260 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



trad 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 ovate 

 (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- 



Atrio-ventric- 

 ular canal 



Inter Ventricular 

 Septum 



Atria- ventricular 

 groove 



R.vent. 



FIG. 253. Oblique transverse sections of heart wall: A, 6 mm.; B, g 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- 



