Mechanism of the Diastole of Contractile Vacuoles

IT is still not known what causes the passage of fluid from the cytoplasm into the contractile vacuole of a protozoan. In a recently published paper¹, Picken has claimed that the rate of this ``infiltration'' of fluid into the vacuole will be proportional, among other things, to the difference between the hydrostatic pressure of the cytoplasm (which is maintained by tension on the pellicle) and the colloid osmotic pressure of the cytoplasm, the former tending to produce infiltration and the latter to prevent it. The system which Picken has envisaged appears to resemble the excretory mechanism of vertebrates and possibly of certain higher invertebrates, in which fluid is filtered, from blood vessels under pressure from the heart through colloid-retaining membranes (for example, the glomerulus) into renal tubules.