694 MANUAL OF PHYSIOLOGY. 



formed into blood corpuscles, and thus it is hollowed out. A 

 layer of endothelial cells lines the cavity, and becomes the endo- 

 cardium. 



The primitive heart is connected at its upper end with the two 

 aortse, and at its lower end with the omphalo-mesenteric veins. 



After a time the tube shows signs of division into three parts ; 

 the upper part becomes the aortic bulb, next to which is formed 

 the cavity of the ventricle, continuous with which is the auricular 

 space. The tube also, which at first lies in a straight line, now 

 becomes twisted on itself, the auricular part becoming posterior 



FIG. 282. 



Transverse section through the region of the heart of a rabbit's embryo of nine days 

 old. (KOlliker.) jj. Jugular veins, ao. Aorta, ph. Fore gut. bl. Blastoderm, hp. 

 Body wall reflected in ect. ent. Hypoblast. e'. Prolongation of hypoblast between the 

 two halves of the heart, ah. Outer wall of the heart, p. Cavity of the pericardium. 

 ih. Inner lining of the heart, ect. Epiblast. df. Visceral mesoblast. 



and superior, whilst the ventricle, with the aortic bulb, remains 

 anterior and somewhat below. 



Each primitive cavity of the heart is divided into two by the 

 gradual growth of partitions, and thus the four permanent heart 

 cavities are developed. 



Externally, a notch shows the division of the ventricle into 

 right and left cavities, whilst from the inside of the right wall 

 there grows a projection which subdivides the ventricle internally. 

 This septum is, however, not at once complete at its upper part, a 





