698 



TEXT-BOOK OF PHYSIOLOGY. 



Placenta Formation. The chorionic villi increase rapidly in size and 

 number and usually surround the whole fetal sac, giving it a peculiar shaggy 

 appearance. Blood-vessels now proceed from the embryo along the belly- 

 stalk (not the allantois in higher forms as formerly stated) . These, the 

 umbilical arteries and veins, pass to the chorionic villi and send branches to 

 those of the placental area; these vascularized villi constitute the chorion 

 frondosum, while the avascular villi form the chorion leve. The vitli of the 

 latter disappear during the second month, leaving the chorionic membrane 

 smooth. The villi of the chorion frondosum now penetrate the uterine glands 

 of the decidua basilaris, which by this time have been denuded of epithelium, 

 and have gained connection with the blood-vessels of themucosa; in this 



rt f Compact 

 layer. 



3 Cavernous/ 

 layer. { 



Muscularis. 





I Chorionic villi. 



Intervillous spaces. 

 Floating villus. 



!> Attached villi. 

 Vein. 



Spiral artery. 

 Gland. 



Vein. 



FIG. 352. DIAGRAM OF HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. (Schaper.) 



manner these uterine glands have become converted into blood sinuses. The 

 chorionic villi either attach themselves to the tunica propria of the mucosa 

 ( fixed villi) or remain free (floating villi). At the edge of the placental area 

 very few villi develop, leaving a circular channel called the marginal sinus. 

 This attachment of villi becomes marked from the third month on and 

 this is considered the beginning of placentation. From this time on to term 

 there is merely an increase in number of villi and vessels with thus a cor- 

 responding increase in the size of the placenta. (Fig. 352.) 



The Placenta. Of all the embryonic structures the placenta is the 

 most important. It is formed by the end of the third month, after which it 

 gradually increases in size up to the end of the eighth month, by which 

 time it is fully developed. It then measures from 1 8 to 24 cm. in diameter 

 and weighs from 400 to 600 grams. It is most frequently attached to the 

 upper and back part of the uterine wall. Though exceedingly complex 

 in structure it consists essentially of two portions, a fetal and a maternal. 



The fetal portion consists primarily of those villi on the chorion in relation 

 with the decidua basilaris. These structures gradually increase in size and 



