Acrasin, the Chemotactic Agent in Cellular Slime Moulds

1. A study has been made of acrasin, the agent inducing chemotaxis in the amoebae of cellular slime moulds. 2. A method has been developed for subjecting sensitive amoebae to a fluctuating gradient set up by an artificial source that can be renewed at intervals of as little as a few seconds with fresh test solution. 3. Amoebae orient to a gradient maintained with the cell-free liquid freshly obtained from the immediate surroundings of a natural source. 4. Acrasin solution as secreted loses its activity very rapidly at room temperature. 5. A highly active stable solid is obtained by drying methanolic culture extracts; it resists boiling and exposure to acids and alkalis. Its solubility decreases rapidly in passing up the alcohol series. 6. The original instability has been shown to be due to the presence of another extracellular slime-mould product, possibly an enzyme; it, unlike acrasin, cannot pass rapidly across a dialysis membrane, is heat labile, and can be precipitated by ammonium sulphate. 7. The advantages of the organism's itself inactivating acrasin are considered. 8. Some of the advantages of a source's releasing acrasin in pulses are discussed; but it is not essential for orientation for it to do so. 9. Sensitive amoebae not only are oriented by an acrasin solution but are caused to secrete acrasin: this is the basis of a chemotactic relay system.