This monograph reviews critically the work on the mechanisms of cell division. The distribution of chromosomes to new cells is a complicated but well ordered process, a thorough understanding of which is essential to advance in the study of problems of cell behavior. A number of hypotheses have been advanced to explain the movements of chromosomes, but to the author no final solution is in sight. The trouble seems to be "that nearly all the hypotheses have been built around the idea that a certain, single type of force underlies all mitotic activity." It now seems clear that "mitosis is comprised of a great complex of different mechanisms." The review ends constructively in the recommendation that at present research better be concentrated on single, limited phases of mitosis in favorable species rather than directed to the immediate complete analysis of the whole mitotic cycle.