80 THE SECOND DAY. [CHAP. 



uniting- above the alimentar_y canal in the back of the embryo 

 to form the single dorsal aorta, which travelling backwards in 

 the median line divides near the tail into two main branches. 

 From each of tlie two primitive aorta^, or from each of the 

 two branches into which the single aorta divides, there is 

 given off on either side a large branch. These have been 

 already spoken of as the omphalo-mesaraic arteries. At 

 this stage they are so large that by far the greater part of 

 the blood passing down the aorta finds its way into them, 

 and a small remnant only pursues a straight course into the 

 continuations of the aorta towards the tail 



Each omphalo-njesaraic artery leaving the aorta at nearly 

 right angles (at a point some little way behind the backward 

 limit of the splanchnopleure fold which is forming the ali- 

 mentary canal), runs outwards beneath the protovertebraj in 

 the lower range of the mesoblast, close to the hypoblast. 

 Consequently, when in its course outwards it reaches the 

 point where the mesoblast is cleft to form the somatopleure 

 and splanchnopleure, it attaches itself to the latter. Travel- 

 ling along this, and dividing rapidly into branches, it reaches 

 the vascular area in whose network of small vessels (and also 

 to a certain extent in the similar small vessels of the pellucid 

 area) it finally loses itself. 



The terminations of the omphalo-mesaraic arteries in the 

 vascular and pellucid areas are further connected with the 

 heart in two different ways. From the network of capillaries, 

 as we may call them, a number of veins take their origin, 

 and finally unite into two main trunks, the omphalo-mesaraic 

 veins. These have already been described as running along the 

 folds of the splanchnopleure to form the venous roots of the 

 heart. Their course is consequently more or less parallel to 

 that of the omphalo-mesaraic arteries, but at some little dis- 

 tance nearer the head, inasmuch as the arteries run in that 

 part of the splanchnopleure which has not yet been folded in 

 to form the alimentary canal. Besides forming the direct 

 roots of the omphalo-mesaraic veins, the terminations of 

 the omphalo-mesaraic arteries in the vascular area are also 

 connected with the sinus terminalis spoken of above as run- 

 ning almost completely round, and forming the outer margin 

 of the vascular area. This (Fig. 23, s.v), may be best de- 

 scribed as composed of two semicircular canals, which nearly 



