EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 253 



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 and with it forms a great part of the atrial septum of the late fetal 

 and adult heart. 



Aoria, 



Sept. II 



Aorta 



Fig. 261. — ^Lateral dissections of the human heart viewed from the left side: A, 6 mm.; B, 9 mm.; 



C, 12 mm. (B is based on a reconstruction by Tandler.) X about 38. Cor. sin., Coronary sinus; 



D. end. c, dorsal endocardial cushion; For. ov., foramen ovale; Int. for., interatrial foramen; /. v. c, 

 inferior vena cava; L. air., left atrium; L. va. s. v., left valve of sinus venosus; L. vent., left ventricle; 

 Put. a., pulmonary artery; Pul. v., pulmonary vein; Sept. I, Sept. II, septum primum, septum secundum; 

 Sup. 0. v., superior vena cava; V. end. c, ventral endocardial cushion. 



Sinus Venosus and its Valves. — The opening of the sinus venosus into the 

 dorsal wall of the right atrium is guarded by two valves (Fig. 260). Along the 

 dorsal and cephalic wall of the atrium these unite to form the septum spurium. 

 Caudally the valves flatten out on the floor of the atrium, but, as stated pre- 

 viously, the left valve later fuses with the atrial septum II. In embryos of 10 

 to 20 mm. the atria increase rapidly in size and the lagging right horn of the 

 sinus venosus is taken up into the wall of the right atrium. By this absorption 

 the superior vena cava now opens directly into the cephalic wall of the atrium, 

 the inferior vena cava into its caudal wall (Fig. 261 C). The transverse portion 

 of the sinus venosus, persisting as the coronary sinus in part, opens into the pos- 

 terior wall of the atrium. 



