Earlier titles in this series have directed attention to selected questions of current theoretical or practical interest in developmental biology, without special emphasis upon individual biological systems. Tho choose giant polytene chromosomes as the common denominator of a new volume may at first sight seem to set rather arbitrary restrictions upon a coherent discussion of the basic problem of cellular differentiation. However, the reader will find out for hirnself that, on the contrary, such a limitation, though dicta ted by the material, focuses the analysis upon one essential point: the differential release of genetic information from operational subunits of the eukaryotic chromosome. In the organization of this volume we have not striven for completeness - some, indeed, may fee 1 that the work on Sciarids is underrepresented - but the various contributors have made every effort to render a thorough and up-to-date picture of the situation in their particular field, and on this basis to discuss all possible angles of general biological interest. Authors and editors have agreed to dedicate this volume to the memory of JACK SCHULTZ.