26o Vertebrate Embryology 



slight, but they produce remarkable changes in 

 the course of the circulation, and convert the 

 embryonic circulation just described into that 

 of the adult chick. 



The ductus Botalli or ductus arteriosus, it 

 will be remembered, is the part of the fifth 

 aortic arch between the dorsal aorta and the 

 point of origin of the vessel that runs to the 

 lung (Fig. 76, A^'). So long as this vessel re- 

 mains open, the blood from the right ventri- 

 cle can pass into the dorsal aorta and thence 

 to the hinder part of the body. At the time 

 of hatching, the ductus Botalli, on each side 

 of the body, closes up entirely, so that the 

 blood from the right ventricle must now pass 

 through the pulmonary arteries to the lungs, 

 and thence, by the pulmonary veins, back to 

 the left auricle. 



The pulmonary circulation is now estab- 

 lished ; and the allantoic circulation, being no 

 longer necessary nor possible, ceases, and the 

 allantoic veins and arteries disappear. The 

 yolk being entirely absorbed or withdrawn into 

 the now completed digestive tract there is no 

 further use for the vitelline veins and arteries, 

 and they also disappear. By the disappear- 

 ance of the allantoic and vitelline vessels, the 



