VI.] THE HEAET. 169 



the embryo) that part which has already been marked 

 off by the lateral bulgings as the auricular portion (au). 



That part of the heart which is turned to the right, 

 including the point of doubling up, is the ventricular 

 portion, and is even at this stage separated from the 

 auricular portion by a slight neck. This external con- 

 striction corresponds to an internal narrowing of the 

 lumen of the heart, and marks the position of the future 

 canalis auricularis. 



The ventricular portion is, on the other hand, like- 

 wise separated by a fainter constriction from the ante- 

 rior continuation of the heart which forms the bulbus 

 arteriosus. The projecting part where the doubling 

 takes place is at this stage still quite round ; we shall 

 see that later on it becomes pointed and forms the apex 

 of the heart. 



The whole venous portion of the heart (if we may so 

 speak of it, though of course at this stage blood of the 

 same quality passes right along the whole cardiac canal) 

 lies in a plane which is more dorsal than the arterial por- 

 tion. The point at which the venous roots of the heart, 

 i.e. the two vitelline trunks, unite into a single canal, is 

 on this day carried farther and farther away from the 

 heart itself By the end of the day there is a consider- 

 able distance between the auricular portion of the actual 

 heart and the point where the venous roots separate, 

 each to pursue its course along the splanchnopleure-fold 

 of its own side. This distance is traversed by a single 

 venous trunk, of which the portion close to the auricles 

 is called the sinus venosus, and the more distant the 

 ductus venosus. We shall give to the whole trunk the 

 name used by the older observers, the meatus venosus. 



