86
ARTHUR HUGHES
flexion precedes leg extension, an action which becomes capable of repetition with increasing facility.
C. The Ventral Horn
The correspondence between these two species is not, however, confined to these features of their developing behavior, but extends to histogenetic changes within the spinal cord, concerning the motor nuclei which
innervate the limbs, the lumbar ventral horns. In Anura, each ventral
horn from its first appearance is a separate mass of cells. In Xenopus at
stage 53, immediately prior to limb movement, it consists of a group of
neuroblasts, ventrolateral to the mantle layer of the cord, opposite the
spinal ganglia which supply the limb. There is evidence from various
types of experiment (Hughes and Tschumi, 1958, 1960) that cell differentiation within these centers depends in some way on the presence of
the early limb bud, and that this influence of periphery on center is
mediated through sensory pathways.
The discrete nature of the anuran ventral horn and the relatively
small number of cells which compose it, makes possible a quantitative
approach to the problems of its development. Inspection of sections
through larvae of Xenopus of different ages suggests that as development
advances, and the ventral horn cells enlarge, they become fewer in number. Counts in each section through the whole group of cells show that,
whereas each ventral horn at its first appearance contains some 5000 or
6000 cells, by metamorphosis some 60 days later this number is reduced
to about 1200, and then remains constant into adult life. As the cell
content of the ventral horn declines, pycnotic cells appear among the
normal neurons. At the same time, acid phosphatase, presumably of lysosomal origin, appears within the ventral horn (Palkama and Prestige,
1964). A similar correspondence with the onset of cell degeneration is
seen within the developing dorsal root ganglia.
Figure 1 shows that the incidence of degeneration among ventral horn
cells in Xenopus is highest at the time when the total population is falling most rapidly. Yet it seems that the removal of cells by degeneration
is on a scale too large to account solely for the reduction in total numbers,
as is shown by calculating the total number of cells which are lost during
development of the ventral horn. To make this estimate it is necessary
to know the time it takes for a ventral horn cell to degenerate, then the
absolute number of degenerating cells at any time may be converted
into a cell death rate per hour.
If the early limb bud is amputated, then within the next 3 days an
ARTHUR HUGHES
flexion precedes leg extension, an action which becomes capable of repetition with increasing facility.
C. The Ventral Horn
The correspondence between these two species is not, however, confined to these features of their developing behavior, but extends to histogenetic changes within the spinal cord, concerning the motor nuclei which
innervate the limbs, the lumbar ventral horns. In Anura, each ventral
horn from its first appearance is a separate mass of cells. In Xenopus at
stage 53, immediately prior to limb movement, it consists of a group of
neuroblasts, ventrolateral to the mantle layer of the cord, opposite the
spinal ganglia which supply the limb. There is evidence from various
types of experiment (Hughes and Tschumi, 1958, 1960) that cell differentiation within these centers depends in some way on the presence of
the early limb bud, and that this influence of periphery on center is
mediated through sensory pathways.
The discrete nature of the anuran ventral horn and the relatively
small number of cells which compose it, makes possible a quantitative
approach to the problems of its development. Inspection of sections
through larvae of Xenopus of different ages suggests that as development
advances, and the ventral horn cells enlarge, they become fewer in number. Counts in each section through the whole group of cells show that,
whereas each ventral horn at its first appearance contains some 5000 or
6000 cells, by metamorphosis some 60 days later this number is reduced
to about 1200, and then remains constant into adult life. As the cell
content of the ventral horn declines, pycnotic cells appear among the
normal neurons. At the same time, acid phosphatase, presumably of lysosomal origin, appears within the ventral horn (Palkama and Prestige,
1964). A similar correspondence with the onset of cell degeneration is
seen within the developing dorsal root ganglia.
Figure 1 shows that the incidence of degeneration among ventral horn
cells in Xenopus is highest at the time when the total population is falling most rapidly. Yet it seems that the removal of cells by degeneration
is on a scale too large to account solely for the reduction in total numbers,
as is shown by calculating the total number of cells which are lost during
development of the ventral horn. To make this estimate it is necessary
to know the time it takes for a ventral horn cell to degenerate, then the
absolute number of degenerating cells at any time may be converted
into a cell death rate per hour.
If the early limb bud is amputated, then within the next 3 days an