PRIMITIVE BLOOD VASCULAR SYSTEM 



275 



growth of the dorsal and ventral walls of the aorta. The mesenteric arteries are displaced 

 caudad only three segments, probably in the same way. 



The Umbilical and Iliac Arteries. — As previously described, the umbilical 

 arteries arise in young human embryos of 2 to 2.5 mm. from the primitive aortae 

 opposite the fourth cervical segment. They take origin from a plexus of ventral 

 vessels of the vitelline series (Fig. 263), and are gradually shifted caudalward until 

 they arise from the dorsal aorta opposite the twenty-third segment (fourth lum- 

 bar). In s mm. embryos the umbilical arteries develop secondary lateral con- 

 nections with the aorta (Fig. 270, A). The new vessels pass lateral to the mes- 



78* SeqmeniaJ arkry 

 Cbeliac artery 



10 th Dorsal segmental artery 

 V- pancreas I „ , , 



1 r Dorsal aorta 



Coeliae axis 



YolH-stalK 

 Vitelline artery 



Mesonephrtc arteries 



Dorsal aorta 



/?. Umbilical artery 

 Cloaca- 



Common iliac artery 



Vitellhi artery 



A ■ B 



Fig. 270. — Reconstructions showing the development of the umbilical and iliac arteries: A, 5 mm. 

 embryo; B, 9 mm. embryo (after Tandler). 



onephric ducts, and in 9 mm. embryos the primitive ventral umbilical artery has 

 disappeared. From the newly formed vessel an artery arises which becomes the 

 external iliac artery of the adult. The new lateral umbilical trunk from the aorta 

 to the origin of the external iliac now becomes the common iliac artery, and shifts 

 its position to the ventral side of the aorta. The remainder of the umbilical 

 trunk constitutes the hypogastric artery. 



Arteries of the Extremities. — It is assumed that in man, as in mammals, the 

 first vessels of the limb buds form a capillary plexus. 



Upper Extremity. — The capillary plexus takes its 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 intersegmental artery, forming the ventral branch of this 

 artery and its lateral ramus. The portion of this vessel in what will become the free arm is 



