EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 25 1 



This fold, which projects into the atrium, is the right valve of the sinus venosus. 

 Later, a smaller fold forms the left valve of the sinus venosus (Fig. 260 B) . The 

 atrium is constricted dorsally by the gut, ventrad by the bulbus. It therefore 

 must enlarge laterally and in so doing forms the right and left atria (Fig. 258 A , 

 B) with the distal portion of the bulb between them. The deep external groove 

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

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

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

 (Fig. 259), leaving the proximal portion of the bulb and the ventricular hmb 

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

 is thus composed of three undivided chambers: (1) the sinus venosus, opening 

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



Pulmonary artery Aorta 



Aorta— -A 



Bulbus— 



R. mntride 



■Atrium 



I— L. ventricle 



Fig. 259. — Diagrams to show the reduction of the bulbo-ventricular fold (represented by diagonal lines) 

 due to its retarded development. (Modified after Keith.) 



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 venous blood circulates 

 on the right, and arterial blood on the left. In amphibians and reptiles transi- 

 tional types occur. 



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

 are: (1) the complete division of the atrium and ventricle, each into right and left 

 chambers; (2) the division of the bulb and its distal continuation, the truncus 

 arteriosus, into the aorta and pulmonary artery; (3) the incorporation 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 



