Rotmann, 1933) influence
digital shape.
Most of the early studies of limb development stem from the pioneering experiments of Ross G. Harrison, who provided some of the earliest
and still most rigorous analyses. The data of Harrison and his students,
largely obtained from embryos of Ambystoma punctatum, stress the
importance of the mesoderm for limb development. There were some
dissenting viewpoints based on experiments with urodeles. Filatow
(1928), using Ambystoma mexicanum, grafted limb mesoderm from a
stage in which the limb bud is just barely evident into a pocket of host
epidermis and obtained no limb structures. When he grafted the overlying epithelium, cleaned of all mesoderm, to a non-limb site, limbs
developed in about half of his operated animals. Balinsky (1931)
obtained no limbs from grafts of limb mesoderm (between Triturus
taeniatus and Ambystoma mexicanum) taken from fairly late (stages
29-36) limb buds; the grafted mesoderm was resorbed. The overwhelming weight of consistent and positive evidence, however, came
from the work of the American group and the mesoderm was established
as the chief determiner of limb properties.
It is interesting, though, that the experiments which most strongly
indicate a morphogenetic role for the ectoderm were done with later
stages than those used by the Yale group. Thus Steiner (1928) used limb
buds (not discs) in experiments in which the 'ektodermkappe' of Rana
fusca and Triturus taeniatus buds was injured by cautery. Limb development stopped until the 'ektodermkappe' regenerated. Balinsky's (1931)
ideas about co-operative relations between ectoderm and mesoderm are
based on experiments in which ectoderm of limb buds, following
rotation of 180°, seemed to influence the position of the limbs, while
similar rotation of ectoderm of younger stages had no effect and the
limbs developed in accordance with the position of the mesoderm.
None of the experiments done with chick embryo limb buds (the chief
amniote embryo on which such experiments have been done) during
this early period made a significant contribution to an understanding of
the factors involved in limb morphogenesis. This is not surprising since
the period of the early and late 1920's represents one in which the basic
techniques for manipulating and growing graft tissues were developed.
Early extirpation, chorio-allantoic grafting and culture experiments
(Murray and Huxley, 1925; Murray, 1926; Selby and Murray, 1928;
Fell and Canti, 1934) indicated that young chick embryo limb buds