110
ARTHUR HUGHES
under similar circumstances, though deafferentation of the lumbar cord
in Anura does not inhibit the early phases of limb motility (Taylor,
1944; Weiss, 1941). There is evidence, however, in Xenopus,
of the
necessity for sensory pathways for the earliest steps in the differentiation of ventral horn cells (Hughes and Tschumi, 1958). In the late
embryo of Eleutherodactylus
(Hughes, 1965c), the removal of the
central processes of the lumbar ganglia results in paralysis of the limb
and degeneration of fibers within its nerves—a result that has also been
obtained with larvae of Bufo marinus and Xenopus laevis at stages near
metamorphosis.
In the adult Anuran, the effect of deafferentation has been described
as little (Gray, 1950) or none (Weiss, 1941). Brookhart and Fadiga
(1960) have shown that in the isolated spinal cord of the frog, motor
neurons can be excited by stimuli applied to the lateral column but not
by any given to dorsal roots. Furthermore, the fibers of descending tracts
make synaptic contacts with the cell body of the motor neuron, whereas
those of incoming dorsal root fibers are confined to dendrites. It can be
surmised that in Anura the first synapses on ventral horn cells belong to
the former category, for early limb movements are closely related to
activity of the trunk. Further study of how these synaptic patterns are
built up during development is the key to further understanding of the
functional development of limb innervation.
IV. Summary
1. The anatomical pattern of the nerves is established in the early
limb bud by the ingrowth of relatively few fibers. The limb nerves are
not formed in relation to other structures; their development is part of
the primary morphogenetic processes within the limb.
2. Subsequent differentiation of ventral horn cells continuously sends
into the limb fresh axons that are guided by the primary fibers, soon
greatly outnumbered by new arrivals. In the differentiation of the anuran
ventral horn, there is evidence that the cranial half is in advance of the
caudal. Motility of the limb segments develops in proximodistal order.
There is evidence that these facts are related and that thigh muscles are
supplied by relatively maturer and more cranial neurons than those that
innervate more distal segments.
3. This rough linear order of development of center and periphery in
Anura results in many transient nerve-muscle contacts that do not
conform to the final pattern of relationships. Stimulation of developing
lumbar nerves in Xenopus
tadpoles at first evokes a wide range of
ARTHUR HUGHES
under similar circumstances, though deafferentation of the lumbar cord
in Anura does not inhibit the early phases of limb motility (Taylor,
1944; Weiss, 1941). There is evidence, however, in Xenopus,
of the
necessity for sensory pathways for the earliest steps in the differentiation of ventral horn cells (Hughes and Tschumi, 1958). In the late
embryo of Eleutherodactylus
(Hughes, 1965c), the removal of the
central processes of the lumbar ganglia results in paralysis of the limb
and degeneration of fibers within its nerves—a result that has also been
obtained with larvae of Bufo marinus and Xenopus laevis at stages near
metamorphosis.
In the adult Anuran, the effect of deafferentation has been described
as little (Gray, 1950) or none (Weiss, 1941). Brookhart and Fadiga
(1960) have shown that in the isolated spinal cord of the frog, motor
neurons can be excited by stimuli applied to the lateral column but not
by any given to dorsal roots. Furthermore, the fibers of descending tracts
make synaptic contacts with the cell body of the motor neuron, whereas
those of incoming dorsal root fibers are confined to dendrites. It can be
surmised that in Anura the first synapses on ventral horn cells belong to
the former category, for early limb movements are closely related to
activity of the trunk. Further study of how these synaptic patterns are
built up during development is the key to further understanding of the
functional development of limb innervation.
IV. Summary
1. The anatomical pattern of the nerves is established in the early
limb bud by the ingrowth of relatively few fibers. The limb nerves are
not formed in relation to other structures; their development is part of
the primary morphogenetic processes within the limb.
2. Subsequent differentiation of ventral horn cells continuously sends
into the limb fresh axons that are guided by the primary fibers, soon
greatly outnumbered by new arrivals. In the differentiation of the anuran
ventral horn, there is evidence that the cranial half is in advance of the
caudal. Motility of the limb segments develops in proximodistal order.
There is evidence that these facts are related and that thigh muscles are
supplied by relatively maturer and more cranial neurons than those that
innervate more distal segments.
3. This rough linear order of development of center and periphery in
Anura results in many transient nerve-muscle contacts that do not
conform to the final pattern of relationships. Stimulation of developing
lumbar nerves in Xenopus
tadpoles at first evokes a wide range of