~o working hypothesis amounts to much until it has been tested on suitable material. Indeed, the choice of an appropriate experimental system has often been the key to the solution of a problem. The present volume is devoted to insect imaginal disks. These groups of larval cells are the primordia of precisely characterized adult counterparts, without apparent function in larvae. At the onset of metamorphosis, the subtle interplay of hormonal signals brings growth to a halt, and differentiation begins. In the fruitfly, a host of mutations are known to affect the development of disks; these provide ample material for analysis. It was largely ERNST HADORN'S ingenuity that directed the attention of many scientists around the world to this promising experimental system, and to him this volume is dedicated. All the contributors have been associated with him at one time or another, as graduate students, postdoctoral fellows, or colleagues. Each author has attempted to cover comprehensively the topic assigned to him. This has inevitably led to some overlapping, for which the editors should be blamed, not the authors, as this results from the way the topic was subdivided at the outset. We believe this volume will be a welcome sourcebook for the specialist in the field, and a provocative monograph for the uninitiated scientist interested in the exciting area of cell determination.

The informational content of cells is encoded in the nucleotide sequences of their DNA. The restrictions on base pairing- A pairing with T (U), and G pairing with C - in nature assures the fidelity of replication of DNA in cell division, and of transcription. In the test tube, these restrictions can be exploited for ascertaining similarities and dissimilarities of nucleic acids of varying origin by measuring the kinetics of reassociation of polynucleotides to double-stranded molecules in DNA DNA renaturation or RNA-DNA hybridization experiments, and by determining the thermal stability and other physical-chemical properties of the resulting hybrid molecules. This method has enormous potential for developmental biology. It offers a more direct approach to the ever-present question of the genetic identity of different cell types in an individual organism, and a more direct test of the hypothesis of diffe rential gene function. It offers the possibility of localizing genes on chromosomes without the use of Mendelian genetics. It is an indispensable tool in the isolation, purification, and characterization of genes. This volume brings together six articles by investigators actively working on various aspects of developmental biology who use nucleic acid hybridization as a tool in their research. Sound in theory, the method is in a honing phase as regards the technical detail. This is expressed in the hesitation with which some of the conclusions are rightly drawn."

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 ."