308 Vertebrate Embryology 



passage of blood from the mother to the foetus, 

 as is sometimes said to be the case. 



As growth proceeds, the body-stalk becomes 

 more and more elongated to form the umbilical 

 cord (Fig. 105). At birth the human unbilical 

 cord is usually about 50 cm. long and 10 to 12 

 mm. in diameter. It has a smooth, glistening, 

 white surface, and appears to be spirally twisted. 

 The spirals vary from about three to thirty in 

 number, and are usually, though not always, 

 from left to right ; the cause of this spiral twist 

 is not certainly known. Proximally the cord 



An Air 



B 



CDE- 



FIG, 100. SECTIONS OF TWO HUMAN UMBILICAL CORDS. 

 Minot.) 



(From 



A, from an embryo of 21 mm. B, from an embryo of sixty-four to sixty-nine 

 days. ^//, allantois. Ar, umbilical artery. Coe, coelom. V, umbilical vein, 

 V, yolk-stalk. 



