130
B. M. SHAFFER
inhibitor of founder differentiation; and the distribution of new centres
suggests that it is present in a gradient extending outwards from established centres. This would certainly explain the limitations on the numbers of centres and fruiting bodies in large populations, which apparently
apply to violaceum (Sussman, 1958; Bonner, privately), and to other
species (Section III, B). But the existence of a gradient has yet to be
established. Inhibition is only temporary; and the distribution of new
centres may merely reflect the progressive shrinkage in the area of the
underlying aggregations as they age.
Founders differ from I-cells in their morphology, behaviour, origin,
and fate, in almost every respect in which these are diagnostic, as well
as in their attributed function and time of action. And although founders
obviously initiate aggregations, they are to be distinguished from Sussman's initiators for the following reasons. First, the quantitative approach has not provided a valid proof that initiators are single cells.
Second, there is no evidence that founders are latently different before
the aggregation stage. Third, founders do not constitute a fixed proportion of the population: the inhibition mechanism by itself ensures that
their numbers are highly dependent on density; and their occurrence
is also influenced profoundly by the noncellular environment. Certainly
their proportion may be several times greater than the maximum
centre-to-cell ratio of 1 : 180 that Sussman (1958) found in preliminary
experiments on this species. Fourth, it is still doubtful to what extent
single-cell founders occur elsewhere. In the three large Dictyostelium
species, the cells form small clumps before they show sustained radial
orientation (Ennis and Sussman, 1958b; Shaffer, 1959; Bonner, 1960).
A small number of fractionations of discoideum clumps and young
aggregations did not reveal any cells with properties of founders, the
dispersed cells being strongly reattracted only after considerable delay
during which they re-formed small clumps (Shaffer, 1959). But it was
not clear whether clumps had to develop first, or whether there were
single-celled founders, which were not ready to secrete again for some
time, and clumps formed first because the cells had become slightly
adhesive. By carefully plotting individual paths in D. purpureum,
Samuel (1961) has now found that in the earliest stages of an aggregation,
the separate cells orient periodically towards one particular motile cell,
which cannot otherwise be distinguished by its morphology or behaviour; and he compares it to a violaceum founder. Because of the
labour involved, he has not been able to analyse enough aggregations
to determine how regularly this happens. In any case, we may expect to
find the series of violaceum founders of decreasing development and
permanence extended in other species; but obviously it must become
ever harder to detect such cells. And it is still possible that in all species
B. M. SHAFFER
inhibitor of founder differentiation; and the distribution of new centres
suggests that it is present in a gradient extending outwards from established centres. This would certainly explain the limitations on the numbers of centres and fruiting bodies in large populations, which apparently
apply to violaceum (Sussman, 1958; Bonner, privately), and to other
species (Section III, B). But the existence of a gradient has yet to be
established. Inhibition is only temporary; and the distribution of new
centres may merely reflect the progressive shrinkage in the area of the
underlying aggregations as they age.
Founders differ from I-cells in their morphology, behaviour, origin,
and fate, in almost every respect in which these are diagnostic, as well
as in their attributed function and time of action. And although founders
obviously initiate aggregations, they are to be distinguished from Sussman's initiators for the following reasons. First, the quantitative approach has not provided a valid proof that initiators are single cells.
Second, there is no evidence that founders are latently different before
the aggregation stage. Third, founders do not constitute a fixed proportion of the population: the inhibition mechanism by itself ensures that
their numbers are highly dependent on density; and their occurrence
is also influenced profoundly by the noncellular environment. Certainly
their proportion may be several times greater than the maximum
centre-to-cell ratio of 1 : 180 that Sussman (1958) found in preliminary
experiments on this species. Fourth, it is still doubtful to what extent
single-cell founders occur elsewhere. In the three large Dictyostelium
species, the cells form small clumps before they show sustained radial
orientation (Ennis and Sussman, 1958b; Shaffer, 1959; Bonner, 1960).
A small number of fractionations of discoideum clumps and young
aggregations did not reveal any cells with properties of founders, the
dispersed cells being strongly reattracted only after considerable delay
during which they re-formed small clumps (Shaffer, 1959). But it was
not clear whether clumps had to develop first, or whether there were
single-celled founders, which were not ready to secrete again for some
time, and clumps formed first because the cells had become slightly
adhesive. By carefully plotting individual paths in D. purpureum,
Samuel (1961) has now found that in the earliest stages of an aggregation,
the separate cells orient periodically towards one particular motile cell,
which cannot otherwise be distinguished by its morphology or behaviour; and he compares it to a violaceum founder. Because of the
labour involved, he has not been able to analyse enough aggregations
to determine how regularly this happens. In any case, we may expect to
find the series of violaceum founders of decreasing development and
permanence extended in other species; but obviously it must become
ever harder to detect such cells. And it is still possible that in all species