Measurements on the chemotactic aggregation of the amoebae of the cellular slime mould Dictyostelium minutum are reported. With the aid of previous theoretical results, the observed patterns of early aggregation on 2- and 3-dimensional substrates are interpreted. There is no statistically significant difference in the patterns in the 2 cases. The results suggest that the simplest plausible chemotactic law for this system is the relative gradient law in which the response velocity of the amoebae is proportional to the relative acrasin gradient. The results also suggest that the differentiation of the founder cell, which initiates aggregation, occurs before all cells have acquired chemotactic sensitivity. A diffusible chemical may be involved in the development of the latter.

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