By Ferritin, the iron storage protein, is made up of a roughly spherical apo-protein shell enclosing a micelle of composition (FeOOH)s(FeO.OP03H2) (Granick and Hahn 1944; Farrant 1954). Electron microscopy (Farrant 1954; van Bruggen, Wiebenga, and Gruber 1960) showed that the micelle is approximately 55 A in diameter.