136
B. M. SHAFFER
neighbouring streams or centres, perhaps only because they cannot set
up an adequate gradient; but it will respond to them if its front end
becomes exposed as a result of its own stream either dispersing or merely
breaking just ahead of it. In the latter case, cytoplasm advances in the
direction of the maximal gradient at the cell's front end even though
the acrasin concentration outside its back end must be higher. Probably the thrust of the cells behind helps to prevent it reversing its
polarity.
As long as aggregating cells stay separate, they appear to be guided
exclusively by chemotaxis: they remain unaffected by an oriented substratum (Bonner, 1947) and do not make any detectable, nondiffusible,
extracellular material. This material is first seen in D. discoideum as a
slime sheath round the older centres; and even in P. violaceum it cannot
be essential for stream organization, as it is absent from young aggregations. Once aggregating cells that adhere strongly come in contact, a
new form of guidance can operate. A cell that arrives at right angles to a
naked stream still tends to remain on the agar rather than crawl on top
of the stream (though a stream does become multilayered, sooner or
later according to the density). If the cell first makes contact only with
the stationary side surface of a stream cell, it may initially turn either
towards or away from the centre, or it may do both; but when, as
the stream cell advances, any part of the lateral surface to which the
newcomer is adhering becomes back end and is then withdrawn towards
the centre, the new cell follows it and is thus for a time L-shaped (Shaffer,
1961b). This channelling of cytoplasmic outflow effected by an adherent
surface only when it moves may be called contact following. In contrast,
ordinary contact guidance (Weiss, 1934, 1958) is not polarized, and contact
promotion, which Curtis (1960) believes to operate in the reaggregation
of cells of multicellular animals when their surface reaches a certain
state, tends to increase the area of cell contact but influences direction only in so far as it discourages cells from moving on to another substratum. Contact following can continue to operate within a stream,
correcting any incipient deviations of the parallel cells within it, however many layers of cells there may be.
There is no satisfactory evidence that the cells of different strains
ever have such incompatible surfaces that they cannot initially join a
common aggregate. Though Raper and Thorn (1941) did not observe
any of their mixed-up Dictyostelium and Polysphondylium cells entering
foreign aggregations, cells of one of these types do not find a stream of
the other an impassable barrier, and both types readily associate when
attracted towards old Polysphondylium centres (Shaffer, 1953a, 1957b).
And though cells of certain species may be attracted towards certain
foreign streams and on reaching them form separate streams alongside
B. M. SHAFFER
neighbouring streams or centres, perhaps only because they cannot set
up an adequate gradient; but it will respond to them if its front end
becomes exposed as a result of its own stream either dispersing or merely
breaking just ahead of it. In the latter case, cytoplasm advances in the
direction of the maximal gradient at the cell's front end even though
the acrasin concentration outside its back end must be higher. Probably the thrust of the cells behind helps to prevent it reversing its
polarity.
As long as aggregating cells stay separate, they appear to be guided
exclusively by chemotaxis: they remain unaffected by an oriented substratum (Bonner, 1947) and do not make any detectable, nondiffusible,
extracellular material. This material is first seen in D. discoideum as a
slime sheath round the older centres; and even in P. violaceum it cannot
be essential for stream organization, as it is absent from young aggregations. Once aggregating cells that adhere strongly come in contact, a
new form of guidance can operate. A cell that arrives at right angles to a
naked stream still tends to remain on the agar rather than crawl on top
of the stream (though a stream does become multilayered, sooner or
later according to the density). If the cell first makes contact only with
the stationary side surface of a stream cell, it may initially turn either
towards or away from the centre, or it may do both; but when, as
the stream cell advances, any part of the lateral surface to which the
newcomer is adhering becomes back end and is then withdrawn towards
the centre, the new cell follows it and is thus for a time L-shaped (Shaffer,
1961b). This channelling of cytoplasmic outflow effected by an adherent
surface only when it moves may be called contact following. In contrast,
ordinary contact guidance (Weiss, 1934, 1958) is not polarized, and contact
promotion, which Curtis (1960) believes to operate in the reaggregation
of cells of multicellular animals when their surface reaches a certain
state, tends to increase the area of cell contact but influences direction only in so far as it discourages cells from moving on to another substratum. Contact following can continue to operate within a stream,
correcting any incipient deviations of the parallel cells within it, however many layers of cells there may be.
There is no satisfactory evidence that the cells of different strains
ever have such incompatible surfaces that they cannot initially join a
common aggregate. Though Raper and Thorn (1941) did not observe
any of their mixed-up Dictyostelium and Polysphondylium cells entering
foreign aggregations, cells of one of these types do not find a stream of
the other an impassable barrier, and both types readily associate when
attracted towards old Polysphondylium centres (Shaffer, 1953a, 1957b).
And though cells of certain species may be attracted towards certain
foreign streams and on reaching them form separate streams alongside