EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD-VESSELS 



259 



between the atria and the bulbo-ventricular part of the heart is the coronary 

 sulcus. As the bulbo-ventricular region increases in size, the duplication of the 

 wall between the two limbs lags behind in development and finally disappears 

 (Fig. 252 a, b), leaving the proximal portion of the bulb and the ventricular limb 

 to form a single chamber, the primitive ventricle. In an embryo of 5 mm. the heart 

 is thus composed of three undivided chambers: (i) the sinus venosus opening 

 dorsad into the right dilation of the atrium; (2) the bilaterally dilated atrium 

 opening by the single transverse atrial canal into (3) the primitive undivided ven- 

 tricle. The three-chambered heart is persistent in adult fishes, but in birds and 

 mammals a four-chambered heart is developed in which circulates venous blood 

 on the right and arterial blood on the left. 



The important changes leading to the formation of the four-chambered heart 



FIG. 252. Reduction of the bulbo-ventricular fold of the heart (Keith). Ao, aortic bulb; Au, atrium; 

 B, bulbus cordis; RV, right ventricle; LV, left ventricle; P (in b) pulmonary artery. 



are: (i) the complete division of the atrium and ventricle, each into right and left 

 chambers; (2) the division of the bulb and truncus arteriosus into the aorta and 

 pulmonary artery; (3) the absorption of the sinus venosus into the wall of the 

 right atrium; (4) the development of the semilunar and atrio-ventricular valves. 

 The first of these changes is completed only after birth. 



Endocardial Cushions and Atrial Septa. In embryos of 5 to 7 mm. there 

 develops a thin sickle-like membrane from the mid-dorsal wall of the atrium (Figs. 

 253 and 254). This is called the atrial septum primum (/). Simultaneously, 

 endothelial thickenings appear in the dorsal and ventral walls of the atrial canal 

 (Fig. 254, A, B). These are the endocardial cushions which later fuse, thus divid- 

 ing the single atrial canal into right and left atrio-ventricular canals. The atrium 

 is now partly divided into right and left atria which, however, communicate ven- 



