320
Β. Μ. SHAFFER
an excess of water excretion over water uptake can be achieved
less readily than in air. Another possible factor is the absence of an
interface that would normally select and orientate molecules at the
cell surface. Such selection was advanced by Weiss (1953) as a general
mechanism of differentiation, and Gerisch (1960) has made the as yet
untested suggestion that D. discoideum cannot form grex under water
because it cannot produce slime sheath that is (externally) hydrophobic.
The fact that both spores and stalk can be formed in liquid paraffin
(Potts, 1902) does not decide conclusively between these alternatives.
The protraction of the lying phase on hypotonic agar (Slifkin and
Bonner, 1952; Bonner and Shaw, 1957) may perhaps be due not to the
absence of a specific stimulus to culminate but rather to inhibition of
culmination, in much the same way as aggregation is delayed in
hypotonic solutions (Hirschberg and Rusch, 1950). Although water
relations may play a part in both these inhibitions, absence of salts in
itself is likely to be of great importance ; this is confirmed by the fact
that osmotic equivalents of non-electrolytes are relatively ineffective in
terminating migration (Slifkin and Bonner, 1952).
Acknowledgements
I am most grateful to Dr Κ. E. Machin for his critical reading of the
manuscript and to Professor Κ. B. Raper for permission to use his
illustration.
Summary
This is a list solely of some previously unpublished observations, reinterpretations, and
conclusions, many highly tentative. Cell distribution in the solitary phase in an initially
uniform environment depends on the balance between mutual repulsion, attraction
to and retardation by food, and the effect of monolayering. In theory a cell can chemically
attract distant cells and repel nearer ones ; this may explain the clouds of separate, unoriented cells that attract outlying cells in early aggregation in Dictyostelium mucoroides
(strain 11). Secretion of the attractor can be induced by a diffusible factor, which also
regulates its own secretion. Differentiation of the primary inducing cells, or founders, can
be inhibited by a diffusible product of existing aggregations. A simple control mechanism
can allow narrow zones with steep acrasin gradients to be repeatedly propagated across
a field of separate cells. Acrasin output per cell is inversely related to aggregate size. As
the cell surface except at the ends of a cell remains stationary relative tot he substratum,
aggregated cells can move on top of one another with the same ground speed. Cells in a
single chain adhere by their more permanent end surfaces. Cells joining or within aggregation streams are not directed by the stationary sides of their neighbours, but 'contact
follow' their rear ends. An aggregate's shape is denned with increasing precision throughout development. An aggregation is a collection of individual cells, its shape depending
on their initial distribution. It develops into an elongated, multicellular individual—a
grex. The grex axis is initially perpendicular to the substratum; it may be formed without contact with an air-water interface, though under water it may be inverted. Later
orientation responses include mutual repulsion in air, and the ability to return to an erect
Β. Μ. SHAFFER
an excess of water excretion over water uptake can be achieved
less readily than in air. Another possible factor is the absence of an
interface that would normally select and orientate molecules at the
cell surface. Such selection was advanced by Weiss (1953) as a general
mechanism of differentiation, and Gerisch (1960) has made the as yet
untested suggestion that D. discoideum cannot form grex under water
because it cannot produce slime sheath that is (externally) hydrophobic.
The fact that both spores and stalk can be formed in liquid paraffin
(Potts, 1902) does not decide conclusively between these alternatives.
The protraction of the lying phase on hypotonic agar (Slifkin and
Bonner, 1952; Bonner and Shaw, 1957) may perhaps be due not to the
absence of a specific stimulus to culminate but rather to inhibition of
culmination, in much the same way as aggregation is delayed in
hypotonic solutions (Hirschberg and Rusch, 1950). Although water
relations may play a part in both these inhibitions, absence of salts in
itself is likely to be of great importance ; this is confirmed by the fact
that osmotic equivalents of non-electrolytes are relatively ineffective in
terminating migration (Slifkin and Bonner, 1952).
Acknowledgements
I am most grateful to Dr Κ. E. Machin for his critical reading of the
manuscript and to Professor Κ. B. Raper for permission to use his
illustration.
Summary
This is a list solely of some previously unpublished observations, reinterpretations, and
conclusions, many highly tentative. Cell distribution in the solitary phase in an initially
uniform environment depends on the balance between mutual repulsion, attraction
to and retardation by food, and the effect of monolayering. In theory a cell can chemically
attract distant cells and repel nearer ones ; this may explain the clouds of separate, unoriented cells that attract outlying cells in early aggregation in Dictyostelium mucoroides
(strain 11). Secretion of the attractor can be induced by a diffusible factor, which also
regulates its own secretion. Differentiation of the primary inducing cells, or founders, can
be inhibited by a diffusible product of existing aggregations. A simple control mechanism
can allow narrow zones with steep acrasin gradients to be repeatedly propagated across
a field of separate cells. Acrasin output per cell is inversely related to aggregate size. As
the cell surface except at the ends of a cell remains stationary relative tot he substratum,
aggregated cells can move on top of one another with the same ground speed. Cells in a
single chain adhere by their more permanent end surfaces. Cells joining or within aggregation streams are not directed by the stationary sides of their neighbours, but 'contact
follow' their rear ends. An aggregate's shape is denned with increasing precision throughout development. An aggregation is a collection of individual cells, its shape depending
on their initial distribution. It develops into an elongated, multicellular individual—a
grex. The grex axis is initially perpendicular to the substratum; it may be formed without contact with an air-water interface, though under water it may be inverted. Later
orientation responses include mutual repulsion in air, and the ability to return to an erect