DEVELOPMENT OF THE FETAL MEMBRANES AND THE PLACENTA 59 



IBryce and Teacher 1 the point of entrance was visible as a small gap closed by a 

 mass of fibrin and leucocytes; in the ovum described by Peters 2 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 

 in the trophoblastic network are filled with maternal blood; these spaces com- 

 municate freely with one another and become greatly distended and form the 

 intervillous space. 



tr. a.Q. tr. 



u.q. 



u.e. 





FIG. 32. Section through ovum imbedded in the uterine decidua. Semidiagrammatic. (After Peters.) am. 

 Ammptic cavity, b.c. Blood-clot, b.s. Body-stalk, ect. Embryonic ectoderm, ent. Entoderm. mes. Mesoderm. 

 m.r. 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 



1 Contribution to the study of the early development and imbedding of the human ovum, 1908. 



2 Die Embettung des menschlichen Eies, 1899. 



