V.] THE VASCULAR SYSTEM. 105 



point opposite the head the end of each semicircle is 

 connected with vessels (Fig. 36), which run straight in 

 towards the heart along the fold of the splanchnopleure, 

 and join the right and left vitelline veins. At the 

 point opposite the tail there is at this stage no such 

 definite connection. At the two sides, midway between 

 their head and tail ends, the two semicircles are espe- 

 cially connected with the vitelline arteries. 



The circulation of the blood then during the latter 

 half of the second day may be described as follows. The 

 blood brought by the vitelline veins falls into the 

 twisted cavity of the heart, and is driven thence through 

 the bulbus arteriosus and aortic arches into the aorta. 

 From the aorta, by far the greater part of the blood 

 flows into the vitelline arteries, only a small remnant 

 passing on into the caudal terminations. From the 

 capillary net-work of the vascular and pellucid areas 

 into which the vitelline arteries discharge their 

 contents, part of the blood is gathered up at once 

 into the lateral or direct trunks of the vitelline 

 veins. Part however goes into the middle region 

 of each lateral half of the sinus terminalis, and there 

 divides on each side into two streams. One stream, 

 and that the larger one, flows in a forward direction 

 until it reaches the point opposite the head, thence it 

 returns by the veins spoken of above, straight to the 

 vitelline trunks. The other stream flows backward, 

 and becomes lost at the point opposite to the tail. 

 This is the condition of things during the second day; 

 it becomes considerably changed on the succeeding day. 



At the time that the heart first begins to beat the 

 capillary system of the vascular and pellucid areas is 



