The theories described in the previous chapter are certainly numerous and varied and, upon a little reflection, we must expect that most of them are wrong. The principal activity that divides the cell was placed in different regions by the different groups of hypothetical mechanisms; therefore, some progress in elimination of alternative hypotheses is possible by identification of the cell regions or structures that are essential for division. With few exceptions, identification of essential or nonessential components was not possible by simple observation of the normal process, and the need for experimentation was recognized. In the most successful of the early experiments, the importance of a structure or region was assessed by removing or disrupting it. If the cell divided, the affected region was, by definition, not essential. The results of such an experiment are clear and certain, and the experimental design constituted a method of proceeding by disproof. Interpretation of division failure as a result of experimentation is neither simple nor clear. Failure may be due to removal of an essential component, or it may be due to some other deficiency or damage that the experimenter unknowingly inflicted. In such cases, the design of control experiments is important. All the major regions of the cell – the mitotic apparatus, the surface, and the region between the mitotic apparatus and the surface – were, in different theories, considered to play an essential role, and evaluation of the degree of dependency of division upon the integrity of a particular region provided a way to test hypotheses.