THE ACRASINA
315
deviations from these theoretical paths to the elasticity of the sheath.
The situation is nevertheless quite complicated. Some of his records
show that the slime near the extreme apex expands very slowly. This
would be explained if here the slime adhered to the relatively permanent
front surfaces of the underlying cells (Vol. 2, p. 170, Fig. 19). The sheath
expands most just behind the apex, and in consequence the elastic
tension might be considerable in this region; but equally the sheath
might be at its weakest here. Certainly the tension might be greatly
reduced by the secretion of more slime.
The less the forward transmission and summation of the thrust
generated by each cell, the less the instability that would be introduced.
The simplest model of this sort would be if the rear cells did no more
than fill the slime sheath as laid down by the tip ; but if that were all,
random changes in the size of the tip would be unopposed, and a slug
would not have any preferred proportions. However, we should not
expect the tip to be autonomous, but to be under the control of the
whole slug, and indeed there are indications that this is so. Francis
(1959) has shown that if a disproportionately small tip is grafted onto a
decapitated slug, it grows to an appropriate size. Bonner (1957) has
already discussed the need for communication between the parts of a
lying grex in relation to the problem of regulating the proportion of
prestalk to prespore cells. He has suggested that a fixed fraction of the
cells in the rear or prespore zone continuously advances to the front of
the grex, releasing a metabolite used up there, and that there is a
compensatory return movement of the slower cells. Such movements
are held to be responsible for the restoration of the anterior end after
sectioning the grex. However, one striking feature of this system that
Bonner has clearly established is that the proportion of prestalk to
prespore cells is fixed and quite independent of grex shape. Consequently,
in so far as shape and length are controlled, the cells must communicate
in some other way, possibly less efficient and certainly less mandatory.
The leading cells may use more energy not only because of their altered
metabolism (Bonner et al., 1955) and their relation to the slime sheath,
but also perhaps because of their change in shape. If they can satisfy
even a few per cent of their total requirements by draining certain
metabolites from cells that remain at the rear, this could be the basis of
a dynamic equilibrium of slug length. For the front would be slowed
down if the distance between it and the rear began to increase, and it
would be accelerated if this distance began to decrease. An equilibrium
of this kind could not exist if the support provided by the rear were
purely mechanical, as in the additive-push model.
Unfortunately the question has been too little studied to determine to
what extent and in what circumstances such an equilibrium does exist.
315
deviations from these theoretical paths to the elasticity of the sheath.
The situation is nevertheless quite complicated. Some of his records
show that the slime near the extreme apex expands very slowly. This
would be explained if here the slime adhered to the relatively permanent
front surfaces of the underlying cells (Vol. 2, p. 170, Fig. 19). The sheath
expands most just behind the apex, and in consequence the elastic
tension might be considerable in this region; but equally the sheath
might be at its weakest here. Certainly the tension might be greatly
reduced by the secretion of more slime.
The less the forward transmission and summation of the thrust
generated by each cell, the less the instability that would be introduced.
The simplest model of this sort would be if the rear cells did no more
than fill the slime sheath as laid down by the tip ; but if that were all,
random changes in the size of the tip would be unopposed, and a slug
would not have any preferred proportions. However, we should not
expect the tip to be autonomous, but to be under the control of the
whole slug, and indeed there are indications that this is so. Francis
(1959) has shown that if a disproportionately small tip is grafted onto a
decapitated slug, it grows to an appropriate size. Bonner (1957) has
already discussed the need for communication between the parts of a
lying grex in relation to the problem of regulating the proportion of
prestalk to prespore cells. He has suggested that a fixed fraction of the
cells in the rear or prespore zone continuously advances to the front of
the grex, releasing a metabolite used up there, and that there is a
compensatory return movement of the slower cells. Such movements
are held to be responsible for the restoration of the anterior end after
sectioning the grex. However, one striking feature of this system that
Bonner has clearly established is that the proportion of prestalk to
prespore cells is fixed and quite independent of grex shape. Consequently,
in so far as shape and length are controlled, the cells must communicate
in some other way, possibly less efficient and certainly less mandatory.
The leading cells may use more energy not only because of their altered
metabolism (Bonner et al., 1955) and their relation to the slime sheath,
but also perhaps because of their change in shape. If they can satisfy
even a few per cent of their total requirements by draining certain
metabolites from cells that remain at the rear, this could be the basis of
a dynamic equilibrium of slug length. For the front would be slowed
down if the distance between it and the rear began to increase, and it
would be accelerated if this distance began to decrease. An equilibrium
of this kind could not exist if the support provided by the rear were
purely mechanical, as in the additive-push model.
Unfortunately the question has been too little studied to determine to
what extent and in what circumstances such an equilibrium does exist.