THE ACRASINA
311
According to Francis (1962), however, speed does increase with volume
and approaches an asymptotic value. Francis himself has proposed that
the factor chiefly responsible for this increase is the diminishing
importance of the resistance of the apical slime sheath, though Bonner
(quoted by Francis) has pointed out that the fact the slimmer slugs are
faster than the fatter ones may rather be due to a difference in the
physiological condition of the cells. Clearly, if the slime sheath is a
membrane at the apex, the cells can do work stretching it; but as we
have seen, it is difficult to measure just how much mechanical work has
to be done, because the sheath is continuously growing; this growth too
must involve some of the cells—presumably the anterior peripheral
ones—in the expenditure of energy.
We still do not know to what extent the sheath does slow down the
cells in an ordinary slug. This problem cannot be solved by comparing
the speeds of cells in a slug with the speeds of cells that are crawling
over agar either separately or in aggregation streams or even aggregation
rings, because of the differences in cell shape and environment. A
possible approach would be to measure the velocity of rotation of
Francis's ring slugs. As these rings lack tips, we cannot compare even
these velocities with those of ordinary slugs to determine the influence
of the apical resistance. What we can compare are the velocities of rings
of different size. If the average motor energy output per cell is constant
in a normal slug and is partly expended against an apical resistance, all
rings should have the same speed whatever their size. If they do not, we
must seek another explanation for the different speeds of ordinary slugs.
In practice it might be slightly tedious to make measurements on the
rings, even if some of their cells had been vitally stained. If, by a technique suggested above, linear decapitated slugs could be made to continue
migration without immediately reconstructing apical slime sheath, one
might readily see if those of different size had the same velocity. It
might also be illuminating to study the behaviour of the rear sections of
slugs of different sizes permanently ligatured at various distances behind their tips, and of cells advancing through a partial ligature into an
incompletely filled slime sheath.
However, there is at least one observation that is not easily interpreted
in terms of a purely mechanical nexus : despite the change in behaviour
at the cut surface of a decapitated slug, the rear end continues to
advance at approximately the same speed as before (Francis, 1962).
Perhaps cell speed is determined to some extent by non-mechanical
factors in the internal environment, transmissible but changing only
slowly, and themselves determined by the collective activity of the
cells. In so far as they were uniform throughout the slug, they could help
to make all the cells travel at the same speed. Such factors could also
311
According to Francis (1962), however, speed does increase with volume
and approaches an asymptotic value. Francis himself has proposed that
the factor chiefly responsible for this increase is the diminishing
importance of the resistance of the apical slime sheath, though Bonner
(quoted by Francis) has pointed out that the fact the slimmer slugs are
faster than the fatter ones may rather be due to a difference in the
physiological condition of the cells. Clearly, if the slime sheath is a
membrane at the apex, the cells can do work stretching it; but as we
have seen, it is difficult to measure just how much mechanical work has
to be done, because the sheath is continuously growing; this growth too
must involve some of the cells—presumably the anterior peripheral
ones—in the expenditure of energy.
We still do not know to what extent the sheath does slow down the
cells in an ordinary slug. This problem cannot be solved by comparing
the speeds of cells in a slug with the speeds of cells that are crawling
over agar either separately or in aggregation streams or even aggregation
rings, because of the differences in cell shape and environment. A
possible approach would be to measure the velocity of rotation of
Francis's ring slugs. As these rings lack tips, we cannot compare even
these velocities with those of ordinary slugs to determine the influence
of the apical resistance. What we can compare are the velocities of rings
of different size. If the average motor energy output per cell is constant
in a normal slug and is partly expended against an apical resistance, all
rings should have the same speed whatever their size. If they do not, we
must seek another explanation for the different speeds of ordinary slugs.
In practice it might be slightly tedious to make measurements on the
rings, even if some of their cells had been vitally stained. If, by a technique suggested above, linear decapitated slugs could be made to continue
migration without immediately reconstructing apical slime sheath, one
might readily see if those of different size had the same velocity. It
might also be illuminating to study the behaviour of the rear sections of
slugs of different sizes permanently ligatured at various distances behind their tips, and of cells advancing through a partial ligature into an
incompletely filled slime sheath.
However, there is at least one observation that is not easily interpreted
in terms of a purely mechanical nexus : despite the change in behaviour
at the cut surface of a decapitated slug, the rear end continues to
advance at approximately the same speed as before (Francis, 1962).
Perhaps cell speed is determined to some extent by non-mechanical
factors in the internal environment, transmissible but changing only
slowly, and themselves determined by the collective activity of the
cells. In so far as they were uniform throughout the slug, they could help
to make all the cells travel at the same speed. Such factors could also