=The Umbilical Cord.= The cord, or _funis_, is a bluish white cord about three-quarters of an inch in diameter, twisted and tortuous throughout its length of about twenty inches. It is the one actual link between the mother and her unborn child, one end being attached to the abdomen of the fetus, about midway between the ensiform and the pubis, and the other to the inner surface of the placenta. The cord is derived from the _abdominal pedicle_ and is merely an extension of the caudal or tail end of the embryo. It is covered with a layer of ectoderm which is continuous with the ectodermal covering of the fetus. The cord consists of a gelatinous mass known as Wharton’s jelly, in the centre of which are embedded three blood vessels; two arteries through which the vitiated blood flows to the placenta, where it gives up its ash; and one vein which carries oxygenated, nourishment-bearing blood back to the fetus. The life of the fetus, therefore, is absolutely contingent upon an uninterrupted, two-way flow of blood through the cord. =The Fetus.= In tracing the development of the ovum after its implantation in the uterine lining, we begin, as previously stated, with a shaggy-looking vesicle, containing fluid, with a clump of cells hanging toward the centre from their point of attachment on the inner surface of the sac. This clump develops into the embryo. =During the first month= the mass increases in size, becomes somewhat elongated and curved upon itself with the two extremities almost in contact. The abdominal pedicle, which later becomes the umbilical cord, appears; the alimentary canal exists as a straight tube and the thymus, thyroid, lungs and liver are recognizable. The heart, eyes, nose, ears, and brain appear in rudimentary form and the extremities begin to be evident as tiny, bud-like projections on the surface of the embryo. =By the end of the fourth week= the sac is about the size of a pigeon’s egg and has two walls. The outer wall, or chorion, as we have already seen, is covered with villi, and the amnion, or inner wall, is smooth; the contained embryo is surrounded by amniotic fluid and measures about 10 millimetres or 4 inches in length. [Illustration: FIG. 25.—Embryo, about 5.5 centimetres long in amniotic sac; uterine wall incised, chorion split and turned back. Drawn by Max Brodel. (From The Umbilicus and Its Diseases, by Thomas R. Cullen, M.D.) ]