THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



189 



collected by tributaries of the vitelline veins and carried to the heart. Thus 

 the vitelline (yolk) circulation is completed (Fig. 160). From this time on, 

 the area vasculosa gradually enlarges, as the germ layers extend farther and 

 farther around the yolk, until it eventually surrounds the whole yolk mass. 

 In mammals, as in the chick, the vascular rudiments develop first in the 

 extraembryonic portion of the mesoderm as clusters of cells which give the 

 area opaca a mottled appearance on surface view. This soon changes to a 

 reticulated appearance as the cell clusters give rise to primitive blood spaces 

 which join one another to form a plexus of channels. This plexus gradually 



Aortic arches 



Heart 



Sinus terminalis 



Ant. cardinal 

 vein 



Aorta 



Sinus 

 venosus 



Right vitelline vein 



Right vitelline artery 



Duct of Cuviev 

 Post, cardinal vein 

 Left vitelline artery 



Left vitelline vein 



FIG. 1 60. Diagram of the vitelline (yolk) circulation of a chick embryo at the end of 

 the third day of incubation. Ventral view. Balfour. 



extends across the area pellucida toward the embryo and terminates in a 

 natural selvage as the primitive aorta on each side of the median line. The 

 vitelline arteries and veins are formed out of the plexus and, with the heart, 

 aortic arches and dorsal aorta as in the chick, constitute the vitelline cir- 

 culatory system (Fig. 161). The vascular area in some mammals gradually 

 enlarges until it embraces the "entire yolk sac (Fig. 162). 



It is seen from the foregoing account that the earliest circulation is asso- 

 ciated with the yolk sac. In animals below the mammals, where a large 

 amount of yolk is present in the sac, the vitelline circulation is of prime 



