According to L. Meyer, under the influence of ovulation, a continual production of substances, oophorines, necessary for the growth of the foetus, takes place. These substances circulate in the blood, and, when they are present in large amounts, cause a stimulation of the entire nervous system. This explains the nervous irritation and other phenomena of this period. During menstruation these substances are excreted. G. Klein (Münch. med. Wochenschr. 1911, p. 997) thinks that the oophorines cause a chemical change of the uterine mucosa and of the blood circulating in the latter, and are themselves chemically changed. They then leave the body simultaneously with the menstrual blood. Hence menstruation is a real catharsis, in the sense of Hypokrates, a freeing of the body from the toxic effects of the oophorines. The presence of the oophorines during this period accounts for the different smell of menstrual blood from ordinary blood. The peculiar smell from the mouths of some menstruating women may also be attributed to the presence of the oophorines in the body. When pregnancy has once been established, menstruation ceases for the entire period the mother is nursing her offspring, be the nursing within the uterus during gestation or later on during lactation, with the breast. Both periods comprise the time when the mother has to give her vital fluids for the nutrition of her offspring. [H] The Hindus recommend the marriage of the girl before the first menstruation has set in. Among many savages the first menstruation is the signal for marrying off the girl. Such women become impregnated with each succeeding child immediately after they have finished nursing the preceding one. By the time propagation ceases, the climacerium sets in. In this way these women only menstruate a few times during their lives. [I] Another reason why menstruation is so rarely observed among animals is their posture. If the preparations preceding menstruation are for the purpose of ingrafting the fertilized ovum, it follows that menstruation must occur in all those animals in which the fertilized egg is fully developed to complete maturity within the interior of the mother. This is indeed what happens. All animals with so intimate an attachment of the foetal and maternal organisms that the foetal placenta cannot be detached from the mother without a haemorrhage, show the phenomenon of menstruation.