196
GEORGE SZÉKELY
B. Determination of the Limb-Moving Segmental Apparatus in
the Spinal Cord
It is known from a beautiful series of experiments made by Detwiler
and his associates (Detwiler, 1920, 1936; Detwiler and Carpenter, 1929)
on Ambystoma
that a grafted limb innervated by spinal segments which
normally do not supply limbs does not exhibit coordinated movement
unless a branch from the brachial plexus, however small, contributes to
the innervation of the grafted limb. Nerve formation under such circumstances seems to be fairly normal (Detwiler, 1920; Piatt, 1956), so that
failure of nerve formation and innervation of the muscles cannot be
blamed for the result. It is more probable that the limb-innervating
segments might alone possess the central apparatus required to move
limbs in a coordinated manner. Rogers' (1934) experiment gives support
to this assumption. He grafted a section of the brachial cord into the
myotome segments just above the limb bud in an Ambystoma
embryo
and could record coordinated movements of the limb innervated by the
isolated piece of cord. A section of the trunk cord, in otherwise identical
circumstances, could not bring the limb into coordinated movements. His
results suggest that the determination of this spinal system might
happen in the embryo.
To study the process of determination of this limb-moving spinal
apparatus, a series of experiments was made on embryos of Pleurodeles
waltlii and Triturus vulgaris (Székely, 1963; Straznicky and Székely,
1966). The trunk cord was removed and replaced by an equal length of
brachial cord taken from another embryo. After the onset of feeding, a
pair of fore- or hindlimbs was grafted into the myoseptal region, close
to the transplanted cord segments. Normal coordinated movements of
the supernumerary limb were found when the cord was transplanted immediately after the closure of the medullary tube. If the establishment
of the capacity of these segments to move limbs may be called the
"functional determination
,, of the participating neurons, this result resembles the establishment of the functional polarity of the eye primordium, which also occurs at that age. To see if any axes could be
found along which the functional determination takes place separately
in time, as in the case of the eye primordium, the brachial section of the
cord was craniocaudally rotated before transplantation. No difference
in the locomotion of the extra pair of limbs was found, even when the
operation was done at the latest larval stage. The substitution of the
trunk cord for the brachial segments in embryos of increasing age,
however, revealed a sign of progression. Trunk segments could substitute
for brachial segments in every respect if they were transplanted before
GEORGE SZÉKELY
B. Determination of the Limb-Moving Segmental Apparatus in
the Spinal Cord
It is known from a beautiful series of experiments made by Detwiler
and his associates (Detwiler, 1920, 1936; Detwiler and Carpenter, 1929)
on Ambystoma
that a grafted limb innervated by spinal segments which
normally do not supply limbs does not exhibit coordinated movement
unless a branch from the brachial plexus, however small, contributes to
the innervation of the grafted limb. Nerve formation under such circumstances seems to be fairly normal (Detwiler, 1920; Piatt, 1956), so that
failure of nerve formation and innervation of the muscles cannot be
blamed for the result. It is more probable that the limb-innervating
segments might alone possess the central apparatus required to move
limbs in a coordinated manner. Rogers' (1934) experiment gives support
to this assumption. He grafted a section of the brachial cord into the
myotome segments just above the limb bud in an Ambystoma
embryo
and could record coordinated movements of the limb innervated by the
isolated piece of cord. A section of the trunk cord, in otherwise identical
circumstances, could not bring the limb into coordinated movements. His
results suggest that the determination of this spinal system might
happen in the embryo.
To study the process of determination of this limb-moving spinal
apparatus, a series of experiments was made on embryos of Pleurodeles
waltlii and Triturus vulgaris (Székely, 1963; Straznicky and Székely,
1966). The trunk cord was removed and replaced by an equal length of
brachial cord taken from another embryo. After the onset of feeding, a
pair of fore- or hindlimbs was grafted into the myoseptal region, close
to the transplanted cord segments. Normal coordinated movements of
the supernumerary limb were found when the cord was transplanted immediately after the closure of the medullary tube. If the establishment
of the capacity of these segments to move limbs may be called the
"functional determination
,, of the participating neurons, this result resembles the establishment of the functional polarity of the eye primordium, which also occurs at that age. To see if any axes could be
found along which the functional determination takes place separately
in time, as in the case of the eye primordium, the brachial section of the
cord was craniocaudally rotated before transplantation. No difference
in the locomotion of the extra pair of limbs was found, even when the
operation was done at the latest larval stage. The substitution of the
trunk cord for the brachial segments in embryos of increasing age,
however, revealed a sign of progression. Trunk segments could substitute
for brachial segments in every respect if they were transplanted before