106
ARTHUR
HUGHES
undergo an extensor tetanus. The whole animal then begins to move
rapidly in an uncontrolled manner for about half a minute, after which
the legs relax and the animal appears normal once more. At high concentrations, which also affect the action of the trunk muscles, the legs
remain permanently extended in tetanus.
Larvae of Rana temporaria and of Bufo vulgaris react to strychnine
in essentially the same manner. In Eleutherodactylus,
far higher concentrations of strychnine are necessary, and the general permeability of
this embryo seems much lower than in larval Anura. At concentrations
of 1/2000 to 1/10,000 w/v, embryos at stages from 9.5 days onward extend
the legs rigidly in a typical tetanus within a few minutes of immersion.
This condition is continuously maintained. Before this stage, however,
such concentrations of the drug are without obvious effects on the limbs,
even at 8.5 days, when the limb can retract but is not yet able to extend.
The retraction reflex of the leg being thus insensitive to the drug, corresponds to the flare reaction of the Xenopus larva. Extension of the leg
is sensitive to strychnine both in Xenopus and in
Eleutherodactylus.
The observation that tetanus in the muscles of the limb is not evident
until the leg is able to extend shows that by then inhibitory mechanisms
similar to those of the adult animal are in operation. The achievement
of a separate innervation of opposed muscles must precede this stage,
though presumably only by a brief interval, for muscles of one group
could hardly operate effectively without the inhibition of their antagonists.
III. Comparison with Mammalian Development
In higher vertebrates, the ventrolateral somatic motor column of the
spinal cord is at first a continuous tract of cells and is later broken up
into discrete horns for each limb, as Hamburger and Levi-Montalcini
(1950) have described in the chick embryo. Still later, each ventral horn
becomes subdivided into columns of cells such that each is thought to
innervate muscles of similar function (Romanes, 1951, 1964). In mammalian development, this aspect of differentiation of the ventral horn
has provided a special opportunity for the study of the development of
the relationships between the groups of motor neurons and the muscles
which they innervate. Romanes (1941) has described the development
in the rabbit fetus of the brachial ventral horn, which he has correlated
with the course of innervation of the muscles of the forelimb and the
appearance of motility therein. On the fourteenth day, the ventral horn
begins to divide into its subsidiary groups of cells. As these first become
ARTHUR
HUGHES
undergo an extensor tetanus. The whole animal then begins to move
rapidly in an uncontrolled manner for about half a minute, after which
the legs relax and the animal appears normal once more. At high concentrations, which also affect the action of the trunk muscles, the legs
remain permanently extended in tetanus.
Larvae of Rana temporaria and of Bufo vulgaris react to strychnine
in essentially the same manner. In Eleutherodactylus,
far higher concentrations of strychnine are necessary, and the general permeability of
this embryo seems much lower than in larval Anura. At concentrations
of 1/2000 to 1/10,000 w/v, embryos at stages from 9.5 days onward extend
the legs rigidly in a typical tetanus within a few minutes of immersion.
This condition is continuously maintained. Before this stage, however,
such concentrations of the drug are without obvious effects on the limbs,
even at 8.5 days, when the limb can retract but is not yet able to extend.
The retraction reflex of the leg being thus insensitive to the drug, corresponds to the flare reaction of the Xenopus larva. Extension of the leg
is sensitive to strychnine both in Xenopus and in
Eleutherodactylus.
The observation that tetanus in the muscles of the limb is not evident
until the leg is able to extend shows that by then inhibitory mechanisms
similar to those of the adult animal are in operation. The achievement
of a separate innervation of opposed muscles must precede this stage,
though presumably only by a brief interval, for muscles of one group
could hardly operate effectively without the inhibition of their antagonists.
III. Comparison with Mammalian Development
In higher vertebrates, the ventrolateral somatic motor column of the
spinal cord is at first a continuous tract of cells and is later broken up
into discrete horns for each limb, as Hamburger and Levi-Montalcini
(1950) have described in the chick embryo. Still later, each ventral horn
becomes subdivided into columns of cells such that each is thought to
innervate muscles of similar function (Romanes, 1951, 1964). In mammalian development, this aspect of differentiation of the ventral horn
has provided a special opportunity for the study of the development of
the relationships between the groups of motor neurons and the muscles
which they innervate. Romanes (1941) has described the development
in the rabbit fetus of the brachial ventral horn, which he has correlated
with the course of innervation of the muscles of the forelimb and the
appearance of motility therein. On the fourteenth day, the ventral horn
begins to divide into its subsidiary groups of cells. As these first become