The Viscosity Concept.- The Measurement of Protoplasmic Viscosity.- Absolute Viscosity.- Plant Protoplasm.- Protozoan Protoplasm.- Muscle Protoplasm.- Nerve Protoplasm.- The Protoplasm of Marine Eggs.- A General Statement.- The Effect of Various Physiological Factors.- The Action of Temperature.- The Action of Pressure and of Electricity.- Radiation Effects.- Visible Light.- Ultraviolet Radiation.- X Rays and Beta Rays.- The Action of Salts, Acids and Bases.- Hypertonic and Hypotonic Solutions.- Fat Solvents.- Cyanide and Azide.- Clotting and Anticlotting Substances.- Substances Used in the Treatment of Cancer.- Various Drugs.- Injury and Injury Substances.- The Significance of the Viscosity Data.