FCETAL MEMBRANES. 



597 



connects the embryo with the chorion, and incorporated with it to form the 

 umbilical cord (Figs. 81 and 82). 



The umbilical cord thus consists of : (Fig. 503) : 



1. Amnion. This is attached to the embryo at the navel. It is at first 

 loosely connected with the underlying tissue of the cord so that it is easily 

 peeled off; later it becomes firmly adherent. The epithelium of the amniotic 

 covering of the cord is stratified and is described by some (Minot, McMurrich) 

 as of embryonic ectodermic origin instead of as part of the amnion. 



2. What may be called the ground substance or substantia propria of the 

 cord. This is an embryonic connective tissue often described as "mucous 



Umbilical vein 



Amnion 



Umbilical Allantoic 

 arteries stalk 



FlG. 503. Transverse section of umbilical cord of a pig embryo six inches in length. Photograph. 



tissue." It consists of a soft gelatinous intercellular substance and irregular, 

 branching stellate cells. On account of its consistency it has been called 

 "Wharton's jelly." 



3. Three umbilical vessels two arteries and one vein. All these vessels 

 are thick walled and the developing smooth muscle is in bundles separated by 

 considerable connective tissue. The two umbilical arteries carry venous blood 

 from the foetus to the placenta where their branches ultimately give rise to the 

 capillaries of the chorionic villi. From the villi the blood enters the terminals 

 of the umbilical vein and returns as arterial blood to the foetus (Fig. 504). 



As they traverse the cord the arteries make a number of spiral turns around 

 the vein and give to the cord the appearance of being spirally twisted. The 



