154
B. M. SHAFFER
distance between clumps. A repellent may be expected to affect the
rates both of collision and escape.
Finally, it is as well to remember that our understanding of aggregation patterns is still very far from being complete. It is perhaps symbolic
that the one photograph of 'typical' aggregation in Bonner's monograph
—one of Raper's pictures of D. discoideum—shows a phenomenon that
has never been described, let alone explained. Certainly it has been
pointed out that between the centre of a stippled aggregation and an
area where there are lengths of weil-formed streams moving towards it,
the cells may be at a very much lower density, or less advanced in
stream building, or both (Shaffer, 1957a, 1958); but it has not been
explicitly stated that, at least in some conditions, as in the photograph
in question, unattached lengths of clearly defined streams regularly
develop all around the peripheries of the clouds, while the cells within
them, centres excepted, are still essentially separate. Possibly the outer
cells are simply slowed down less severely. If so, the considerable density independence of this effect, suggested by the earlier observations,
rather tells against the repellent being responsible.
C. Acrasin
Bonner (1947) defined acrasin as 'a type of substance consisting either
of one or numerous compounds responsible for stimulating and directing
aggregation'. Such a factor, by analogy with several other physiological
agents, may appropriately be called a synagogue. But perhaps it is more
useful to fractionate it by function into an attractor, a component inducing secretion of the attractor and of itself, and aninducer of increased
adhesiveness.
Theoretical arguments (Section IV, A, 1) and failure to induce aggregation towards centres or material that had been in contact with living
centres, suggested that the attractor disappeared rapidly after secretion,
or had to be secreted in pulses, or both. By sandwiching sensitive cells
between a small agar block and glass, they could be exposed to an artificial gradient that could be renewed as often as every few seconds
simply by adding fresh test material to the edge of the block, because
this material diffused into the interior without mechanically disturbing
the existing gradient. Cell-free liquid that had bathed aggregates could
orient such cells, but lost its potency in a few minutes. Stable attractor
was obtained by dialysing it away from the large molecules, and later
by extracting D. discoideum cultures with methanol or simply by boiling them; the original instability was attributed to the action of a protein (Shaffer, 1953a, 1956a, b). Stable attractor was then also prepared
by acid extraction; this material was fractionated to give a component
active by itself, an inhibitor, and an inhibitor of this inhibitor; and
B. M. SHAFFER
distance between clumps. A repellent may be expected to affect the
rates both of collision and escape.
Finally, it is as well to remember that our understanding of aggregation patterns is still very far from being complete. It is perhaps symbolic
that the one photograph of 'typical' aggregation in Bonner's monograph
—one of Raper's pictures of D. discoideum—shows a phenomenon that
has never been described, let alone explained. Certainly it has been
pointed out that between the centre of a stippled aggregation and an
area where there are lengths of weil-formed streams moving towards it,
the cells may be at a very much lower density, or less advanced in
stream building, or both (Shaffer, 1957a, 1958); but it has not been
explicitly stated that, at least in some conditions, as in the photograph
in question, unattached lengths of clearly defined streams regularly
develop all around the peripheries of the clouds, while the cells within
them, centres excepted, are still essentially separate. Possibly the outer
cells are simply slowed down less severely. If so, the considerable density independence of this effect, suggested by the earlier observations,
rather tells against the repellent being responsible.
C. Acrasin
Bonner (1947) defined acrasin as 'a type of substance consisting either
of one or numerous compounds responsible for stimulating and directing
aggregation'. Such a factor, by analogy with several other physiological
agents, may appropriately be called a synagogue. But perhaps it is more
useful to fractionate it by function into an attractor, a component inducing secretion of the attractor and of itself, and aninducer of increased
adhesiveness.
Theoretical arguments (Section IV, A, 1) and failure to induce aggregation towards centres or material that had been in contact with living
centres, suggested that the attractor disappeared rapidly after secretion,
or had to be secreted in pulses, or both. By sandwiching sensitive cells
between a small agar block and glass, they could be exposed to an artificial gradient that could be renewed as often as every few seconds
simply by adding fresh test material to the edge of the block, because
this material diffused into the interior without mechanically disturbing
the existing gradient. Cell-free liquid that had bathed aggregates could
orient such cells, but lost its potency in a few minutes. Stable attractor
was obtained by dialysing it away from the large molecules, and later
by extracting D. discoideum cultures with methanol or simply by boiling them; the original instability was attributed to the action of a protein (Shaffer, 1953a, 1956a, b). Stable attractor was then also prepared
by acid extraction; this material was fractionated to give a component
active by itself, an inhibitor, and an inhibitor of this inhibitor; and