236 THE MESODERMAL DERIVATIVES 



The Circulatory System. 



The blood vessels arise from the fusion or confluence of blood 

 islands which develop first in the splanchnic mesoderm and later in 

 scattered mesenchyme. These are isolated groups of yolk-laden cells 

 which form endothelial pockets containing blood-forming cells. These 

 lacunae ("litde lakes") merge to form the blood vessels, and eventu- 

 ally the embryo acquires a complete and closed blood vascular sys- 

 tem, continuous with the heart. This occurs even before there is any 

 rhythmic pulsation of the blood or the heart. The blood islands give 

 rise to the early embryonic blood, but this blood is later derived from 

 the spleen and bone marrow and possibly the liver. 



The Arterial System. The dorsal aorta arises in splanchnic 

 mesoderm dorsal to the enteron. Above the pharynx it is double and 

 the vessels are known as the supra-branchial aortae or dorsal aortae. 

 Aortic arches develop within the mesenchyme of the visceral arches 

 (III to VI). They arise first as blood islands which merge to form 

 blood vessels. The afferent branchial portion of each arch joins the 

 ventral aortae, while the efferent branchial portion joins the dorsal 

 aortae. This junction does not occur in either the mandibular or the 

 hyoid arches, but it does occur in all the others. A closed circuit is 

 established, therefore, between the ventral heart and the dorsal aortae 

 by way of the viscerally located aortic arches. 



The third, fourth, and fifth visceral arches (branchial arches I, 11, 

 and III) now give rise to secondary blood vessels or capillary loops. 

 These connect dorsally (efferent branchial artery) and ventrally 

 (afferent branchial artery) with the corresponding aortic arch. The 

 intermediate section of each new vessel then grows out into the devel- 

 oping external gill. That portion arising from the ventral part of the 

 aortic arch, and therefore nearest the heart, is then known as the 

 afferent branchial loop. The returning portion from the gill which is 

 connected with the dorsal part of the aortic arch is the efferent bran- 

 chial loop. These loops coil through the filaments of the developing 

 external gills, but the original portion of the aortic arch in each of the 

 visceral arches remains for a brief time. The blood may therefore 

 flow through either of two courses (i.e., the arch or the branchial 

 loop), during the early functioning of the external gills. Usually no 

 external gill grows outward from the sixth visceral arch and as a 

 result an external branchial vessel never develops. The aortic arches 



