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TEXT-BOOK OF PHYSIOLOGY 



one liter at term; it serves to protect the fetus during gestation and at 

 parturition it dilates the os cervicis, and flushes the birth canal. This liquid 

 is derived mainly from the blood as it contains albumin, sugar, fat and in- 

 organic salts. Traces of urea indicate that some of its constituents are de- 

 rived from the embryo itself. 



The caudal end of the embryonic area is left connected with the chorion 

 by a heavy band of mesoderm termed the belly -stalk, to which the caudal 

 part of the amnion is attached. The entoderm is invaginated into the 

 belly-stalk for a short distance constituting the allantois of higher forms. 

 In oviparous forms the allantois grows out between the closing somatopleuric 

 folds that form the body-wall and constitutes a free sac upon which vessels 

 (aUantoic arteries and veins) develop from the embryo. This sac then 

 spreads beneath the white shell membrane forming the organ of nutrition 

 and respiration of these forms during the last half of their incubation periods. 

 In mammals the extra-embryonic portion of the allantois is of little 

 importance (Fig. 329). 



Placenta Formation. The chorionic villi increase rapidly in size and 

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



Chorionic villi. 



Compact 

 layer. 



Cavernous f 

 layer. \ 



Muscularis 



Intervillous spaces. 



Floating villus. 



Attached villi. 

 Vein. 



Spiral artery! 

 Gland. 



Vein. 



FIG. 330. DIAGRAM OF HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. (Schdper.) 



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 villi 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 the mucosa; in this 

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

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



