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B. M. SHAFFER
the substratum. So far as is known, the grex at no stage orients in a
external acrasin gradient, but instead it may be guided by a variety of
differentials that do not affect aggregating cells. Some of these responses
have been intensively investigated, but unfortunately—perhaps because so much attention has been paid to that grossly seductive body,
the D. discoideum slug—we know very little about the most important
of them: erection. Sponge (H. V. Wilson, 1907) and coelenterate aggregates (Child, 1928), as well as regenerates and normal developmental
stages of many organisms, fix their main axes perpendicular to the
substratum, by making use of a variety of chemical and physical differentials, some due to their own activity, others not, and many of them
only postulated (Child, 1941). In slime moulds, a clue may be provided
by the apparently bizarre observation that when P. violaceum aggregates on a glass surface under water, it is still able to form elongated
grex (though stalk is suppressed), and the axes are again initially perpendicular to the substratum, but they may all be inverted (Shaffer,
1961b). This development can be realized when the central portions of
the still inflowing streams are covered with slime sheath, because this
allows them to be lifted away from the glass as the grex lengthen, and
to act like guy ropes to prevent them toppling over. It is not yet clear
whether the normal differential is still available but inverted, or whether
the grex respond to an abnormal one in its absence. But it seems rather
unlikely that metabolic gradients resulting from the asymmetry of
access or escape of various substances owing to contact with the substratum would actually be reversed under water. Neither light nor a
temperature gradient is the primary factor. If during a grex's inverted
erection, its 'base' breaks the surface of the water, the cells there slowly
start to form a normally oriented grex, which extends up into the air at
the expense of the original one. This is the only case known in which all
the cells in a grex are unequivocally compelled to reverse their orientation.
There remains the question whether an aggregate can produce an
axis autonomously. Bonner (1950) reported that D. discoideum cells
kept suspended in water on a shaker clumped together and that the
aggregates eventually became pear-shaped. However, Gerisch (1960)
found that such aggregates remained spherical indefinitely under water.
Only when they were brought into the interface with the air did they
elongate—again, perpendicularly to the surface. He suggested that
Bonner's grex formed on the glass above the waterline and then fell in.
It does indeed seem unlikely that the axis was established while submerged, because a slug cannot even maintain its elongation if it is
covered with water, but soon becomes approximately spherical again
(Shaffer, 1961b). However, this observation also shows that the aqueous
B. M. SHAFFER
the substratum. So far as is known, the grex at no stage orients in a
external acrasin gradient, but instead it may be guided by a variety of
differentials that do not affect aggregating cells. Some of these responses
have been intensively investigated, but unfortunately—perhaps because so much attention has been paid to that grossly seductive body,
the D. discoideum slug—we know very little about the most important
of them: erection. Sponge (H. V. Wilson, 1907) and coelenterate aggregates (Child, 1928), as well as regenerates and normal developmental
stages of many organisms, fix their main axes perpendicular to the
substratum, by making use of a variety of chemical and physical differentials, some due to their own activity, others not, and many of them
only postulated (Child, 1941). In slime moulds, a clue may be provided
by the apparently bizarre observation that when P. violaceum aggregates on a glass surface under water, it is still able to form elongated
grex (though stalk is suppressed), and the axes are again initially perpendicular to the substratum, but they may all be inverted (Shaffer,
1961b). This development can be realized when the central portions of
the still inflowing streams are covered with slime sheath, because this
allows them to be lifted away from the glass as the grex lengthen, and
to act like guy ropes to prevent them toppling over. It is not yet clear
whether the normal differential is still available but inverted, or whether
the grex respond to an abnormal one in its absence. But it seems rather
unlikely that metabolic gradients resulting from the asymmetry of
access or escape of various substances owing to contact with the substratum would actually be reversed under water. Neither light nor a
temperature gradient is the primary factor. If during a grex's inverted
erection, its 'base' breaks the surface of the water, the cells there slowly
start to form a normally oriented grex, which extends up into the air at
the expense of the original one. This is the only case known in which all
the cells in a grex are unequivocally compelled to reverse their orientation.
There remains the question whether an aggregate can produce an
axis autonomously. Bonner (1950) reported that D. discoideum cells
kept suspended in water on a shaker clumped together and that the
aggregates eventually became pear-shaped. However, Gerisch (1960)
found that such aggregates remained spherical indefinitely under water.
Only when they were brought into the interface with the air did they
elongate—again, perpendicularly to the surface. He suggested that
Bonner's grex formed on the glass above the waterline and then fell in.
It does indeed seem unlikely that the axis was established while submerged, because a slug cannot even maintain its elongation if it is
covered with water, but soon becomes approximately spherical again
(Shaffer, 1961b). However, this observation also shows that the aqueous