THE ACRASINA
143
cent stream, or to the cells behind it, in which case it loops backwards to
form a ring; and by suitable out-of-phase cuts, two parallel streams may
be made to turn themselves into a single continuous wave form.
In ordinary agar cultures, P. violaceum, in contrast to D. discoideum
for example, does not have a stippled phase: cells pack themselves round
the founder, and then streams, if any, grow outwards essentially in
continuous lines (Fig. 9). No pulses can be seen in Bonner's (1960) film.
The only gradient in the direction the stream is flowing is not within it
but just beyond its outer edge (Fig. 10(b)), confined to a narrow zone, into
which the separate cells crowd themselves, and which, of course, is
propagated outwards as the stream lengthens. The pattern of aggregation rather suggests that the cells are not stimulated to acquire the
properties of stream cells, that is, to become strongly secreting and
adhesive, until they actually make contact with a stream or centre.
But appearances are misleading: acrasin is again the real stimulus, and
the cells will undergo this transformation at a distance from the main
aggregate if prevented from reaching it. The difference between continuous and stippled stream growth depends on the relative rates at
which this transformation is relayed outwards and the cells move inwards ; the rate of relay in turn depends both on the speed of response
and the effective range of the stimulus; and all these factors depend on
the species, the physiological age of the cells, and the conditions of
culture. If violaceum is sandwiched between agar, or between glass and
agar, the relay system may function more rapidly and yield stippled
streams to a very limited extent (Shaffer, 1961a). This does not appear
to be due to increased acrasin secretion, and may perhaps be attributed
to a lowering of the inducer threshold.
Most observers have noted that areas of oriented cells may show a
variety of configurations in which, for a time at least, definite centres
are lacking; and this happens most commonly when the agar plate is
damp, or when the cells are under water (Arndt, 1937; Raper, 1941a;
Bonner, 1950; Shaffer, 1957a, c, 1958). Gerisch (1961a) has obtained
such aggregations regularly, in water, and has concluded that temporal
differences in secretion are not involved in producing centres, and that a
new interpretation is obviously needed. His conclusion is due partly to a
misunderstanding of my views, and partly to his suggested, new definition of a centre as that which is called a young grex in this paper—a
definition that seems unsatisfactory, as it leaves the centre of a typical
radiate aggregation without any status.
There are really several quite separate points involved. First, there is
the visible pattern of aggregation. In my belief, stippled configurations
of oriented cells, whether radial, circular, parallel, or reticular, depend
on the time sequence of secretion, and in fact are visible records of the
143
cent stream, or to the cells behind it, in which case it loops backwards to
form a ring; and by suitable out-of-phase cuts, two parallel streams may
be made to turn themselves into a single continuous wave form.
In ordinary agar cultures, P. violaceum, in contrast to D. discoideum
for example, does not have a stippled phase: cells pack themselves round
the founder, and then streams, if any, grow outwards essentially in
continuous lines (Fig. 9). No pulses can be seen in Bonner's (1960) film.
The only gradient in the direction the stream is flowing is not within it
but just beyond its outer edge (Fig. 10(b)), confined to a narrow zone, into
which the separate cells crowd themselves, and which, of course, is
propagated outwards as the stream lengthens. The pattern of aggregation rather suggests that the cells are not stimulated to acquire the
properties of stream cells, that is, to become strongly secreting and
adhesive, until they actually make contact with a stream or centre.
But appearances are misleading: acrasin is again the real stimulus, and
the cells will undergo this transformation at a distance from the main
aggregate if prevented from reaching it. The difference between continuous and stippled stream growth depends on the relative rates at
which this transformation is relayed outwards and the cells move inwards ; the rate of relay in turn depends both on the speed of response
and the effective range of the stimulus; and all these factors depend on
the species, the physiological age of the cells, and the conditions of
culture. If violaceum is sandwiched between agar, or between glass and
agar, the relay system may function more rapidly and yield stippled
streams to a very limited extent (Shaffer, 1961a). This does not appear
to be due to increased acrasin secretion, and may perhaps be attributed
to a lowering of the inducer threshold.
Most observers have noted that areas of oriented cells may show a
variety of configurations in which, for a time at least, definite centres
are lacking; and this happens most commonly when the agar plate is
damp, or when the cells are under water (Arndt, 1937; Raper, 1941a;
Bonner, 1950; Shaffer, 1957a, c, 1958). Gerisch (1961a) has obtained
such aggregations regularly, in water, and has concluded that temporal
differences in secretion are not involved in producing centres, and that a
new interpretation is obviously needed. His conclusion is due partly to a
misunderstanding of my views, and partly to his suggested, new definition of a centre as that which is called a young grex in this paper—a
definition that seems unsatisfactory, as it leaves the centre of a typical
radiate aggregation without any status.
There are really several quite separate points involved. First, there is
the visible pattern of aggregation. In my belief, stippled configurations
of oriented cells, whether radial, circular, parallel, or reticular, depend
on the time sequence of secretion, and in fact are visible records of the