DEVELOPMENT OF THE FROG. 



553 



the blood is driven by afferent branchials to the gills, from 

 these it collects in efferent vessels which unite on each side 

 to form the two aortae. The aortae send arteries to the head, 

 and passing backwards unite to form the single dorsal aorta 

 which supplies the body. For a time there are two dorsal 

 aortae. When the external gills are replaced by the internal, 

 a new set of gill capillaries are developed, but otherwise the 

 circulation remains the same. As in Ceratodus, a pul- 

 monary artery arises from the fourth efferent branchial. At 

 the time when the hind legs begin to be developed, a direct 

 communication is established between afferent and efferent 

 branchial vessels, so that blood can pass from the heart to 

 the dorsal aorta without going through the gills. As the 

 pulmonary circulation becomes increasingly important, the 

 single auricle of the heart becomes divided into two by a 

 septum, and the pulmonary veins are established. At the 

 time of the metamorphosis an increasing quantity of blood 

 avoids the gills in the manner indicated above, and these, 

 being thrown out of connection with the rest of the body, soon 

 atrophy, while the lungs become the important respiratory 

 organs. The fate of the various branchial arteries may be 

 gathered from the table which follows. 



Before, however, all these internal changes have taken 

 place, the external form undergoes a striking metamorphosis. 



