THE HEART AND ARTERIES. 303 



Up to about the twelfth day the interauricular septum 

 remains very imperfect, and there is free communication between 

 the two auricles through a large aperture, the foramen ovale. 

 About the twelfth day this communication narrows considerably. 



On the sixteenth day the Eustachian valve is formed as a 

 fold projecting into the right auricle, between the openings of the 

 posterior vena cava and the right anterior vena cava. Up to 

 this time the blood from both these vessels has passed from the 

 right auricle, through the foramen ovale, into the left auricle and 

 so to the left ventricle. The effect of the Eustachian valve is to 

 direct the blood from the right anterior vena cava into the 

 right ventricle, while still allowing the blood from the posterior 

 vena cava to pass through the foramen ovale into the left 

 auricle. From this time the two auricles are about equal in 

 size. 



Shortly before hatching, the foramen ovale becomes partially 

 blocked up by a membranous, valve-like fold ; the completion 

 of this stoppage is effected shortly after the time of hatching, 

 from which time the structure of the heart is practically that of 

 the adult bird. 



The thickening of the ventricular wall, which is a marked 

 feature of the later stages of development, is effected by inwardly 

 projecting ridges of the muscular wall, which ultimately form a 

 system of anastomosing muscular trabecule, from which, by 

 further thickening, the columns earner and musculi papillares 

 are developed. 



The thickening of the wall of the auricles is effected in 

 similar fashion, but is not carried to so great an extent. 



The wall of the truncus arteriosus thickens by simple increase 

 in the thickness of the muscular and other layers composing it. 



3. The Arteries. 



a. The Aortic Arches. The truncus arteriosus divides right 

 and left, as in the frog, into the aortic arches, which run round 

 the sides of the pharynx to its dorsal surface ; here they open 

 into the dorsal aorta?, by which the blood is carried all over the 

 body of the embryo, as well as to the yolk-sac and the allantois 

 (cf. Figs. 113 and 128). 



The aortic arches of the chick are developed in order, from 

 before backwards. The first, or most anterior pair (Fig. 127, ai), 



