THE DEVELOPMENT OF INNERVATION IN
TETRAPOD
LIMBS
ARTHUR HUGHES
Zoology Department,
University of Bristol, Bristol, England
I. Introduction
79
II. The Anura
83
A. The Prefunctional Period .
.
.
83
B. The Development of Limb Movement
84
C. The Ventral Horn
86
D. Innervation of the Limb at Functional Stages
96
III. Comparison with Mammalian Development
106
IV. Summary
HO
References
HI
I. Introduction
The demonstration that the nerve fiber is formed by outgrowth (His,
1879; Ramon y Cajal, 1890; Harrison, 1910) established the fact that
the nervous system develops from cellular units. This basic fact by itself,
however, sheds no light on the problems of how the immensely complex
pattern of the nervous system is built up by the growth of individual
nerve fibers. Indeed, the recognition that a nerve fiber, however long its
course, is the product of a single cell rendered such problems still more
mysterious. The earlier theory (Hensen, 1868, 1876; Held, 1909) that
protoplasmic filaments, locally developed within the tissues of the
embryo, serve as the precursors of the nerve fiber had provided a simple
basis for an explanation of nervous pathways; none of this, however,
could survive the advent of the doctrine of nervous outgrowth. More
recently, systems of preformed pathways (Weiss, 1934) at micellar of
other subcellular orders of magnitude have been postulated. This hypothesis again shifts the problem of patterning of the nervous system back
to an early stage of ontogeny. Such orientated tracts preceding the
growth of fibers within the embryo still remain to be demonstrated.
Meanwhile there are difficulties in relating the results of experiments on
the orientation of nerve fibers in tissue culture to the growth of nerves
within the embryo (Hughes, 1960).
79
TETRAPOD
LIMBS
ARTHUR HUGHES
Zoology Department,
University of Bristol, Bristol, England
I. Introduction
79
II. The Anura
83
A. The Prefunctional Period .
.
.
83
B. The Development of Limb Movement
84
C. The Ventral Horn
86
D. Innervation of the Limb at Functional Stages
96
III. Comparison with Mammalian Development
106
IV. Summary
HO
References
HI
I. Introduction
The demonstration that the nerve fiber is formed by outgrowth (His,
1879; Ramon y Cajal, 1890; Harrison, 1910) established the fact that
the nervous system develops from cellular units. This basic fact by itself,
however, sheds no light on the problems of how the immensely complex
pattern of the nervous system is built up by the growth of individual
nerve fibers. Indeed, the recognition that a nerve fiber, however long its
course, is the product of a single cell rendered such problems still more
mysterious. The earlier theory (Hensen, 1868, 1876; Held, 1909) that
protoplasmic filaments, locally developed within the tissues of the
embryo, serve as the precursors of the nerve fiber had provided a simple
basis for an explanation of nervous pathways; none of this, however,
could survive the advent of the doctrine of nervous outgrowth. More
recently, systems of preformed pathways (Weiss, 1934) at micellar of
other subcellular orders of magnitude have been postulated. This hypothesis again shifts the problem of patterning of the nervous system back
to an early stage of ontogeny. Such orientated tracts preceding the
growth of fibers within the embryo still remain to be demonstrated.
Meanwhile there are difficulties in relating the results of experiments on
the orientation of nerve fibers in tissue culture to the growth of nerves
within the embryo (Hughes, 1960).
79