THE ACRASINA
119
ambient bacteria, if only because of their physical inaccessibility. In the
plasmodial slime moulds, the transition from vegetative to fruiting
stage is irreversible (Rusch, 1959).
B. The Initiators
It is necessary to review the work of Sussman and his associates at
considerable length not only because of the extent of their contribution,
but also because Sussman (1959) has complained that a recent monograph has slighted the work without giving enough facts for the reader
to evaluate it himself.
Large-population experiment. Sussman and Noel (1952) found that the
ratio of centres to cells increased with cell density to a maximum at
what was called the optimum density and then declined. The maximum
was characteristic for each species, as were the optimum and threshold
densities. In D. discoideum, the species with which, except where otherwise indicated, we are exclusively concerned in this subsection (Section
III, B), Sussman's (1958) latest value for the maximum is 1 centre per
2,200 cells.
Comment. The authors briefly mentioned that, with very big populations, the number of centres formed was largely independent of the number of cells, when these were confined to a standard area. This rather
remarkable observation has never been referred to since. However, I
believe it provides the clue to most of these quantitative relationships.
From the data in their discoideum graph, we can calculate that the number of centres per unit area also reached a maximum at the 'optimum'
density, but then remained constant up to a much higher density. Within
this density range, the mean area of an aggregation was 8-6 mm
2 , which
is equal to a circle of diameter 3-3 mm. This value was species specific: in
D. purpureum, the diameter was 2*3 mm under the same conditions. We
may suppose that a centre was able, on average, to inhibit another centre
from forming within this distance of itself, regardless of cell density.
The inhibition of new organization centres is, of course, a general feature
of development in other organisms. We may, for example, contrast the
situation in the insect Rhodnius: Wigglesworth (1940) determined that
its existing bristle plaques prevented new ones from forming, not
within some absolute distance of themselves, but less than a certain
number of cells away. Interestingly enough, well over thirty years ago,
Arndt (1937) noticed that the numbers of aggregation centres and of
fruiting bodies of D. mucoroides per unit area remained constant over a
wide range of cell densities, though he did not publish any figures; and
Bonner (privately) has found this to be true in all the species he has
examined. Now, inhibition itself, without additional hypotheses, will
account for the existence of a maximum centre-to-cell ratio. However,
119
ambient bacteria, if only because of their physical inaccessibility. In the
plasmodial slime moulds, the transition from vegetative to fruiting
stage is irreversible (Rusch, 1959).
B. The Initiators
It is necessary to review the work of Sussman and his associates at
considerable length not only because of the extent of their contribution,
but also because Sussman (1959) has complained that a recent monograph has slighted the work without giving enough facts for the reader
to evaluate it himself.
Large-population experiment. Sussman and Noel (1952) found that the
ratio of centres to cells increased with cell density to a maximum at
what was called the optimum density and then declined. The maximum
was characteristic for each species, as were the optimum and threshold
densities. In D. discoideum, the species with which, except where otherwise indicated, we are exclusively concerned in this subsection (Section
III, B), Sussman's (1958) latest value for the maximum is 1 centre per
2,200 cells.
Comment. The authors briefly mentioned that, with very big populations, the number of centres formed was largely independent of the number of cells, when these were confined to a standard area. This rather
remarkable observation has never been referred to since. However, I
believe it provides the clue to most of these quantitative relationships.
From the data in their discoideum graph, we can calculate that the number of centres per unit area also reached a maximum at the 'optimum'
density, but then remained constant up to a much higher density. Within
this density range, the mean area of an aggregation was 8-6 mm
2 , which
is equal to a circle of diameter 3-3 mm. This value was species specific: in
D. purpureum, the diameter was 2*3 mm under the same conditions. We
may suppose that a centre was able, on average, to inhibit another centre
from forming within this distance of itself, regardless of cell density.
The inhibition of new organization centres is, of course, a general feature
of development in other organisms. We may, for example, contrast the
situation in the insect Rhodnius: Wigglesworth (1940) determined that
its existing bristle plaques prevented new ones from forming, not
within some absolute distance of themselves, but less than a certain
number of cells away. Interestingly enough, well over thirty years ago,
Arndt (1937) noticed that the numbers of aggregation centres and of
fruiting bodies of D. mucoroides per unit area remained constant over a
wide range of cell densities, though he did not publish any figures; and
Bonner (privately) has found this to be true in all the species he has
examined. Now, inhibition itself, without additional hypotheses, will
account for the existence of a maximum centre-to-cell ratio. However,