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.
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.)
By the end of the second month, or eighth week, the head end of the embryo has greatly increased in size and is about as large as the rest of the body. Bone centres appear in the rudimentary clavicles; the kidneys and supra-renal bodies are formed; the limbs are more developed, webbed hands and feet are formed, the external genitalia are apparent but the sex is not distinguishable. The amnion is distended with fluid, but it is not yet in contact with the chorion; the chorionic villi have become more luxuriant on that part of the chorion resting on the decidua basalis, the future site of the placenta. The approximate weight of the embryo is 4 grams and its length 25 millimetres or an inch.
By the end of the third month, or twelfth week, centres of ossification have appeared in most of the bones, the fingers and toes are separated and bear nails in the form of fine membranes; the umbilical cord has definite form, has increased in length and begun to twist. The neck is longer, teeth are forming and the eyes have lids. The amnion and chorion are now in contact, and the villi have disappeared excepting at one point where a small, but complete placenta has developed. The embryo is about 9 centimetres long and weighs about 30 grams.
Fig. 26.—Diagram showing appearance of fetus at different stages in its development.