166
B. M. S H A F F E R
Acrasis is another genus described by van Tieghem (1880) and subsequently lost. Happily it has been rediscovered by L. Olive and Stoianovitch (1960), and its fruiting bodies present a pretty problem in morphogenetic movement. They consist of one or more columns of stalk cells,
which resemble those of the Guttulinidae rather than the Dictyosteliidae
in remaining full of cytoplasm and germinating readily, surmounted by
one or more straight or branched chains of spore cells. These chains are
formed from the lobed mass of prespores in under half an hour. Now,
the Dictyosteliidae in fruiting can extrude a single file of cells from one
end of a mass; and there seems to be no reason why the same system
should not work in an inverted manner, with the mass remaining on the
ground, and the cell column being freely projected into the air, which
would offer no resistance. This method, however, requires a rather high
degree of organization, for it is the sheath that, besides providing support, defines the stalk. And it is hard for a less organized mass to pull
itself out into a single file, even if this lies on the ground. It might seem
that the fast cells have only to draw away from the slower ones, or the
cells at opposite ends to move in opposite directions; but if the cells are
sufficiently free to intercalate themselves in the remaining files as these
diminish in number, it is difficult for the last file to retain its integrity.
This problem does not arise if the cells push themselves out into line,
that is, if active movement in the main is not parallel to the axis, but
transverse, as it normally is when an unorganized group of men are
ordered to fall in, in single line. The Acrasis situation has yet to be
studied experimentally, but Olive and Stoianovitch noticed that the
cells in general did not crawl to the free end of the chain and join on
there. What then must be a cell's 'instructions'? Certainly it must be
guided by a gradient perpendicular to the axis of the mass. But it is
not enough that it should turn till it is stimulated symmetrically with
respect to its own axis, as do aggregating cells of the Dictyosteliidae,
for these do not force their way into a stationary stream, when they
arrive perpendicular to its surface. The Acrasis cell must not rest
till another condition is satisfied: both its ends must be stimulated
equally.
2. Speed
If the back or front of a vitally stained D. discoideum slug is grafted
on to the complementary half of an unstained one, the stained cells
retain their position during further migration (Raper, 1940b). However,
Bonner (1952, 1957; Bonner and Adams, 1958) has found that a slug's
cells are not distributed through it at random, for if they are grafted
into a new position along its axis, they soon find their way back to their
old one. The cells largely sort themselves into their equilibrium positions
B. M. S H A F F E R
Acrasis is another genus described by van Tieghem (1880) and subsequently lost. Happily it has been rediscovered by L. Olive and Stoianovitch (1960), and its fruiting bodies present a pretty problem in morphogenetic movement. They consist of one or more columns of stalk cells,
which resemble those of the Guttulinidae rather than the Dictyosteliidae
in remaining full of cytoplasm and germinating readily, surmounted by
one or more straight or branched chains of spore cells. These chains are
formed from the lobed mass of prespores in under half an hour. Now,
the Dictyosteliidae in fruiting can extrude a single file of cells from one
end of a mass; and there seems to be no reason why the same system
should not work in an inverted manner, with the mass remaining on the
ground, and the cell column being freely projected into the air, which
would offer no resistance. This method, however, requires a rather high
degree of organization, for it is the sheath that, besides providing support, defines the stalk. And it is hard for a less organized mass to pull
itself out into a single file, even if this lies on the ground. It might seem
that the fast cells have only to draw away from the slower ones, or the
cells at opposite ends to move in opposite directions; but if the cells are
sufficiently free to intercalate themselves in the remaining files as these
diminish in number, it is difficult for the last file to retain its integrity.
This problem does not arise if the cells push themselves out into line,
that is, if active movement in the main is not parallel to the axis, but
transverse, as it normally is when an unorganized group of men are
ordered to fall in, in single line. The Acrasis situation has yet to be
studied experimentally, but Olive and Stoianovitch noticed that the
cells in general did not crawl to the free end of the chain and join on
there. What then must be a cell's 'instructions'? Certainly it must be
guided by a gradient perpendicular to the axis of the mass. But it is
not enough that it should turn till it is stimulated symmetrically with
respect to its own axis, as do aggregating cells of the Dictyosteliidae,
for these do not force their way into a stationary stream, when they
arrive perpendicular to its surface. The Acrasis cell must not rest
till another condition is satisfied: both its ends must be stimulated
equally.
2. Speed
If the back or front of a vitally stained D. discoideum slug is grafted
on to the complementary half of an unstained one, the stained cells
retain their position during further migration (Raper, 1940b). However,
Bonner (1952, 1957; Bonner and Adams, 1958) has found that a slug's
cells are not distributed through it at random, for if they are grafted
into a new position along its axis, they soon find their way back to their
old one. The cells largely sort themselves into their equilibrium positions