104 THE SECOND DAY. [CHAP. 



backward limit of the splanchnopleure fold which is 

 forming the alimentary canal), runs outwards beneath 

 the mesoblastic somites in the lower range of the meso- 

 blast, close to the hypoblast. Consequently, when in its 

 course outwards it reaches the point where the meso- 

 blast is cleft to form the somatopleure and splanchno- 

 pleure, it attaches itself to the latter. Travelling 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 vitelline 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 

 vitelline 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 conse- 

 quently more or less parallel to that of the viteUine 

 arteries, but at some little distance nearer the head, 

 inasmuch as the arteries run in that part of the splanch- 

 nopleure which has not yet been folded in to form the ali- 

 mentary canal. Besides forming the direct roots of the 

 vitelline veins, the terminations of the vitelline arteries 

 in the vascular area are also connected with the sinus 

 termiruilis spoken of above as running almost completely 

 round, and forming the outer margin of the vascular 

 area. This (Fig. 36, ST.), may be best described as 

 composed of two semicircular canals, which nearly meet 

 at points opposite the head and opposite the tail, thus all 

 but encircling the vascular area between them. At the 



