THE ACRASINA
141
of material. For although it is true that if the attractor were inactivated
sufficiently rapidly any given relative concentration gradient could be
established at any distance from a single source that could secrete
enough of it, in fact it is impossible for one source to extend its range to
great distances in this way: not merely would it need a vast amount of
time and chemical, but the increasing concentration around the source
would presumably overload the responding cells' receptor mechanism,
and orientation would be lost when the cells got too near. It is interesting that in the acrasin tests (Section IV, C) substances do become less
active at sufficiently high concentrations (Sussman, Lee, and Kerr,
1956; Wright, privately), and aggregation begins at increasing distances
from the source as concentration is raised (Hostak, 1960; Shaffer,
1961b).
Furthermore, without invoking a propagated front, it is difficult to
explain a zone of abrupt orientation and rapid centripetal movement
such as is seen in Bonner's (1944, 1960) time-lapse films of the larger
species of Dictyostelium moving outwards from a centre over initially
separate cells or small clumps at about 0-5 mm a minute. (Various types
of propagated waves were strikingly revealed in Arndt's film made in
1929; but as the density was so high that the cells were in contact, their
origin could have been purely mechanical.)
Such factors as cell shape, spacing, velocity, and polarity, will not be
symmetrical on the two sides of the crest of the front; and secretion
and responsiveness need not be. Thus several influences may tend,
statistically, to preserve orientation after the wave front has passed.
But the same cells may be successively reoriented by different centres,
which shows that they can propagate secretion fronts repeatedly, and
suggests that they may do so even when they continue to aggregate to
the original centre. Films reveal that an aggregation may indeed support
a succession of waves of rapid oriented movement at intervals of as little
as 5 minutes; and as long as the cells are separate, these will be useful in
reinforcing the authority of the pacemaker (Fig. 10(a)).
This raises further problems in the control of secretion. Whether, as
we have supposed, output is not directly proportional to external inducer
concentration, or whether it is, it is hard to see how successive fronts
can be propagated unless there is a decrease in responsiveness, a sort of
refractory period, after each of them has passed. If the cells were stimulated continuously, and the length of this period determined the interval
between successive fronts, one would expect these always to be transmitted in the same direction. In fact, they may travel in opposite directions. The cells must therefore recover well before they propagate the
next front. This implies that secretion declines in the interim—that
the fronts are pulses. And it provides further support for the hypothesis
141
of material. For although it is true that if the attractor were inactivated
sufficiently rapidly any given relative concentration gradient could be
established at any distance from a single source that could secrete
enough of it, in fact it is impossible for one source to extend its range to
great distances in this way: not merely would it need a vast amount of
time and chemical, but the increasing concentration around the source
would presumably overload the responding cells' receptor mechanism,
and orientation would be lost when the cells got too near. It is interesting that in the acrasin tests (Section IV, C) substances do become less
active at sufficiently high concentrations (Sussman, Lee, and Kerr,
1956; Wright, privately), and aggregation begins at increasing distances
from the source as concentration is raised (Hostak, 1960; Shaffer,
1961b).
Furthermore, without invoking a propagated front, it is difficult to
explain a zone of abrupt orientation and rapid centripetal movement
such as is seen in Bonner's (1944, 1960) time-lapse films of the larger
species of Dictyostelium moving outwards from a centre over initially
separate cells or small clumps at about 0-5 mm a minute. (Various types
of propagated waves were strikingly revealed in Arndt's film made in
1929; but as the density was so high that the cells were in contact, their
origin could have been purely mechanical.)
Such factors as cell shape, spacing, velocity, and polarity, will not be
symmetrical on the two sides of the crest of the front; and secretion
and responsiveness need not be. Thus several influences may tend,
statistically, to preserve orientation after the wave front has passed.
But the same cells may be successively reoriented by different centres,
which shows that they can propagate secretion fronts repeatedly, and
suggests that they may do so even when they continue to aggregate to
the original centre. Films reveal that an aggregation may indeed support
a succession of waves of rapid oriented movement at intervals of as little
as 5 minutes; and as long as the cells are separate, these will be useful in
reinforcing the authority of the pacemaker (Fig. 10(a)).
This raises further problems in the control of secretion. Whether, as
we have supposed, output is not directly proportional to external inducer
concentration, or whether it is, it is hard to see how successive fronts
can be propagated unless there is a decrease in responsiveness, a sort of
refractory period, after each of them has passed. If the cells were stimulated continuously, and the length of this period determined the interval
between successive fronts, one would expect these always to be transmitted in the same direction. In fact, they may travel in opposite directions. The cells must therefore recover well before they propagate the
next front. This implies that secretion declines in the interim—that
the fronts are pulses. And it provides further support for the hypothesis