260 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



trad through the interatrial foramen. Next in embryos of 9 mm. the septum I 

 thins out dorsad and cephalad and a second opening appears, the foramen ovale 

 (Figs. 253 and 254, B). The atria are now connected by two openings, the inter- 

 atrial foramen and the foramen ovale. Soon (embryos of 10 to 12 mm.) the ven- 



l/alves of 



Sinus venosus 



Septum! 



■Atrio-yenlricuJai 

 opening 



Valves of Sinus venesui • 

 tyt.I 



■Foramen ovale 



Interventricular 

 Septum 



tSinus venos i 

 R.va/ve sinus venosus yij 



Endocardial £ 

 Cushion 



fl. common cardinal vein 



Septum U 

 L.valve sinuj, venosus 



Afrio-irentricutar 

 groove 



R.Vent. 



-■>- 



L.Atria-venf. 

 • \ opening 



L.Yent 



Fig. 253. — Oblique transverse sections of heart wall: A, 6 mm.; B, 9 mm.; C, 12 mm. (A and B are 

 \ based on figures of Tandler). 



tral and caudal edge of septum I fuses with the endocardial cushions, which have 

 in turn united with each other (Figs. 253 and 254, C). The interatrial foramen 

 is thus obliterated, but the foramen ovale persists until after birth. In embryos 

 of 9 mm. the septum secundum is developed from the dorsal and cephalic wall 

 of the atrium just to the right of the septum primum (Fig. 253, C). It is impor- 



