116
B. M. SHAFFER
largest cells in my cultures of D. discoideum may be flat and produce
lopopodia continuously and very actively (the characteristics of Ennis
and Sussman's (1958b) I-cells) but they may also be highly domed and
relatively inactive for considerable periods. If covered with water,
however, the domed forms may produce numerous papillate pseudopods on their free surfaces, and appear crinkled under low magnification.
The dark ring, nearly the width of the cell, which is visible in some of the
published photographs of I-cells, and apparently considered by the
authors to be heavy granulation, is not in fact granular, but represents
a rather remarkable kind of pseudopodial activity—a pattern of surface
ridges maintained in very rapid flux for several hours. As judged by
my cultures, it occurs in only a small fraction of the large cells and does
not seem to be related to subsequent aggregation.
B. Organization
It is generally stated that the solitary phase is wholly lacking in
pattern and organization, although Arndt (1937) did notice that when
a thick bacterial layer in one of his cultures had been almost completely
converted into a layer of amoebae, this developed expanding craters.
If a single cell is introduced into a continuous layer of bacteria, its
daughters at first scatter irregularly, having nothing to influence them
but mutual repulsion, and this effect seems rather weak in the presence
of food (Shaffer, 1961b). Once they have largely cleared an area of
bacteria, they crowd together into a feeding front at its periphery (Figs.
3 and 4). This distribution may be explained by factors already demonstrated or postulated. Firstly, the cells will be sparser in the middle
because they are faster. Secondly, repellent accumulated in the central
area will drive all the cells outwards; that made in the feeding front will
tend to drive those in the van outwards, those in the rear inwards.
Thirdly, the bacteria will attract all the cells outwards. Fourthly, the
progressive deceleration as the cells encounter more and more bacteria will oppose an increase in the front's depth as it advances. Lastly,
although random movements will favour the front's dispersal, contact
inhibition may perhaps slow up cells within it and trap them there. The
relative strength of these factors will determine the front's definition:
the extent to which scattered cells move ahead of it, and the proportion
of cells that fall behind it. This is partly under genetic control: some
aggregateless mutants produce sharply defined plaques, others produce
diffuse ones (Ennis and Sussman, 1958a); and Kessler's Guttulina may
lack any recognizable feeding front (Shaffer, 1961b). On the other hand,
in my experience, different plate cultures of the same strain may present
quite a different appearance. When a small, isolated bacterial colony
has been almost completely consumed, the resultant 'feeding aggrega-
B. M. SHAFFER
largest cells in my cultures of D. discoideum may be flat and produce
lopopodia continuously and very actively (the characteristics of Ennis
and Sussman's (1958b) I-cells) but they may also be highly domed and
relatively inactive for considerable periods. If covered with water,
however, the domed forms may produce numerous papillate pseudopods on their free surfaces, and appear crinkled under low magnification.
The dark ring, nearly the width of the cell, which is visible in some of the
published photographs of I-cells, and apparently considered by the
authors to be heavy granulation, is not in fact granular, but represents
a rather remarkable kind of pseudopodial activity—a pattern of surface
ridges maintained in very rapid flux for several hours. As judged by
my cultures, it occurs in only a small fraction of the large cells and does
not seem to be related to subsequent aggregation.
B. Organization
It is generally stated that the solitary phase is wholly lacking in
pattern and organization, although Arndt (1937) did notice that when
a thick bacterial layer in one of his cultures had been almost completely
converted into a layer of amoebae, this developed expanding craters.
If a single cell is introduced into a continuous layer of bacteria, its
daughters at first scatter irregularly, having nothing to influence them
but mutual repulsion, and this effect seems rather weak in the presence
of food (Shaffer, 1961b). Once they have largely cleared an area of
bacteria, they crowd together into a feeding front at its periphery (Figs.
3 and 4). This distribution may be explained by factors already demonstrated or postulated. Firstly, the cells will be sparser in the middle
because they are faster. Secondly, repellent accumulated in the central
area will drive all the cells outwards; that made in the feeding front will
tend to drive those in the van outwards, those in the rear inwards.
Thirdly, the bacteria will attract all the cells outwards. Fourthly, the
progressive deceleration as the cells encounter more and more bacteria will oppose an increase in the front's depth as it advances. Lastly,
although random movements will favour the front's dispersal, contact
inhibition may perhaps slow up cells within it and trap them there. The
relative strength of these factors will determine the front's definition:
the extent to which scattered cells move ahead of it, and the proportion
of cells that fall behind it. This is partly under genetic control: some
aggregateless mutants produce sharply defined plaques, others produce
diffuse ones (Ennis and Sussman, 1958a); and Kessler's Guttulina may
lack any recognizable feeding front (Shaffer, 1961b). On the other hand,
in my experience, different plate cultures of the same strain may present
quite a different appearance. When a small, isolated bacterial colony
has been almost completely consumed, the resultant 'feeding aggrega-