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The Umbilical Cord and Bodystalk. The umbilical cord (Fig. 28) attaches the fetus to the placenta; its length at full time, as a rule, is about equal to the length of the fetus, i.e., about 50 cm., but it may be greatly diminished or increased. The rudiment of the umbilical cord is represented by the tissue which connects the rapidly growing embryo with the extraembryonic area of the ovum. Included in this tissue are the body-stalk and the vitelline duct —the former containing the allantoic diverticulum and the umbilical vessels, the latter forming the communication between the digestive tube and the yolk-sac. • The body-stalk • is the posterior segment of the embryonic area, and is attached to the chorion. It consists of a plate of mesoderm covered by thickened ectoderm on which a trace of the neural groove can be seen, indicating its continuity with the embryo. Running through its mesoderm are the two umbilical arteries and the two umbilical veins, together with the canal of the allantois—the last being lined by entoderm (Fig. 31). The body-stalk Its dorsal surface is covered by the amnion, while its ventral surface is bounded by the extraembryonic celom, and is in contact with the vitelline duct and yolk-sac. • With the rapid elongation of the embryo and the formation of the tail fold, the body stalk comes to lie on the ventral surface of the embryo (Figs. 27 and 28), where its mesoderm blends with that of the yolk-sac and the vitelline duct. The cord is covered by a layer of ectoderm which is continuous with that of the amnion, and its various constitutents are enveloped by embryonic gelatinous tissue, jelly of Wharton. • The vitelline vessels and duct, together with the right umbilical vein, undergo atrophy and disappear; and thus the cord, at birth, contains a pair of umbilical arteries and one (the left) umbilical vein. The Decidua • Before the fertilized ovum reaches the uterus, the mucous membrane of the body of the uterus undergoes important changes and is then known as the decidua. • The thickness and vascularity of the mucous membrane are greatly increased; its glands are elongated and open on its free surface by funnelshaped orifices, while their deeper portions are tortuous and dilated into irregular spaces. • The interglandular tissue is also increased in quantity, and is crowded with large round, oval, or polygonal cells, termed decidual cells. • These changes are well advanced by the second month of pregnancy, when the mucous membrane consists of the following strata (Fig. 33): • (1) stratum compactum, next the free surface; in this the uterine glands are only slightly expanded, and are lined by columnar cells; • (2) stratum spongiosum, in which the gland tubes are greatly dilated and very tortuous, and are ultimately separated from one another by only a small amount of interglandular tissue, while their lining cells are flattened or cubical; • (3) a thin unaltered or boundary layer, next the uterine muscular fibers, containing the deepest parts of the uterine glands, which are not dilated, and are lined with columnar epithelium; it is from this epithelium that the epithelial lining of the uterus is regenerated after pregnancy. stratum compactum stratum spongiosum unaltered or boundary layer • Distinctive names are applied to different portions of the decidua. • The part which covers in the ovum is named the decidua capsularis; • the portion which intervenes between the ovum and the uterine wall is named the decidua basalis or decidua placentalis; it is here that the placenta is subsequently developed. • The part of the decidua which lines the remainder of the body of the uterus is known as the decidua vera or decidua parietalis. decidua placentalis decidua vera decidua capsularis • Coincidently with the growth of the embryo, the decidua capsularis is thinned and extended and the space between it and the decidua vera is gradually obliterated, so that by the third month of pregnancy the two are in contact. By the fifth month of pregnancy the decidua capsularis has practically disappeared, while during the succeeding months the decidua vera also undergoes atrophy, owing to the increased pressure. • The glands of the stratum compactum are obliterated, and their epithelium is lost. • In the stratum spongiosum the glands are compressed and appear as slit-like fissures, while their epithelium undergoes degeneration. • In the unaltered or boundary layer, however, the glandular epithelium retains a columnar or cubical form. the process of Implantation or Imbedding of the Ovum.— The fertilization of the ovum occurs in the lateral or ampullary end of the uterine tube and is immediately followed by segmentation. On reaching the cavity of the uterus the segmented ovum adheres like a parasite to the uterine mucous membrane, destroys the epithelium over the area of contact, and excavates for itself a cavity in the mucous membrane in which it becomes imbedded. • and the ovum is then completely surrounded by the uterine mucous membrane. The structure actively concerned in the process of excavation is the syncytiotrophoblast of the ovum, which possesses the power of dissolving and absorbing the uterine tissues. The trophoblast proliferates rapidly and forms a network of branching processes which cover the entire ovum and invade and destroy the maternal tissues and open into the maternal blood vessels, with the result that the spaces in the trophoblastic network are filled with maternal blood; these spaces communicate freely with one another and become greatly distended and form the intervillous space.