40 



HEART AND VASCULAR SYSTEM. 



almost continuous chain. These red points are small groups of coloured nucleated 

 blood corpuscles which have been developed within certain of the niesoblast cells in 

 the manner explained in another portion of this work (see Histology, Development of 

 blood-corpuscles and blood- vessels). The niesoblast cells in question form the blood- 

 vessels of the vascular area by becoming united with one another into a capillary 

 network, which becomes connected mesially with branches of the primitive aortas 



Fig. 43. VASCULAR AREA OF THE RABBIT OP 11 DAYS. (v. Beneden and Julin. ) 



The arteries are represented red, the veins blue ; the capillaries are not shown. 

 In both the stages illustrated, the terminal sinus is seen to be arterial. 



(vitelline arteries), and peripherally with a circular vessel (terminal sinus), arterial in 

 mammals but venous in the chick, which forms the circumferential boundaiy of the 

 vascular area. From the capillary network of the vascular area the blood is collected 

 into two vitelline veins, which course backwards and inwards to carry the blood of the 

 area to the venous roots of the heart. This is the first circulation, or the circulation 

 of the vascular area. It is also called the vitelline circulation, because the vascular 

 area is developed in the niesoblast of the splanchnopleure layer which encloses the 

 vitellus, and its capillaries are an important meaiis of bringing the food material of 

 the vitellus to the embryo. 



