84
ARTHUR
HUGHES
instance did nerve fibers penetrate the transplanted limb. Although the
anatomical pattern of the limb nerves can be recognized at an early
stage of the innervation of the limb, it does not necessarily follow that
the pioneer nerve fibers of which the nerves are then composed are distributed within the limb bud in any ordered fashion. The function of
these fibers, it may be suggested, is primarily to bring in Schwann cells,
which lay down tracks along which grow later generations of nerve
fibers that seem unable to reach their terminal stations without such aid.
B. The Development of Limb Movement
Before the limb of the Eleutherodactylus
embryo shows any movement
at all, a positive Koelle reaction for acetylcholinesterase can be detected
within most of the developing muscle masses. In the same way, the dyecoupled esterase reaction described by Lewis (1958), which in embryonic
muscles can be regarded as indicating the presence of cholinesterase
(Lewis and Hughes, 1960), is positive within the upper half of the hind
limb of Xenopus at stage 53, whereas the first movements are not seen
until the following stage.
In Eleutherodactylus,
the first movements begin at about 7.5 days,
whai myofibrils are seen to one side of the nucleus in the muscle cells of
the limb. These, however, are as yet without cross-striations. The first
movements are only seen during periods of trunk activity. They are
uncoordinated and consist of an uncertain wiping and clapping action
of the feet. From this point two separate patterns of movement can be
traced in embryos freed from their envelopes and developing in water.
One is the reflex behavior shown in response to light touch over the body
surface (Hughes, 1965a); the other is the pattern of spontaneous movements seen in continuous observation of unstimulated embryos (Hughes,
1966a). The first leg movement independent of that of the trunk is seen
at 8-8.5 days. By then cross-striations are apparent in all groups of
limb muscles. The first reflex movement is a withdrawal, but to this is
soon added a subsequent rapid extension of the limb, a movement which
has been termed the "kick," usually at first on the opposite side to that
stimulated. In the following days, leg extension develops into powerful
and symmetrical thrusts that, when capable of being sustained, result in
continuous bilateral swimming, in coordination with the action of the
forelimbs.
By contrast, the spontaneous behavior of the embryo builds up into
diagonal ambulation. Movements in unstimulated embryos are seen in
head, trunk, tail, and limbs, either singly or in any combination. During
ARTHUR
HUGHES
instance did nerve fibers penetrate the transplanted limb. Although the
anatomical pattern of the limb nerves can be recognized at an early
stage of the innervation of the limb, it does not necessarily follow that
the pioneer nerve fibers of which the nerves are then composed are distributed within the limb bud in any ordered fashion. The function of
these fibers, it may be suggested, is primarily to bring in Schwann cells,
which lay down tracks along which grow later generations of nerve
fibers that seem unable to reach their terminal stations without such aid.
B. The Development of Limb Movement
Before the limb of the Eleutherodactylus
embryo shows any movement
at all, a positive Koelle reaction for acetylcholinesterase can be detected
within most of the developing muscle masses. In the same way, the dyecoupled esterase reaction described by Lewis (1958), which in embryonic
muscles can be regarded as indicating the presence of cholinesterase
(Lewis and Hughes, 1960), is positive within the upper half of the hind
limb of Xenopus at stage 53, whereas the first movements are not seen
until the following stage.
In Eleutherodactylus,
the first movements begin at about 7.5 days,
whai myofibrils are seen to one side of the nucleus in the muscle cells of
the limb. These, however, are as yet without cross-striations. The first
movements are only seen during periods of trunk activity. They are
uncoordinated and consist of an uncertain wiping and clapping action
of the feet. From this point two separate patterns of movement can be
traced in embryos freed from their envelopes and developing in water.
One is the reflex behavior shown in response to light touch over the body
surface (Hughes, 1965a); the other is the pattern of spontaneous movements seen in continuous observation of unstimulated embryos (Hughes,
1966a). The first leg movement independent of that of the trunk is seen
at 8-8.5 days. By then cross-striations are apparent in all groups of
limb muscles. The first reflex movement is a withdrawal, but to this is
soon added a subsequent rapid extension of the limb, a movement which
has been termed the "kick," usually at first on the opposite side to that
stimulated. In the following days, leg extension develops into powerful
and symmetrical thrusts that, when capable of being sustained, result in
continuous bilateral swimming, in coordination with the action of the
forelimbs.
By contrast, the spontaneous behavior of the embryo builds up into
diagonal ambulation. Movements in unstimulated embryos are seen in
head, trunk, tail, and limbs, either singly or in any combination. During