This book consists of a series of fourteen articles which represent, in expanded form, papers contributed to a symposium that was held at the University of Chicago on Nov. 13, 1942, in honor of the seventy-fifth birthday of Prof. R. R. Bensley. The general subject is the chemical structure of cytoplasm, which is seen to present a variety of interesting aspects, both theoretic and practical. Among the former are the isolation of mitochondria for analysis, the study of protoplasmic fibrils by means of the electron microscope and the use of the ultracentrifuge and of electrocataphoresis in fresh discoveries of substances that lie in the twilight zone between living and dead material. Among the practical aspects are carcinogenesis in epithelium and chromatolysis in motoneurons. The series is introduced by a foreword by Hoerr and an appreciation by Cowdry and closes with a review of the chemistry of cytoplasm by Professor Bensley.