DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
93
axotomy undergo chromatolysis, with the swelling of the cell body and the
loss of Nissl substance. Ultimately these cells become pycnotic and die.
The stages in the differentiation of the ventral horn cell which are
marked by changes in reaction to amputation are distinguished as Phases
I, II, and III. In normal development there is evidence that Phase II is
a relatively short one in the life of the developing neuron. Cells entering
this phase during the turnover period either soon degenerate or go on to
Phase III with further differentiation. Prestige suggests that the high
mortality of Phase II cells both in normal development and after limb
amputation relates to the exhaustion of some essential substance within
the cell, larger reserves of which are present within the more resistant
and mature Phase III cell. There is evidence that among the Phase II
cells which degenerate as a result of amputation, not all had sent an axon
into the limb. They must thus undergo cell death through some contagious effect, spreading from one cell body to another.
The same sequence of reactions can be recognized in
Eleutherodactylus,
where the development of the nervous system seems to advance by relatively clear-cut steps. The change from Phase II to Phase III in the cells
of the ventral horn is abrupt. It occurs at about the eighth day and thus
early in the development of limb movement (Fig. 4). It can be shown
that this change in reaction of ventral horn cells to amputation comes
later in cells related to distal than to proximal segments of the limb.
Such evidence is found in comparing the effects of amputation of the
limb at different levels. Before the eighth day, cell amputations evoke
a prompt and drastic fall in the number of ventral horn cells. After this
stage, however, the effect of the operation depends upon the level at
which the limb was severed. In Fig. 5 is shown a serial analysis of the
ventral horns of two embryos of Eleutherodactylus
that were amputated
at 8 days, respectively, at the knee and at the hip, and were both fixed 2
days later. For the latter operation there is now little difference in the
number of ventral horn cells on the two sides, and it may be surmised
that those which reacted to amputation by prompt degeneration in Phase
II were balanced by others in Phase III, in which normal degeneration
was thereby postponed. With the embryo amputated at the knee, however, there is a considerable loss of cells on the operated side, particularly
in the caudal region of the ventral horn. Here, therefore, there was a
larger number of cells in Phase II than of older cells in Phase III. All
these, however, are related to distal segments of the leg, for in this embryo the nerves and muscles of the thigh are still intact. Thus we see
that at the stage when the operations were performed, the muscles of the
93
axotomy undergo chromatolysis, with the swelling of the cell body and the
loss of Nissl substance. Ultimately these cells become pycnotic and die.
The stages in the differentiation of the ventral horn cell which are
marked by changes in reaction to amputation are distinguished as Phases
I, II, and III. In normal development there is evidence that Phase II is
a relatively short one in the life of the developing neuron. Cells entering
this phase during the turnover period either soon degenerate or go on to
Phase III with further differentiation. Prestige suggests that the high
mortality of Phase II cells both in normal development and after limb
amputation relates to the exhaustion of some essential substance within
the cell, larger reserves of which are present within the more resistant
and mature Phase III cell. There is evidence that among the Phase II
cells which degenerate as a result of amputation, not all had sent an axon
into the limb. They must thus undergo cell death through some contagious effect, spreading from one cell body to another.
The same sequence of reactions can be recognized in
Eleutherodactylus,
where the development of the nervous system seems to advance by relatively clear-cut steps. The change from Phase II to Phase III in the cells
of the ventral horn is abrupt. It occurs at about the eighth day and thus
early in the development of limb movement (Fig. 4). It can be shown
that this change in reaction of ventral horn cells to amputation comes
later in cells related to distal than to proximal segments of the limb.
Such evidence is found in comparing the effects of amputation of the
limb at different levels. Before the eighth day, cell amputations evoke
a prompt and drastic fall in the number of ventral horn cells. After this
stage, however, the effect of the operation depends upon the level at
which the limb was severed. In Fig. 5 is shown a serial analysis of the
ventral horns of two embryos of Eleutherodactylus
that were amputated
at 8 days, respectively, at the knee and at the hip, and were both fixed 2
days later. For the latter operation there is now little difference in the
number of ventral horn cells on the two sides, and it may be surmised
that those which reacted to amputation by prompt degeneration in Phase
II were balanced by others in Phase III, in which normal degeneration
was thereby postponed. With the embryo amputated at the knee, however, there is a considerable loss of cells on the operated side, particularly
in the caudal region of the ventral horn. Here, therefore, there was a
larger number of cells in Phase II than of older cells in Phase III. All
these, however, are related to distal segments of the leg, for in this embryo the nerves and muscles of the thigh are still intact. Thus we see
that at the stage when the operations were performed, the muscles of the