DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
97
veloping muscle fibers at related levels within the limb. We may further
assume that functional contact between nerve fiber and muscle fiber are
only made at a particular stage in their differentiation, possibly one of
short duration. The first functional contacts between nerve and muscle
fibers at which cholinesterase can be detected are, as far as can be judged
from the use of the light microscope, mere approximations of one element
with the other. There is evidence that the enzyme can be detected in
more proximal regions earlier than in distal segments.
It seems clear that the first fibers that transmit impulses to limb
muscles are only a minority of those present in each nerve of supply.
There is a close correlation between the time of the onset of function in
the muscles of the triceps femoris complex in Eleutherodactylus
when the
knee joint is first able to extend and that of an increase in diameter of a
few of the nerve fibers that supply these muscles (Hughes, 1965a). The
caliber of nerve fibers in Eleutherodactylus
is in proportion to the general
scale of the animal. Before the limb begins to move, the fibers of limb
nerves appear to be well under a micron in diameter. Growth in caliber
first begins near the point of entry of the nerve into the muscle, and
when the leg first kicks with vigor, a few "large" fibers of 2.0 jx in
diameter can be seen in the ramus profundus anterior from which the
muscle is supplied. There are then some five or six of these fibers. In the
adult Eleutherodactylus,
most of the fibers in the nerve to the triceps
femoris are relatively large and of similar caliber.
There is evidence that limb movement can still develop if the number
of fibers that enter the muscles is reduced well below the normal level.
When embryos of Eleutherodactylus,
before the stage of motile limbs,
are deprived of thyroid hormones, either by thyroidectomy or by hypophysectomy, fibers disappear from the nerve that supplies the triceps
femoris, though limb movement nevertheless develops normally (Hughes,
1966b). Such embryos even give an impression of hyperactivity. In a
fully hypophysectomized embryo the growth in volume of the muscle
does not lag behind that of an "operational control" in which some
pituitary tissue remained after the operation. Yet there is a marked difference in the number of nerve fibers in each group of operated embryos
(Fig. 7).
The beginnings of limb movement were seen in limbs of Bufo
marinus
that had been xenografted on hosts of Eleutherodactylus
martinicensis
(Hughes, 1964a)— an indication of the unspecific nature of the first
contacts between motor axons and muscle fibers of the limb, These
97
veloping muscle fibers at related levels within the limb. We may further
assume that functional contact between nerve fiber and muscle fiber are
only made at a particular stage in their differentiation, possibly one of
short duration. The first functional contacts between nerve and muscle
fibers at which cholinesterase can be detected are, as far as can be judged
from the use of the light microscope, mere approximations of one element
with the other. There is evidence that the enzyme can be detected in
more proximal regions earlier than in distal segments.
It seems clear that the first fibers that transmit impulses to limb
muscles are only a minority of those present in each nerve of supply.
There is a close correlation between the time of the onset of function in
the muscles of the triceps femoris complex in Eleutherodactylus
when the
knee joint is first able to extend and that of an increase in diameter of a
few of the nerve fibers that supply these muscles (Hughes, 1965a). The
caliber of nerve fibers in Eleutherodactylus
is in proportion to the general
scale of the animal. Before the limb begins to move, the fibers of limb
nerves appear to be well under a micron in diameter. Growth in caliber
first begins near the point of entry of the nerve into the muscle, and
when the leg first kicks with vigor, a few "large" fibers of 2.0 jx in
diameter can be seen in the ramus profundus anterior from which the
muscle is supplied. There are then some five or six of these fibers. In the
adult Eleutherodactylus,
most of the fibers in the nerve to the triceps
femoris are relatively large and of similar caliber.
There is evidence that limb movement can still develop if the number
of fibers that enter the muscles is reduced well below the normal level.
When embryos of Eleutherodactylus,
before the stage of motile limbs,
are deprived of thyroid hormones, either by thyroidectomy or by hypophysectomy, fibers disappear from the nerve that supplies the triceps
femoris, though limb movement nevertheless develops normally (Hughes,
1966b). Such embryos even give an impression of hyperactivity. In a
fully hypophysectomized embryo the growth in volume of the muscle
does not lag behind that of an "operational control" in which some
pituitary tissue remained after the operation. Yet there is a marked difference in the number of nerve fibers in each group of operated embryos
(Fig. 7).
The beginnings of limb movement were seen in limbs of Bufo
marinus
that had been xenografted on hosts of Eleutherodactylus
martinicensis
(Hughes, 1964a)— an indication of the unspecific nature of the first
contacts between motor axons and muscle fibers of the limb, These