The development of spermatozoa has been for the last twenty years the subject of prolonged research and controversy, which cannot yet be said to have resulted in satisfactory solution of the involved problems. In view of the intrinsic difficulties of the subject, and the discrepant results and nomenclature of different authorities, it is the object of the present paper to recapitulate the history of investigation, to collate the results and the nomenclature, and to propose a theory which will explain and rationalise the maze of apparently conflicting observations.