_L. Mayer_, who made observations on the type of menstruation in 5,671 women, and tabulated his results, distinguishes between constant and inconstant intervals. Among the constant intervals he enumerates those forms, both regular and irregular, which do not during the whole life of the individual undergo transformation into another form, but remain always of the same type. If, for instance, in any individual the interval is always either two or eight weeks, in that woman menstruation is indeed irregular, but constant in type. If, however, for some years she menstruates at intervals either of two or of eight weeks, and then proceeds to menstruate at intervals of four weeks, her menstruation is of the inconstant type. _Mayer_ found among his 5,671 cases The constant type in 4,981 women, that is in 87.83 per cent. The inconstant type in 690 women, that is in 12.16 per cent. Of the cases in which the type was constant there were 69.68 per cent. in which the regular period of four weeks obtained, and 20.31 per cent. in which it was irregular in the sense above defined. Among these latter, the commonest periods were 15 to 21 days and 22 to 27 days. The same author observed the irregular type of menstruation in nearly one-fourth of the women belonging to the well-to-do classes. According to the observations of _Krieger_ on 481 cases in which the periods were regular, that is, in which the intervals in each case were equal in duration, the time from the commencement of one period to the commencement of the next was: 28 days in 70.80 per cent. 30 days in 13.74 per cent. 21 days in 1.66 per cent. 27 days in 1.45 per cent. As regards the season in which menstruation first appears, _Krieger_ states that in one-half of the women examined by him menstruation had begun in the autumn season, in the month of September, October, or November. _Szukits_, as a result of an investigation into the menstrual functions of Austrian women, determined that among 1,013 women menstruation occurred: Every 28 to 30 days in 642 women. Every 8 to 21 days in 169 women. Every 35 to 56 days in 128 women. And was quite irregular in 74 women. In 500 Jewish women, _Hirsch_ found that menstruation occurred: