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B. M. SHAFFER
extent they can flow forward only when these move out of the way.
This applies, for example, when a new side stream is trying to enter a main
one. It may also explain why the cells in a stream may be slower than
just before they join it; and rather less ambiguously, since the majority
of cells remain in much the same environment, how the section of a
stream beyond a gap may manage to catch up the central stump. In the
latter case, the cells in the front rank, exposed to a greater acrasin concentration at their rear, must presumably be impelled forward by the
cells following them. But some actions, such as the slowing of the cells
initially in a stream swelling, must be self-determined. Moreover, cells
may change their relative position within a stream, and such changes
may not reflect the relative speeds of the same cells just before entering
it (Shaffer, 1961b), which emphasizes how subtle the relationship is
between cytoplasmic outflow and environment.
3. Shape
Separate cells lengthen, though to a very variable extent, in an acrasin
gradient; and some cells may respond in more than one place, and thus
yield two or even three elongated lobes (Shaffer, 1961a). The shape of
individual stream cells is to some extent determined by the pulls exerted
by their neighbours and by adhesion to the substratum. Bonner (1950)
noted that an isolated section of an underwater stream contracted because of the rounding up of its components, but this might have been due
mainly to the check to forward movement. However, an underwater
stream still flowing into a centre also shortens considerably within a few
minutes if it is freed from the substratum, as is seen most easily with a
sheathed P. violaceum stream because this does not readhere to the
glass (Shaffer, 1961b).
B. Organization
1. Stipple and Stream
Though aggregation is basically a single movement, in the higher
Acrasina it yields the most elaborate and beautiful patterns, which
have proved particularly accessible to analysis (Shaffer, 1956a, b ;
1957a-c; 1958) because in culture they are essentially two-dimensional.
In 'stippled' aggregation (Figs. 7 and 8), the cells become oriented while
they are still very largely separate from one another. Gradually they
join up into short chains and then into continuous streams. In the early
stages, they can be guided only by chemotaxis. Initially, there may or
may not be an obvious centre, but if there is, its direct range of attraction is severely limited. Moreover, although the advantages of secreting
B. M. SHAFFER
extent they can flow forward only when these move out of the way.
This applies, for example, when a new side stream is trying to enter a main
one. It may also explain why the cells in a stream may be slower than
just before they join it; and rather less ambiguously, since the majority
of cells remain in much the same environment, how the section of a
stream beyond a gap may manage to catch up the central stump. In the
latter case, the cells in the front rank, exposed to a greater acrasin concentration at their rear, must presumably be impelled forward by the
cells following them. But some actions, such as the slowing of the cells
initially in a stream swelling, must be self-determined. Moreover, cells
may change their relative position within a stream, and such changes
may not reflect the relative speeds of the same cells just before entering
it (Shaffer, 1961b), which emphasizes how subtle the relationship is
between cytoplasmic outflow and environment.
3. Shape
Separate cells lengthen, though to a very variable extent, in an acrasin
gradient; and some cells may respond in more than one place, and thus
yield two or even three elongated lobes (Shaffer, 1961a). The shape of
individual stream cells is to some extent determined by the pulls exerted
by their neighbours and by adhesion to the substratum. Bonner (1950)
noted that an isolated section of an underwater stream contracted because of the rounding up of its components, but this might have been due
mainly to the check to forward movement. However, an underwater
stream still flowing into a centre also shortens considerably within a few
minutes if it is freed from the substratum, as is seen most easily with a
sheathed P. violaceum stream because this does not readhere to the
glass (Shaffer, 1961b).
B. Organization
1. Stipple and Stream
Though aggregation is basically a single movement, in the higher
Acrasina it yields the most elaborate and beautiful patterns, which
have proved particularly accessible to analysis (Shaffer, 1956a, b ;
1957a-c; 1958) because in culture they are essentially two-dimensional.
In 'stippled' aggregation (Figs. 7 and 8), the cells become oriented while
they are still very largely separate from one another. Gradually they
join up into short chains and then into continuous streams. In the early
stages, they can be guided only by chemotaxis. Initially, there may or
may not be an obvious centre, but if there is, its direct range of attraction is severely limited. Moreover, although the advantages of secreting