DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
87
5000
4000
3000
2000
\000\Length of hind limbs
Viable cells
Degenerations
„_
Q>
ells
V
ble c
f vial
o
jmbei
2
Days
51 I 53 I 55 I 57 I 59 I 61 I 63 I 651
52
54
56
58
60
62
64
66
35
•30
25
20
•15
10
•5
0
Adults
FIG. 1. Counts of viable and degenerating cells in lumbar ventral horns of
Xenopus larvae during development, together with curves fitted by eye. Stages of
development, length of hind limb, and approximate time scale are also indicated.
The sketches of animals are at stages 51, 56, 60, and 64, respectively. The scale of
cell death rate is based on a time of degeneration of 3.2 hr. (From Hughes and
Fozzard, 1961.)
extra number of pycnotic cells appear within the ventral horn, on such
a scale that the total number of cells is temporarily depleted. On the
basis that the enhanced rate of cell loss is due to the extra degenerations
provoked by amputation, it was calculated that the duration of degeneration of a ventral horn cell in larvae of Xenopus
is 3.2 hr. This estimate
involves two assumptions, namely, first, that the extra degenerations
occupy the same time as normal histogenetic degenerations, and, second,
Number of degenerations
Cell death rate per hour