Two years ago, about twenty people gathered informally in our institute to discuss mitosis. We took this opportunity to try to separate the "hard" facts of mitosis which are accepted by most people, from the "soft" ones which are still open for discussion. Surprisingly few "hard" facts survived with their reputation still intact. This result led us to orga­ nize a similar meeting on a larger scale. The outcome was the workshop "Mitosis: Facts and Questions", which was held at the German Cancer Research Center in Heidelberg from April 25-29, 1977. An introductory lecture was given for each of nine major topics, followed by an extensive discussion of facts, questions and future experiments. Further details were provided by posters. The proceedings of the meeting are published in this volume. We feel that many open questions and facts described here will provide stimulating ideas and a basis for further investigation of this fundamental process. The success of the workshop would not have been possible without the help of many people. We are very grateful to the German Cancer Research Center for its interest and assistance, and for the support of the Verein zur Forderung der Krebsforschung in Deutschland represented by Prof. Dr. h.c. K.H. Bauer, the ECBO (European Cell Biology Organization) and the Deutsche Gesellschaft fur Zellbiologie. Our sincere thanks are also extended to our students and technicians for their enthusiastic help, and to Mrs. Joa for typing the manuscripts.

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Session I: The Timing of Cell Cycle Events.- Discussion Session I: Cell Cycle, Timing of Events, Chromosome Cycle.- Session II: Surface Signals and Cellular Regulation of Growth.- Discussion Session II: Membranes.- Session III: Ultrastructure of Mitotic Cells.- Discussion Session III: Fine Structure of Mitotic Cells.- Session IV: Microtubule-Organizing Centers of the Mitotic Spindle.- Discussion Session IV: Tubulin and Microtubules.- Session V: Nontubulin Molecules in the Spindle.- Discussion Session V: Nontubulin Molecules.- Session VI: Mitosis in Differentiation, Morphogenesis, and Cancer.- Discussion Session VI: Mitosis in Differentiation, Morphogenesis, and Cancer.- Session VII: Chromosome Movement: Facts and Hypotheses.- Discussion Session VII: Chromosome Movement: Facts and Hypotheses.- Session VIII: Mitosis in vitro: Isolates and Models of the Mitotic Apparatus.- Discussion Session VIII: Mitosis in vitro.- Session IX: Future Research on Mitosis.- Discussion Session IX: Future Research on Mitosis.- Poster Abstracts.