The discussion centers on the evolution of aggregation and the mechanism of chemotaxis. The biochemistry of aggregation in Polysphondylium violaceum seems very different from that of the large species of Dictyostelium, yet the aggregation process itself seems similar, and differences are mainly in details. It is not possible to answer if this is a case of convergent evolution or if there has been biochemical evolution or transition. An important question is raised concerning the mechanism of negative vs. positive chemotaxis; are there separate sites for the repellant? Is there a different signal from the receptors to the interior parts of the cell? All the known naturally occurring chemotactic agents in cellular slime molds are small molecules which can diffuse relatively rapidly and whose gradients can be effectively controlled if the substance is stable and there is a specific inactivating enzyme. Pulses are considered as alternate ways of producing uneven distribution of signal molecules along the axis of a cell. The understanding of chemotaxis will probably have important implications for many fundamental aspects of cell physiology.