430 OUTLINES OF CHORDATE DEVELOPMENT 



ated, the amnion and chorion being brought into contact with 

 one another over their mesodermal surfaces, and finally they 

 may fuse together. The amnionic cavity is filled with a fluid 

 known as the liquor amnii, now thought to be formed by the 

 amnionic epithelium. It contains solids, mostly albumins, 

 grape sugar, and urea, to the extent of about 1 per cent. The 



FIG. 179. Diagrams illustrating the formation of the umbilical cord and the 

 relations of the allantois and yolk-sac in the human embryo. From McMurrich 

 (Development of the Human Body). The heavy black line represents the 

 embryonic ectoderm; the dotted line marks the line of the transition of the body 

 (embryonic) ectoderm into that of the amnion. Stippled areas, mesoderm. Ac, 

 Amnionic cavity; Al, allantois; Be, exocoelom; Bs, body stalk; Ch, chorion; 

 P, placenta; Uc, umbilical cord; V, chorionic (trophodermic) villi; Ys, yolk-sac. 



amount of the liquor amnii in man varies considerably, but usu- 

 ally between 0.5 and 1.0 liter at the period of its maximum 

 amount, which is some time before parturition. At parturition, 

 of course, both the amnion and chorion normally are ruptured 

 and the amnionic fluid escapes with or before the foetus. 



The more important aspects of the chorion are those con- 



