Robertson found wide occurrence of such trilaminar composition in most of the membranes of cellular organelles and in 1959, he propounded the concept of the "unit membrane". A unit membrane has the protein-lipid-protein arrangement. In 1938 Harvey and Danielle produced a hypothetical model of the plasma membrane which has shown a biomolecular lipid structure sandwiched by two layers of protein molecules. The electron microscope studies have confirmed this protein-lipid-protein arrangement in the plasma membrane. The trilaminar nature of plasma membrane has been proposed by Danielli and Davson in 1935 by certain indirect evidences.