Development of the Third Day 201 



and 76, VU)] this transverse vein is called the 

 ductus Cuvieri, or Cuvierian vein. 



In order to understand fully the evolution 

 of the complex avian circulation from the 

 simple and fish-like circulation of the embry- 

 onic chick, it is important that each stage in 

 the development should be clearly understood. 

 A brief description of the course of the circu- 



FiG. 66. — Diagram of the venous cikcula- 

 TION OF THE THIRD DAY. (After Foster and Bal- 

 four.) 



//, heart. _/, jugular or anterior cardinal vein f, inferior 

 or posterior cardinal vein. O/^ vitelline vein. dc, ductus 

 Cuvieri. 



lation of the blood at this period will, there- 

 fore, be given. 



The blood, on entering the heart, is forced, 

 by the contraction of its walls, through this 

 much-twisted but, as yet, undivided tube, to 

 the bulbus arteriosus ; from the bulbus it 

 passes dorsalward, around each side of the 

 pharynx, through the three pairs of aortic 

 arches to the two dorsal aortse which lie 



