100 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



transverse section of the body-stalk shows within it the same 

 elementary structures as are seen in a transverse section of the 

 embryo. The cord must be regarded as a real part of the 

 embryo. The umbilicus, which marks the point of attachment of 

 the cord, is situated in the adult on the ventral surface, but 

 before it was thrust into this position by the development of the 

 caudal and perineal regions of the body, it represented the 

 posterior termination of the embryo (Figs. 74 and 75). The 

 amniotic somatopleure rises from the sides of the body-stalk and 

 encloses it just as it rises from, and encloses, the embryo (Fig. 72), 

 and, as is shown in the next paragraph, the lateral folds of the 

 somatopleure unite in the ventral line of the body-stalk as in 

 the ventral line of the belly (Fig. 78). 



mmon 



cauity of amnion 



( continuation of 

 medullary yrooue 



umb. veins 



-somatopleure 



umb. artery 



coelom 



•cauity of allantois 



Vitelline duct. 



Fig. 7S.— Diagrammatic section showing the structures which go to form the 

 Umbilical Cord. (After His.) 



Transverse Section of the Umbilical Cord (Fig. 78). A 



section of the cord shows : 



(1) Two umbilical arteries (continuations of the primitive 

 dorsal aortae). 



(2) One umbilical vein, formed by the fusion of the two 

 original veins. 



(3) The cavity of the allantois formed from the hind gut. 

 Within the cord its lumen becomes obliterated early. 



(4) The vitelline duct, the stalk of the yolk sac, communica- 

 ting with the intestine and yolk sac. It becomes obliterated in 

 the 3rd month. 



