316
Β. Μ. SHAFFER
It would be interesting to examine the behaviour of partially ligatured
slugs or of slugs induced to push their way through apertures narrower
than themselves. The one published sequence of drawings of a slug
(Bonner et al., 1953) shows that after it shortened at the start of migration—presumably because of the bringing into play of factors different
from those involved in its formation—its length remained virtually
unchanged despite a fivefold reduction in volume, a threefold reduction
in speed, and the lapse of 3 days. This surprising constancy has apparently gone unremarked, and so we cannot know how commonly the
factors controlling length remain in so fine a balance while the cells
undergo such a considerable change in condition. In the present state of
ignorance it would seem idle to stretch our hypotheses to account for it.
There remains the question of the taper of the grex tip. This has been
considered mainly by Francis (1962), who has suggested that it may be
controlled by the slime sheath selectively restricting the advance of the
cells. He has also observed that the tip becomes longer and narrower as
speed increases. As just discussed, not only may the properties of the
slime sheath vary over the tip, but also its relations with the tip cells
may change from place to place. In addition, there are cell interactions
to be considered. If the anterior cells owe their position to their competitive overtaking, we should expect a change in shape if competition
increased. As already pointed out (Vol. 2, p. 174), changes in the apical
slime sheath directly produced by environmental differentials might be
responsible for changes of direction. An interesting suggestion (Francis,
1962) is that not only such bends but also other departures from the
basic cartridge shape, including the rare beaded and spiral forms and
even longitudinal fission, could be due to appropriate changes in the
strength of the sheath. But in so far as we have to postulate a change in
the pattern of slime secretion by the underlying cells, we must bear in
mind that an alteration in a variety of other cell properties and activities
could be responsible for changes in slug shape. A more extreme form of
beading regularly occurs in Polysphondylium
grex, for here the beads
actually separate from one another. As these are cut off successively
from the rear of the grex and left behind at intervals along the stalk, in
this case changes in the slime sheath can hardly be causal.
There is one major change in shape that affects every D. discoideum
grex (Figs. 25 c-f ; Fig. 26 c, d) : after it has stopped migrating and
become erect again, it decreases in height and increases correspondingly
in girth (Bonner, 1944). Bonner (1952) was able to show by vital
staining that this happens when the stalk that has started to form
within the upper part of the grex is travelling down through the
prespore mass towards the substratum; and only after it has reached
this does the grex grow taller again (Raper and Fennell, 1952). Bonner
Β. Μ. SHAFFER
It would be interesting to examine the behaviour of partially ligatured
slugs or of slugs induced to push their way through apertures narrower
than themselves. The one published sequence of drawings of a slug
(Bonner et al., 1953) shows that after it shortened at the start of migration—presumably because of the bringing into play of factors different
from those involved in its formation—its length remained virtually
unchanged despite a fivefold reduction in volume, a threefold reduction
in speed, and the lapse of 3 days. This surprising constancy has apparently gone unremarked, and so we cannot know how commonly the
factors controlling length remain in so fine a balance while the cells
undergo such a considerable change in condition. In the present state of
ignorance it would seem idle to stretch our hypotheses to account for it.
There remains the question of the taper of the grex tip. This has been
considered mainly by Francis (1962), who has suggested that it may be
controlled by the slime sheath selectively restricting the advance of the
cells. He has also observed that the tip becomes longer and narrower as
speed increases. As just discussed, not only may the properties of the
slime sheath vary over the tip, but also its relations with the tip cells
may change from place to place. In addition, there are cell interactions
to be considered. If the anterior cells owe their position to their competitive overtaking, we should expect a change in shape if competition
increased. As already pointed out (Vol. 2, p. 174), changes in the apical
slime sheath directly produced by environmental differentials might be
responsible for changes of direction. An interesting suggestion (Francis,
1962) is that not only such bends but also other departures from the
basic cartridge shape, including the rare beaded and spiral forms and
even longitudinal fission, could be due to appropriate changes in the
strength of the sheath. But in so far as we have to postulate a change in
the pattern of slime secretion by the underlying cells, we must bear in
mind that an alteration in a variety of other cell properties and activities
could be responsible for changes in slug shape. A more extreme form of
beading regularly occurs in Polysphondylium
grex, for here the beads
actually separate from one another. As these are cut off successively
from the rear of the grex and left behind at intervals along the stalk, in
this case changes in the slime sheath can hardly be causal.
There is one major change in shape that affects every D. discoideum
grex (Figs. 25 c-f ; Fig. 26 c, d) : after it has stopped migrating and
become erect again, it decreases in height and increases correspondingly
in girth (Bonner, 1944). Bonner (1952) was able to show by vital
staining that this happens when the stalk that has started to form
within the upper part of the grex is travelling down through the
prespore mass towards the substratum; and only after it has reached
this does the grex grow taller again (Raper and Fennell, 1952). Bonner