118
B. M. SHAFFER
tion' may be mistaken for a fruiting one (Fig. 5); but in most cases, such
clumps, and feeding fronts as well, are not directly transformed into
fruiting aggregations but disperse, because though both processes are
stimulated by the exhaustion of the food supply, the time lag before
'true' aggregation is greater.
I I I . The Onset of the Social Phase
A. Food Shortage
The fruiting phase begins after the supply of edible and accessible
bacteria has been depleted and usually exhausted, the delay depending
on the precise conditions (Raper, 1940b; Hirschberg and Rusch, 1950;
Shaffer, 1958). Thus, the cells escaping from the back of a feeding front
are usually still seeking food; but not finding it, their antisocial responses
are gradually replaced or overruled by social ones. This happens sooner
if the cells can join existing aggregations than if they have to found new
ones (Arndt, 1937; Shaffer, 1957b). However, at times many species
start fruiting aggregations within the feeding front, and D. minutum
often does so (Shaffer, 1961b).
Potts (1902) delayed aggregation by adding fresh supplies of bacteria.
I t could not be postponed indefinitely, but this might have been due
simply to the inaccessibility of the food. Whereas it has been known
since the time of van Tieghem (1880) that aggregated cells mechanically
dispersed in the absence of nutrient will reaggregate, there has been
much dispute about whether they will return to the vegetative stage if
nutrient is present. Van Tieghem thought they would; E. W. Olive
(1902) disagreed. When the process of feeding was better understood,
results acquired more weight but not more uniformity. Arndt (1937)
found that most of the cells taken from any aggregation or grex of D.
mucoroides would reaggregate, even if they were completely separated
and there was no residue of the original organization, provided that the
cells were not too scattered and the bacteria were not too numerous,
although always a few cells started to feed. He concluded that once
food shortage had sufficiently altered a cell's properties, food became
a less effective stimulus, though it could still be decisive if present
in sufficient quantity. But Raper (1940b) found that if aggregations and grex of D. discoideum were covered with bacteria, any cells
that were separated or simply stirred up so that they lost their original
positions returned to the vegetative stage, whereas those that were left
undisturbed continued with their development. The critical factor was
thus believed to be intercellular organization. Recent incidental observations on P. violaceum support Arndt's view (Shaffer, 1961a). This is
not to deny that cells within an aggregate will be more resistant to
B. M. SHAFFER
tion' may be mistaken for a fruiting one (Fig. 5); but in most cases, such
clumps, and feeding fronts as well, are not directly transformed into
fruiting aggregations but disperse, because though both processes are
stimulated by the exhaustion of the food supply, the time lag before
'true' aggregation is greater.
I I I . The Onset of the Social Phase
A. Food Shortage
The fruiting phase begins after the supply of edible and accessible
bacteria has been depleted and usually exhausted, the delay depending
on the precise conditions (Raper, 1940b; Hirschberg and Rusch, 1950;
Shaffer, 1958). Thus, the cells escaping from the back of a feeding front
are usually still seeking food; but not finding it, their antisocial responses
are gradually replaced or overruled by social ones. This happens sooner
if the cells can join existing aggregations than if they have to found new
ones (Arndt, 1937; Shaffer, 1957b). However, at times many species
start fruiting aggregations within the feeding front, and D. minutum
often does so (Shaffer, 1961b).
Potts (1902) delayed aggregation by adding fresh supplies of bacteria.
I t could not be postponed indefinitely, but this might have been due
simply to the inaccessibility of the food. Whereas it has been known
since the time of van Tieghem (1880) that aggregated cells mechanically
dispersed in the absence of nutrient will reaggregate, there has been
much dispute about whether they will return to the vegetative stage if
nutrient is present. Van Tieghem thought they would; E. W. Olive
(1902) disagreed. When the process of feeding was better understood,
results acquired more weight but not more uniformity. Arndt (1937)
found that most of the cells taken from any aggregation or grex of D.
mucoroides would reaggregate, even if they were completely separated
and there was no residue of the original organization, provided that the
cells were not too scattered and the bacteria were not too numerous,
although always a few cells started to feed. He concluded that once
food shortage had sufficiently altered a cell's properties, food became
a less effective stimulus, though it could still be decisive if present
in sufficient quantity. But Raper (1940b) found that if aggregations and grex of D. discoideum were covered with bacteria, any cells
that were separated or simply stirred up so that they lost their original
positions returned to the vegetative stage, whereas those that were left
undisturbed continued with their development. The critical factor was
thus believed to be intercellular organization. Recent incidental observations on P. violaceum support Arndt's view (Shaffer, 1961a). This is
not to deny that cells within an aggregate will be more resistant to