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TEXT-BOOK OF EMBRYOLOGY. 



of new vessels so long as any tissue of the body continues to grow and become vascular. In 

 the formation of the larger vascular trunks in the body there is evidence to support the 

 view that the vessels arise independently, for in some cases at least cavities, lined by flat 

 cells appear in the mesenchyme, which are at first quite unconnected, and only secondarily 

 unite to form a continuous vessel. On the other hand, in the case of the formation of granu- 

 lation tissue or any new tissue, including pathological growths, it is usually held that the 



FlG. 212. Surface view of chick embryo with 18 pairs of primitive segments, including the 



area vasculosa. Riickert, Hertwig. 



The reticulum indicates the blood vessels; the dark spots in the vessels being blood islands The 

 darker line at the border of the figure represents the sinus terminalis. 



new vessels arise as outgrowths from other vessels: The endothelium of a vessel proliferates 

 and grows out as a slender process which becomes hollow and thus forms a capillary con- 

 tinuous with the original vessel from which it is an outgrowth. 



Some of the larger channels of the area vasculosa converge to form a single 

 vessel on each side, which enters the embryonic body through the splanchno- 

 pleure and joins the caudal end of the heart (p. 224). These two vessels are 

 known as the omphalomesenteric (vitelline) veins (Fig. 213). Other channels of 



