THE MUSCULAR SYSTEM. 143 



these direct communications enlarge, so that an increasing 

 amount of blood takes the direct short passage, and reaches the 

 aorta without having passed through the gills. Additional 

 work is thus thrown on the lungs and skin, which consequently 

 receive a larger supply of blood : the gills rapidly atrophy, 

 though remnants of them usually persist, in a functionless con- 

 dition, until the end of the first year ; and the change from the 

 gill-breathing to the air-breathing condition is completed. 



The further changes necessary to convert the circulation into 

 that of the adult are slight. Of the four aortic arches present 

 at the metamorphosis, Fig. 34, the first, in the first branchial 

 arch, persists as the carotid arch of the adult frog : the lingual 

 artery is a branch from the ventral end of the eflerent vessel of 

 the arch, and is present from an early stage of development 

 (Fig. 34) : and the external and internal carotid arteries, are 

 already present. The carotid gland, CG, is not, as sometimes 

 stated, a persistent portion of a gill, but is formed by further 

 ■elaboration of the direct communication between the afferent 

 and efferent branchial vessels of the first branchial arch. 



The second aortic arch, in the second branchial arch, becomes 

 the systemic arch of the frog. Its dorsal end remains connected 

 with the carotid arch, though the connection may in the adult 

 become closed and ligamentous {Cf. Fig. 5, p. 27). 



The third aortic arch, in the third branchial arch, loses its 

 connection with the aorta, and finally disappears altogether. 



The fourth aortic arch, in the fourth branchial arch, also 

 loses its connection with the aorta, but persists as the pulmo- 

 outaneous arch of the adult, from which both pulmonary and 

 cutaneous arteries arise. 



L. Development of the Muscular System and the Coelom. 



The splitting of the mesoblast into outer or somatopleuric, 

 and inner or splanchnopleuric layers has already been described. 

 {Cf. Fig. 27, p. 124). 



In the body the mesoblast becomes very early divided on 

 each side into (1) a vertebral plate, which is more dorsally 

 situated, and lies alongside the spinal cord and notochord; and 

 (2) a lateral plate, which surrounds the side of the body. 



The vertebral plate very early becomes divided transversely 

 into muscle-segments or myotomes, which form a row of hollow 

 •and somewhat cubical bodies, lying along each side of the 



