It is indeed a pleasure and an honour for me to be invited to address on a subject on which the Dutch Schools have performedsuch outstanding work in the last ten years.We wish to express our admiration of their continuous and successful efforts and we give our thanks particularly t o Professor J. J. VAN LOGHEM for his advice so generously given to us since 1949. With JON V A N ROOD, our scientific competition has taken paraUe1 pathways in a new and &fficult field of research which we have and we shall always pursue in a spirit of friendship and understanding.We also wish t o mention the contribution of other teams, such as