1. Cellular-slime-mould amoebae that adhere poorly to one another on contact and effectively do not secrete the chemotactic agent acrasin may become strongly adhesive and start to secrete it. This change, which is particularly important for their aggregation, has been called integration, and the reverse change, disintegration.
2. In Polysphondylium violaceum a cell may receive the integrative stimulus at some distance from the acrasin source that attracts it, but usually it does not change till it has moved close to it or actually reached it; so either an aggregation is simply a heap without any tributary cell-streams, or virtually continuous streams are slowly built out from the centre.
3. In Dictyostelium discoideum the spread of integration, in comparison with cell velocity, may be so rapid that the inflowing streams very soon reach their ultimate extent, though if the population density is not too high their cells only slowly establish contact with one another.
4. As the peripheral cells become integrated, they ‘relay’ the centre's influence. Whether the streams are uninterrupted or ‘stippled’, there need be no continuous and finely graded centrifugal decrement in acrasin secretion: orderly aggregation is ensured by the sequence in which secretion is induced, which results in the centrifugal propagation of one or more comparatively narrow zones in which the gradient is adequate for orientation.
5. A spontaneous or an experimentally produced decrease in the strength of the integrative stimulus, or adaptation to the stimulus, or both, may induce some integrated cells to revert to the unintegrated state.
6. The spread of disintegration, too, in comparison with cell velocity, tends to be more rapid in D. discoideum than in P. violaceum: in the former species all the cells in a considerable length of stream may begin to separate at almost the same time; in the latter they may detach themselves in succession.
7. As disintegration may affect any part of an aggregation, various patterns result therefrom. If it slowly spreads from the centre of a P. violaceum aggregation while the streams are still growing at their outer ends, a ‘fairy ring’ is formed, in which cell movement remains polar.
8. Populations of a D. mucoroides strain, if not too sparse, aggregate at a comparatively fixed time after consuming the available food; but those of D. discoideum at widely different densities and degrees of starvation produce synchronous outbursts of aggregation when transferred from darkness to light.
9. The development and distinctive properties of the initiators of aggregation centres are considered. These cells can release acrasin into the medium without there being any there already or without there being sufficient to induce the remaining cells to secrete.
10. Much evidence is against aggregation being basically a sexual phenomenon.