148
B. M. SHAFFER
and guiding them towards it along little-used side branches. After an
hour, extensive streams lead to the food (Aschner and Kogan, 1959).
At low magnification these may almost be mistaken for those of Acrasina.
Aschner and Kogan suggest that as excitation does not cross a gap
(minimum width undefined) in the net, it may actually travel along the
filaments and successively activate all the cells adhering to them;
alternatively, it may only appear to travel along them, because the cells
are confined to them, and really be transmitted by cell to cell relay.
They point out that the first method is less liable to be upset by turbulence when operating in the ocean. They do not discuss how the net
could offer polarized guidance; but if it does do so, this will be of great
morphogenetic interest. Possibly it might act only during brief pulses
rather than continuously. The authors do not comment on the radial
expansion, but presumably this is another example of repulsion; that
the cells are not regularly separate might be due to their adhesiveness
or to the production of repellent only during feeding. We may perhaps
attribute the cells' dramatic increase in speed when streaming towards
food partly to the fact that they can follow existing tracks instead of
having to make new ones.
As a last and much more complex example, it may be recalled that
there are some indications that outgrowing nerve fibres have solved
many of their problems of orientation and patterning by relayed changes
in properties and a combination of guidance mechanisms that have elsewhere (Shaffer, 1957a) been compared in detail with those found in the
Acrasina.
It has generally been thought that once a cell has joined a slimemould aggregation, it typically proceeds to the centre without interruption. But this is much too simple a view. Several exceptions to this rule
have been noted by a number of observers, particularly Arndt (1937)
and Raper (1940a, 1941b), in several species; and a detailed study
(Shaffer, 1957c, 1958) has shown that quite apart from differences due
to density (Raper, 1940b, 1941a, 1956b), the course of aggregation is
remarkably variable (Fig. 1). For example, any short part of a stream
may start to turn itself into an independent, hemispherical 'secondary
centre', and may or may not succeed. Most striking of all, in Yeats's
words:
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world.
The cells largely or completely stop secreting attractor and lose much or
all of their increased adhesiveness, so that they remain bound together
in loose groups or separate entirely. The vigour with which the cells
msh apart, best seen in time-lapse films, as well as their orientation,
B. M. SHAFFER
and guiding them towards it along little-used side branches. After an
hour, extensive streams lead to the food (Aschner and Kogan, 1959).
At low magnification these may almost be mistaken for those of Acrasina.
Aschner and Kogan suggest that as excitation does not cross a gap
(minimum width undefined) in the net, it may actually travel along the
filaments and successively activate all the cells adhering to them;
alternatively, it may only appear to travel along them, because the cells
are confined to them, and really be transmitted by cell to cell relay.
They point out that the first method is less liable to be upset by turbulence when operating in the ocean. They do not discuss how the net
could offer polarized guidance; but if it does do so, this will be of great
morphogenetic interest. Possibly it might act only during brief pulses
rather than continuously. The authors do not comment on the radial
expansion, but presumably this is another example of repulsion; that
the cells are not regularly separate might be due to their adhesiveness
or to the production of repellent only during feeding. We may perhaps
attribute the cells' dramatic increase in speed when streaming towards
food partly to the fact that they can follow existing tracks instead of
having to make new ones.
As a last and much more complex example, it may be recalled that
there are some indications that outgrowing nerve fibres have solved
many of their problems of orientation and patterning by relayed changes
in properties and a combination of guidance mechanisms that have elsewhere (Shaffer, 1957a) been compared in detail with those found in the
Acrasina.
It has generally been thought that once a cell has joined a slimemould aggregation, it typically proceeds to the centre without interruption. But this is much too simple a view. Several exceptions to this rule
have been noted by a number of observers, particularly Arndt (1937)
and Raper (1940a, 1941b), in several species; and a detailed study
(Shaffer, 1957c, 1958) has shown that quite apart from differences due
to density (Raper, 1940b, 1941a, 1956b), the course of aggregation is
remarkably variable (Fig. 1). For example, any short part of a stream
may start to turn itself into an independent, hemispherical 'secondary
centre', and may or may not succeed. Most striking of all, in Yeats's
words:
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world.
The cells largely or completely stop secreting attractor and lose much or
all of their increased adhesiveness, so that they remain bound together
in loose groups or separate entirely. The vigour with which the cells
msh apart, best seen in time-lapse films, as well as their orientation,