252 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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 (/). Simultaneously, 

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

 (Figs. 261 A, B). There are the endocardial cushions, which later fuse, thus 



Values of sinus venosus 

 Septum I 



Atrio-wntricular canal 



Valves of sinus venosus 



'Septum I 



iForamen ovale 



A trio-ventric- 

 ular canal 



Interventricular 

 septum 



Sinus venosus 



R. valve of sinus venosus' 



Endocardial cushioii. 



Atrio-ventricular groove 



R. ventricle' 



R. common cardinal vein 



■Septum II 



L. valve of sinus venosus 



L. atrio-ventricular opening 

 L. ventricle 



Fig. 260.— Horizontal sections through the chambers of the human heart; A, 6 mm.; B, 9 mm.; C, 12 

 mm. {A and B are based on figures of Tandler.) X about 50. 



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, how- 

 ever, communicate ventrad through the interatrial foramen. Next, in embryos 

 of 9 mm. , the septum I thins out dorsad and cephalad and a second opening appears, 



