GENERAL EMBRYOLOGY , 163 1 



amongst other structures, it contains the bloodvessels which convey 

 (i) the impure blood from the body of the foetus to the placenta for 

 purification, and {2) the blood, when purified in the placenta, from 

 that organ back into the foetal body. Functionally it replaces the 

 allantoic stalk. It extends, as a ' cord,' between the umbilical 

 orifice and the foetal part of the placenta. In the later stages of 

 pregnancy its average length is about 22 inches. This length is only 

 attained after expansion of the amniotic sac, and it permits of 

 ample freedom of movement on the part of the foetus in the amniotic 

 fluid. Under these circumstances the placenta is not subjected to 

 any influence of a dragging nature. The average length, however, 

 may be exceeded, or it may be very much curtailed. The cord 

 presents a spiral appearance, principally due to the winding of the 

 two umbilical arteries around the umbilical vein. 

 The constituents of the imibilical cord are as follows: 



1. Two umbilical arteries. 



2. One umbilical vein. 



3. A remnant of the allantoic stalk. 



4. A remnant ol the yolk-sac. 



5. A remnant of the vitelline duct. 



6. Mesoderm and Wharton's jelly. 



7. An ectodermic envelope. 



As stated in connection with the allantois, the allantoic stalk 

 consists of mesoderm, and contains two allantoic arteries and two 

 allantoic veins, which bloodvessels it serves to conduct tc and 

 from the placental area of the chorion (chorion frondosum). The 

 two umbiUcal arteries replace the allantoic arteries ; and the single 

 umbilical vein of the umbilical cord represents the two allantoic 

 veins. 



The nmbilical arteries, within the abdomen of the foetus, are 

 known as the hypogastric arteries, and the proximal li inches of 

 these vessels represent the permanent internal iliac arteries. The 

 remainder of each vessel, for the most part, constitutes the perma- 

 nent obliterated hypogastric artery. The h\T50gastric and umbilical 

 arteries carry impure blood from the foetus to the placenta. The 

 umbilical vein undergoes no change in name on entering the abdomen 

 of the foetus. It conveys pure blood from the placenta to the 

 foetus, and, after entering the abdomen, it passes to the umbilical 

 fissure of the liver. As regards its subsequent history, it is repre- 

 sented by (i) the round ligament of the liver, and (2) the ligamentum 

 ductus venosi. 



The remnant ol the allantoic stalk blends with the chorion fron- 

 dosum. In rare cases the lumen of the allantoic diverticulum may 

 persist for some time, thus giving rise to the condition which is 

 known as an umbilical urinary fistula. 



The yolk-sac and vitelline duct are present as mere remnants. 



Mesoderm forms a large part of the umbilical cord. It supports 

 and connects the various constituent elements, and consists of 

 stellate cells, so arranged as to form a reticulum. The meshes of 



