182 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



exception of one feature in the 

 amniotes. In these, as a result 

 of the great size of the yolk, the 

 heart at first appears as a pair of 

 widely separated tubes (Fig. 195), 

 which later approach and then 

 unite to form the single tube, 

 which then undergoes the twist- 

 ing and differentiation already 

 described. 



The ventral aorta extends for- 

 ward from the heart beneath the 

 pharynx. It is a part of the same 

 primitive tube from which the 

 heart arises. From this tube 

 there are given off vessels right 

 and left (aortic arches) which 

 pass outward in the tissue between 

 the gill slits, then up on either 

 side of the pharynx, and at last 

 those of each side unite dorsally 

 to form a vessel (radix gtortae) 

 above the pharynx. Behind the 

 region of the gill slits the radices 

 of the two sides unite to form a 

 tube (dorsal aorta), running back- 

 ward between the notochord (ver- 

 tebral column) and the alimentary 

 canal to the end of the body. 



From these arterial vessels 

 smaller arteries are given off to 

 FIG. 196. Diagram of early supply the various regions of the 



arterial circulation. <z, auricle ; #<?, 



anterior abdominal artery; *, bul- b d y- FrOm the first Or anterior 



bus arteriosus; <r, conus arteriosus; 



ca, caudal artery; ci, common iliac artery; col, cceliac axis; f/, carotids; d, dorsal 



aorta; /, femoral artery; i, iliac artery; im, inferior mesenteric arteries ; J, jugular 



vein; m, metameric (intercostal) arteries; o, omphalomesaraic (hepatic) veins; /, 



postcardinal veins; ra t radix aortae; s, sinus venosus ; sc, subclavian artery; sm, 



superior mesenteric artery ; v y ventricle ; va, ventral aorta ; zV, vertebral artery. 



