THE ACRASINA
131
the first body to be attractive is sometimes multicellular, and that in
some species, including discoideum, it always is.
D. Sexuality
Many have observed an amoeba partially or completely surrounding
another one, but the site, frequency and significance of this action have
been much disputed. Both C. M. Wilson and Ross have used the terms
engulfment and syngamy synonymously, but not everyone considers
their published accounts of either cytoplasmic or nuclear fusion to be
adequate. Wilson (1952, 1953), following Skupienski (1918), believed
that aggregations were composed of zygotes, though he differed in finding meiosis during aggregation, not sporulation. Wilson and Ross (1957)
conceded it was possible that not all haploid cells had to pass through a
sexual phase, but nevertheless still believed that the majority of them
did so. They examined multiple isolates of four species, including P .
violaceum, and in all of them found meiotic divisions 'soon after the
first zygotes formed a center of aggregation'. Ross (1960) reported that
in D. discoideum syngamy was common in about half the cultures of
stable haploid strains, and absent from stable diploid strains (except
apparently for a few haploid cells they contained). But he asserted that
on rare occasions, all the cells in all the plates lying on the bench changed
their ploidy on successive days: in one case, for example, from diploid to
haploid, back to diploid, and then to a stable mixture of the two.
Meiosis or mitosis—according to whether syngamy had occurred or not
—was confined to the initial elongation of the grex.
Whereas Wilson and Ross observed engulfment only among cells
entering or about to enter aggregations, Labudde (1956) found it only
in the vegetative stage. R. R. Sussman et al. (1960) reported that about
half the I-cells always contained engulfed cells, whereas it was only
after growth had stopped that a low percentage of the (haploid) R-cells
did so, which reconciled the previous results to some extent; but what
happened before or during aggregation was not mentioned. In my own
cultures of these species, and especially of P . violaceum, at a sufficiently
low density for all the cells to remain individually observable under
phase contrast after entering the streams, and in which young centres
could be dispersed so as to allow their component cells to be re-examined,
it was extremely rare to see either a healthy cell engulfed by another one,
or any form of cell division, either within a centre or a stream, or during
their formation (Shaffer, 1958, 1961a).
Labudde (1956) found as many as 6 amoebae within a single cell;
Wilson and Ross (1957) saw several examples of 'multiple fusion', that
is, 2-3 engulfed cells, which were written off as incompatible gametes
(an explanation that can hardly apply when cells stuff themselves with
131
the first body to be attractive is sometimes multicellular, and that in
some species, including discoideum, it always is.
D. Sexuality
Many have observed an amoeba partially or completely surrounding
another one, but the site, frequency and significance of this action have
been much disputed. Both C. M. Wilson and Ross have used the terms
engulfment and syngamy synonymously, but not everyone considers
their published accounts of either cytoplasmic or nuclear fusion to be
adequate. Wilson (1952, 1953), following Skupienski (1918), believed
that aggregations were composed of zygotes, though he differed in finding meiosis during aggregation, not sporulation. Wilson and Ross (1957)
conceded it was possible that not all haploid cells had to pass through a
sexual phase, but nevertheless still believed that the majority of them
did so. They examined multiple isolates of four species, including P .
violaceum, and in all of them found meiotic divisions 'soon after the
first zygotes formed a center of aggregation'. Ross (1960) reported that
in D. discoideum syngamy was common in about half the cultures of
stable haploid strains, and absent from stable diploid strains (except
apparently for a few haploid cells they contained). But he asserted that
on rare occasions, all the cells in all the plates lying on the bench changed
their ploidy on successive days: in one case, for example, from diploid to
haploid, back to diploid, and then to a stable mixture of the two.
Meiosis or mitosis—according to whether syngamy had occurred or not
—was confined to the initial elongation of the grex.
Whereas Wilson and Ross observed engulfment only among cells
entering or about to enter aggregations, Labudde (1956) found it only
in the vegetative stage. R. R. Sussman et al. (1960) reported that about
half the I-cells always contained engulfed cells, whereas it was only
after growth had stopped that a low percentage of the (haploid) R-cells
did so, which reconciled the previous results to some extent; but what
happened before or during aggregation was not mentioned. In my own
cultures of these species, and especially of P . violaceum, at a sufficiently
low density for all the cells to remain individually observable under
phase contrast after entering the streams, and in which young centres
could be dispersed so as to allow their component cells to be re-examined,
it was extremely rare to see either a healthy cell engulfed by another one,
or any form of cell division, either within a centre or a stream, or during
their formation (Shaffer, 1958, 1961a).
Labudde (1956) found as many as 6 amoebae within a single cell;
Wilson and Ross (1957) saw several examples of 'multiple fusion', that
is, 2-3 engulfed cells, which were written off as incompatible gametes
(an explanation that can hardly apply when cells stuff themselves with