114
B. M. SHAFFER
Raper, 1961; Shaffer, 1961b). They may crawl preferentially in grooves
on an agar plate (Shaffer, 1953b), but probably this is simply because
there may be more bacteria there, and more generally because pseudopodial activity is promoted by the wetter conditions, for under water
they show no tendency to follow glass fibres or scratches (Bonner, 1947),
unlike fibroblasts (Weiss, 1945). Fibroblasts may also be guided by their
own, oriented, extracellular products and thus may sometimes follow
the paths taken by other cells (Moscona, 1960). However, pure cultures
of amoebocytes do not yield such materials; and though the misnomer
slime mould seems to have misled several workers into believing otherwise, myxamoebae do not appear to yield these substances either, at
least as long as they remain solitary (Shaffer, 1961b). Attempts to stain
such materials have failed (Bonner, 1947; Bonner, Chiquoine and
Kolderie, 1955); the cell tracks that Paddock (1953) thought might be
deformations of the agar but believed to be slime are unquestionably
deformations; and the fine threads stretching between cells that DeHaan
(1959) thought might be cytoplasm, but believed to be external sticky
material, are almost certainly cytoplasm. There may, of course, be ultramicroscopic deposits, but there is no evidence that they can influence
the direction of movement.
The most important surface is that of another cell. When one fibroblast meets another, its advance is halted and movement in the opposite
direction is favoured. This contact inhibition (Abercrombie and Heaysman, 1953) and contact retraction (Weiss, 1958) readily account for the
characteristic spreading of these cells in culture, which differs from that
of amoebocytes and myxamoebae in that the cell processes tend to
remain adherent and so to form a network. When myxamoebae on an
agar plate come in contact, there is inhibition to the extent that they
rarely crawl over one another, but frequently the same parts continue
to lead, and the cells flow round one another. Adhesion is so slight that
they readily separate again, but suspension in saline with gentle agitation is sufficient to make the cells clump. These clumps, which Gerisch
has called agglutinates, at first disperse completely if they are allowed to
settle, but if kept suspended, they turn into true aggregates at about
the same time as the cells would have aggregated on agar (Bonner, 1950;
Gerisch, 1959, 1960; Shaffer, 1961b). To what extent, if at all, contact
inhibition by myxamoebae contributes to the phenomenon provisionally
labelled mutual repulsion has not been assessed.
2. Speed
It is immediately apparent that myxamoebae move very slowly when
they are feeding, and Bonner's recent time-lapse film strikingly records
the acceleration that occurs when the bacteria in a given area have been
B. M. SHAFFER
Raper, 1961; Shaffer, 1961b). They may crawl preferentially in grooves
on an agar plate (Shaffer, 1953b), but probably this is simply because
there may be more bacteria there, and more generally because pseudopodial activity is promoted by the wetter conditions, for under water
they show no tendency to follow glass fibres or scratches (Bonner, 1947),
unlike fibroblasts (Weiss, 1945). Fibroblasts may also be guided by their
own, oriented, extracellular products and thus may sometimes follow
the paths taken by other cells (Moscona, 1960). However, pure cultures
of amoebocytes do not yield such materials; and though the misnomer
slime mould seems to have misled several workers into believing otherwise, myxamoebae do not appear to yield these substances either, at
least as long as they remain solitary (Shaffer, 1961b). Attempts to stain
such materials have failed (Bonner, 1947; Bonner, Chiquoine and
Kolderie, 1955); the cell tracks that Paddock (1953) thought might be
deformations of the agar but believed to be slime are unquestionably
deformations; and the fine threads stretching between cells that DeHaan
(1959) thought might be cytoplasm, but believed to be external sticky
material, are almost certainly cytoplasm. There may, of course, be ultramicroscopic deposits, but there is no evidence that they can influence
the direction of movement.
The most important surface is that of another cell. When one fibroblast meets another, its advance is halted and movement in the opposite
direction is favoured. This contact inhibition (Abercrombie and Heaysman, 1953) and contact retraction (Weiss, 1958) readily account for the
characteristic spreading of these cells in culture, which differs from that
of amoebocytes and myxamoebae in that the cell processes tend to
remain adherent and so to form a network. When myxamoebae on an
agar plate come in contact, there is inhibition to the extent that they
rarely crawl over one another, but frequently the same parts continue
to lead, and the cells flow round one another. Adhesion is so slight that
they readily separate again, but suspension in saline with gentle agitation is sufficient to make the cells clump. These clumps, which Gerisch
has called agglutinates, at first disperse completely if they are allowed to
settle, but if kept suspended, they turn into true aggregates at about
the same time as the cells would have aggregated on agar (Bonner, 1950;
Gerisch, 1959, 1960; Shaffer, 1961b). To what extent, if at all, contact
inhibition by myxamoebae contributes to the phenomenon provisionally
labelled mutual repulsion has not been assessed.
2. Speed
It is immediately apparent that myxamoebae move very slowly when
they are feeding, and Bonner's recent time-lapse film strikingly records
the acceleration that occurs when the bacteria in a given area have been