the engulfing by the cell surface of fluid and substances that the fluid contains. Pinocytosis is one of the principal mechanisms by which high-molecular-weight compounds enter the cell, particularly proteins and carbohydrate-protein complexes. The phenomenon of pinocytosis was discovered by the American scientist W. Lewis in 1931. During pinocytosis short, slender outgrowths on the plasma membrane surround a drop of fluid. Then the region of the plasma membrane that contains these outgrowths invaginates and pinches off to form a bubble inside the cell. The formation of pinocytotic bubbles, whose diameters do not exceed 2 microns (μ), has been traced by phase-contrast microscopy and microcinematography. With the aid of the electron microscope, bubbles with diameters that range from 0.07 to 0.1 μ can be discerned. Pinocytotic bubbles are capable of freely moving within the cell or of merging with each other or with other intracellular membranous structures. The most active pinocytosis is observed in amoebas, in epithelial cells of the intestines and renal tubules, and in the endothelium of blood vessels and growing oocytes. The level of pinocytotic activity depends on the physiological state of the cell and on the environment. Active inducers of pinocytosis are γ-globulin, gelatin, and certain salts.