62 Vertebrate Embryology 



systemic or venous, and pulmonary or arterial cavities. 

 Simultaneously with this, valves are formed in the trun- 

 cus arteriosus, by which the streams of venous and arte- 

 rial blood are kept apart to a certain extent. At the 

 time of the metamorphosis the gill circulation is cut off, 

 by the establishment of direct communications between 

 the afferent and efferent branchial vessels (Fig. 26), and 

 the pulmonary circulation becomes of much greater im- 

 portance than before." ' 



The branchial blood vessels, or aortic arches, 

 are six in number, and lie in the visceral arches, 

 the afferent vessel lying parallel and posterior 

 to the efferent vessel (Fig. 26). Of the 

 branchial vessels, only those lying in the first, 

 second, third, and fourth arches are func- 

 tional, the vessels of the mandibular and hyoid 

 arches being in a rudimentary condition. 



Although the afferent and efferent vessels 

 lie so close together, there is at first, as has 

 been said, no communication between them 

 except through the gill capillaries (Fig. 23) 

 which are given off from their sides, first to 

 the external and then to the internal gills. As 

 the direct communication between the afferent 

 and efferent vessels, which lies near the ven- 

 tral end of the arch (Fig. 26), becomes larger 

 it is evidently easier for the blood to flow 



' Marshall. 



