THE ACRASINA
149
strongly suggests that the repellent is still active, a point also made by
Samuel (1961). If so, it will tend to obscure residual acrasin secretion, if
any. Disintegration has been much less studied than the act of aggregating, partly because its occurrence is far less predictable; but clearly the
disturbance must be transmitted to all the cells in the centre. Its propagation is shown still more strikingly when it attacks a stream, affecting
all the cells in a large section at nearly the same time, or as in P .
violaceum, spreading along it very slowly and almost exclusively outwards, detaching the cells in succession—a difference that recalls the
two extreme types of stream growth. The whole of the middle of a
violaceum aggregation may be progressively eaten away while it continues to grow at its periphery, yielding a 'fairy ring' of separate lengths
of radial streams that simply conveys separate non-co-operative cells
from outside it to a higher density inside it. Disintegration in one stream
increases the probability of a neighbour being affected; this implicates
a diffusible agent. This is also the most probable explanation of the
finding that sometimes after a culture has produced a rash of minute
streamless centres, a large proportion of them disintegrate in the same
hour (though it could be maintained that they were acting independently
at a critical age).
When and where disintegration occurs must be determined partly
by small internal differences between cells or cell groups, partly by the
chemical climate provided by all the neighbouring cells. The shut-down
in attractor output, spontaneous or induced, is perhaps due to diversion
of precursors, or increased production of some inhibitor or inactivating
enzyme, or to the external inducer falling below threshold, and may well
involve some form of adaptation—in its physiological sense—to the
inducer. The cells released by disintegration, just like those that have
never aggregated, may be induced to form streams, or they may directly
enter an existing centre or a new one that they start themselves; and the
cycle of 'integration' and disintegration may be repeated many times.
If no acrasin sources present themselves, the cells disperse and may even
return to being insensitive.
2. Cloud and Clump
The changes in the distribution in some of the Dictyosteliidae before
any centres or streams are visible, despite or rather because of their
simplicity, have been little studied, and our analysis must be frankly
speculative. When cells of P . violaceum are suspended over aggregations,
they may be drawn into a uniform cloud above each of them without
clumping on top of the centres and streams (Section III, B ; Shaffer,
1961b). This is hard to explain unless, when the cells are directly overhead, a gradient of repellent, for which there is in any case independent
F
A . C . 2
149
strongly suggests that the repellent is still active, a point also made by
Samuel (1961). If so, it will tend to obscure residual acrasin secretion, if
any. Disintegration has been much less studied than the act of aggregating, partly because its occurrence is far less predictable; but clearly the
disturbance must be transmitted to all the cells in the centre. Its propagation is shown still more strikingly when it attacks a stream, affecting
all the cells in a large section at nearly the same time, or as in P .
violaceum, spreading along it very slowly and almost exclusively outwards, detaching the cells in succession—a difference that recalls the
two extreme types of stream growth. The whole of the middle of a
violaceum aggregation may be progressively eaten away while it continues to grow at its periphery, yielding a 'fairy ring' of separate lengths
of radial streams that simply conveys separate non-co-operative cells
from outside it to a higher density inside it. Disintegration in one stream
increases the probability of a neighbour being affected; this implicates
a diffusible agent. This is also the most probable explanation of the
finding that sometimes after a culture has produced a rash of minute
streamless centres, a large proportion of them disintegrate in the same
hour (though it could be maintained that they were acting independently
at a critical age).
When and where disintegration occurs must be determined partly
by small internal differences between cells or cell groups, partly by the
chemical climate provided by all the neighbouring cells. The shut-down
in attractor output, spontaneous or induced, is perhaps due to diversion
of precursors, or increased production of some inhibitor or inactivating
enzyme, or to the external inducer falling below threshold, and may well
involve some form of adaptation—in its physiological sense—to the
inducer. The cells released by disintegration, just like those that have
never aggregated, may be induced to form streams, or they may directly
enter an existing centre or a new one that they start themselves; and the
cycle of 'integration' and disintegration may be repeated many times.
If no acrasin sources present themselves, the cells disperse and may even
return to being insensitive.
2. Cloud and Clump
The changes in the distribution in some of the Dictyosteliidae before
any centres or streams are visible, despite or rather because of their
simplicity, have been little studied, and our analysis must be frankly
speculative. When cells of P . violaceum are suspended over aggregations,
they may be drawn into a uniform cloud above each of them without
clumping on top of the centres and streams (Section III, B ; Shaffer,
1961b). This is hard to explain unless, when the cells are directly overhead, a gradient of repellent, for which there is in any case independent
F
A . C . 2