THE AORTIC SYSTEM. 



99 



ing stage in the dog. The veins are much more symmetrical than in the chick, and 

 have the same general plan; the sinus terminalis belongs to the venous system, so 

 that the connection with the arterial circulation, found later, is secondary; the aorta 

 of the embryo is double, and gives off on each side (segmentally arranged?) trans- 

 verse branches, one of which develops into the large trunk shown in figure 54; the 

 network of small vessels forms two layers, of which the upper is connected with 

 the arteries, the lower with the veins. The change from the earlier condition to 

 the later has still to be followed. 



FIG. 54. AREA VASCULOSA OF A RABBI?, PRESUMABLY OF ABOUT TWELVE DAYS. (After Van Beneden and Julin.) 



The arrangement of the main vessels in the area vasculosa at a later stage 

 in the rabbit is quite different. The sinus terminalis forms a complete ring (Fig. 

 54), and is connected with the arterial system by a single trunk, which corresponds 

 to the left omphalic artery of the bird. For some time the connection between the 

 embryonic arteries and the area vasculosa is entirely through capillaries, and the 

 arterial trunk on the vascular area does not appear in the rabbit for several days. 

 There are two veins, one arising from each side of the body and passing out on 

 to the area vasculosa over the back of the embryo; they are the two large upper 

 vessels in the figure. 



The Aortic System. 



In early stages the aortic end of the heart terminates under the ventral floor 

 of the pharynx. The endothelial heart continues as the aorta, which almost at 



