This volume contains papers presented at the Fourth International Altschul Sympo­ sium, held June 27-29, 1996, at the University of Saskatchewan in Saskatoon, Saskatche­ wan, Canada. The Altschul Symposia Series are held in memory of Rudolf Altschul, a graduate of the University of Prague and a pioneer in the fields of vascular and nervous system biology. Dr. Altschul was head of the Department of Anatomy at the University of Saskatchewan from 1955 to 1963. The Altschul Symposia are made possible by an endow­ ment left by Anni Altschul, Dr. Altschul's wife, and by other contributions given by the sponsors listed at the end of this Preface.