UMBILICAL CORD 381 



The umbilical arteries arise in young embryos as the main terminal branches into 

 which the dorsal aorta bifurcates. These vessels curve ventrally on either side of 

 the pelvis and pass out through the cord to the chorion; they are equidistant from the 

 allantois which they accompany. In the adult the parts of these vessels near the 

 aorta are known as the common iliac arteries, and the small offshoots from them which 



FIG. 382. CROSS SECTION OF UMBILICAL CORDS. 



A, from an embryo of two months, X 20; B, at birth, X 3. aL, Allantois; art., artery; coe., coelom; ?.. 



vein; y. s. t yolk stalk. 



have grown down the limbs, have become the external iliac arteries. The distal course 

 of the original vessels may still be followed through the hypogastric arteries (internal 

 iliacs) up on either side of the median line to the navel; toward the navel the vessels 

 have become reduced to slender cords. The umbilical vein, within the cord, represents 

 the fusion of a pair. On entering the body it conveys the blood from the placenta, 

 through the persistent left umbilical vein, di- 

 rectly to the under side of the liver, which it 

 crosses as the ductus venosus, and then empties 

 into the vena cava inferior. In the adult, its 

 former course is marked by the round ligament of 

 the liver and the ligament of the ductus venosus. 



The allantois, which the umbilical ves- 

 sels accompany, at first extends the entire 

 length of the cord as a slender epithelial 

 tube. Its condition at three months is 

 shown in Fig. 383. At birth, it has be- 

 come reduced to a very slender, and gen- 

 erally interrupted, solid strand of epithe- 

 lial cells. That it may retain its con- 

 tinuity is stated by Ahlfeld (Arch. f. 

 Gynak., 1876, vol. 10, p. 81). This remnant may be sought for near the 

 body of the embryo, and its tendency to retain its original position equi- 

 distant from the umbilical arteries is the best guide for locating it. By 

 the use of Mallory's connective tissue stain, the epithelial cells may be 



FIG. 383. CROSS SECTION OF THE AL- 

 LANTOIC DUCT, FROM THE UMBILICAL 

 CORD OF A HUMAN EMBRYO OF THREE 

 MONTHS. X34O. (Minot.) 



Ent., Entodermal epithelium; mes., mes- 

 enchyma. 



