DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
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movements were later lost as an increasing incompatibility of these tissues led to degeneration of the host nerves which had penetrated into the
grafted limbs.
When motility of the limb is first seen in the normal embryo, first in
flexion at the hip joint and later at the knee, nerve-muscle contacts
within the limb, relatively few in number, are being achieved in proximodistal order. This by itself would be expected to lead only to the comparatively weak action of the limb which then occurs. It is likely that
the return of the limb to a position of rest after such movements is then
wholly elastic, and as yet owes nothing to separate innervation of
antagonist muscles.
Further evidence of the nature of nerve-muscle relationships at this
period comes from experiments in which shocks from an electronic
stimulator were applied to muscles and to nerves of the hind limb of
larvae of Xenopus in which the spinal cord had been severed from the
brain (Hughes and Prestige, 1967). During the earlier phases of the development of the hind limb, and before the appearance of any movement,
no response is seen by stimulating either cord or lumbar ganglia or by
applying an electrode directly to the limb. In the earliest phases of limb
motility, between stages 53 and 54, however, some uncoordinated movement is seen when muscles are stimulated through the skin. At this period
the limb does not respond to touch, and movement is only occasionally
seen when stimuli are applied to the cord or ganglia, though such responses develop during the course of the flare stage of motility, during
stages 54 and 55.
The first reactions of the limb to shocks applied to the cord or the
ganglia are variable for several reasons. In the first place it is a matter
of chance which groups of fibers are excited by an electrode placed in
their proximity, and different responses were seen with larvae up to stage
56 on repeated applications of an electrode to what appeared to be the
same spot. Later, however, the random placing of the electrode had less
effect on the result obtained. Second, the immature neurons are very
readily fatigued. This loss of response was much more marked in younger
larvae where it occurred after three or four bursts of impulses had been
delivered. At stage 54, only occasionally do shocks provoke the flexion
at the hip which at this period is the sole movement of the limb in normal
behavior. Stimuli applied to the cord may result either in uncoordinated
action of the whole limb, rotation of the foot to and fro in its own plane,
or a simultaneous tetanic contraction of all thigh and leg muscles. At
other times the foot waves about at random. Commonly seen is a rotation
99
movements were later lost as an increasing incompatibility of these tissues led to degeneration of the host nerves which had penetrated into the
grafted limbs.
When motility of the limb is first seen in the normal embryo, first in
flexion at the hip joint and later at the knee, nerve-muscle contacts
within the limb, relatively few in number, are being achieved in proximodistal order. This by itself would be expected to lead only to the comparatively weak action of the limb which then occurs. It is likely that
the return of the limb to a position of rest after such movements is then
wholly elastic, and as yet owes nothing to separate innervation of
antagonist muscles.
Further evidence of the nature of nerve-muscle relationships at this
period comes from experiments in which shocks from an electronic
stimulator were applied to muscles and to nerves of the hind limb of
larvae of Xenopus in which the spinal cord had been severed from the
brain (Hughes and Prestige, 1967). During the earlier phases of the development of the hind limb, and before the appearance of any movement,
no response is seen by stimulating either cord or lumbar ganglia or by
applying an electrode directly to the limb. In the earliest phases of limb
motility, between stages 53 and 54, however, some uncoordinated movement is seen when muscles are stimulated through the skin. At this period
the limb does not respond to touch, and movement is only occasionally
seen when stimuli are applied to the cord or ganglia, though such responses develop during the course of the flare stage of motility, during
stages 54 and 55.
The first reactions of the limb to shocks applied to the cord or the
ganglia are variable for several reasons. In the first place it is a matter
of chance which groups of fibers are excited by an electrode placed in
their proximity, and different responses were seen with larvae up to stage
56 on repeated applications of an electrode to what appeared to be the
same spot. Later, however, the random placing of the electrode had less
effect on the result obtained. Second, the immature neurons are very
readily fatigued. This loss of response was much more marked in younger
larvae where it occurred after three or four bursts of impulses had been
delivered. At stage 54, only occasionally do shocks provoke the flexion
at the hip which at this period is the sole movement of the limb in normal
behavior. Stimuli applied to the cord may result either in uncoordinated
action of the whole limb, rotation of the foot to and fro in its own plane,
or a simultaneous tetanic contraction of all thigh and leg muscles. At
other times the foot waves about at random. Commonly seen is a rotation