When the ovum reaches the finished state, which is called "maturity," it leaves the ovary, and is carried along the fallopian tube (see accompanying figure) into the uterus, where it usually finds a lodgment in the upper part, as shown in Figure I. Once the minute ovum has been caught in the projections of the velvety inner surface of the uterus, this thick velvety lining of the uterus in the neighborhood of the ovum begins a rapid growth, gradually enveloping the rapidly expanding ovum, as shown in Figures I and II of the accompanying plate. Within the ovum there are taking place some of the most marvelous changes in the whole life history of the individual. The nucleus of the fertilized egg, and the protoplasm which surrounds it, divide into two cells, then into four, eight, sixteen, etc. These divisions follow each other in such rapid succession that there are many hundreds of cells by the end of the first twenty-four hours. These cells soon begin to arrange themselves into layers and groups, which, step by step, develop the different tissues and organs of the body. By the end of thirty days the little embryo, about as large as one inch of the end of a lead pencil, would be recognized as the embryo of some mammalian animal, but it would be quite impossible to say whether it would develop into a human being or some other animal, if it were seen quite apart from its immediate surroundings. By the end of another thirty days, however, the little embryo has multiplied its size several times, and has reached a form instantly recognizable as the young of the human kind, as shown in Figure IV. It still, however, retains the vestige of a little tail, which within the next thirty days will have been completely absorbed. [Illustration: THE FEMALE ORGANS OF GENERATION] Note that the little two months embryo has projecting from its abdomen a large structure which is labeled "cord." This cord is a part of what is called the umbilical cord, and it is this that joins the embryo to the mother. Note in Figure III the large stalk of this cord passing upward from the body of the embryo and merging into the structures in the top of the uterus. Note further that there are little branching structures passing from the base of this stalk up into the base of the uterus. These branching structures are loops of blood-vessels, and they form part of the placenta, or "afterbirth." Through this cord the embryo receives its nourishment from the mother. The blood of the mother bathes these loops of blood-vessels, and the embryo absorbs from the mother's blood the nourishment which builds its bones, muscles, brain, spinal-cord, and glands. From the same source the embryo receives the oxygen necessary for the maintenance of its life.