THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 571 



are established on the two sides of the fore-gut (permanently in 

 the gill-breathing Fishes, Dipnoi, and a part of the Amphibia, 

 transitorily in the higher Vertebrates), so also there are developed 

 at the corresponding places on the part of the vascular system five 

 pairs of vascular arches* (fig. 316 i-). They take their origin 

 from the truncus arteriosus (figs. 316, 317), which runs forward 

 under the fore-gut, then follow along the visceral arches up to the- 

 dorsal surface of the embryo, and here unite on either side of the 

 vertebral column into longitudinal vessels, the two primitive aortse 

 (fig. 317 ad). On this account they are called aortic arches, but 

 they are more appropriately designated as visceral-arch vessels. 



In the Vertebrates that breathe by 

 means of gills, the vessels of the 

 visceral arches become of importance 

 in the process of respiration, and early 

 lose their simple structure. From 

 their ventral initial portions there 

 arise numerous lateral branches run- 

 ning to the branchial lamellae, which 

 have arisen in large numbers from 

 the mucous membrane investing the 

 visceral arches ; here they are resolved 

 into fine capillary networks. From 



r * Fig. 316. -Diagram of the arrange- 



these the blood IS re-colkcted into ment of the vessels of the visceral 



venous branches, which open into the SSrtlfvIrtlte^?" 1 * f *" 



Xipper end Of the visceral-arch Vessels. 15, First to fifth aortic arches ; ad 



The larger the ventral and dorsal SS2|ESSE 



lateral branches, the more incon- , subciavia ; p, puimonaiis. 



spicuous does the middle part of the 



vessel of the visceral arch become. At length it has separated into- 

 an initial part, the branchial artery, which is distributed to the 

 branchial lamellae in numerous branches, and an upper part, the 

 branchial vein, into which the blood is re-collected. The two are 

 connected with each other by means of the close network only, 

 which, from its superficial position in the mucous membrane, presents 

 a suitable condition for the removal of the gases from the blood. 



Since in the Amniota there are no branchial lamellae produced, 

 branchial arteries and veins also fail to be developed, the vessels of 



* [The existence of site pairs of vascular arches has recently been shown to be 

 the typical condition, the newly discovered pair, situated between the fourth and 

 fifth pairs of RATHKE'S scheme (fig. 316), being of short duration in Amniota.]. 



