976 HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 



tissue contains large round cells (decidual cells), which in the depth are often 

 transformed into fibrillar and spindle-shaped cells; in addition, leukocytes are 

 present. The uterine glands, which, at the beginning of pregnancy, are enor- 

 mously developed, undergo a transformation between the third and the fourth 

 month to large, noncellular dilated tubes. In the last months these become 

 indistinct and their epithelium (which, according to Friedlander, Lott, and Hen- 

 nig was originally ciliated), disappears progressively toward the depth. 



The capsular decidua, much thinner than the true decidua, is devoid of epi- 

 thelium and also of vessels and glands from the middle of pregnancy. Toward 

 the end of pregnancy both deciduas unite completely with each other. 



The basilar decidua and likewise the uterine placenta consist of a compact 

 layer (pars caduca) , which is detached during labor, and of a deeper spongy layer, 

 in which the process of detachment takes place and of which a portion remains 

 upon the surface of the muscularis (pars fixa) . From the latter the regeneration 

 of the new mucosa after labor takes place. Also the tubes exhibit during preg- 

 nancy hyperplasia of the mucosa and of the muscularis. 



The ovum reaches the endometrium as a vesicle without villi. The 

 mucosa has become softened and hyperemic, and the ovum sinks into 

 its tissue, quickly to become completely encapsulated. In the basal 

 decidua there form lacunar maternal blood-passages, which undergo 

 progressive enlargement With the formation of the amnion, there 

 occurs, after its closure, the production from the epiblast of a special 

 entirely closed vesicle, which passes over the embryo, the amnion and 

 the umbilical vesicle, and thus lies next to the primitive chorion ; this 

 is the serous capsule (Fig. 376, VII, S), which applies itself closely to 

 the chorion. The allantois, rich in vessels, passes out of the navel, and 

 lies directly upon the ovular membrane; its vesicle atrophies about 

 the second month in man, but its vascular layer grows rapidly and 

 lines the entire interior of the ovular cavity, where it can be found on 

 the eighteenth day. From the fourth week the vessels, together with 

 a connective-tissue framework, form many intricately branching villi t 

 while the original ovular membrane (prochorion or primitive chorion) 

 disappears (in dogs it is absorbed and serves for nourishment). There 

 is thus reached a stage of general vascularization of the chorion; the 

 derivative of the zona pellucida is now replaced as the ovular mem- 

 brane by the villous vascular layer of the allantois, which is covered 

 by the cells of the serous capsule (derived from the epiblast). The 

 chorionic villi grow downward in the direction toward the decidual 

 vascular spaces. The villi are separated from the vascular space by 

 two layers of specialized cells: the chorionic epithelium, or the layer 

 of Langhans (derived from the fetal ectoderm), and a second layer 

 designated syncytium, whose cells with large nuclei and indefinite 

 outline are, according to most recent investigators, derived from trans- 

 formed uterine epithelium. In the protoplasm of the latter vacuoles 

 appear and the cilia atrophy when the existing spaces are filled with 

 blood. The ovum adheres to the syncytium after the disappearance 

 of the zona pellucida. The stage of general vascularization continues, 

 however, until the third month; at that time the vegetation of the 

 vascular villi ceases upon the entire ovular membrane that is in relation 

 with the decidua reflexa. On the other hand, those villi of the chorion 

 that are in direct contact with the decidua vera become larger and more 

 branched. There thus results the distinction between the chorion 

 laeve and the chorion frondosum. 



The chorion laeve, which has a connective-tissue stroma and is covered by a 

 epithelium, possesses besides at great intervals diminutive villi 



