THE DEVELOPMENT OF THE VASCULAR SYSTEM 



223 



In mammals in general the allantois is a rudimentary structure incapable 

 of receiving the total waste of the embryo. The umbilical (allantoic) 

 vessels develop, however, as in reptiles and birds but become associated 

 through the belly stalk with the placenta which establishes communication 

 between the embryo and the mother (Fig. 201). The vessels within the 

 embryo are at first disposed in the same manner as in the lower forms, 



Int. carotid artery 



Vertebral artery 



Vitelline vein 

 Vitelline artery 



Umbilical vein 



Umbilical 



arteries 



Duct of Cuvier 



Post, cardinal 

 vein 



\ 



'Aorta 



Post, cardinal vein 



FIG. 202. Reconstruction of a human embryo of 7 mm. Mall. 



Arteries represented in black. A.V., Auditory vesicle; B, bronchus;!,, liver; K, anlage of 

 kidney; T, thyreoid gland; III-XII, cranial nerve roots; i, 2, 3, 4, branchial grooves; i, 

 8, 12, 5 (on spinal nerve roots), ist and 8th cervical, i2th dorsal, 5th lumbar spinal nerves 

 respectively. Dotted outlines represent limb buds. 



the umbilical arteries arising from the caudal portion of the aorta and the 

 umbilical veins passing forward in the ventro-lateral body wall to join the 

 ducts of Cuvier. With the formation of the umbilical cord the two umbilical 

 veins within this structure fuse into a single vessel (Fig. 202). The later 

 changes in the umbilical veins are most conveniently considered subsequently. 

 In mammals in general the umbilical (allantoic) circulatory system 

 performs a two-fold function. The blood carries to the placenta the waste 



