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Β. Μ. SHAFFER
at least in one strain the slugs are always long and thin (Francis, 1962).
Although individual cells and small groups are shed from the back end
(Raper, 1935; Bonner, 1957), a slug in a constant environment may
migrate for a considerable period without increasing in length. Why
should the two ends of a slug move at the same speed despite normal
physiological variation in cell behaviour? If all the cells were locked
rigidly together, there would be no great problem, but in reality their
relative mobility may be very high (Bonner, 1952). In the last section,
we discussed this matter in relation to the speed of the slug as a whole.
Here we are primarily interested in the slug's shape. If the slug were
pulled from the front we should expect it to get steadily longer and
thinner, and if pushed from the back, steadily shorter and stouter. We
have already been led to the view that, except in special circumstances,
the anterior cells are not in fact faster than the posterior ones (Vol. 2,
p. 167f), and also that the cells' motor energy output is controlled by
the internal environment. So can we simply suppose that in slugs of all
sizes the motive power of all the cells is fixed at such a level that, if there
were no resistance to their movement ahead of them, they could travel
faster than they do? If the forward push exerted by each cell were
transmitted to the front of the slug, such a slug could be compared to a
three-dimensional array of trains moving on interconnected parallel
tracks and composed exclusively of locomotives. Because the front end
of the slug is tapered, the trains would increase in length the nearer they
were to the axis. If their advance were opposed by a uniform anterior
resistance, there would be a similarly graded increase in speed, and the
slug would get progressively longer and thinner as the axial trains
recruited more and more of the peripheral cell locomotives and got
faster and faster. The process would tend to continue till the cells were in
single file, unless they had previously reached their maximum speed.
The initial instability would be increased, firstly, if the anterior locomotives were in fact more powerful, because it is they that account for
the initial differences in the length of the trains ; and secondly, if the
anterior resistance increased from the axis to the periphery, as a result
of the strengthening of the slime sheath—even if the situation were not
quite so extreme as it would be if the slime at the very apex were only a
solution.
What force could oppose elongation? Elastic tension in the sheath
could do so if it increased towards the apex, but whether it does this or
not is not clear. Francis (1962) has studied the behaviour of the sheath
near the tip by following the movements of markers sticking to its
outside. Pointing out that if the sheath were equally extensible everywhere the markers would follow orthogonal trajectories, he has attributed
t Vol. 2, p. 167, line 21, 'cannot' was a misprint for 'can'.
Β. Μ. SHAFFER
at least in one strain the slugs are always long and thin (Francis, 1962).
Although individual cells and small groups are shed from the back end
(Raper, 1935; Bonner, 1957), a slug in a constant environment may
migrate for a considerable period without increasing in length. Why
should the two ends of a slug move at the same speed despite normal
physiological variation in cell behaviour? If all the cells were locked
rigidly together, there would be no great problem, but in reality their
relative mobility may be very high (Bonner, 1952). In the last section,
we discussed this matter in relation to the speed of the slug as a whole.
Here we are primarily interested in the slug's shape. If the slug were
pulled from the front we should expect it to get steadily longer and
thinner, and if pushed from the back, steadily shorter and stouter. We
have already been led to the view that, except in special circumstances,
the anterior cells are not in fact faster than the posterior ones (Vol. 2,
p. 167f), and also that the cells' motor energy output is controlled by
the internal environment. So can we simply suppose that in slugs of all
sizes the motive power of all the cells is fixed at such a level that, if there
were no resistance to their movement ahead of them, they could travel
faster than they do? If the forward push exerted by each cell were
transmitted to the front of the slug, such a slug could be compared to a
three-dimensional array of trains moving on interconnected parallel
tracks and composed exclusively of locomotives. Because the front end
of the slug is tapered, the trains would increase in length the nearer they
were to the axis. If their advance were opposed by a uniform anterior
resistance, there would be a similarly graded increase in speed, and the
slug would get progressively longer and thinner as the axial trains
recruited more and more of the peripheral cell locomotives and got
faster and faster. The process would tend to continue till the cells were in
single file, unless they had previously reached their maximum speed.
The initial instability would be increased, firstly, if the anterior locomotives were in fact more powerful, because it is they that account for
the initial differences in the length of the trains ; and secondly, if the
anterior resistance increased from the axis to the periphery, as a result
of the strengthening of the slime sheath—even if the situation were not
quite so extreme as it would be if the slime at the very apex were only a
solution.
What force could oppose elongation? Elastic tension in the sheath
could do so if it increased towards the apex, but whether it does this or
not is not clear. Francis (1962) has studied the behaviour of the sheath
near the tip by following the movements of markers sticking to its
outside. Pointing out that if the sheath were equally extensible everywhere the markers would follow orthogonal trajectories, he has attributed
t Vol. 2, p. 167, line 21, 'cannot' was a misprint for 'can'.