EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 249 



The heart is now an unpaired endothelial tube lying in the folds of the 

 splanchnic mesoderm (Fig. 190 A). Soon the ventral attachment of the meso- 

 derm disappears, leaving the heart suspended by a temporary dorsal mesocardium 

 in the single pericardial chamber (Fig. 255 C). The endothelial tube forms the 

 endocardium, the splanchnic mesoderm later gives rise to the epicardium and 

 ^ myocardium (muscle layer of heart). 



This type of heart occurs in human 

 embryos of 2 mm. (5 or 6 somites, 

 Fig. 256) and shows three regions: 

 (1) the atrium, which receives the 

 blood from the primitive veins; (2) 

 the ventricle; (3) the bulh, from 

 which is given off the ventral aorta. 



■f'ft^'R ■■■^--vjiir^^ 



Fig. 255. — Diagrams to illustrate the origin of 

 the mammalian heart. Ect., Ectoderm; End., endo- 

 thelial tubes; Enl., entoderm; Fg.,- fore-gut; Msc.d., 

 dorsal mesocardium; Ms.spl., splanchnic mesoderm 

 (epi- and myocardium). 



Fig. 256. — The heart of a 2 mm. human 

 embryo in ventral view (Mall). X 65. The 

 open tube is the fore-gut. 



As the cardiac tube grows faster than the pericardial cavity in which it hes 

 it bends to the right, the bulbus and ventricle forming a U-shaped loop (Fig. 

 257). Four regions may now be distinguished: (1) the sinui venosus; (2) the 

 atrium, also thin walled and lying cranial to the sinus; (3) the thick- walled ventricu- 

 lar limb, ventrad and caudad in position ; (4) the bulbar limb, cranial to the ventric- 

 ular limb and separated from it by the bulbo-ventricular cleft. Next, in embryos 



