DEVELOPMENT OF THE BASIC FEATURES OF THE ARTERIOVENOUS SYSTEM 727 



to the forming pharyngeal area of the gut tube (figs. 234; 236D, E; 332F-M). 

 This encirclement of the primitive blood capillaries by the splanchnic layers 

 of the hypomeric mesoderm produces the rudimentary tubular heart, com- 

 posed within of two fused subintestinal capillaries and without of modified 

 fused portions of the hypomeric mesoderm. The modified portions of the 

 hypomeric mesoderm form the epimyocardial rudiment of the heart, while the 

 fused capillaries within establish the rudimentary endocardium (fig. 332F-M). 



Proceeding anteriad from the area of primitive tubular heart, the blood 

 capillaries establish the primitive ventral aortae (fig. 332A). 



From the primitive ventral aortae, the two capillaries move forward toward 

 the anterior end of the foregut where they diverge and pass dorsally, one on 

 either side of the foregut, as the first or mandibular pair of aortal arches. In 

 the dorsal area of the foregut the two primitive aortal arches pass inward 

 toward the median plane and each aortal arch joins a primitive capillary 

 which runs antero-posteriorly along the upper aspect of the developing gut 

 tube. These two supraintestinal blood vessels are the rudiments of the future 

 dorsal aorta and they are known as the dorsal aortae. They lie above the 

 primitive gut and below the notochord. In the region where the mandibular 

 pair of aortal arches joins the dorsal aortae, each primitive dorsal aorta sends 

 a capillary sprout toward the developing eye region and the brain. This 

 capillary forms the rudiment of the anterior end of the internal carotid artery. 

 About the midregion of the developing midgut, each of the dorsal aortae 

 sends off a lateral branch which connects with a series of capillaries in the 

 yolk or yolk-sac area of the deveoping midgut. The vessels which diverge 

 from the dorsal aorta to the yolk-sac region form the rudiments of the two 

 vitelline arteries. The capillary network in the yolk region or yolk-sac area 

 of the midgut in turn connect with the two subintestinal capillaries, previously 

 mentioned, which enter the forming heart. The two latter blood vessels con- 

 stitute the vitelline veins (fig. 332B). Meanwhile, successive pairs of aortal 

 arches are formed posterior to the first pair, connecting the ventral aortae 

 with the dorsal aortae (fig. 332D). These aortal arches pass through the 

 substance of the visceral arches, as mentioned in Chapters 14 and 15. 



3. Formation of the Primitive Blood Vessels Associated 

 WITH THE Mesodermal and Neural Areas 



The system of blood vessels described above (fig. 232A) is developed 

 in relation to the primitive gut tube. Very shortly, however, another system 

 of vessels is established dorso-laterally to the mesodermal tubes. This second 

 system of blood capillaries forms the beginning of the cardinal system (fig. 

 232B). The cardinal system is composed of two anterior cardinal veins 

 which begin as a series of small capillaries on either side over the forming 

 brain; from whence these veins proceed backward, one on either side over 

 the branchial mesoderm, and lateral to the forming somites. These vessels 



