_Krieger_ has collected data relating to the duration of the individual periods. He found the duration constant in the great majority of cases, _i. e._, 93.285 per cent.; but variable in a small minority, _i. e._, 6.715 per cent. The periods in which the duration was regular did not always last precisely the same number of days, the duration in many cases being 3 to 4 days, 5 to 6 days, etc.; but the same duration recurred regularly at each successive period, so that all these instances must be reckoned among the periods of regular duration. The duration must be regarded as irregular or variable in those cases in which the variation was from 2 to 4 days, 3 to 8 days, etc. Sometimes a regular three-day or five-day period becomes transformed into an eight-day period; or conversely an eight-day period into a four-day period. Among the cases in which the duration was regular, it amounted Most frequently to 8 days, in 26.695 per cent. Next in frequency was a duration of 3 days, in 20.762 per cent. Next, a duration of 4 days, in 16.949 per cent. Next, a duration of 5 days, in 11.864 per cent. _L. Mayer_ has also drawn a distinction between constant and variable duration of the menstrual periods. Among 4,927 women, he found 4,542 (92.185%) in whom the duration was constant, and 385 (7.815%) in whom it was variable. Of the constant periods, the duration was: 8 days in 1182 women, that is in 26.024 per cent. 4 days in 829 women, that is in 18.252 per cent. 3 days in 731 women, that is in 16.094 per cent. 5 days in 730 women, that is in 16.072 per cent. An extremely short duration, less than 24 hours, was found in 70 women, an extremely long duration, 7 to 14 days, was found in 175 women, and finally a duration exceeding 14 days was found in 19 women. The mean duration in these cases was 5.387 days. The results obtained by _Szukits_, who investigated the duration of the periods in 1,013 women, are somewhat divergent from the above. He found: A duration of a few hours only in 95 women, that is in 9.38 per cent. A duration of 1 to 2 days in 66 women, that is in 6.51 per cent. A duration of 3 days in 407 women, that is in 40.17 per cent. A duration of 4 days in 171 women, that is in 16.88 per cent. A duration of 5 to 6 days in 115 women, that is in 11.35 per cent. A duration of 7 to 8 days in 118 women, that is in 11.63 per cent. A duration of 9 days and upwards in 41 women, that is in 4.05 per cent. The mean duration in these cases was 3.87 days. The mean duration of the menstrual flow is: In Paris 5 days. In London 4.6 days. In Berlin 4.5 days. In Copenhagen 4.3 days (according to Mayer, 5.3 days). In Austria 3.8 days.