CHAPTER X. 



THE DEVELOPMENT OF THE VASCULAR SYSTEM. 

 THE BLOOD VASCULAR SYSTEM. 



The blood vessels constitute such an extensive and complex system that 

 it is obviously beyond the scope of this book to consider the entire system in 

 detail. Consequently attention must be directed only to the develop- 

 ment of the main channels, including the heart, and to the principles of 

 vessel formation. 



FIG. 156. Surface views of chick blastoderms. Ruckert, Her twig. 



a, Blastoderm with primitive streak and head process; showing blood islands (dark spots in 



crescent-shaped area in lower part of figure). 



b, Blastoderm with 6 pairs of primitive segments. Reticulated appearance is due to blood 



islands (dark spots) and to developing vessels, the entire reticulated area being the area 

 vasculosa. 



The formation of blood vessels in all the higher vertebrates including 

 mammals begins in the opaque area of the blastoderm (area opaca) while 

 the germ layers still lie flat. Toward the end of the first day of incubation 

 in the chick, about the time the primitive streak reaches the height of its 



185 



