102
ARTHUR HUGHES
This theory postulates that the turnover of cells within the ventral horn
is to some extent selective. During the period when limb motility is most
rapidly developing in Xenopus, a wider range of limb movements than
is seen in the development of normal behavior can be elicited in the
spinal tadpole by the application of any type of stimulus. This fact may
suggest that the central synaptic patterns through which limb movements are initiated and inhibited exert a selective effect on peripheral
pathways. On the other hand, Weiss's theory of myotypic specification
of motor neurons by muscles, which does not take account of cell or fiber
turnover during development, maintains the opposite view of the interrelationships between center and periphery during development.
There is evidence for a further selective mechanism that operates at
still later stages of development among the fibers of nerves of supply.
Toward the end of embryonic life in Eleutherodactylus
there are striking
changes in the number of nerve fibers that supply the main muscles of
the limb, whereas smaller muscles receive a constant number of fibers.
The two largest muscles of the leg, the triceps femoris complex and the
adductor magnus are innervated by a larger number of fibers than are
any other leg muscles. In the nerves to both muscles there is a peak in
the number of fibers at the twelfth to thirteenth day, after which there
is a steep fall. This peak corresponds with one seen in counts of nerve
fibers at more central positions, in the lumbar nerve roots, particularly
the dorsal, and also in the sciatic nerve trunk. There is a similar
maximum in the largest cutaneous nerve, the crural. The largest nerves
within the limb follow the variations in number of fibers in the lumbar
roots, though these changes have less influence on the composition of the
smaller nerves of supply (Fig. 10). In Xenopus, there is a comparable
decrease in the number of fibers in lumbar roots toward the approach of
metamorphosis.
The general significance of these peaks in Eleutherodactylus
is not
easy to discern. They tend to flatten when the embryo is growing under
slightly unnatural conditions, as, for instance, when removed from the
envelope and immersed in water or saline. They apparently reflect some
general changes within the whole embryo, for they also affect the volume
of the largest leg muscles. Within the peripheral nervous system, however, it seems that toward the end of development the decrease in number of nerve fibers is associated with a withdrawal of elements irrelevant
to the plan of the fully developed nervous system. Such evidence comes
from the study of the innervation and movement of grafted limbs
(Hughes, 1962, 1964b). This was derived in the first place from the
ARTHUR HUGHES
This theory postulates that the turnover of cells within the ventral horn
is to some extent selective. During the period when limb motility is most
rapidly developing in Xenopus, a wider range of limb movements than
is seen in the development of normal behavior can be elicited in the
spinal tadpole by the application of any type of stimulus. This fact may
suggest that the central synaptic patterns through which limb movements are initiated and inhibited exert a selective effect on peripheral
pathways. On the other hand, Weiss's theory of myotypic specification
of motor neurons by muscles, which does not take account of cell or fiber
turnover during development, maintains the opposite view of the interrelationships between center and periphery during development.
There is evidence for a further selective mechanism that operates at
still later stages of development among the fibers of nerves of supply.
Toward the end of embryonic life in Eleutherodactylus
there are striking
changes in the number of nerve fibers that supply the main muscles of
the limb, whereas smaller muscles receive a constant number of fibers.
The two largest muscles of the leg, the triceps femoris complex and the
adductor magnus are innervated by a larger number of fibers than are
any other leg muscles. In the nerves to both muscles there is a peak in
the number of fibers at the twelfth to thirteenth day, after which there
is a steep fall. This peak corresponds with one seen in counts of nerve
fibers at more central positions, in the lumbar nerve roots, particularly
the dorsal, and also in the sciatic nerve trunk. There is a similar
maximum in the largest cutaneous nerve, the crural. The largest nerves
within the limb follow the variations in number of fibers in the lumbar
roots, though these changes have less influence on the composition of the
smaller nerves of supply (Fig. 10). In Xenopus, there is a comparable
decrease in the number of fibers in lumbar roots toward the approach of
metamorphosis.
The general significance of these peaks in Eleutherodactylus
is not
easy to discern. They tend to flatten when the embryo is growing under
slightly unnatural conditions, as, for instance, when removed from the
envelope and immersed in water or saline. They apparently reflect some
general changes within the whole embryo, for they also affect the volume
of the largest leg muscles. Within the peripheral nervous system, however, it seems that toward the end of development the decrease in number of nerve fibers is associated with a withdrawal of elements irrelevant
to the plan of the fully developed nervous system. Such evidence comes
from the study of the innervation and movement of grafted limbs
(Hughes, 1962, 1964b). This was derived in the first place from the