DEVELOPMENT OF THE HEART 



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 mesoderm disappears, leaving the heart 

 suspended by a temporary dorsal meso- 

 cardium in the single pericardia! chamber 

 (Fig.ass C). The endothelial tube forms 

 the endocardium, the splanchnic mesoderm 

 later gives rise to the epicardium and myo- 

 cardium. This type of heart occurs in 

 human embryos of 2 mm. (5 or 6 somites, 

 Fig. 256) and shows three regions: (i) the 

 atrium, which receives the blood from the 

 primitive veins; (2) the ventricle; (3) the 

 bulb, from which is given off the ventral aorta. 



As the cardiac tube grows faster than 

 the pericardial cavity in which it lies, it bends 

 to the right, the bulbus and ventricle form- 

 ing a U-shaped loop (Fig. 257). Four 

 regions may now be distinguished; (i) the 

 sinus venosus; (2) the atrium, also thin- 

 walled and lying cranial to the sinus; (3) 

 the thick-walled ventricular limb, ventrad 



and caudad in position; (4) the bulbar limb, cranial to the ventricular 

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



B 



FIG. 256. The heart of a 2 

 mm. human embryo in ventral 

 view (Mall). X 65. .The open 

 tube is the fore-gut. 



FIG. 257. A, Heart of human embryo of 2.15 mm. :o, Bulbus cordis; b, primitive ventricle; 

 c, atrial portion. B, Heart of human embryo of about 3 mm.: a, Bulbus cordis; b, atrial por- 

 tion (behind); c, primitive ventricle (in front). Ventral views (HisV 



embryos of 3 to 4 mm., the bulbo-ventricular loop shifts its position until 

 its base is directed caudad and ventrad (Fig. 257 B). At the same 

 time the sinus venosus is brought dorsal to the atrium, which in turn is 



