The Development of the Frog 67 



from the bulbus arteriosus directly around the 

 pharynx, through the four aortic arches, into 

 the dorsal aorta. Previous to this time a 

 branch has grown out from the fourth aortic 

 arch to the lung, the pulmonary artery (Fig. 



26, AP), and as the gills diminish more and 

 more in size, this vessel becomes larger and 

 larger until it carries all of the blood that 

 formerly went to the gills for purification. 

 From the lungs the blood is brought back di- 

 rectly to the heart by the pulmonary veins. 



The first aortic arch, on the completion of 

 metamorphosis, becomes the carotid arch of 

 the frog, which carries blood to the head (Fig. 



27, i). That portion of the dorsal aorta be- 

 tween the openings of the first and second 

 aortic arches may remain open, but more com- 

 monly becomes entirely obliterated. 



The second aortic arch of the tadpole be- 

 comes the systemic arch of the adult frog 

 (Fig. 27, 2). 



The third aortic arch gradually diminishes 

 in size, and eventually entirely disappears. 



The fourth aortic arch of the tadpole be- 

 comes the pulmo-cutaneous arch of the frog, 

 carrying blood to the lungs and skin, as the 

 name would indicate (Figs. 26 and 27, 3). 



