DEVELOPMENT OF THE ARTERIES 



267 



The Umbilical and Iliac Arteries. As previously described, the 

 umbilical arteries arise in young embryos of 2 to 2.5 mm. from the primi- 

 tive aortae opposite the fourth cervical segment. They take origin from a 

 plexus of ventral vessels of the vitelline series (Fig. 270), and are gradually 

 shifted caudad until they arise from the dorsal aorta opposite the twenty- 

 third segment (fourth lumbar). In 5 mm. embryos, the umbilical arteries 

 develop secondary lateral connections with the aorta (Fig. 278 A). The 

 new vessels pass lateral to the mesonephric ducts, and, in 7 mm. embryos, 

 the primitive ventral stem-artery has disappeared. The segment of this 



Seventh inter segmental 

 artery 



Ccdiac artery 

 V. pancreas 



Tenth dorsal inter segmental artery 

 Dorsal aorta 



'celiac axis 



, Vitflline artery 

 ', (Superior 

 mesenteric) 



Mesonephric arteries 

 Dorsal aorta 



R. umbilical artery 

 Cloaca 



Common iliac artery 

 A B 



FIG. 278. Reconstructions, showing the development of the umbilical and iliac arteries (after 

 Tandler): A, 5 mm. human embryo; B, 9 mm. human embryo. 



new trunk, proximal to the origin of the external iliac artery which soon 

 arises from it, become.s the common iliac. The remainder of the umbilical 

 trunk constitutes the hypogastric artery. When the placental circulation 

 ceases at birth, the distal portions of the hypogastric arteries, from pelvis 

 to umbilicus, atrophy, forming the solid obliterated hypogastric arteries 

 of adult anatomy. 



The middle sacral artery is the direct caudal continuation of the aorta. 

 Its dorsal position is the result of secondary growth changes. 



Arteries of the Extremities. It is assumed that in man, as in observed birds and mam- 

 mals, the first vessels of the limb buds form a capillary plexus. 



Upper Extremity. The capillary plexus takes origin by several lateral branches from 

 the aorta. In human embryos of 5 mm. but one connecting vessel remains, and this takes 

 its origin secondarily from the seventh dorsal intersegmental artery, forming the ventral 

 ramus of this artery and its lateral offset (Fig. 274) . The portion of this vessel in what will 



