DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
107
distinct from one another, axons can be traced into the limb, but no
functional connections are as yet established between muscles and nerves.
While the subdivision of the ventral horn continues, fibers enter into a
closer relation with the various muscles of the limb and trunk—a development that "proceeds proximo-distally, and as each segment of the limb
is progressively innervated, a further cell column separates from the main
lateral mass of cells ... in such a way that at the end of this stage all
the cell columns present in the adult are visible." At 16.5 days, "fine cell
processes can be found leaving the trunks of the nerves throughout the
limb. They are most marked in the shoulder muscles and least in the
muscles of the hand," whereas the 19-day embryo "shows a considerable
increase in the number of fine nerve fibers ramifying among the muscles
of the hand." The onset of motility in the limb is at much the same time,
and movements appear in a proximodistal order.
The differentiating musculature of the mammalian fetus has been
found to be excitable by direct electrical stimulation before any normal
movements occur. In the rat fetus, Windle et al. (1935) found that the
developing shoulder muscles responded to shocks at 15.3 days (11 mm
crown-rump length), whereas not until a day later was there any reflex
response to muscle stretch. A table of the genesis of movement due to
E. A. Swenson, quoted by East (1931), agrees in placing the earliest
movements at the sixteenth day. This table states that the limbs can
then adduct and abduct and that the hind foot flexes and extends 3 days
later. In the rabbit fetus, Pankratz (1939) observed that the forelimb
moved with the trunk on the sixteenth day, and independent limb movements developed during the period from the nineteenth to the twentyninth day, with flexion of carpal joints and spreading of the toes. In the
cat, Windle and Griffin (1931) found that proximal muscles in the limb
became functional before distal. These results were confirmed by Coronios
(1933), and similar sequences were seen in the sheep fetus by Barcroft
and Barron (1939), in which, as in the ventral horn of the rabbit, the
"cell columns are . . . in the process of segregation at the time the first
reactivity of the embryo occurs" (Barron, 1941).
This event is preceded by the division of the originally continuous
ventrolateral column of motor neuroblasts in the cord into separate
ventral horns for each limb—a process which in the fetal mouse begins on
the twelfth day (Harris, 1965). This author has studied the process
quantitatively, counting both viable and degenerating cells from the
eleventh to the fifteenth day. She finds that degenerations appear within
the column on the twelfth day and rise to a peak within the next 2 days,
107
distinct from one another, axons can be traced into the limb, but no
functional connections are as yet established between muscles and nerves.
While the subdivision of the ventral horn continues, fibers enter into a
closer relation with the various muscles of the limb and trunk—a development that "proceeds proximo-distally, and as each segment of the limb
is progressively innervated, a further cell column separates from the main
lateral mass of cells ... in such a way that at the end of this stage all
the cell columns present in the adult are visible." At 16.5 days, "fine cell
processes can be found leaving the trunks of the nerves throughout the
limb. They are most marked in the shoulder muscles and least in the
muscles of the hand," whereas the 19-day embryo "shows a considerable
increase in the number of fine nerve fibers ramifying among the muscles
of the hand." The onset of motility in the limb is at much the same time,
and movements appear in a proximodistal order.
The differentiating musculature of the mammalian fetus has been
found to be excitable by direct electrical stimulation before any normal
movements occur. In the rat fetus, Windle et al. (1935) found that the
developing shoulder muscles responded to shocks at 15.3 days (11 mm
crown-rump length), whereas not until a day later was there any reflex
response to muscle stretch. A table of the genesis of movement due to
E. A. Swenson, quoted by East (1931), agrees in placing the earliest
movements at the sixteenth day. This table states that the limbs can
then adduct and abduct and that the hind foot flexes and extends 3 days
later. In the rabbit fetus, Pankratz (1939) observed that the forelimb
moved with the trunk on the sixteenth day, and independent limb movements developed during the period from the nineteenth to the twentyninth day, with flexion of carpal joints and spreading of the toes. In the
cat, Windle and Griffin (1931) found that proximal muscles in the limb
became functional before distal. These results were confirmed by Coronios
(1933), and similar sequences were seen in the sheep fetus by Barcroft
and Barron (1939), in which, as in the ventral horn of the rabbit, the
"cell columns are . . . in the process of segregation at the time the first
reactivity of the embryo occurs" (Barron, 1941).
This event is preceded by the division of the originally continuous
ventrolateral column of motor neuroblasts in the cord into separate
ventral horns for each limb—a process which in the fetal mouse begins on
the twelfth day (Harris, 1965). This author has studied the process
quantitatively, counting both viable and degenerating cells from the
eleventh to the fifteenth day. She finds that degenerations appear within
the column on the twelfth day and rise to a peak within the next 2 days,