THE ACRASINA
307
sack, and endlessly frustrated by their somewhat masochistically
extending the sack as fast as they advance. To make quantitative
predictions, it is assumed once again that each cell produces motive
energy at a fixed rate whatever its speed; and further, that part of this
provides a push that is perfectly transmitted to the front end of the slug
where it helps to overcome the resistance due to the apical slime sheath,
whilst all the rest of it acts against the cell's internal viscosity, so that if
all external resistance could be removed a cell would attain its 'free
speed'.
However, this interesting model does not really account for the
observation that the amputated anterior tenth of a slug moves slightly
more slowly than the intact slug (Francis, 1959): it is assumed that the
cells in the rear have a lower free speed than those in the tip ; yet although in a large slug the rear cells must be travelling faster than this
postulated speed limit, they not only fail to act as a brake on the tip,
their presence actually increases the tip's velocity. On the other hand,
if—as additional evidence also suggests (Samuel, 1961 ; Vol. 2, p. 167-8)—
the rear cells are not inherently slower, the fact that they do not have a
greater effect on the tip's speed must mean that each cell's entire push is
not brought to bear against the anterior end, or that a cell's output of
motive energy cannot be treated as a constant. The former in any case
seems to follow from Francis's observation (1962) that slugs do not
elongate even when migrating against a centrifugal force of 45 g, from
which he concluded that the cells have sufficiently rigid ' Vails" for each
to experience only its own increase in weight.
We must pursue the attempt to account for the unity or wholeness of
slug motor behaviour in terms of the behaviour of a slug's components,
but we may expect that this will include cellular interactions that are
very much more complicated. If these are absent, our analysis will
have little to contribute to the problem of tissue organization in general.
Now, whether or not the work done in locomotion is constant—and
there are no good reasons for supposing that it is—cell movement is
obviously strongly dependent on environmental conditions. Thus
Samuel (1961) found the velocity of separate cells crawling over agar to
be influenced by a variety of diffusible substances and even to be
increased several-fold if the agar was merely somewhat wetter than
usual; likewise Konijn (1961) found their movement to be greatly
affected by very slight differences in preparing the agar. Movement
within a grex will be controlled by the environment its cells have created
for themselves, which must be profoundly different from the external
one, much more difficult to analyse, and presumably able to vary
independently of it. It has been suggested (Vol. 2, pp. 167-8) that the
cell sorting discovered by Bonner (1952) is based on the cells' differential
307
sack, and endlessly frustrated by their somewhat masochistically
extending the sack as fast as they advance. To make quantitative
predictions, it is assumed once again that each cell produces motive
energy at a fixed rate whatever its speed; and further, that part of this
provides a push that is perfectly transmitted to the front end of the slug
where it helps to overcome the resistance due to the apical slime sheath,
whilst all the rest of it acts against the cell's internal viscosity, so that if
all external resistance could be removed a cell would attain its 'free
speed'.
However, this interesting model does not really account for the
observation that the amputated anterior tenth of a slug moves slightly
more slowly than the intact slug (Francis, 1959): it is assumed that the
cells in the rear have a lower free speed than those in the tip ; yet although in a large slug the rear cells must be travelling faster than this
postulated speed limit, they not only fail to act as a brake on the tip,
their presence actually increases the tip's velocity. On the other hand,
if—as additional evidence also suggests (Samuel, 1961 ; Vol. 2, p. 167-8)—
the rear cells are not inherently slower, the fact that they do not have a
greater effect on the tip's speed must mean that each cell's entire push is
not brought to bear against the anterior end, or that a cell's output of
motive energy cannot be treated as a constant. The former in any case
seems to follow from Francis's observation (1962) that slugs do not
elongate even when migrating against a centrifugal force of 45 g, from
which he concluded that the cells have sufficiently rigid ' Vails" for each
to experience only its own increase in weight.
We must pursue the attempt to account for the unity or wholeness of
slug motor behaviour in terms of the behaviour of a slug's components,
but we may expect that this will include cellular interactions that are
very much more complicated. If these are absent, our analysis will
have little to contribute to the problem of tissue organization in general.
Now, whether or not the work done in locomotion is constant—and
there are no good reasons for supposing that it is—cell movement is
obviously strongly dependent on environmental conditions. Thus
Samuel (1961) found the velocity of separate cells crawling over agar to
be influenced by a variety of diffusible substances and even to be
increased several-fold if the agar was merely somewhat wetter than
usual; likewise Konijn (1961) found their movement to be greatly
affected by very slight differences in preparing the agar. Movement
within a grex will be controlled by the environment its cells have created
for themselves, which must be profoundly different from the external
one, much more difficult to analyse, and presumably able to vary
independently of it. It has been suggested (Vol. 2, pp. 167-8) that the
cell sorting discovered by Bonner (1952) is based on the cells' differential