DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
89
of most Amphibia is laid down during the development of the oocyte
within the ovary, and is, thus, quite separate in origin and time of appearance from that developed within the embryo from the melanocytes of
neural crest origin. The ovarian pigment is distributed through the cells
formed during the cleavage of the egg and is still the only melanin present
in the embryo up to late neurula stages. The neural epithelium receives
its share of pigment, but within its cells the granules become progressively diluted as mitosis proceeds and the neural tube grows in size.
Consequently those cells that cease to divide and differentiate relatively
early contain more melanin than do those that belong to later generations
of cells. In the early larva of well-pigmented Amphibia are seen heavily
loaded cells at the outer edge of the mantle layer in a transverse section
of the cord, among which the primary motor cells of the cord are conspicuous (Hughes, 1963).
Neuroblasts of the early ventral horn also contain melanin granules
in amounts varying from one species to another. The proportion of cells
in which melanin can be seen decreases during development, mainly
through degeneration; and some estimate of the degree of cell turnover
can thereby be derived, if one makes the assumption that individual
granules do not pass from one cell to another. This is most likely to be
true where no cell contains more than a very few granules. Quantitative
estimation has been attempted for only one species, the Trinidadian
Hyla punctatissima,
in which the granules are relatively large. In larvae
of this Hyla, it is possible to count four categories of ventral horn cells,
namely, viable cells with and without pigment, together with the corresponding groups of degenerating cells. The results can be shown in serial
analysis, in which the whole ventral horn is divided into successive equal
blocks of sections, in each of which the number of cells in each caetegory
is plotted on a longitudinal axis. Such an analysis is shown in Fig. 2 for
three stages of development, namely, first, a larva of 40 mm in total
length, a second of 46 mm, and a third animal at metamorphosis.
In Fig. 2 the number of cells in each category is shown serially in consecutive groups of equal numbers of sections through the lumbar ventral
horns of three stages of development of Hyla punctatissima.
In the
earliest larva, 40 mm in length, the number of degenerating cells is still
relatively low, whereas in the second (46 mm in length) degenerations
are frequent, though the number of cells in the ventral horn has not yet
fallen. At metamorphosis, cell death has almost ceased and the total
number of cells is by then about one-eighth of the earlier figure. From
Fig. 2 it can be seen that between the stages of 40 and 46 mm the number
89
of most Amphibia is laid down during the development of the oocyte
within the ovary, and is, thus, quite separate in origin and time of appearance from that developed within the embryo from the melanocytes of
neural crest origin. The ovarian pigment is distributed through the cells
formed during the cleavage of the egg and is still the only melanin present
in the embryo up to late neurula stages. The neural epithelium receives
its share of pigment, but within its cells the granules become progressively diluted as mitosis proceeds and the neural tube grows in size.
Consequently those cells that cease to divide and differentiate relatively
early contain more melanin than do those that belong to later generations
of cells. In the early larva of well-pigmented Amphibia are seen heavily
loaded cells at the outer edge of the mantle layer in a transverse section
of the cord, among which the primary motor cells of the cord are conspicuous (Hughes, 1963).
Neuroblasts of the early ventral horn also contain melanin granules
in amounts varying from one species to another. The proportion of cells
in which melanin can be seen decreases during development, mainly
through degeneration; and some estimate of the degree of cell turnover
can thereby be derived, if one makes the assumption that individual
granules do not pass from one cell to another. This is most likely to be
true where no cell contains more than a very few granules. Quantitative
estimation has been attempted for only one species, the Trinidadian
Hyla punctatissima,
in which the granules are relatively large. In larvae
of this Hyla, it is possible to count four categories of ventral horn cells,
namely, viable cells with and without pigment, together with the corresponding groups of degenerating cells. The results can be shown in serial
analysis, in which the whole ventral horn is divided into successive equal
blocks of sections, in each of which the number of cells in each caetegory
is plotted on a longitudinal axis. Such an analysis is shown in Fig. 2 for
three stages of development, namely, first, a larva of 40 mm in total
length, a second of 46 mm, and a third animal at metamorphosis.
In Fig. 2 the number of cells in each category is shown serially in consecutive groups of equal numbers of sections through the lumbar ventral
horns of three stages of development of Hyla punctatissima.
In the
earliest larva, 40 mm in length, the number of degenerating cells is still
relatively low, whereas in the second (46 mm in length) degenerations
are frequent, though the number of cells in the ventral horn has not yet
fallen. At metamorphosis, cell death has almost ceased and the total
number of cells is by then about one-eighth of the earlier figure. From
Fig. 2 it can be seen that between the stages of 40 and 46 mm the number