The mature ovum, however, is not by itself capable of being converted into the embryo. It requires fecundation by the spermatic fluid of the male, and this may take place immediately on the expulsion of the ovum from the ovary, or during its passage through the Fallopian tube, or, according to Bischoff, Coste, and others, in the cavity of the uterus, or even upon the surface of the ovary. Should impregnation, however, fail, the ovum gradually loses its vitality, and is eventually expelled by the uterine secretions. It occasionally happens that the descent of the impregnated ovum is arrested, and the formation of the embryo commences in the ovary. This is termed _ovarian pregnancy_. Or again, the ovum may be arrested in its passage through the Fallopian tube, causing what is termed _tubal pregnancy_; or, after it has been expelled from the ovary, it may fail to be received by the fimbriated extremity, and escape into the cavity of the abdomen, forming what has been termed _ventral pregnancy_. If the microscopic germ lodges in some slight interstice of fiber, during its passage through the walls of the uterus, it may be detained long enough to fix itself there, and when this occurs, it is termed _interstitial pregnancy_. All these instances of extra-uterine pregnancy may necessitate the employment of surgical skill, in order that they may terminate with safety to the mother. Their occurrence, however, is very rare. The intense nervous excitement produced by the act of coition is immediately followed by a corresponding degree of depression, and a too frequent repetition of it is necessarily injurious to health. The secretions of the seminal fluid being, like other secretions, chiefly under the influence of the nervous system, an expenditure of them requires a corresponding renewal. This renewal greatly taxes the corporeal powers, inducing lassitude, nervousness, and debility. It is a well known fact that the highest degree of mental and bodily vigor is inconsistent with more than a moderate indulgence in sexual intercourse. To ensure strength, symmetry, and high intellectual culture in the human race, requires considerable care. Consideration should be exercised in the choice of a companion for life. Constitutional as well as hereditary ailments demand our closest attention. Age has also its judicious barriers. As before stated, when reproduction commences, growth, as a rule, ceases, therefore, it is inexpedient that matrimony should be consummated before the parties have arrived at mature stature. PREVENTION OF CONCEPTION.