1. When all available food has been consumed, the amoebae of Dictyostelium and Polysphondylium aggregate towards collecting centres; this movement involves chemotaxis.
2. Just after they have finished feeding, the amoebae adhere little or not at all to one another if they come in contact; they do not secrete the chemotactic agent acrasin, and cannot orient in an acrasin gradient.
3.Later they become able to respond to an acrasin gradient, if this is great enough. In the absence of one, they may be sufficiently adhesive to form loose amorphous associations, or, depending on the species and the environment, still be quite separate. Amoebae with similar properties may be released by the spontaneous break-up of aggregated masses. In some species, amoebae that are unaggregated may lose their sensitivity to acrasin after a few hours.
4.The elongated amoebae attracted to a source of acrasin may condense into streams. Stream amoebae move at varying speeds; they adhere strongly to others of the same species and to a variable extent to those of different species. They secrete the acrasin characteristic of their own species. Along a stream, there is often no continuous centrifugal decrement in secretion, as far as can be detected. The amoebae in it can be guided by the direction in which their adherent neighbours are flowing, though if they are exposed to an adequate acrasin gradient they will respond to it. Commonly, an adjacent stream or a centre planted beside them is not strong enough to attract them, unless they are freed to some extent from the influence of their neighbours, especially those ahead of them.
5. Except for their being effectively non-motile and more nearly isodiametric, the amoebae in a centre are similar to those in a stream and may secrete acrasin at much the same concentrations as these do. With both types, there may be short-term fluctuations in secretion, and in at least one species a long-lasting decrease.
6. The dimensions and gross appearance of an aggregation at the population densities used are partly determined by the time intervals over which the amoebae become acrasin-sensitive and centres are initiated, and by the relationship between these.