176 



EMBRYOLOGY 



differentiate in the splanchnic mesoderm. These masses of mesoderm cells 

 form both blood and the lining of the blood vessels. The innermost cells of 

 the blood islands differentiate into blood cells, while an outer layer forms 

 a sac. Many of these sacs fuse and by growth and elongation are converted 

 into blood vessels. This process results in a complicated network of vessels in 

 an area surrounding the embryo — the area vasculosa. Within the embryo two 



NEURAL TUBE 



NOTOCHORD 



SOMATIC MESODERM 

 SPLANCHNIC MESODERM 



DORSAL MESODERM 



ECTODERM 



A 



ENDOCARDIAL VESICLES 



^T 



FOREGUT 



Fig. 104. The formation of the heart by fusion of the two rudiments. Top: 

 The two rudiments of the endocardium are thin-walled vessels, endocardial vesi- 

 cles. Two thickenings in the splanchnic mesoderm form the future heart muscle. 

 Center: The endocardial vesicles have come together, thereby pinching through 

 the endoderm. The epimyocardium envelops the endocardium by closing over its 

 upper and lower surface. Bottom: The epimyocardium forms an envelope around 

 the endocardium and is continuous with the somatic mesoderm dorsally. Thus the 

 heart is suspended by a dorsal mesocardium. The endocardium joins with the 

 vitelline veins, 



