Page:Anatomy of the Human Body (1918).djvu/59

Rh Bryce and Teacher the point of entrance was visible as a small gap closed by a mass of fibrin and leucocytes; in the ovum described by Peters the opening was covered by a mushroom-shaped mass of fibrin and blood-clot (Fig. 32), the narrow stalk of which plugged the aperture in the mucous membrane. Soon, however, all trace of the opening is lost and the ovum is then completely surrounded by the uterine mucous membrane.

The structure actively concerned in the process of excavation is the trophoblast 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 bloodvessels, with the result that the spaces n the trophoblastic network are filled with maternal blood; these spaces communicate freely with one another and become greatly distended and form the intervillous space.

<!--{{fis|Fig. 32.—Section through ovum imbedded in the uterine decidua. Semidiagrammatic. (After Peters.) am. Ammotic cavity, 'h.c. Blood-clot. h.s. Body-stalk, ect. Embryonic ectoderm, ent. Entoderm, mes. Mesoderm. 'm.x. Maternal vessels, tr. Trophoblast. u.e. Uterine epithelium, u.g. Uterine glands, y.s.' Yolk-sac.

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 funnel-shaped 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 {{smallrefs}}