CYTOPLASM CYTOPLASM is neither a true liquid nor a true solid, but has an interesting combination of characters which place it between the two states. It may contain as much as 97 per cent water, and in some circumstances exhibits streaming, or flow; in contrast, it has some measure of rigidity, tensile strength, and elasticity. It is not perfectly elastic, however, but undergoes plastic deformation; the surface is sticky, and the material may be drawn into fine threads. To these properties must be added thixotropy (a change from the semi-solid to the semi-liquid state caused by stirring), imbibition, semipermeability, and optical anisotropy. Earlier attempts to explain the properties of cytoplasm, for example, the ‘alveolar’, ‘granular’, and ‘fibrillar’ hypotheses, have proved inadequate, and have been discarded in favour of a gel structure: whereas the former postulated structural units of microscopic dimensions, the latter involves a structure beyond microscopic resolution. A colloid is described as having a dispersed phase in a continuous dispersion medium. Ostwald describes nine possible combinations, since either of the two phases may be liquid, solid, or gaseous, and eight of these are recognized in Nature (that is, all except the gas-gas combination). A consideration of the properties of each fails to explain all the properties of cytoplasm, and it would appear that the substance cannot be an ordinary colloid with dispersed particles.