_Menstruation._—The main function of the uterus is to serve as a couch or resting place for the fertilized ovum during its development. For this end a wound has to be set in the lining of the uterus to facilitate the grafting of the fertilized ovum, just as the gardner makes a slit for ingrafting the young shoot. The preparation of the ingrafting of the young animal is accomplished by the monthly changes in the uterus preceding menstruation. The monthly general congestion of the genital organs, necessary for the ovum to leave the Graafian follicle, also causes the uterine mucous membrane to swell from one to three millimeters in the thickness.[G] The mucosa becomes thus turgid and velvety. The epithelial cells of the endometrium swell and multiply. The openings of the uterine glands are enlarged, and a whitish, opaque mucus is poured out. The glandular cells are enlarged, and there is a multiplication of the round cells of the stroma. The cells are cloudy and filled with fat granules. This fatty degeneration involves the glandular cells, the cells of the inter-glandular tissue, and the cells of the blood vessels which are distended with blood. The endometrium or lining is thus prepared for the reception of the impregnated ovum. These alterations develop independently of the haemorrhage. The swelling begins a long time before the period and reaches its maximum only at this time. If the ovum has become impregnated it is ready to respond to these preparations. If, however, nature’s attempt to reproduce an individual of the species has been frustrated, and the ovum has not been impregnated, the latter has to be cast out, and all the preparations are eliminated. The exuberant epithelial cells are exfoliated. The delicate capillaries, engorged with blood, sweat drops of blood by diapedesis or burst and discharge their contents. The congested and engorged glands secrete and excrete profusely. This mingled mass of epithelial cells, blood and mucus leaves the body in form of menstrual discharges. Menstruation, therefore, is a retrograde process, in consequence of the failure of the impregnation of the ovum. The monthly flow is like the shedding of the leaves by the tree, the getting rid of some material the function of which has been frustrated and for which the economy has no more use. Menstruation is thus the manifest phenomenon of the retrograde metamorphosis which ensues at the failure of the ovum to become fertilized. Where there is fertilization there is no menstruation. Hence, those women who marry immediately after menstruation sets in, at puberty,[H] and are impregnated and those women who are impregnated immediately after they finished nursing their babies, will seldom show the phenomenon of menstruation. This is the reason why menstruation is so rarely observed among animals.[I]