246 Vertebrate Embryology 



ventral ends of these arches (Fig. 76, A P). 

 The dorsal end of the fifth arch between the 

 point of union of the pulmonary artery and 

 the dorsal aorta (Fig. 76, A^^ is known as 

 the ductus Botalli. During almost the entire 

 period of incubation, the ductus' Botalli re- 

 mains open and offers the blood from the 

 right side of the heart an easy passage into 

 the dorsal aorta, so that little, if any, of it 

 passes through the lung capillaries. At the 

 time of hatching, the ductus Botalli begins to 

 shrivel up, and finally becomes entirely closed, 

 so that all of the blood from the right side of 

 the heart must pass into the pulmonary cir- 

 culation. The lower half of the fifth aortic 

 arch then becomes the pulmonary artery. 



To recapitulate briefly. The dorsal ends of 

 the first and second aortic arches persist as the 

 carotid arteries: the ventral ends of these two 

 arches persist as the lingual arteries. The ven- 

 tral ends of the third arches persist as the sub- 

 clavian arteries. The fourth arch, on the right 

 side, persists as the systemic arch ; the fourth 

 arch, on the left side, disappears. The ventral 

 ends of the fifth arches persist as the pulmon- 

 ary arteries. 



As has been described, the two dorsal aortse, 



