It is now possible to turn to an investigation which, although of very limited extent, serves to place the question of a male menstrual cycle for the first time on a sound basis. If there is such a cycle analogous to menstruation in women, it must be a recurring period of nervous erethism, and it must be demonstrably accompanied by greater sexual activity. In the _American Journal of Psychology_ for 1888, Mr. Julius Nelson, afterward Professor of Biology at the Rutgers College of Agriculture, New Brunswick, published a study of dreams in which he recorded the results of detailed observations of his dreams, and also of seminal emissions during sleep (by him termed "gonekbole" or "ecbole"), during a period of something over two years. Mr. Nelson found that both dreams and ecboles fell into a physiological cycle of 28 days. The climax of maximum dreaming (as determined by the number of words in the dream record) and the climax of maximum ecbole fell at the same point of the cycle, the ecbolic climax being more distinctly marked than the dream climax. The question of cyclic physiological changes is considerably complicated by our uncertainty regarding the precise length of the cycle we may expect to find. Nelson finds a 28-day cycle satisfactory. Perry-Coste, as we shall see, accepts a strictly lunar cycle of 29½ days. Fliess has argued that in both women and men, many physiological facts fall into a cycle of 23 days, which he calls male, the 28-day cycle being female. (W. Fliess, _Die Beziehungen zwischen Nase und weiblichen Geschlechts-Organen_, 1897, pp. 113 et seq.) Although Fliess brings forward a number of minutely-observed cases, I cannot say that I am yet convinced of the reality of this 23-day cycle. It is somewhat curious, however, that at the same time as Fliess, though in apparent independence, and from a different point of view, another worker also suggested that there is a 23-day physiological cycle (John Beard, _The Span of Gestation and the Cause of Birth_, Jena, 1897). Beard approaches the question from the embryological standpoint, and argues that there is what he terms an "ovulation unit" of about 23½ days, in the interval from the end of one menstruation to the beginning of the next. Two "ovulation units" make up one "critical unit," and the length of pregnancy, according to Beard, is always a multiple of the "critical unit;" in man, the gestation period amounts to six critical units. These attempts to prove a new physiological cycle deserve careful study and further investigation. The possibility of such a cycle should be borne in mind, but at present we are scarcely entitled to accept it.