for helpful criticisms. My gratitude to Allen Phillips who helped immensely with the photographic work, Molly Hubbard, whose artistic ability is self-evident in the following figures, and Evelyn Hall, whose typing skills made this manuscript possible. My appreciation goes further to all my good friends in Dr. Hans Läufer's laboratory and especially to Jeanne Best and Regina Kuliawat, each of whom shared with me experiences, dissapointments, moments of happiness and gradual personal development.And finally, I want to direct my sweetest thoughts to the three most important people in my life, my parents, Ann and Constantine Vafopoulos, and my husband, Dr. George Mandalos, who, all, have motivated me, loved me and cared about me during my years of growth.