THE HEAET AND AETEEIES. SOS 



Up to about the twelfth day the interanricular 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 valv e is formed as a 

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

 posterior vena cava and the right anterior vena c ava. 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 

 bloc ked up by a me mbranous, valve-like fol d ; 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 trabeculee, from which, by 

 further thickening, the columnEe earner and musculi papil]ares 

 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 tru ncu s arterio sus divides rig ht 

 an d left, as in the frog, i nto tlie aortic ar ches, whi ch run round 

 the sides nf _the pha.ryny tft its dorsal surfa ce ; here they open 

 into the dorsal aortse, by wh ich the_blo od is carried all ov er the 

 body of the embryo, as well as to th e Yolk=sac- a j id t b £ allant ois 

 (c/. 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), 



