THE ACRASINA
169
suspected subtlety in the factors controlling the relative movement of
myxamoebae, though at present these remain unknown quantitative
and qualitative differences that may or may not be bound to the surface, but which influence cytoplasmic outflow only through intercellular
contact.
Apart from innate variability, a cell's velocity is determined by its
environment, both cellular and noncellular, large-scale and local. For
most of the time, it is made to move at the same speed as the grex.
Regional influences may be either chemical, as just described, or geometrical, which exert most effect when stalk is being formed. If the
apical papilla and the stalk sheath were both cylindrical, and there were,
for example, as many cells inside the sheath per unit length as prestalk
cells outside it, and those inside it were stationary, the prestalk cells
would have to move just twice as fast as the stalk grew. But the situation is complicated by the terminal expansion of the stalk sheath, the
continued advance of the cells inside it (Section V, A, 1), and the grex's
external taper. The prestalk zone has the shape of a volcano, and its
cells must therefore rapidly accelerate as they pass from the base up to
the rim and then decelerate to rest after being rammed into the crater.
3. Shape
An increasing proportion of the prestalk or anterior cells of the lying
or standing grex become 'transversely' oriented (Bonner, 1944), or, as
here contended, simply flattened from front to back, like coins. Raper
and Fennell (1952) believed this showed that during culmination these
were being pushed into position and that the primary lifting force involved was generated inside the stalk. But even if such pressure were in
fact able, as suggested, to make the grex slide up the stalk and to expand the top of the stalk sheath into the shape of a wineglass (Section V,
A, 1), it is not obvious how it could make the prestalk cells 'transverse'.
And one would still have to explain how the anterior cells got their shape
when no stalk was being formed. Besides, a group of transverse cells
may be found right at the back of some grex (Raper and Fennell, 1952;
Bonner, 1957). And in Acytostelium, not only is the stalk acellular, but
all the cells are transverse (Raper and Quinlan, 1958). This shape must
therefore be actively acquired; but how? In a file of stream cells, adhesion will tend to prevent the end surfaces of the cells from shearing
on one another (Fig. 19(a)) and to restrict surface membrane expansion
and retraction to more lateral positions (Fig. 19(b)). And whereas the side
surfaces of a cell must be completely remade every time it advances its
own length, the end membranes can be relatively permanent, and can
therefore presumably be more firmly bonded. If the area of static,
adherent end surface were greatly increased (cf. Schmitt, 1941; Mookerjee
169
suspected subtlety in the factors controlling the relative movement of
myxamoebae, though at present these remain unknown quantitative
and qualitative differences that may or may not be bound to the surface, but which influence cytoplasmic outflow only through intercellular
contact.
Apart from innate variability, a cell's velocity is determined by its
environment, both cellular and noncellular, large-scale and local. For
most of the time, it is made to move at the same speed as the grex.
Regional influences may be either chemical, as just described, or geometrical, which exert most effect when stalk is being formed. If the
apical papilla and the stalk sheath were both cylindrical, and there were,
for example, as many cells inside the sheath per unit length as prestalk
cells outside it, and those inside it were stationary, the prestalk cells
would have to move just twice as fast as the stalk grew. But the situation is complicated by the terminal expansion of the stalk sheath, the
continued advance of the cells inside it (Section V, A, 1), and the grex's
external taper. The prestalk zone has the shape of a volcano, and its
cells must therefore rapidly accelerate as they pass from the base up to
the rim and then decelerate to rest after being rammed into the crater.
3. Shape
An increasing proportion of the prestalk or anterior cells of the lying
or standing grex become 'transversely' oriented (Bonner, 1944), or, as
here contended, simply flattened from front to back, like coins. Raper
and Fennell (1952) believed this showed that during culmination these
were being pushed into position and that the primary lifting force involved was generated inside the stalk. But even if such pressure were in
fact able, as suggested, to make the grex slide up the stalk and to expand the top of the stalk sheath into the shape of a wineglass (Section V,
A, 1), it is not obvious how it could make the prestalk cells 'transverse'.
And one would still have to explain how the anterior cells got their shape
when no stalk was being formed. Besides, a group of transverse cells
may be found right at the back of some grex (Raper and Fennell, 1952;
Bonner, 1957). And in Acytostelium, not only is the stalk acellular, but
all the cells are transverse (Raper and Quinlan, 1958). This shape must
therefore be actively acquired; but how? In a file of stream cells, adhesion will tend to prevent the end surfaces of the cells from shearing
on one another (Fig. 19(a)) and to restrict surface membrane expansion
and retraction to more lateral positions (Fig. 19(b)). And whereas the side
surfaces of a cell must be completely remade every time it advances its
own length, the end membranes can be relatively permanent, and can
therefore presumably be more firmly bonded. If the area of static,
adherent end surface were greatly increased (cf. Schmitt, 1941; Mookerjee