THE UMBILICAL CORD. Ill 



embryo is known as the umbilicus. This attachment to the chorion is in the pla- 

 cental region. 



The twisting of the cord is well marked externally at the time of birth by 

 the spiral ridges, within each of which a large blood-vessel runs. The number 

 of spirals varies from 3 to 32, the turns beginning at the embryo, though usually 

 from left to right, but sometimes from right to left. The twisting begins about 

 the middle of the second month. Its cause is unknown, but there is no reason to 

 assume that it is due to revolutions of the embryo. The cord is covered by a 

 layer of epithelium which is continuous at the distal end with the epithelium of 

 the amnion, and at the proximal end with the epidermis of the embryo. It con- 

 tains typically no capillaries, and, except in the immediate neighborhood of the 

 embryo, no nerve-fibers. 



