THE ACRASINA
167
at the end of aggregation, though a few cells can in fact be seen changing
their positions throughout migration, perhaps having altered their
properties. Some cell character must thus show a gradient along the
length of the slug, but what?
Bonner believes that the anterior cells are fast, and the posterior ones
slow. If this were so, we might expect that just as a group of longdistance runners becomes progressively extended as the race continues,
so a slug would lengthen throughout its migration, perhaps even tenfold in the first day if the rate at which cells return to position after
displacement (Bonner, 1952) were maintained. Migration may be made
to continue for as much as 20 days (Slifkin and Bonner, 1952), yet except for a slight decrease initially (Krivanek, 1956), the length of a slug
remains remarkably constant (Bonner, Koontz, and Paton, 1953).
Bonner has not discussed what factors might be responsible for preventing slow elongation, but it is difficult to see how either cell adhesion
or the acrasin gradient experienced by surface cells could be effective
if the velocity differences were real. The anterior cells must indeed meet
a resistance, mainly because they have to expand the slime sheath to
advance, but there is nothing comparable to prevent the back end
lagging.
It might then be supposed that in the slowest slugs, the 'fast' cells
are held back so severely that the 'slowest' ones cannot keep up with
them; but if so, the rear cells ought to stay in the front if moved there
and not fall backwards. Moreover, on this basis we should expect the
rate of slug elongation to increase with the speed at which the tip advances, whereas in fact it remains zero. A slug's velocity increases with
its size (Bonner, Koontz, and Paton, 1953), and the 'slowest' cells in the
largest slugs must be eight times faster than the 'fastest' ones in the
smallest slugs. Yet we should not expect the rear cells from a large slug
to move forwards when grafted into a small one.
A further difficulty arises from the work of Francis (1959) who
successfully joined the ends of a slug together to produce a rotating
ring. No slime-sheath barrier then impeded the 'faster' cells, and had
they been racing, they would have lapped the slower ones and so become
progressively more evenly distributed round the ring. But though the
movements of labelled cells have yet to be examined, it seems unlikely
that this happened; for otherwise, it is hard to explain how, instead of
the ring rotating indefinitely, one region was invariably able to differentiate into a new tip a few hours after the operation, and thus transform
the ring into a linear slug again, even if migration subsequently continued for days.
These difficulties disappear if a slug is organized rather like a squad
of adolescent soldiers on a long march, who are made to position them-
167
at the end of aggregation, though a few cells can in fact be seen changing
their positions throughout migration, perhaps having altered their
properties. Some cell character must thus show a gradient along the
length of the slug, but what?
Bonner believes that the anterior cells are fast, and the posterior ones
slow. If this were so, we might expect that just as a group of longdistance runners becomes progressively extended as the race continues,
so a slug would lengthen throughout its migration, perhaps even tenfold in the first day if the rate at which cells return to position after
displacement (Bonner, 1952) were maintained. Migration may be made
to continue for as much as 20 days (Slifkin and Bonner, 1952), yet except for a slight decrease initially (Krivanek, 1956), the length of a slug
remains remarkably constant (Bonner, Koontz, and Paton, 1953).
Bonner has not discussed what factors might be responsible for preventing slow elongation, but it is difficult to see how either cell adhesion
or the acrasin gradient experienced by surface cells could be effective
if the velocity differences were real. The anterior cells must indeed meet
a resistance, mainly because they have to expand the slime sheath to
advance, but there is nothing comparable to prevent the back end
lagging.
It might then be supposed that in the slowest slugs, the 'fast' cells
are held back so severely that the 'slowest' ones cannot keep up with
them; but if so, the rear cells ought to stay in the front if moved there
and not fall backwards. Moreover, on this basis we should expect the
rate of slug elongation to increase with the speed at which the tip advances, whereas in fact it remains zero. A slug's velocity increases with
its size (Bonner, Koontz, and Paton, 1953), and the 'slowest' cells in the
largest slugs must be eight times faster than the 'fastest' ones in the
smallest slugs. Yet we should not expect the rear cells from a large slug
to move forwards when grafted into a small one.
A further difficulty arises from the work of Francis (1959) who
successfully joined the ends of a slug together to produce a rotating
ring. No slime-sheath barrier then impeded the 'faster' cells, and had
they been racing, they would have lapped the slower ones and so become
progressively more evenly distributed round the ring. But though the
movements of labelled cells have yet to be examined, it seems unlikely
that this happened; for otherwise, it is hard to explain how, instead of
the ring rotating indefinitely, one region was invariably able to differentiate into a new tip a few hours after the operation, and thus transform
the ring into a linear slug again, even if migration subsequently continued for days.
These difficulties disappear if a slug is organized rather like a squad
of adolescent soldiers on a long march, who are made to position them-