DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM. 



Dorsal aortse 



1st aortic 

 arch 



Common trunk formed 



by umbilical and 



yolk-sac veins 



umbilicalis 

 impar 

 mbilical arteries 



Vitelline arteries 



FIG. 818. SCHEMA OF CIRCULATION OF AN EMBRYO, 1'35 MM. LONG, WITH 

 Six SOMITES. (After Felix, modified. ) 



THE ARTERIES AND THE HEART. 



In the general account of the development of the primitive vascular system and the establish- 

 ment of the foetal circulation, given in a previous chapter, it was pointed out that the germ of 

 the whole blood -vascular system appears in the wall of the yolk-sac as a series of strands of cells 

 which constitute the 

 angioblast. Some of the Dorsal intersegmental branches 



angioblast cells remain 

 in situ and form the 

 blood-vessels of the walls 

 of the yolk-sac and the 

 corpuscular contents of 

 the blood-vessels ; other 

 angioblastic cells wander 

 into the embryonic area 

 and form the blood- 

 vessels of the embryo ; 

 whilst still others be- 

 come located in the de- 

 veloping liver and other 

 organs, where they be- 

 come foci for the 

 formation of new blood 

 corpuscles. 1 



The first blood- 

 vessels developed in the 

 embryonic area are the 

 primitive aortae. They 



appear, either just before the embryonic area begins to be folded into the form of the embryo 

 or as the folding is commencing, in the pericardial or anterior region of the embryonic area, 

 where they are continuous with the earlier-formed vessels on the wall of the yolk-sac. From 

 the pericardial region they extend caudal wards, one on each side of the notochord, and as they 



pass caudal wards they 

 give off a series of dorsal 

 and ventral branches. 

 The dorsal branches are 

 intersegmental in 

 arrangement, inasmuch 

 as they lie in the inter- 

 vals between the meso- 

 dermal somites. The 

 ventral branches are 

 more irregular, and are 

 neither strictly seg- 

 mental nor interseg- 

 mental in arrangement ; 

 moreover, they are not 

 only distributed to the 

 wall of the alimentary 

 canal, but they also pass 

 across it to the yolk-sac. 

 Further, those which are 

 situated nearest the tail 

 anastomose together, on 

 the side wall of the 

 hind-gut area, forming 



_ lexus, and it is from that plexus, on each side, that the umbilical artery is prolonged along 

 the body-stalk to the chorion (Figs. 818, 819). 



As the head fold forms, the cephalic part of each primitive aorta is bent into the form of a 

 loop, and the whole vessel assumes a hook-shaped form. The long or dorsal limb of the hook, 



Dorsal intersegmental branches 

 ,' Dorsal aortae 



rtic arcl 



Heart 



Stem formed by union of' 



lateral umbilical arid 



vitelline veins 



Vena umbilicalis 

 | impar 

 Umbilical arteries 



Vitelline veins 



819. SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO, 2 '6 MM. LONG, 

 WITH FOURTEEN SOMITES. (Arteries after Felix, modified.) 



1 See note, p. 1059. 

 1025 



66 



