302
Β. Μ. SHAFFER
still unknown. One possibility considered by Francis was that light
directly altered some resistance to the movement of the slug as a whole,
such as the enveloping slime sheath. Another was that it speeded up the
slug cells, though in fact in preliminary observations he could not detect
any such effect on cells isolated from a slug. Illuminating half the tip of
a group of coherent cells removed from a slug gave indecisive results. Of
course, it may well be that cells entering a grex do acquire a new
response, and such a response might or might not be lost again as soon
as they were removed from their co-operatively maintained, internal
grex environment. On the other hand, because the relevant response is
to the light's intensity rather than its direction, it remains possible that it
can be shown by separate cells even before aggregation. All that has been
so far demonstrated is that these do not move towards the light when
the culture is illuminated from the side (Samuel, 1961; Francis, 1962).
No one has yet shone a light on only half the tip of a single slime-mould
cell. When this is done to Amoeba (Mast, 1932), it turns and moves out of
the light, just like a slug, and this is because light inhibits forward
streaming. (If a whole Amoeba is illuminated, its speed first decreases,
later increases (Mast and Stahler, 1937).) This raises the question of
whether light could make the slug turn not by speeding the cells up but
by slowing them down. The answer depends on the mechanics of slug
movement. If the moving parts have to keep in line abreast, as do the
wheels of a turning car or the men in each rank of a wheeling squad,
those on the outside of the curve do indeed have to move faster ; but if
the parts can change their relative positions, as we know that slug cells
can do, those on the outside of the curve can be slower. In much the
same way, local growth of any structure may lead to either of two
contrasting results : it may make it bend towards the opposite side or
extend it to the same side. What we want to know is whether slugs
wholly in the light move faster than those in the dark.
2. Speed and the Mechanism of Movement
Although there is abundant evidence that cells in the interior of a
D. discoideum
slug advance by their own 'amoeboid' movement
(Bonner, 1950, 1952, 1957; Bonner et al, 1953; Francis, 1962), because
of ignorance of the mechanism of this movement it has not been clear
how they obtain the necessary traction. In general it has been assumed
that a slime-mould amoeba is just a smaller edition of a large solitary
amoeba like Amoeba itself. In the latter, the granular cytoplasm
alternately travels forwards as an endoplasmic core and forms a
stationary ectoplasmic tube. It might therefore seem that any number
of such cells piled on top of one another could move forward merely by
treating their neighbours as if they were the ground. There is, however, a
Β. Μ. SHAFFER
still unknown. One possibility considered by Francis was that light
directly altered some resistance to the movement of the slug as a whole,
such as the enveloping slime sheath. Another was that it speeded up the
slug cells, though in fact in preliminary observations he could not detect
any such effect on cells isolated from a slug. Illuminating half the tip of
a group of coherent cells removed from a slug gave indecisive results. Of
course, it may well be that cells entering a grex do acquire a new
response, and such a response might or might not be lost again as soon
as they were removed from their co-operatively maintained, internal
grex environment. On the other hand, because the relevant response is
to the light's intensity rather than its direction, it remains possible that it
can be shown by separate cells even before aggregation. All that has been
so far demonstrated is that these do not move towards the light when
the culture is illuminated from the side (Samuel, 1961; Francis, 1962).
No one has yet shone a light on only half the tip of a single slime-mould
cell. When this is done to Amoeba (Mast, 1932), it turns and moves out of
the light, just like a slug, and this is because light inhibits forward
streaming. (If a whole Amoeba is illuminated, its speed first decreases,
later increases (Mast and Stahler, 1937).) This raises the question of
whether light could make the slug turn not by speeding the cells up but
by slowing them down. The answer depends on the mechanics of slug
movement. If the moving parts have to keep in line abreast, as do the
wheels of a turning car or the men in each rank of a wheeling squad,
those on the outside of the curve do indeed have to move faster ; but if
the parts can change their relative positions, as we know that slug cells
can do, those on the outside of the curve can be slower. In much the
same way, local growth of any structure may lead to either of two
contrasting results : it may make it bend towards the opposite side or
extend it to the same side. What we want to know is whether slugs
wholly in the light move faster than those in the dark.
2. Speed and the Mechanism of Movement
Although there is abundant evidence that cells in the interior of a
D. discoideum
slug advance by their own 'amoeboid' movement
(Bonner, 1950, 1952, 1957; Bonner et al, 1953; Francis, 1962), because
of ignorance of the mechanism of this movement it has not been clear
how they obtain the necessary traction. In general it has been assumed
that a slime-mould amoeba is just a smaller edition of a large solitary
amoeba like Amoeba itself. In the latter, the granular cytoplasm
alternately travels forwards as an endoplasmic core and forms a
stationary ectoplasmic tube. It might therefore seem that any number
of such cells piled on top of one another could move forward merely by
treating their neighbours as if they were the ground. There is, however, a