CH. LIX.] DEVELOPMENT OF VASCULAR SYSTEM 



Development of the Vascular System. 



849 



We have already seen that at an early stage of development, blood- 

 vessels begin to form in the splanchnic mesoblast on the wall of the 

 yolk-sac, outside the embryo, in an area called the area vasculosa. 

 From the cephalic end of this fcrea two longitudinal vessels run back- 

 wards through the embryonic region, and they terminate posteriorly in 

 the caudal part of the area vasculosa (fig. 644). As they run through the 

 embryonic region, which is still outspread on the surface of the ovum, 

 they pass beneath the pericardium, and then beneath the inner parts 

 of the protovertebrae, not far from the sides of the notochord. As the 



FIG. 644. Diagram representing the arrangement of the primitive blood-vessels before the embryo is 

 folded off from the ovum. 1, Primitive vessel of leffside ; 2, protovertebra ; 8, primitive streak ; 

 4, vascular area of yolk-sac ; 5, non-vascular area of yolk-sac ; 6, splanchnic mesoblast ; 7, somatic 

 mesoblast ; 8, epiblast ; 9, pericardium. 



head and the tail folds of the embryo form, these longitudinal 

 vascular tubes are bent, both in front and behind, and, after the 

 bending, each consists of five parts. A dorsal part which extends 

 along the dorsal wall of the alimentary canal ; two ventral parts, one 

 in front of the umbilicus and one behind that orifice, and two arches, a 

 cephalic and a caudal, connecting the dorsal portion of each vessel with 

 the anterior and posterior ventral portions respectively (fig. 645). The 

 blood flows from the anterior part of the yolk-sac wall into the anterior 

 ventral parts of these primitive embryonic vessels by two channels, 

 which are called the omphalo-mesenteric veins. The anterior ventral 

 vessels into which the omphalo-mesenteric veins pass, lie, now that 

 the folding of the embryo has taken place, in the dorsal wall of the 



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