DEVELOPMENT OF THE VASCULAR SYSTEM. 



375 



299, abr). From each side of this, from five to seven gill- 

 arteries proceed ; these rise between the gill-openings (s) 

 to the gill-arches, encircle the throat, and combine above 

 into a common aorta-stem, the continuation of which, 

 passing backward above the intestine, corresponds to the 

 dorsal vessel of Worms. As the arched arteries distribute 

 themselves in a respiratory capillary net over the gill- 

 arches, they thus contain venous blood in their lower part 

 (as arterial gill-arches), and arterial blood in their upper 

 part (as aorta-arches). The points at which separate aorta- 

 arches unite, which occur on the right and left sides, are 

 called aorta-roots. Of an originally greater number of 

 aorta-arches, only five pairs are retained, and from these 

 five (Fig. 300), in all higher Vertebrates, the most im- 

 portant parts of the arterial system develop. 



Fig. 299. — Head of an embryonic Fish, with the rudiment and the 

 blood-vessel system ; seen from the left side : do, Cuverian duct (point of 

 union of the front and hind main veins) ; sv, venous sinus (enlarged 

 terminal portion of the Cuverian duct) ; a, auricle ; v, main chamber ; 

 abr, gill-artery stem ; s, gill-openings (between the arterial arches) ; ad, 

 aorta ; c', head-artery (carotis) ; n, nose-groove. (After Gegenbaur.) 



The appearance of the lungs, connected with the respi- 

 ration of air, which first occurs in the Dipneusta, is most 

 important in the further developement of the arterial 



