420 



METAZOAN PHYLA 



By the development of the amniotic cavity and the extra-embryonic 

 coelom a considerable space is produced between the body of the embryo 

 and the placenta, the two being connected only by a body stalk, which 

 represents the original connection between the trophoderm and the 

 inner cell mass. In some cases the allantois grows into this body stalk 

 and assists in the formation of the placenta. Such a placenta, accord- 

 ingly, is termed an allantoic placenta. If the allantois remains rudimen- 

 tary and does not enter the placenta, the latter is called a chorionic 



Placental 



Chorion 



Amniotic 

 cavity 



Fig. 288. — Diagram of the embryonic membranes and circulation of a mammal. Stage 

 between I and K in Fig. 287. For comparison with Fig. 257. Arteries crosslined, veins 

 black. Arrows show direction of blood flow. {From Wilder, "History of the Human 

 Body," by the courtesy of Henry Holt & Company.) 



placenta; in this case the mesodermal layer of the chorion becomes 

 much thickened and very vascular. Whether the placenta is allantoic 

 or chorionic the blood vessels of the mother come into intimate contact 

 with those of the embryo, though the two sets of vessels never actually 

 communicate. As a result of this condition a free interchange of sub- 

 stances in solution occurs. From the maternal blood oxygen and food 

 are passed into the fetal circulation, while carbon dioxide and waste 

 substances pass in the opposite direction. In the case of the human 

 embryo and of other forms the amniotic cavity becomes very large, the 

 extra-embryonic coelom becoming correspondingly reduced, or even 

 eliminated, and the embryo comes to lie suspended in the amniotic cavity 

 by the body stalk, which is called the umbilical cord. This amniotic 



