240 HUMAN PHYSIOLOGY. 



and some mammals the amnion is derived from the somatopleure in the 

 folding off of the body. In amniotes the amniotic cavity is at first small, but 

 rapidly increases in size. It contains a clear fluid, the amniotic fluid, which 

 amounts at term to about one quart. It serves to protect the fetus during 

 gestation, and at parturition it dilates the os cervis and flushes the birth 

 canal. This liquid is derived mainly from the blood as it contains albumin, 

 sugar, fat and inorganic salts. Traces of urea indicate that some of its 

 constituents are derived 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; the allantois 

 grows out between the closing somatopleure folds forming the body wall 

 and constitutes a free sack upon which vessels, allantoic arteries and veins, 

 develop from the embryo. This sack then spreads beneath the white shell 

 membrane forming the organ for 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. 



The Formation of the Placenta. The chorionic villi increase rapidly 

 in size and number and usually surround the whole fetal sack 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) . 

 There 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 leva. 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 (fixed villi) or remain free, floating villi. At the 

 edge of the plancental area very few villi develop leaving a circular channel 

 called the marginal sinus. This attachment of the villi becomes marked 

 from the third month on and is considered the beginning of placentation. 

 From this time on to full term there is merely an increase in number 

 of villi and vessels and thus an increase in the size of the placenta. 



The placenta is the most important of the fetal structures. As it develops, 

 conditions are established which permit of a free exchange of material be- 

 tween mother and child. Whether by osmosis or by an act of secretion, the 



