242 Vertebrate Embryology 



latter now lie nearly as far forward as the 

 ventricles. There is probably no division of 

 the auricles into two chambers, though the 

 external appearance would indicate such a di- 

 vision. This appearance is due to the great 

 development of the auricular appendages. 1 

 The vascular system. Since, by the end of 



FIG. 75. HEART OF A CHICK ON THE FOURTH 



DAY OF INCUBATION VIEWED FROM THE VENTRAL 



SURFACE. (After Foster and Balfour.) 



/., left auricular appendage. C.A, canalis auricularis. z, 

 ventricle. , bulbus arteriosus. 



this day, the vascular system reaches a state 

 of considerable complexity, and since its fur- 

 ther development is difficult to determine in 

 the laboratory, it will be the best to give, at 

 this place, the entire history of the vascular 

 system, from the fourth day to the establish- 

 ment of the adult circulation. The condition 

 of the vascular system at the close of the 



1 Marshall states that the interauricular septum is formed on this 

 day. 



