THE FEMALE EEPEODUCTIVE ORGANS 537 



implanted ovum the decidual tissues close up over the ovum which is 

 thus surrounded by the so-called decidua reflexa or decidua capsularis, 

 which, after the early months of pregnancy, is gradually obliterated by the 

 increasing growth of the fetus, and is finally replaced, its functions being 

 progressively usurped by the newly formed placental tissues; 3, all the 

 remaining portions of the decidual mucosa, those which line the greater 

 part of the uterine cavity, collectively form the decidua vera, with whose 

 surface, in the later months of pregnancy, the fetal chorion is intimate in 

 relation. 



The mucosa of the cervix uteri meanwhile becomes greatly hyper- 



A B c 



FIG. 471. CHORIONIC VILLUS AT VARIOUS STAGES OF DEVELOPMENT. 



A, chorionic villus at third week; B, at fourth month; C, at term. (After Wil- 

 liams.) X 225. 



trophied and its glands much enlarged. This portion of the uterine mucosa 

 does not, however, enter into the formation of the decidua vera; the 

 changes occurring in its tissues, though similar, are much less pro- 

 nounced. 



The Chorionic Villi. These innumerable processes form the greater 

 portion of the placental tissues. They vary in size from the broad main 

 stems to the very slender terminal branches of the floating villi. They 

 consist of a core of mesoderm covered with a variable layer of ectoderm. 

 In the early condition of the placenta (fourth or fifth month of pregnancy) 

 the villi are clothed with a double epithelial layer, of which the superficial 

 takes the form of a syncytium (plasmoditrophoblast), while the deeper 

 consists of a cellular layer, the cells of Langhans (cytotrophoblast). At 

 later periods (seventh month to full term) the syncytium is found to have 

 undergone a peculiar alteration, having become much thinner, and having 

 even completely disappeared from considerable portions of the villi, it being 

 replaced by canalized fibrin; at other points the syncytial cytoplasm is 

 much thickened and the nuclei appear to be bunched or grouped within 

 the thickened portions; these areas are known as cell-knots or proliferation 

 islands. Here and there the degenerated cell-knots have been replaced by 



