1HE VASCULAR SYSTEM 



321 



them and the vitelline arteries become reversed; and, as this 

 part of the embryo expands and develops legs, tail, and pelvic 

 organs, these latter become supplied by secondary branches 

 from this main trunk. A similar arterial supply to the head 

 region is furnished by the artery which develops anteriorly 

 from the arterial gill-arch. In the figure it appears as a mere 

 stump, but is destined to become the carotid artery, which in- 



FIG. 89. Early circulation of vertebrate embryo. 



(A) First appearance of definite vessels. (B) Later stage, during the formation 

 of gill arteries. 



AC, carotid artery; A, dorsal aorta; AV , vitelline artery; VV , vitelline vein; H, 

 heart. 



creases in size and the complexity of its branches in exact 

 proportion to the development of the part which it supplies. 

 As these last two vessels, carotid artery and posterior aorta, 

 distribute the blood ouside of the main channel, a new set of 

 vessels must be developed to bring it back again and thus 

 complete the circuit. Those appear in the form of the four 

 cardinal veins, two anterior and two posterior (not shown in 



