Total membrane mass and membrane lipid increase during differentiation. A novel but simple method of graphic analysis disclosed that each polar lipid class of vertebrate and invertebrate organs and fungal mycelia occurs at only seven different percentages except in organs in which relative proportions of the lipid classes change with age. Analysis of changes in relative proportions of lipid classes in human brain showed that, during myelination, cerebroside and sphingomyelin replace phosphatidyl choline, sulfatide replaces phosphatidyl ethanolamine, and phosphatidyl serine replaces other acidic phospholipids. Data for aorta, the lens of the eye, and the fruit fly disclosed, however, that phosphatidyl ethanolamine, phosphatidyl choline, and sphingomyelin can substitute for each other. Thus, there are only two general substitution groups. A new model for cell membrane structure consists of four lipid layers (each two molecules thick) and two protein layers in the sequence lipid-protein-lipid-lipid-protein-lipid. Membrane lipid class composition is determined in part