DEVELOPMENT OF THE HEART 251 



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 duplication 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 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 dilatation of the atrium; (2) the bilaterally 

 dilated atrium, opening by the single transverse atrial canal into (3) the 

 primitive, undivided ventricle. 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 side and arterial 

 blood on the left. In amphibians and reptiles, transitional types occur. 



The important changes next to be considered, leading to the forma- 

 tion of the four-chambered heart are: (i) 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. 



Origin of the Right and Left Atria. In human embryos of 5 to 7 

 mm. there develops a thin, sickle-like membrane from the mid-dorsal 

 wall of the atrium (Figs. 260 and 261). This is called the atrial septum 

 primum (I). Simultaneously, endothelial thickenings appear in the dorsal 

 and ventral walls of the atrial canal (Figs. 261 A, B). These are the 

 endocardial cushions, which later fuse, thus dividing the single atrial canal 

 into right and left atrio-ventricular canals (Fig. 266). The atrium is now 

 partly divided into right and left atria, which, however, communicate 

 ventrad through the inter atrial foramen. Next, in embryos of 9 mm., the 

 septum I thins out dorsad and cephalad and a second opening appears, 

 the foramen ovale (Figs. 260 and 261 B). The atria are now connected 

 by two openings, the interatrial foramen and the foramen ovale. Soon 

 (embryos of 10 to 12 mm.), the ventral and caudal edge of septum I fuses 

 with the endocardial cushions, which have in turn united with each other 

 (Figs. 260 and 261 C). The interatrial foramen is thus obliterated, but 

 the foramen ovale persists until after birth. In embryos of 9 mm. the 

 septum secundum (II) is developed from the dorsal and cephalic wall of 

 the atrium, just to the right of the septum primum (Fig. 260 C). It is 

 important, as it later fuses with the left valve of the sinus venosus, whence 

 the two form a great part of the atrial septum of the late fetal and adult 

 heart. 



