DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
85
the latter part of development, simultaneous movement of both arms and
legs become increasingly diagonalwise, and this tendency is already clear
at the time when the leg is first capable of reflex extension. During the
development of both types of behavior, the individual movements are at
first fragmentary. In early attempts at diagonal action of the limbs, the
embryo ceases movement abruptly to hold a bizarre pose from which it
will resume many seconds later to complete the movement, sometimes in
several stages.
In a larva such as Xenopus in which some bodily activity is continuous,
it is less easy to distinguish between reflex and spontaneous activity.
Random trembling motion of the hind limbs is seen between stages 53
and 54, after which is seen the first clear-cut movement, consisting of a
single bilateral flexure at the hip joint, whereby each limb moves outward from the normal position, at that time parallel to the body axis;
then follows a passive relaxation. This action is called by Prestige (1967)
the "flare." It is never seen while the larva is swimming or during periods
of rest, but only immediately after a bout of activity. It cannot be
elicited by touch, for at this time the limb is wholly insensitive.
When the limb has grown to 4 mm in length, a further period of steady
development of limb action begins, which is continuous until stage 58,
some 12 days later. This is marked by the appearance of reciprocal and
alternate stepping movements after a light touch on tail or legs. Up to
four cycles of repeated steps may follow each other. In each leg there is
first flexion at the hip and knee and then extension of both hip and knee
joints. The trunk generally remains still during stepping activity. During the stepping stages of development, bilateral flare movement after
swimming is also seen and becomes even more prominent and less labile.
The duration of swimming necessary to evoke a flare gradually diminishes until only a change in posture is necessary. This movement now involves both hip flexion and knee extension. At stages 60-61 a new phase
of limb action begins in that the legs are then used continuously in
swimming, as well as the tail. In Xenopus, leg swimming is by repeated
bilateral thrusts. At stage 63, all movement ceases in the tail.
Xenopus and Eleutherodactylus
develop on two widely different time
scales, and yet the formative period when the action of the leg is developing is confined to a relatively short period in both—in
Eleutherodactylus for about 36 hr out of a total of 14 days. In Xenopus, the corresponding figures are 15 days out of 58 for development at 22°-24°C
according to the data of Nieuwkoop and Faber (1956). The sequence
seen in both animals shows a general resemblance in that leg abduction or
85
the latter part of development, simultaneous movement of both arms and
legs become increasingly diagonalwise, and this tendency is already clear
at the time when the leg is first capable of reflex extension. During the
development of both types of behavior, the individual movements are at
first fragmentary. In early attempts at diagonal action of the limbs, the
embryo ceases movement abruptly to hold a bizarre pose from which it
will resume many seconds later to complete the movement, sometimes in
several stages.
In a larva such as Xenopus in which some bodily activity is continuous,
it is less easy to distinguish between reflex and spontaneous activity.
Random trembling motion of the hind limbs is seen between stages 53
and 54, after which is seen the first clear-cut movement, consisting of a
single bilateral flexure at the hip joint, whereby each limb moves outward from the normal position, at that time parallel to the body axis;
then follows a passive relaxation. This action is called by Prestige (1967)
the "flare." It is never seen while the larva is swimming or during periods
of rest, but only immediately after a bout of activity. It cannot be
elicited by touch, for at this time the limb is wholly insensitive.
When the limb has grown to 4 mm in length, a further period of steady
development of limb action begins, which is continuous until stage 58,
some 12 days later. This is marked by the appearance of reciprocal and
alternate stepping movements after a light touch on tail or legs. Up to
four cycles of repeated steps may follow each other. In each leg there is
first flexion at the hip and knee and then extension of both hip and knee
joints. The trunk generally remains still during stepping activity. During the stepping stages of development, bilateral flare movement after
swimming is also seen and becomes even more prominent and less labile.
The duration of swimming necessary to evoke a flare gradually diminishes until only a change in posture is necessary. This movement now involves both hip flexion and knee extension. At stages 60-61 a new phase
of limb action begins in that the legs are then used continuously in
swimming, as well as the tail. In Xenopus, leg swimming is by repeated
bilateral thrusts. At stage 63, all movement ceases in the tail.
Xenopus and Eleutherodactylus
develop on two widely different time
scales, and yet the formative period when the action of the leg is developing is confined to a relatively short period in both—in
Eleutherodactylus for about 36 hr out of a total of 14 days. In Xenopus, the corresponding figures are 15 days out of 58 for development at 22°-24°C
according to the data of Nieuwkoop and Faber (1956). The sequence
seen in both animals shows a general resemblance in that leg abduction or