THE ACRASINA
147
kept; and it may be that the probability is greatly increased in an
environment conditioned by other cells. Certainly, in D. discoideum, a
secreted factor increases the number of centres (Ennis and Sussman,
1958a; Section III, B). As it is made by mutants that are attracted to
aggregations but are unable to build their own, it cannot be the attractor, and it is unlikely to be the inducer of attractor secretion that operates in the relays. However, it is not clear whether it stimulates trigger
activity directly or only interferes with the inhibition mechanism. In
certain species, even in a seemingly favourable environment, the chance
of cells becoming triggers may be rather low; and the population apparently has to sit around waiting for its members to 'throw a six'—
whatever its equivalent may be in biochemical terms—before aggregation can start.
Arndt (1937) thought cells attracted towards a moving source did not
orient towards its current position but took such a course as would lead
them to intercept it. Prediction would be rather a feat for a chemotactic
system, and in fact his evidence, preserved in his film, does not justify
his conclusion. Nevertheless, aggregation shows that chemotaxis may
reach a considerable degree of subtlety, and suggests that it may function in other less easily analysable systems in which a simpler form has
been deemed inoperative; but whether or not this is the case, the
patterning of many other cell communities must depend on similar
relayed changes in the properties and behaviour of their members. It is
an amusing coincidence that if plates of Bonner's agar dry out very
slowly, the salts may crystallize in patterns so remarkably like aggregations that when the individual crystals are about the size of amoebae,
they may pass, on occasion, even under a dissecting microscope, as new
Acrasina—species of Natrium and Kalium. The resemblance arises because each inorganic cell determines the possible positions of the axis
of the one that forms next to it, so that here too orientation is relayed
centrifugally from the initiating crystal.
Moscona (1960) found that the extracellular mucopolysaccharide
secreted by dispersed, vertebrate, embryonic cells in culture may guide
and organize them, and developed the view that the ground substance
takes a similar role in building up tissues. Such material is not essential
for aggregation in the Acrasina, but it does play a vital part in the
development of Labyrinthula, an organism of problematical affiliations
that is sometimes placed in the Mycetozoa in a separate suborder. The
cells of a vegetative community glide singly or in groups over a network
of mucous filaments that they have previously extruded. The community slowly expands radially while vacating the central area that has
been cleared of food. When any part of the periphery touches a new food
source, an excitation spreads out from it, accelerating the cells 100-fold
147
kept; and it may be that the probability is greatly increased in an
environment conditioned by other cells. Certainly, in D. discoideum, a
secreted factor increases the number of centres (Ennis and Sussman,
1958a; Section III, B). As it is made by mutants that are attracted to
aggregations but are unable to build their own, it cannot be the attractor, and it is unlikely to be the inducer of attractor secretion that operates in the relays. However, it is not clear whether it stimulates trigger
activity directly or only interferes with the inhibition mechanism. In
certain species, even in a seemingly favourable environment, the chance
of cells becoming triggers may be rather low; and the population apparently has to sit around waiting for its members to 'throw a six'—
whatever its equivalent may be in biochemical terms—before aggregation can start.
Arndt (1937) thought cells attracted towards a moving source did not
orient towards its current position but took such a course as would lead
them to intercept it. Prediction would be rather a feat for a chemotactic
system, and in fact his evidence, preserved in his film, does not justify
his conclusion. Nevertheless, aggregation shows that chemotaxis may
reach a considerable degree of subtlety, and suggests that it may function in other less easily analysable systems in which a simpler form has
been deemed inoperative; but whether or not this is the case, the
patterning of many other cell communities must depend on similar
relayed changes in the properties and behaviour of their members. It is
an amusing coincidence that if plates of Bonner's agar dry out very
slowly, the salts may crystallize in patterns so remarkably like aggregations that when the individual crystals are about the size of amoebae,
they may pass, on occasion, even under a dissecting microscope, as new
Acrasina—species of Natrium and Kalium. The resemblance arises because each inorganic cell determines the possible positions of the axis
of the one that forms next to it, so that here too orientation is relayed
centrifugally from the initiating crystal.
Moscona (1960) found that the extracellular mucopolysaccharide
secreted by dispersed, vertebrate, embryonic cells in culture may guide
and organize them, and developed the view that the ground substance
takes a similar role in building up tissues. Such material is not essential
for aggregation in the Acrasina, but it does play a vital part in the
development of Labyrinthula, an organism of problematical affiliations
that is sometimes placed in the Mycetozoa in a separate suborder. The
cells of a vegetative community glide singly or in groups over a network
of mucous filaments that they have previously extruded. The community slowly expands radially while vacating the central area that has
been cleared of food. When any part of the periphery touches a new food
source, an excitation spreads out from it, accelerating the cells 100-fold