THE UMBILICAL COED. 601 



between the embryo and tlie motber at as early a period as 

 possible. 



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

 of tbe yolk-sac (Pig. 254, y'), becomes closely applied to tbe 

 allantoic stalk, and bound/ up witb it in a sbeatb formed by tbe 

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

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



The umbilical cord increases considerably in length during 

 development. About tbe 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 band, 

 may attain a length of 167 cm. 



Tbe umbilical cord is almost invariably twisted spirdly on 

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

 the middle of tbe 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 ; tbe 

 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 tbe arteries appear to twist. 

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

 ing in length more rapidly than tbe 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, tbe embryo must rotate in the liquor amnii. Tbe cord 

 may become twisted round the neck of the foetus, and may 

 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. 



