The first volume of the series, on "The Stability of the Differentiated State" received many favorable reviews from the scientific community. Many readers seem to agree with us that publication of topical volumes is a worthwhile alternative to periodic compilations of rather unrelated, though up-to-date reviews. Production of topical volumes is however, plagued with one great difficulty, that of "author synchronization". This difficulty explains the lag between volumes 1 and 2 of the series. Nevertheless we hope that the present volume will be appreciated as a valuable source of information on its central topic: How do cell organelles originate, and what mechanisms assure their continuity? Tubingen, Berlin, Zurich, /V. BEERMANN, J. REINERT, H. URSPRUNG, Heidelberg H. -W. HA GENS Contents Assembly, Continuity, and Exchanges in Certain Cytoplasmic Membrane Systems by W. GORDON WHALEY, MARIANNE DAUWALDER, aud ]OYCE E. KEPHART 1 I. The Nature of the Membrane. . . . . . H. The Assembly of Membranes . . . . . . 5 III. The Growth and Transfer of Membranes. 6 A. The Nuclear Envelopc . . . 6 B. The Endoplasmic Reticulum 13 C. The Golgi Apparatus . 17 D. The Plasma Membrane 28 E. Vacuoles and Vesicles 31 IV. Concluding Remarks 37 References . . . . . 38 Origin and Continuity of Mitochondria by ROBERT BAXTER 1. Introduction . . . . . . . . . . . . . 46 H. Mitochondrial Biogenesis : thc Machincry 46 III. Limitations of Mitochondrial Autonomy 50 IV. The Replication of Mitochondria 53 V. Discussion and Conclusion 58 Referenccs . . . . . . . . . 59 Origin and Continuity of Plastids by /VILFRIED STUBBE 1. Introduction . . . . . . . . . . . . . 65 II. Arguments for the Continuity of Plastids .