"”It is a remarkable fact, in this connection, that the female of these animals will allow the approaches of the male only during and immediately after the oestral period; that is, just when the egg is recently discharged, and ready for impregnation. At other times, when sexual intercourse would be necessarily fruitless, the instinct of the animal leads her to avoid it; and the concourse of the sexes is accordingly made to correspond in time with the maturity of the egg and its aptitude for fecundation.” “The egg, immediately upon its discharge from the ovary, is rady for impregnation. If sexual intercourse happens to take place about that time, the egg an the spermatic fluid meet in some part of the female generative passages, and fecundation is accomplished. . . . If, on the other hand, coitus do not take place, the egg passes down to the uterus unimpregnated, loses its vitality after a short time, and is finally carried away with the uterine secretions.” “It is easily understood, therefore, why sexual intercourse should be more liable to be followed by pregnancy when it occurs about the menstrual epoch than at other times . . . Before its discharge, the egg is immature, and unprepared for impregnation; and after the menstrual period has passé, it gradually loses its freshness and vitality."