THE DEVELOPMENT OF 1HE VASCULAR SYSTEM. 



215 



lose their segmental character and are replaced or supplemented by longi- 

 tudinal vessels. 



In addition to the dorsal segmental branches of the aorta, which have 

 been described, other branches develop which carry blood to the viscera. 

 A number of these, or possibly all, are also primarily segmental vessels, 

 although they lose every trace of their segmental character during develop- 

 ment. The first of the visceral branches to appear is the omphalomesenteric 

 artery which arises from the ventral side of the aorta and which has been 

 mentioned in connection with the vitelline circulation. Originally it passes 



Int. mammary artery 



tof. epigastric artery 



Umbilical artery 



Femoral artery 



FIG. 187. Diagram of human embryo of 13 mm., showing the mode of development 

 of the internal mammary and inferior epigastric arteries. M all. 



out through the mesentery and follows the yolk stalk to ramify on the surface 

 of the yolk sac. But since the yolk sac is of slight importance, the distal 

 part of the artery soon disappears, while the proximal part becomes the 

 superior mesenteric artery (Fig. 188). The cceliac artery arises from the ventral 

 side of the aorta a short distance cranially to the omphalomesenteric (Fig. 

 1 88) and gives rise in turn to the gastric, hepatic and splenic arteries. The 

 inferior mesenteric artery also arises from the ventral side of the aorta some 

 distance caudal to the omphalomesenteric (Fig. 188). In the early stages 

 these visceral arteries arise relatively much farther cranially than in the 



