DEVELOPMENT OF THE CHICK 139 



and fuse into one, which begins to beat slowly 

 at first and then more rapidly. Into the 

 posterior end of the single tube open two 

 vitelline veins from the vascular area, and 

 from the anterior end leave the two primitive 

 aortce (right and left), which bend upwards 

 along the sides of the fore-gut and then course 

 backwards towards the tail of the embryo. 

 From the aortse arise the two vitelline arteries 

 which carry the blood back to the vascular 

 area. Thus is established a circulation of 

 blood from the yolk-sac to the embryo and 

 from the embryo to the yolk-sac. 



Elongation of the tubular heart, fixed at 

 its two ends by its connection with vessels, 

 causes it to become bent in an S-shaped manner, 

 and so the arterial and venous ends are brought 

 relatively close together. By the growth of 

 septa the cavities of the adult heart are defined, 

 but in the interatrial septum secondary per- 

 forations are formed which result in the pro- 

 duction of a communication between the right 

 and left atria — the foramen ovale — which re- 

 mains patent until hatching has taken place. 

 The septa between the cavities of the heart 

 are completed by the end of the fifth or the 

 beginning of the sixth day. 



