134
B. M. SHAFFER
IV. The Aggregation
A. The Cells
1. Orientation
The earlier observations and speculations on the way in which cells
are led towards an aggregation centre have often been reviewed (e.g.,
Bonner, 1947; Raper, 1956a; Konijn and Raper, 1961). Here we need
note only that it is unjust to Harper to suppose that he ever suggested
that negative hydrotaxis was the main cause of aggregation. He (1929)
pointed out that the cells must be guided by some chemical or physical
stimulus they produced themselves. Bonner's (1947) extensive experiments demonstrated that distant cells were in fact attracted chemotactically by an agent he named acrasin, but were not influenced by a variety
of other stimuli.
We may reasonably suppose that chemotaxis, both positive and
negative, was primarily used for finding food, and that it was only later
in evolution that the cells' reactions to their metabolites became the
basis of social as well as of antisocial behaviour.
If the Weber-Pechner law holds, as Bonner (1947) assumed, and the
minimum percentage concentration difference a cell can detect is independent of concentration, or even if, as is more probable, the threshold
percentage is large at very low concentrations and decreases to a constant value as concentration increases, a point source will have a maximum range however much chemical it produces, if this is stable; but the
range can be greatly extended if the chemical is inactivated (Section IV,
C; Shaffer, 1956b). 'Removal' will be more important still in a limited
medium in preventing the gradient being masked by 'noise'. Crude
preparations of attractor in fact are unstable (Shaffer, 1953a). A second
principle of 'chemotactics' is that it is better to secrete the factor in
pulses rather than at a constant rate if the responding cells can hold their
courses between them.
Cells may become sensitive to attractor before any natural sources
develop (Arndt, 1937; Pfützner-Eckert, 1950; Shaffer, 1957b), and
sensitive cells can respond to unstable or stable artificial sources (Shaffer,
1953a; 1956a, b). Orientation is due to preferential cytoplasmic outflow
of the side of a cell or the side of an individual pseudopod facing a
source. It is not known whether acrasin, or any of the other chemotactic
factors (Section II, A, 1), can influence the cells' distribution by altering
their rate of turning (Ullyott, 1936) when the gradient is too small for
orientation. Bonner (1950) reported that separate aggregating cells
were more strongly polarized than vegetative ones, and that if the
acrasin gradient was reversed, elongated cells would usually make a
B. M. SHAFFER
IV. The Aggregation
A. The Cells
1. Orientation
The earlier observations and speculations on the way in which cells
are led towards an aggregation centre have often been reviewed (e.g.,
Bonner, 1947; Raper, 1956a; Konijn and Raper, 1961). Here we need
note only that it is unjust to Harper to suppose that he ever suggested
that negative hydrotaxis was the main cause of aggregation. He (1929)
pointed out that the cells must be guided by some chemical or physical
stimulus they produced themselves. Bonner's (1947) extensive experiments demonstrated that distant cells were in fact attracted chemotactically by an agent he named acrasin, but were not influenced by a variety
of other stimuli.
We may reasonably suppose that chemotaxis, both positive and
negative, was primarily used for finding food, and that it was only later
in evolution that the cells' reactions to their metabolites became the
basis of social as well as of antisocial behaviour.
If the Weber-Pechner law holds, as Bonner (1947) assumed, and the
minimum percentage concentration difference a cell can detect is independent of concentration, or even if, as is more probable, the threshold
percentage is large at very low concentrations and decreases to a constant value as concentration increases, a point source will have a maximum range however much chemical it produces, if this is stable; but the
range can be greatly extended if the chemical is inactivated (Section IV,
C; Shaffer, 1956b). 'Removal' will be more important still in a limited
medium in preventing the gradient being masked by 'noise'. Crude
preparations of attractor in fact are unstable (Shaffer, 1953a). A second
principle of 'chemotactics' is that it is better to secrete the factor in
pulses rather than at a constant rate if the responding cells can hold their
courses between them.
Cells may become sensitive to attractor before any natural sources
develop (Arndt, 1937; Pfützner-Eckert, 1950; Shaffer, 1957b), and
sensitive cells can respond to unstable or stable artificial sources (Shaffer,
1953a; 1956a, b). Orientation is due to preferential cytoplasmic outflow
of the side of a cell or the side of an individual pseudopod facing a
source. It is not known whether acrasin, or any of the other chemotactic
factors (Section II, A, 1), can influence the cells' distribution by altering
their rate of turning (Ullyott, 1936) when the gradient is too small for
orientation. Bonner (1950) reported that separate aggregating cells
were more strongly polarized than vegetative ones, and that if the
acrasin gradient was reversed, elongated cells would usually make a