THE ACRASINA
135
U-turn, though occasionally they would round up and then put out a
new pseudopod. However, though there certainly is a strong tendency
for the front to remain the front, some cells definitely do produce a new
pseudopod from their rear and directly reverse their movement while
maintaining their elongation; moreover it is not uncommon to find Yor V-shaped cells, with both arms oriented by the gradient (Shaffer,
1961a).
Raper and Thorn (1941) found that whereas in mixed culture the
larger Dictyostelium species aggregated jointly, their aggregations were
quite separate from those of P. violaceum and even overlapped them.
This is due to the specificity of their acrasins (Shaffer, 1953a). Several
other species seem to use specific attractors during aggregation (Shaffer,
1961b); and there often appear to be weak cross reactions at distances
of one or two cell lengths, though at such short range it is more difficult
to distinguish between guided approach and chance encounter; but one
would scarcely expect specificity to be absolute.
Runyon (1942) discovered that cells on one side of a dialysis membrane
duplicated not only the aggregation centres formed by cells on the other
side but also the streams. This showed that these too produced acrasin.
Bonner (1949) drew the same conclusion from their very existence. And
the fact that a centre can attract cells from at most a few hundred micra
(Bonner, 1947), whereas a stream may extend for a distance 100 times
greater, emphasizes the quantitative importance of the stream's acrasin
(Shaffer, 1957a). At first it was naturally supposed that there was a
gradient within a stream guiding its cells towards the centre. But by
using Bonner's (1949) two tests of acrasin emission, adapted to allow
for various complications (Shaffer, 1953b; Section V, A, 1), it can be
shown that there may be no detectable centrifugal decrement in emission of attractor along a whole stream, and indeed that, in the larger
species of Dictyostelium, centres and continuous streams often maintain
much the same concentration at the surface (Shaffer, 1957b). Individual
cells attracted towards a solitary stream from its flank arrive at right
angles to it and eventually turn towards the centre. P. violaceum provides strong evidence that this turn is guided by contact and not by
small, temporary, or local gradients of a diffusible factor. The older
sections of its large streams are covered with slime sheath a fraction of a
micron thick; and single cells attracted to a stream, but unable to penetrate its sheath, wander about at random over its surface, though they
cannot escape from its edge because of the gradient there, transverse to
its axis (Shaffer, 1959). There may, of course, be quantitative but nondiffusable differences between the back of each stream cell and the front
of the one behind influencing cytoplasmic outflow.
A cell within a continuous stream commonly is not attracted by
135
U-turn, though occasionally they would round up and then put out a
new pseudopod. However, though there certainly is a strong tendency
for the front to remain the front, some cells definitely do produce a new
pseudopod from their rear and directly reverse their movement while
maintaining their elongation; moreover it is not uncommon to find Yor V-shaped cells, with both arms oriented by the gradient (Shaffer,
1961a).
Raper and Thorn (1941) found that whereas in mixed culture the
larger Dictyostelium species aggregated jointly, their aggregations were
quite separate from those of P. violaceum and even overlapped them.
This is due to the specificity of their acrasins (Shaffer, 1953a). Several
other species seem to use specific attractors during aggregation (Shaffer,
1961b); and there often appear to be weak cross reactions at distances
of one or two cell lengths, though at such short range it is more difficult
to distinguish between guided approach and chance encounter; but one
would scarcely expect specificity to be absolute.
Runyon (1942) discovered that cells on one side of a dialysis membrane
duplicated not only the aggregation centres formed by cells on the other
side but also the streams. This showed that these too produced acrasin.
Bonner (1949) drew the same conclusion from their very existence. And
the fact that a centre can attract cells from at most a few hundred micra
(Bonner, 1947), whereas a stream may extend for a distance 100 times
greater, emphasizes the quantitative importance of the stream's acrasin
(Shaffer, 1957a). At first it was naturally supposed that there was a
gradient within a stream guiding its cells towards the centre. But by
using Bonner's (1949) two tests of acrasin emission, adapted to allow
for various complications (Shaffer, 1953b; Section V, A, 1), it can be
shown that there may be no detectable centrifugal decrement in emission of attractor along a whole stream, and indeed that, in the larger
species of Dictyostelium, centres and continuous streams often maintain
much the same concentration at the surface (Shaffer, 1957b). Individual
cells attracted towards a solitary stream from its flank arrive at right
angles to it and eventually turn towards the centre. P. violaceum provides strong evidence that this turn is guided by contact and not by
small, temporary, or local gradients of a diffusible factor. The older
sections of its large streams are covered with slime sheath a fraction of a
micron thick; and single cells attracted to a stream, but unable to penetrate its sheath, wander about at random over its surface, though they
cannot escape from its edge because of the gradient there, transverse to
its axis (Shaffer, 1959). There may, of course, be quantitative but nondiffusable differences between the back of each stream cell and the front
of the one behind influencing cytoplasmic outflow.
A cell within a continuous stream commonly is not attracted by