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B. M. SHAFFER
slime-mould spores almost to the point of invisibility); and Ross (1960)
did not describe any multiple engulfment. Again, the report that I-cells
may contain up to 5 other cells, R-cells only 1 (R. R. Sussman et al.,
1960), may help to explain this difference. All authors agree that the
engulfed cells are initially healthy and of normal size, and may become
compact, dense residues; the controversy as to whether they appear to
be in vacuoles or not probably turns simply on the degree of degeneration. I have seen some of the largest D. discoideum cells mauling more
than a dozen ordinary cells in an hour, sometimes completely enclosing
them and later ejecting the remains, sometimes briefly wrapping round
them and then spitting them out more or less maimed, sometimes even
cutting them in two and treating the pieces, whether nucleate or not,
in either of the first two ways. (The presence of such Ί-cells' in very
small populations decreases their chance of aggregating: there are
too few survivors.) After some hours, this rogue behaviour stops, and
then for a time other cells may even crawl over a large-cell's surface with
impunity. It seems likely that the cells that the R. R. Sussman group
(1960) believed to be moving freely through the I-cells' cytoplasm, protruding pseudopods and discharging vacuoles just as if they were freeliving, were in fact on the surface. Occasionally, I have noticed ordinary
cells behaving somewhat like the large ones and flinging long hyaline
pseudopods round one another; but unless they withdraw them quickly,
they are liable to have them torn off. Sussman (1955b), amongst others
has suggested that engulf ment may be cannibalism. It remains possible
that it may sometimes be followed by cytoplasmic fusion, but this has
not been established. The only one of Wilson and Ross's papers based
on living material (Ross, 1960), despite the abundance of so-called
syngamy, merely mentioned that cells with engulfed partners entered
streams, and that their subsequent behaviour was thus obscured.
Ennis and Sussman (1958b) originally suggested that their I-cells
might be identical with the large cells that Skupienski and Wilson and
Ross identified as zygotes. But apart from the attributed differences in
their stage of origin and treatment of other cells, the I-cells are supposed
to be present in D. discoideum in a minute but fixed proportion in the
haploid strain, the zygotes in a large but indefinite one. Moreover,
neither Wilson nor Ross found cells with more than 2 nuclei or more than
2 sets of chromosomes; and they believed that the binucleate cells,
which occurred only in aggregations, were about to undergo nuclear
fusion. The R. R. Sussman group (1960) found that 1% even of nonaggregating R-cells were binucleate and that all of them divided into
uninucleate daughters; whereas the I-cells contained up to 6 nuclei, and
when uninucleate were probably polyploid (perhaps tetraploid), and
often produced 3 or 4 daughters. I have seen 3 large discoideum cells in
B. M. SHAFFER
slime-mould spores almost to the point of invisibility); and Ross (1960)
did not describe any multiple engulfment. Again, the report that I-cells
may contain up to 5 other cells, R-cells only 1 (R. R. Sussman et al.,
1960), may help to explain this difference. All authors agree that the
engulfed cells are initially healthy and of normal size, and may become
compact, dense residues; the controversy as to whether they appear to
be in vacuoles or not probably turns simply on the degree of degeneration. I have seen some of the largest D. discoideum cells mauling more
than a dozen ordinary cells in an hour, sometimes completely enclosing
them and later ejecting the remains, sometimes briefly wrapping round
them and then spitting them out more or less maimed, sometimes even
cutting them in two and treating the pieces, whether nucleate or not,
in either of the first two ways. (The presence of such Ί-cells' in very
small populations decreases their chance of aggregating: there are
too few survivors.) After some hours, this rogue behaviour stops, and
then for a time other cells may even crawl over a large-cell's surface with
impunity. It seems likely that the cells that the R. R. Sussman group
(1960) believed to be moving freely through the I-cells' cytoplasm, protruding pseudopods and discharging vacuoles just as if they were freeliving, were in fact on the surface. Occasionally, I have noticed ordinary
cells behaving somewhat like the large ones and flinging long hyaline
pseudopods round one another; but unless they withdraw them quickly,
they are liable to have them torn off. Sussman (1955b), amongst others
has suggested that engulf ment may be cannibalism. It remains possible
that it may sometimes be followed by cytoplasmic fusion, but this has
not been established. The only one of Wilson and Ross's papers based
on living material (Ross, 1960), despite the abundance of so-called
syngamy, merely mentioned that cells with engulfed partners entered
streams, and that their subsequent behaviour was thus obscured.
Ennis and Sussman (1958b) originally suggested that their I-cells
might be identical with the large cells that Skupienski and Wilson and
Ross identified as zygotes. But apart from the attributed differences in
their stage of origin and treatment of other cells, the I-cells are supposed
to be present in D. discoideum in a minute but fixed proportion in the
haploid strain, the zygotes in a large but indefinite one. Moreover,
neither Wilson nor Ross found cells with more than 2 nuclei or more than
2 sets of chromosomes; and they believed that the binucleate cells,
which occurred only in aggregations, were about to undergo nuclear
fusion. The R. R. Sussman group (1960) found that 1% even of nonaggregating R-cells were binucleate and that all of them divided into
uninucleate daughters; whereas the I-cells contained up to 6 nuclei, and
when uninucleate were probably polyploid (perhaps tetraploid), and
often produced 3 or 4 daughters. I have seen 3 large discoideum cells in