THE ACRASINA
173
environment is simply too unfavourable for the discoideum grex to exist
in it, and Gerisch's conclusion that unilateral contact with the interface
between liquid and air is indispensable for producing an axis, and the
inequality that is the basis of subsequent differentiation, is not of general
validity. Not only does P . violaceum not need this particular differential,
but we still have no information about how far an aggregate will develop when suspended in a medium in which it can turn into a grex:
for example, in mineral oil (Potts, 1902), and for Polysphondylium, of
course, in water too. Admittedly the abolition of all environmental differentials is rather a theoretical concept; but if it could be achieved, it
would not be surprising if one region of an aggregate could become
dominant by exploiting some random internal difference. Contact following might well be able to produce a uniform orientation within an
aggregate, and perhaps even some measure of cell sorting. Certainly,
very elaborate tissue structures may develop within suspended aggregates of vertebrate cells (Moscona, 1961).
Regularly in Acytostelium and the small Dictyostelium species (Raper,
1941a; Raper and Quinlan, 1958), and in most other forms in a dry
environment, the grex proceeds without interruption to turn itself into
a fruiting body, while remaining erect. As it could continue to move in
the direction in which it had elongated simply by preserving its radial
symmetry, one might think there was no positive guidance at this stage.
However, if a culminating grex falls over or is laid on the ground, its
tip makes a sharp right-angled turn to lead it up into the air again. To
what differential the tip responds is quite unknown. It may or may not
be one involved in fixing the axis originally. Harper (1926, 1929, 1932)
discussed this question at greater length than anyone else has done. But
he gave three divergent accounts of the erection of inverted fruits, and
in a single paper expressed his belief that negative geotropism was the
primary factor involved; that the amoebae primarily 'glide in that
direction in which the greatest resistance is offered to their motion' (a
completely different mechanism, which would presumably have made
most of the cells, if acting individually, burrow into the interior of the
aggregate, and the aggregate itself burrow into the ground); that at
the end of aggregation the cells at the top of the rounded mass had
climbed up to the region of greatest downward pull (which is simply not
the case); and that they thereafter maintained a position of maximum
resistance to the downward pull by thrusting upwards in proportion
to their load. However, workers from Brefeld onwards had already
noticed that fruiting bodies tended to be erect whatever the plane of
the substratum; and in Raper's (1935) experience, erection occurred
whether on the bottom or on the side of a dish and whether the dish was
inverted or not. Negative hydrotropism, an explanation suggested by
173
environment is simply too unfavourable for the discoideum grex to exist
in it, and Gerisch's conclusion that unilateral contact with the interface
between liquid and air is indispensable for producing an axis, and the
inequality that is the basis of subsequent differentiation, is not of general
validity. Not only does P . violaceum not need this particular differential,
but we still have no information about how far an aggregate will develop when suspended in a medium in which it can turn into a grex:
for example, in mineral oil (Potts, 1902), and for Polysphondylium, of
course, in water too. Admittedly the abolition of all environmental differentials is rather a theoretical concept; but if it could be achieved, it
would not be surprising if one region of an aggregate could become
dominant by exploiting some random internal difference. Contact following might well be able to produce a uniform orientation within an
aggregate, and perhaps even some measure of cell sorting. Certainly,
very elaborate tissue structures may develop within suspended aggregates of vertebrate cells (Moscona, 1961).
Regularly in Acytostelium and the small Dictyostelium species (Raper,
1941a; Raper and Quinlan, 1958), and in most other forms in a dry
environment, the grex proceeds without interruption to turn itself into
a fruiting body, while remaining erect. As it could continue to move in
the direction in which it had elongated simply by preserving its radial
symmetry, one might think there was no positive guidance at this stage.
However, if a culminating grex falls over or is laid on the ground, its
tip makes a sharp right-angled turn to lead it up into the air again. To
what differential the tip responds is quite unknown. It may or may not
be one involved in fixing the axis originally. Harper (1926, 1929, 1932)
discussed this question at greater length than anyone else has done. But
he gave three divergent accounts of the erection of inverted fruits, and
in a single paper expressed his belief that negative geotropism was the
primary factor involved; that the amoebae primarily 'glide in that
direction in which the greatest resistance is offered to their motion' (a
completely different mechanism, which would presumably have made
most of the cells, if acting individually, burrow into the interior of the
aggregate, and the aggregate itself burrow into the ground); that at
the end of aggregation the cells at the top of the rounded mass had
climbed up to the region of greatest downward pull (which is simply not
the case); and that they thereafter maintained a position of maximum
resistance to the downward pull by thrusting upwards in proportion
to their load. However, workers from Brefeld onwards had already
noticed that fruiting bodies tended to be erect whatever the plane of
the substratum; and in Raper's (1935) experience, erection occurred
whether on the bottom or on the side of a dish and whether the dish was
inverted or not. Negative hydrotropism, an explanation suggested by