THE ACRASINA
137
(Shaffer, 1957a, b), subject to re-examination, it seems possible that
this was because the original streams were covered in slime sheath as
impenetrable as it was unsuspected. Nevertheless we should expect
quantitative and qualitative differences in the cell surface to be responsible for different degrees of contact following, and to supplement the
use of specific acrasins, and perhaps of varying rates of becoming sensitive to the same acrasin, in sorting cells out (Section V, A, 1 and 2).
And in fact I have already found gross differences in the ability of one
species to contact follow another.
2. Speed
It is possible that the attractor only channels cytoplasmic outflow
without influencing its rate; but if chemotaxis is positive, on the simplest
hypothesis one would expect speed to be increased if affected at all.
Bonner's films show that waves of chemotactic orientation spreading
across areas of separate cells are also waves of rapid inward movement.
This is highly suggestive, but no actual measurements have been made
comparing the oriented cells with those moving at random.
Some Dictyostelium species at the start of aggregation form clouds of
separate unoriented cells (Shaffer, 1959). Samuel (1961) has found that
in mucoroides the cells are progressively slowed down as they move into
a cloud and as they later form small clumps within it. The agent must be
diffusible. When one of the clumps becomes a definite centre attracting
the other cells, a gradient of increased speed extends outwards from it.
It is simplest to assume that the attractor is responsible for this acceleration, but one need not do so, and in fact Samuel does not.
The situation is still more difficult to analyse when the cells are
adherent. There may be temporary differences in speed between different parts of a single file, accompanied necessarily by changes in cell
shape; but over a sufficient period of time, as long as it remains unbroken, all its members must cover the same distance. This uniformity
must be imposed by the mechanical effects of the cells ahead and
behind, and the speed must represent the average rate of flow of which
the cells are capable in a given environment. Any stream may be thought
of as essentially a bundle of cell files; and as the side surface of a cell is
stationary, it is at least mechanically possible for them all to move at
the same speed, however many there are side by side or on top of one
another. When two hemispherical swellings, or 'secondary centres',
form on a stream and come to rest, the cells in the connective stream
between them may sometimes be partially or completely arrested for
several hours; but once it parts, those ahead of the gap rush forward
into the next swelling within a few minutes (Shaffer, 1957c). But cells
may also be slowed down by those in front of them, because to a large
137
(Shaffer, 1957a, b), subject to re-examination, it seems possible that
this was because the original streams were covered in slime sheath as
impenetrable as it was unsuspected. Nevertheless we should expect
quantitative and qualitative differences in the cell surface to be responsible for different degrees of contact following, and to supplement the
use of specific acrasins, and perhaps of varying rates of becoming sensitive to the same acrasin, in sorting cells out (Section V, A, 1 and 2).
And in fact I have already found gross differences in the ability of one
species to contact follow another.
2. Speed
It is possible that the attractor only channels cytoplasmic outflow
without influencing its rate; but if chemotaxis is positive, on the simplest
hypothesis one would expect speed to be increased if affected at all.
Bonner's films show that waves of chemotactic orientation spreading
across areas of separate cells are also waves of rapid inward movement.
This is highly suggestive, but no actual measurements have been made
comparing the oriented cells with those moving at random.
Some Dictyostelium species at the start of aggregation form clouds of
separate unoriented cells (Shaffer, 1959). Samuel (1961) has found that
in mucoroides the cells are progressively slowed down as they move into
a cloud and as they later form small clumps within it. The agent must be
diffusible. When one of the clumps becomes a definite centre attracting
the other cells, a gradient of increased speed extends outwards from it.
It is simplest to assume that the attractor is responsible for this acceleration, but one need not do so, and in fact Samuel does not.
The situation is still more difficult to analyse when the cells are
adherent. There may be temporary differences in speed between different parts of a single file, accompanied necessarily by changes in cell
shape; but over a sufficient period of time, as long as it remains unbroken, all its members must cover the same distance. This uniformity
must be imposed by the mechanical effects of the cells ahead and
behind, and the speed must represent the average rate of flow of which
the cells are capable in a given environment. Any stream may be thought
of as essentially a bundle of cell files; and as the side surface of a cell is
stationary, it is at least mechanically possible for them all to move at
the same speed, however many there are side by side or on top of one
another. When two hemispherical swellings, or 'secondary centres',
form on a stream and come to rest, the cells in the connective stream
between them may sometimes be partially or completely arrested for
several hours; but once it parts, those ahead of the gap rush forward
into the next swelling within a few minutes (Shaffer, 1957c). But cells
may also be slowed down by those in front of them, because to a large