THE ACRASINA
113
II. The Non-Social Phase
A. The Cells
1. Orientation
Most workers, myself included, have regularly said that cells in the
solitary phase are wholly independent, and move at random or show no
evidence whatever of directed movement: but in fact their behaviour
has been little studied; a priori, one would expect a number of factors,
both directional and nondirectional, to influence cytoplasmic flow. In
particular, the well-established chemotaxis of solitary amoebae and
amoebocytes (McCutcheon, 1946) towards food suggests that myxamoebae may react similarly. Arndt (1937) thought he could detect this
in his film, and I have noticed that cells may collect over a bacterial
colony though separated from it by a layer of agar. More than one worker
(Wright, privately) has found that myxamoebae can be oriented by
bacterial extracts, using the agar-block test (Shaffer, 1956b), and recently Samuel (1961) plotted their movement towards bacterial clumps
and showed that the chemotactic index (McCutcheon, 1946) is strongly
positive. A less arduous method of determining their paths is to photograph the tracks they leave in agar (Paddock, 1953; Shaffer, 1953b,
1961b), though curiously it has yet to be seriously exploited.
In the absence of bacteria, myxamoebae behave very much like
amoebocytes in culture: they distribute themselves evenly over a limited
surface, and they spread outwards from the point at which they are
deposited. That this is not merely diffusion produced by random movement but involves active repulsion operating at a distance, as with pigment cells (Twitty and Niu, 1954), is suggested in the first case by the
way in which the cells remain almost entirely separate from one another
unless the density is so high that contact cannot be avoided, and in the
second case (Shaffer, 1961b) by the remarkably radial tracks. Samuel
(1961) has now plotted successive positions of dispersing cells and found
the chemotactic index to be strongly negative. So one would expect
that if myxamoebae were deposited in two adjacent heaps, their paths
within the intervening area would be deformed if this were a saddle of
relatively high repellent concentration, but this has not yet been demonstrated ; on the contrary, the present evidence is that the cells still disperse radially until they meet at the midline and then they move at
random (Samuel, privately). Perhaps we may tentatively accept the
existence of a repellent while awaiting more direct proof.
The substratum may affect a cell's direction through static adhesion or
by influencing its spreading. Myxamoebae will not move over unwetted
surfaces and hence may be confined to very small areas (Konijn and
113
II. The Non-Social Phase
A. The Cells
1. Orientation
Most workers, myself included, have regularly said that cells in the
solitary phase are wholly independent, and move at random or show no
evidence whatever of directed movement: but in fact their behaviour
has been little studied; a priori, one would expect a number of factors,
both directional and nondirectional, to influence cytoplasmic flow. In
particular, the well-established chemotaxis of solitary amoebae and
amoebocytes (McCutcheon, 1946) towards food suggests that myxamoebae may react similarly. Arndt (1937) thought he could detect this
in his film, and I have noticed that cells may collect over a bacterial
colony though separated from it by a layer of agar. More than one worker
(Wright, privately) has found that myxamoebae can be oriented by
bacterial extracts, using the agar-block test (Shaffer, 1956b), and recently Samuel (1961) plotted their movement towards bacterial clumps
and showed that the chemotactic index (McCutcheon, 1946) is strongly
positive. A less arduous method of determining their paths is to photograph the tracks they leave in agar (Paddock, 1953; Shaffer, 1953b,
1961b), though curiously it has yet to be seriously exploited.
In the absence of bacteria, myxamoebae behave very much like
amoebocytes in culture: they distribute themselves evenly over a limited
surface, and they spread outwards from the point at which they are
deposited. That this is not merely diffusion produced by random movement but involves active repulsion operating at a distance, as with pigment cells (Twitty and Niu, 1954), is suggested in the first case by the
way in which the cells remain almost entirely separate from one another
unless the density is so high that contact cannot be avoided, and in the
second case (Shaffer, 1961b) by the remarkably radial tracks. Samuel
(1961) has now plotted successive positions of dispersing cells and found
the chemotactic index to be strongly negative. So one would expect
that if myxamoebae were deposited in two adjacent heaps, their paths
within the intervening area would be deformed if this were a saddle of
relatively high repellent concentration, but this has not yet been demonstrated ; on the contrary, the present evidence is that the cells still disperse radially until they meet at the midline and then they move at
random (Samuel, privately). Perhaps we may tentatively accept the
existence of a repellent while awaiting more direct proof.
The substratum may affect a cell's direction through static adhesion or
by influencing its spreading. Myxamoebae will not move over unwetted
surfaces and hence may be confined to very small areas (Konijn and