570 THE. HUMAN EMBRYO. 



SZ, projecting into, and partially blocking up, the aperture be- 

 tween the right and left auricles. This spina vestibuli, accord- 

 ing to Professor His, plays an important part in the formation 

 of both the interauricular and interventricular septa. 



An additional fold, the septum spurium, SB, projects into the 

 cavity of the right auricle, opposite the upper end of the slit-like 

 opening of the sinus venosus ; it is a transient structure, and 

 ultimately disappears completely. 



Of the two auricles, the right one, RA, is at first (Fig. 244) 

 much the larger. The walls of the auricles, like the rest of the 

 heart, consist of two layers, muscular and endothelial ; these 

 are at first some distance apart, but about the twenty-third day 

 they come in contact, and unite firmly to form the definite 

 auricular wall. The connective-tissue elements of the wall are 

 derived apparently from the gelatinous matter which originally 

 separates the muscular and endothelial walls from each other. 



The interauricular septum is formed, according to His, by 

 coalescence of the septum superius (Fig. 244, SK) with the spina 

 vestibuli, SZ ; the latter growing downwards towards the ven- 

 tricle as a thickened plug, which before the end of the fifth 

 week divides the originally single auriculo-ventricular aperture 

 into separate right and left openings. 



It is not quite clear whether the foramen ovale in the human 

 embryo is merely due to the interauricular septum remaining 

 incomplete dorsally ; or whether it is a new aperture formed in 

 the dorsal part of the septum, as described by Born in the case 

 of the rabbit. 



The canalis auricularis, at the beginning of the fourth week 

 (Fig. 243), is a short, straight, and rather narrow tube, connect- 

 ing the auricular and ventricular portions of the heart. In the 

 latter part of the fourth week this portion of the heart shortens 

 somewhat, the auricular and ventricular portions approach each 

 other, and the canalis auricularis becomes telescoped within 

 them (Fig. 244, RK), projecting partly into the auricular and 

 partly into the ventricular cavity, and being no longer visible 

 from the surface except as a sharply marked anmilar constric- 

 tion. 



The lumen of the canalis auricularis becomes at the same 

 time reduced to a narrow transverse slit, its dorsal and ventral 

 walls thickening to form a pair of endothelial cushions, which 



