260 PROFESSOR MARSHALL AND EDWARD J. BLES. 



B. Formation of the Blood Vessels and Blood Corpuscles. 



8. The blood vessels arise as irregular lacunar spaces in the 



mesoblast, formed by widening of the intervals between 

 adjacent mesoblast cells. These spaces open into each 

 other, and so form continuous channels. 



9. The mesoblast cells surrounding these channels become 



converted into the epithelial walls of the vessels. 



10. The further growth of the vessels is sometimes effected by the 



formation of solid cellular cords, continuing the lines of 

 the vessels. The axial cells of these cords break down, 

 thus giving rise to tubular vessels. 



11. There are at first no blood corpuscles. When these appear, 



they are formed from the walls of the vessels themselves. 



12. The blood corpuscles are true cells. They are at first 



spherical in shape, and laden with yolk granules. By 

 the time the mouth opens, the yolk granules are much 

 diminished in size and number • and shortly afterwards 

 they disappear, and the corpuscles acquire their adult 

 shape and appearance. 



C. Development of the Aorta and of the Vessels of the Visceral Arches. 



13. The first vessels to appear, except the vitelline veins and 



heart, are the dorsal aortse. These arise on each side 

 as a number of isolated lacunar spaces along the roof of 

 the pharynx, which open into one another and so form 

 continuous vessels. 



14. In the four branchial arches, blood vessels are formed on a 



definite plan. In the first and second branchial arches, 

 these vessels appear immediately after the dorsal aortse : 

 in the third and fourth branchial arches they arise later. 



15. In each branchial arch there appears a lacunar efferent 



vessel at the level of the external gill ; this extends 

 dorsally, and meets, and opens into a diverticulum of the 

 dorsal aorta. It also extends ventralwards towards the 

 truncus arteriosus, but does not open into this. 



16. Immediately behind the efferent vessel, at the level of the 



gill, an afferent vessel appears ; this is also lacunar, and 

 at first independent • it soon acquires connection with 



