THE UMBILICAL CORD. 601 



between the embryo and the mother at as early a period afl 

 possible. 



In the later stages of development, the yolk-stalk, or pedicle 

 of the yolk-sac (Fig. 254, ?/), becomes closely applied to the 

 allantoic stalk, and bound up with it in a sheath formed by the 

 spreading amnion, am ; and it is to the compound structure 

 formed of these elements that the name umbilical cord is given. 



The umbilical cord increases considerably in length during 

 development. About the middle of gestation it is usually from 

 13 to 21 cm. long, and from 9 to 11 mm. thick. At the time of 

 birth, its average length is from 48 to 60 cm., and its thickness 

 11 to 13 mm. ; but it is liable to very great individual varia- 

 tions. It may be as short as 12 cm.; or, on the other hand, 

 may attain a length of 167 cm. 



The umbilical cord is almost invariably twisted spirally on 

 itself, and the cause of this twisting, which commences about 

 the middle of the second month, has been the subject of much 

 discussion. If examined more closely, it is found that all the 

 constituents of the cord are not twisted to the same extent ; the 

 spirals described by the allantoic arteries being always more 

 numerous, and closer together, than those of the whole cord, or 

 than those of the veins round which the arteries appear to twist. 

 The twisting appears to be due to the allantoic arteries increas- 

 ing in length more rapidly than the other constituents of the 

 cord, and so being compelled to adopt a tortuous instead of a 

 straight course. The allantoic arteries may describe as many as 

 thirty or forty complete turns in passing from the foetus to the 

 placenta. 



As the spiral growth involves the whole umbilical cord, and 

 this cord is fixed at its placental end, it is clear that, as the cord 

 twists, the embryo must rotate in the liquor amnii. The cord 

 may become twisted round the neck of the foetus, and in;iy 

 even be tied in knots : these knots being produced by the 

 cord, at an early stage of development, becoming thrown into a 

 loop, and the embryo then floating through the loop. 



Structure of the umbilical cord, The fully developed umbilical 

 cord consists of the following structures (Fig. 254) : 



1. The sheath formed around it by the amnion. This invests 

 the cord very closely, except at its insertion into the placenta. 



