CHAPTER XVI 

 LATER DEVELOPMENT OF CHICK AND RABBIT 



Chick, 

 Vascular System. 



As we have noted already, even before the somites have been 

 formed, an area vasculosa is marked out in the area opaca, and it 

 appears in surface view as a mottled region. This spreads fairly 

 rapidly, and by 38 hours completely covers the embryonic area, 

 having around its edge a continuous dark line. These patches, 

 which, it should be noted, are extra embryonal and outside the area 

 pellucida, are actually closely packed irregular groups of cells lying 

 between the meso and entoderm, and receive the name of " blood 

 islands." They soon connect up and form an anastomising network. 

 In the stage when three or four somites are laid down, vacuoles 

 appear in these groups, and the outer cells arrange themselves in a 

 flattened layer to form the endothelium of the future blood-vessels, 

 while the remaining cells round off and develop haemoglobin, thus 

 forming the erythrocytes or first blood cells. So that around the 

 periphery of the area where a similar process goes on a continuous 

 limiting vessel, the sinus terminalis, is formed, and all over the 

 remaining parts a network of capillaries in which larger trunks soon 

 appear. These include the anterior vitelline veins running from 

 the anterior margin of the area vasculosa backwards, and the 

 lateral vitelline veins coming in from the side. The vessels thus 

 formed invade, at quite an early stage, the area pellucida, which 

 never possesses distinct blood islands. As the lateral mesoderm 

 splits it will be found that the vessels are confined to the splanchnic 

 layer, while the somatic layer is non- vascular. 



l*he first embryonal vessels to appear are the dorsal aortae, a pair 

 of tubular structures lying in the mesoderm latero-ventrally to the 

 neural tube. They pass forward to the head region, and posteriorly 

 they diverge at about the level of the last somite in an embryo of 

 12 somites, to run out into the vascular area as the vitelline arteries. 



As the head fold is forming, the coelom just behind it enlarges 

 markedly to form a pair of sacs, the amnio-eardiac vesicles. These 



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