V.J THE VASCULAR SYSTEM. 103 



time (consequently some little while after the com- 

 mencement of the heart's pulsation) carried on. 



The two primitive aortce have already been de- 

 scribed as encircling the foregut, and then passing 

 along the body of the embryo immediately beneath 

 the mesoblastic somites on each side of the notochord. 

 They are shewn in Figs. 32 A.o. and 34 a.o in section as 

 two large rounded spaces lined with flattened cells. At 

 first they run as two distinct canals along the whole 

 length of the embryo ; but, after a short time, unite at 

 some little distance behind the head into a single trunk, 

 which lies in the middle line of the body immediately 

 below the notochord (Fig. .57). Lower down, nearer the 

 tail, this single primitive trunk again divides into two 

 aortae, which, getting smaller and smaller, are finally 

 lost in the small blood-vessels of the tail. At this 

 epoch, therefore, there are two aortic arches springing 

 from the bulbus arteriosus, and uniting above the ali- 

 mentary 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 the two primitive aortae, 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 

 vitelline 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 vitelline artery leaving the aorta at nearly 

 right angles (at a point some little way behind the 



