These projections, the so-called Villi, push their way into the mucous membrane of the uterus and serve a two-fold purpose. One of their functions is to fix the ovum in its new abode; and, though the attachment is not at first very secure, it becomes stronger in the course of time and is capable of withstanding whatever tendency the activity of daily life may have to loosen it. The other, and equally important, task of the villi, the majority of which dip into the mother's blood, is to transmit substances to and from the embryo. We have traced thus far the earliest steps in the development of the ovum. One portion, we observed, was promptly set apart for the construction of the future child; this favored portion became inclosed by all the rest of the ovum, which has a more or less spherical form and is technically called the fetal sac. The first duty of the sac is to take root in the womb, and the second, no less vital, is to draw nourishment from the mother. But neither of these functions can be performed without the participation of the uterine mucous membrane, the soil, as it were, in which the ovum is planted. We must now learn how the maternal tissues assume the responsibility placed upon them. THE REACTION OF THE UTERUS.--The womb, which is small before marriage, is converted by pregnancy into the largest organ of the body. The virginal uterus, shaped somewhat like a pear, and placed with apex downward, is carefully protected within the bony basin between the hips, which is commonly called the Pelvis. The upper and larger part of the organ, known as the body, lies at the bottom of the abdominal cavity; the lower part, the neck, projects into the vagina. The cavity inside the womb communicates above with the two oviducts and terminates below in a canal which runs through the neck and opens into the vagina by an orifice known as the mouth of the womb. Pregnancy modifies every portion of the womb in one way or another; but the most profound alterations occur in the body, in the cavity of which the ovum has come to rest. During the forty weeks of gestation the organ grows in weight from two ounces to as many pounds; from three inches in length it increases to fifteen inches; and its capacity is multiplied 500 times.