1. A description is given of the development of certain structures--centre, stream, and ring--formed by the amoebae of four species of Dictyostelium and Polysphondyliumduring aggregation, and of the outcome of experimentally cutting a stream, recombining lengths of stream in simple geometrical relationships, and excising a centre.

2. In all cases there is a high degree of intraspecific variation in behaviour, and it is not possible to predict, except in terms of rough probability, what the future course of development will be.

3. An amoeba at this stage of the life cycle can exist in one of four main states. In the first, it is unaggregated and cannot react chemotactically; in the second, it is unaggregated but can react; in the third, it is in a stream moving towards a centre or has properties similar to those of a stream cell; and in the fourth, it is in a centre and nearly stationary. Though an enormous variety of patterns may be produced by large numbers of interacting amoebae, their behavioural repertory may be considered, without too much simplification, to be restricted to approaching these four states in various sequences and at various rates.

4. Most of these sequences can be observed over a wide range of environmental conditions. Changes in humidity produce the greatest effect: the drier the culture, the stronger the drift from the first state to the fourth.

5. It is suggested that variability is occasioned by the simplicity of the mechanisms controlling development.

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