88
ARTHUR HUGHES
that over a period of 1 or 2 days we can neglect the rate of accretion of
new cells to the ventral horn, which after a further 10 days or more restores the normal number.
The period at which the total number of ventral horn cells is falling
most rapidly is between stages 54 and 59, at precisely the same time as
the development of movement of the limb is mainly concentrated. Again
in Eleutherodactylus
the same is true. Here the number of cells in the
ventral horn during development decreases from about 1200 to 300, and
this fall is most rapid between 7.5 and 9 days.
As soon as these correspondences were revealed it was assumed that
there was some causal connection between them. Close study was, therefore, made of the development of both the ventral horn and of the innervation of the leg, particularly with regard to the manner by which
each grows and differentiates. The large-scale turnover of cells in the
ventral horn and the reduction in total number which has been seen in
all the Anura which have so far been investigated is only one of the
changes which the ventral horn undergoes during development. As the
cells differentiate into mature neurons, they become fewer in number. It
has been shown, however, that these two processes are separable. By depriving anuran tadpoles and embryos of thyroid hormones, the normal
loss of cells continues, while the differentiation of the individual cell is
inhibited (Kollros and Race, 1960; Hughes, 1966b). Under these conditions the total number of cells decreases, the horn increases in length,
with a corresponding reduction in the number of cells at transverse levels.
The growth in length is always concentrated at the caudal pole, a fact
which can be inferred by comparison of the lumbar horns on each side in
an animal at metamorphosis where one limb has been amputated at
about stage 55. The growth of the ipsilateral ventral horn is curtailed,
and so on the unoperated side the motor neurons extend much further
caudally (Hughes, 1961, Fig. 2F). Inspection of serial sections through
the developing ventral horn shows clearly that the youngest cells are at
the caudal end. In quantitative work, only those cells counted as "ventral
horn cells" are clearly more differentiated than those of the more medial
mantle layer, both with respect to nuclear size and the development of
the cytoplasm. Through series of transverse sections this distinction becomes less easy to draw as one approaches the caudal end of the horn.
There is other evidence, however, that the recruitment of cells to the
anuran ventral horn during development is greater at hinder levels. This
depends on a natural marker that in some Anura is found within the first
neurons to differentiate. As is well known, the black pigment of the eggs
ARTHUR HUGHES
that over a period of 1 or 2 days we can neglect the rate of accretion of
new cells to the ventral horn, which after a further 10 days or more restores the normal number.
The period at which the total number of ventral horn cells is falling
most rapidly is between stages 54 and 59, at precisely the same time as
the development of movement of the limb is mainly concentrated. Again
in Eleutherodactylus
the same is true. Here the number of cells in the
ventral horn during development decreases from about 1200 to 300, and
this fall is most rapid between 7.5 and 9 days.
As soon as these correspondences were revealed it was assumed that
there was some causal connection between them. Close study was, therefore, made of the development of both the ventral horn and of the innervation of the leg, particularly with regard to the manner by which
each grows and differentiates. The large-scale turnover of cells in the
ventral horn and the reduction in total number which has been seen in
all the Anura which have so far been investigated is only one of the
changes which the ventral horn undergoes during development. As the
cells differentiate into mature neurons, they become fewer in number. It
has been shown, however, that these two processes are separable. By depriving anuran tadpoles and embryos of thyroid hormones, the normal
loss of cells continues, while the differentiation of the individual cell is
inhibited (Kollros and Race, 1960; Hughes, 1966b). Under these conditions the total number of cells decreases, the horn increases in length,
with a corresponding reduction in the number of cells at transverse levels.
The growth in length is always concentrated at the caudal pole, a fact
which can be inferred by comparison of the lumbar horns on each side in
an animal at metamorphosis where one limb has been amputated at
about stage 55. The growth of the ipsilateral ventral horn is curtailed,
and so on the unoperated side the motor neurons extend much further
caudally (Hughes, 1961, Fig. 2F). Inspection of serial sections through
the developing ventral horn shows clearly that the youngest cells are at
the caudal end. In quantitative work, only those cells counted as "ventral
horn cells" are clearly more differentiated than those of the more medial
mantle layer, both with respect to nuclear size and the development of
the cytoplasm. Through series of transverse sections this distinction becomes less easy to draw as one approaches the caudal end of the horn.
There is other evidence, however, that the recruitment of cells to the
anuran ventral horn during development is greater at hinder levels. This
depends on a natural marker that in some Anura is found within the first
neurons to differentiate. As is well known, the black pigment of the eggs