THE ACRASINA
121
in effect removed cations. But Konijn and Raper (1961), during a most
painstaking repetition of the small-population work, discovered that the
real action was to make the agar more hydrophobic, and that if salts
were not added, the cells did become unhealthy and less able to aggregate. They found that with small, undisturbed, uncentrifuged populations, grown in isolation, the proportion aggregating was indeed very
much greater than could be expected from the 1 : 2,250 ratio. However,
they then went on to show that even when the cells were centrifuged
as in Sussman's laboratory, the proportion did not in fact decrease to
anything like the expected value. At 600 cells/mm
2 , a density used by
Sussman and Noel, virtually all populations of more than 100 centrifuged cells, and about 80% of those of 50-100, aggregated. A further
discrepancy was that the proportion aggregating was strongly dependent
on cell density, a higher density compensating for a fall in numbers.
These discrepancies could not possibly have been due to genetic differences: not only were the above results obtained with Sussman's
haploid stock of strain NC4, but essentially identical ones were given
by Raper's diploid NC4 and 9 other isolates of discoideum from nature
presumed from cell and spore size to be haploid. And they could hardly
have been due to slight differences in laboratory conditions, if it were
true, as Bradley, Sussman and Ennis (1956) reported, that quantitatively aggregation was highly resistant to a wide variety of physical
and chemical changes in the environment—indeed, these workers were
unable to discover a single factor that increased the number of primary
centres. However, Konijn and Raper found that the percentage of
populations aggregating was extremely sensitive to differences in the
agar substratum produced by small variations in preparation or even
in pouring. In one experiment, for example, aggregation was increased
from ^ 2 0 % to ^ 8 0 % when the agar was boiled in an open vessel instead of being autoclaved. Most of these variations were believed to
affect the degree of cell dispersal.
Gerisch (1961a) too found that the percentage of aggregation depended on density and that at about 600 cells/mm
2 all populations of
more than 100 cells of diploid or haploid NC4, deposited on coverslips,
aggregated, whether they had been washed by centrifuging or not.
However, as they were prepared at room temperature, it might be
objected that they had already been stimulated by initiators before
isolation, although the suspensions in Sussman and Noel's experiments
were not kept cold after being spun.
None of these results could really be expressed in terms of centres per
cell, because there was no evidence of a fixed ratio, but if aggregations
were started by predetermined initiator cells, there must have been at
least 10-40 times more of them than the Sussman group found. However,
121
in effect removed cations. But Konijn and Raper (1961), during a most
painstaking repetition of the small-population work, discovered that the
real action was to make the agar more hydrophobic, and that if salts
were not added, the cells did become unhealthy and less able to aggregate. They found that with small, undisturbed, uncentrifuged populations, grown in isolation, the proportion aggregating was indeed very
much greater than could be expected from the 1 : 2,250 ratio. However,
they then went on to show that even when the cells were centrifuged
as in Sussman's laboratory, the proportion did not in fact decrease to
anything like the expected value. At 600 cells/mm
2 , a density used by
Sussman and Noel, virtually all populations of more than 100 centrifuged cells, and about 80% of those of 50-100, aggregated. A further
discrepancy was that the proportion aggregating was strongly dependent
on cell density, a higher density compensating for a fall in numbers.
These discrepancies could not possibly have been due to genetic differences: not only were the above results obtained with Sussman's
haploid stock of strain NC4, but essentially identical ones were given
by Raper's diploid NC4 and 9 other isolates of discoideum from nature
presumed from cell and spore size to be haploid. And they could hardly
have been due to slight differences in laboratory conditions, if it were
true, as Bradley, Sussman and Ennis (1956) reported, that quantitatively aggregation was highly resistant to a wide variety of physical
and chemical changes in the environment—indeed, these workers were
unable to discover a single factor that increased the number of primary
centres. However, Konijn and Raper found that the percentage of
populations aggregating was extremely sensitive to differences in the
agar substratum produced by small variations in preparation or even
in pouring. In one experiment, for example, aggregation was increased
from ^ 2 0 % to ^ 8 0 % when the agar was boiled in an open vessel instead of being autoclaved. Most of these variations were believed to
affect the degree of cell dispersal.
Gerisch (1961a) too found that the percentage of aggregation depended on density and that at about 600 cells/mm
2 all populations of
more than 100 cells of diploid or haploid NC4, deposited on coverslips,
aggregated, whether they had been washed by centrifuging or not.
However, as they were prepared at room temperature, it might be
objected that they had already been stimulated by initiators before
isolation, although the suspensions in Sussman and Noel's experiments
were not kept cold after being spun.
None of these results could really be expressed in terms of centres per
cell, because there was no evidence of a fixed ratio, but if aggregations
were started by predetermined initiator cells, there must have been at
least 10-40 times more of them than the Sussman group found. However,