Nature of Conception.-- Conception, or impregnation, is the union of the germ and the sperm cell, the result of which is a new being. On coition, the semen being received into the female organs, which are at that time in a state of turgescence, the spermatozoa, by means of their own vibratile activity, find their way into the Fallopian tubes, and here come in contact with the ovule. The ovule is a minute cell with a transparent membrane, within which is the yolk containing the germinal vesicle. The spermatozoon penetrates into the ovule and becomes fused with it. The processes of development begin at once to occur. There is congestion of the uterine mucous membrane out of proportion to the rest of the uterus; the ovum finds lodging here, and becomes surrounded by a membrane which incloses it in a separate sac. Pregnancy Defined.-- Pregnancy begins with conception and ends with parturition; it provides for the nutrition and the expulsion of the embryo and for its nutrition for a short time after birth. The average duration of pregnancy is ten lunar months, or two hundred and eighty days. The date of the confinement is calculated by reckoning from the date of the last menstrual flow; count backward three months from the date of the first appearance of the last menses; to this add twelve months and seven days, five days being for the average menstrual duration and two days for the possibility of fecundation. Duration of Pregnancy.-- Many difficulties are experienced in determining the date of the expected confinement. As most pregnancies occur in married women, we cannot base any calculations on a single act of coitus. And even if there was but one, all physiologists agree that there is a variable period in different women, and in the same woman at different times, between insemination and the fertilization of the ovum. It is the moment of fecundation, or the union of the germ and sperm cells, which marks the beginning of pregnancy. The uncertainty becomes still greater owing to our inadequate knowledge as to the length of time during which the sexual elements, the ova and the spermatozoa, retain their vitality after liberation from their respective sources. While it is not certainly known, it is probable that the ovum is capable of impregnation any time during its sojourn within the oviduct and before reaching the uterus, or probably for a period of about one week from the time of its escape from the Graafian follicle. The remarkable vitality of the spermatozoa even under less favorable circumstances-- direct observation shows that these elements retain their movements for over nine days outside of the body-- renders it almost certain that their powers of fertilization are maintained for a long time after they are deposited within the healthy female genital tract; it is believed that the spermatozoa are capable of fertilization after a sojourn of three or more weeks within the oviduct.