THE PLACENTA 



425 



the early months of pregnancy, is gradually obliterated by the in- 

 creasing growth of the fetus, and is finally replaced, its functions 

 being progressively usurped by the newly formed placental tis- 

 sues ; 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 preg- 

 nancy, the fetal chorion is intimate in relation. 



The mucosa of the cervix uteri meanwhile becomes greatly 

 hypertrophied 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 pronounced. 



THE PLACENTA 



The human placenta at full term is formed partly by fetal and 

 partly by maternal tissues, the former of which may be said to be 

 implanted in the superficial layers of the latter. Hence on the 

 fetal side the organ is limited by the fetal membranes, amnion 





FIG. 343. DIAGRAM SHOWING THE STRUCTURE OF THE HUMAN PLACENTA AS SEEN IN 



TRANSECTION. 



The fetal blood vessels and maternal arteries are black ; the maternal veins and inter. 

 villous blood spaces are white, a, amnion; 6, chorion; c, chorionic villi; d, decidua; 

 e, glandular layer of the uterine mucosa ; /, muscular wall of the uterus. 



