DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
81
whether random growth of fibers can ever be assumed when normal
motility is restored after the division of a motor nerve. In the regeneration of the optic nerve, in fish and in Amphibia, the fibers that grow once
more from the ganglion layer of the retina cross the scar wholly in
disorder, yet some process of "unscrambling" occurs between this point
and their entry into the optic tectum (Sperry, 1951). Furthermore, when
normal function is regained after a comparatively short interval, Schwann
cells associated with fibers that have degenerated may persist in sufficiently organized groups to serve as guiding tracks along which new
fibers reach their appropriate destinations; this possibility needs to be
excluded by histological study.
There is evidence that in Anura, the extent to which muscles recover
normal function after regeneration of their nerves of supply decreases
with age. In Rana, Weiss (1941) cross-sutured nerves in the hind limb
and observed a full recovery of function in larvae before metamorphosis,
but observed that this ability was lost in juvenile life. Again, Sperry
(1947) found that in anuran tadpoles, regeneration of the nerve supply
to the extrinsic eye muscles was followed by the return of normal eye
movements only in young larvae, and not at later stages.
In juveniles of Xenopus, division of the sciatic nerve within the thigh
is followed by a slow return of the normal action of the distal segments
of the leg (Hughes and Prestige, unpublished). In postfetal mammals,
however, Sperry (1945) has concluded that there is no evidence for any
regain of normal use in limb muscles where the nerves of supply have
regenerated either at random or after experimental cross-suture, except
for the limited possibilities of conscious re-education in the human
subject.
It thus appears that the diverse results of experiments on nerve
regeneration provide no clear indication how embryonic nerve and
muscle fibers achieve functional contact in the first instance. The growth
of nerve fibers toward groups of muscles and the establishment of functional contacts, either in the first instance during development or later
in regeneration, may proceed in various ways. Of such, the two extremes
are that growth may be entirely random, with definitive neuromuscular
relationships,subsequently established; or, on the other hand, the precise
function of each individual axon may be already determined before it
has made contact with muscle fibers, which it grows toward and makes
contact with in some fashion as yet little understood. In the following
pages will be described what has been learned concerning the development of the innervation of the tetrapod limb by combining observations
81
whether random growth of fibers can ever be assumed when normal
motility is restored after the division of a motor nerve. In the regeneration of the optic nerve, in fish and in Amphibia, the fibers that grow once
more from the ganglion layer of the retina cross the scar wholly in
disorder, yet some process of "unscrambling" occurs between this point
and their entry into the optic tectum (Sperry, 1951). Furthermore, when
normal function is regained after a comparatively short interval, Schwann
cells associated with fibers that have degenerated may persist in sufficiently organized groups to serve as guiding tracks along which new
fibers reach their appropriate destinations; this possibility needs to be
excluded by histological study.
There is evidence that in Anura, the extent to which muscles recover
normal function after regeneration of their nerves of supply decreases
with age. In Rana, Weiss (1941) cross-sutured nerves in the hind limb
and observed a full recovery of function in larvae before metamorphosis,
but observed that this ability was lost in juvenile life. Again, Sperry
(1947) found that in anuran tadpoles, regeneration of the nerve supply
to the extrinsic eye muscles was followed by the return of normal eye
movements only in young larvae, and not at later stages.
In juveniles of Xenopus, division of the sciatic nerve within the thigh
is followed by a slow return of the normal action of the distal segments
of the leg (Hughes and Prestige, unpublished). In postfetal mammals,
however, Sperry (1945) has concluded that there is no evidence for any
regain of normal use in limb muscles where the nerves of supply have
regenerated either at random or after experimental cross-suture, except
for the limited possibilities of conscious re-education in the human
subject.
It thus appears that the diverse results of experiments on nerve
regeneration provide no clear indication how embryonic nerve and
muscle fibers achieve functional contact in the first instance. The growth
of nerve fibers toward groups of muscles and the establishment of functional contacts, either in the first instance during development or later
in regeneration, may proceed in various ways. Of such, the two extremes
are that growth may be entirely random, with definitive neuromuscular
relationships,subsequently established; or, on the other hand, the precise
function of each individual axon may be already determined before it
has made contact with muscle fibers, which it grows toward and makes
contact with in some fashion as yet little understood. In the following
pages will be described what has been learned concerning the development of the innervation of the tetrapod limb by combining observations