DEVELOPMENT OF THE HUMAN HEART 



263 



monale is well formed and its two ridges reach well into the bulbus 

 to the interventricular foramen. Much the same arrangement is 

 found in Embryos No. 136, 116 and 380 which are of the same 

 stage of development as No. 80. 



Fig. 7 Transverse section of the atrial canal and bulb of the heart of an embryo 

 9 mm. long (No. 422). X 40. 



Fig. 8 Transverse section of the heart of an embryo 7 mm. long (No. 2). X 40. 



In Nos. 241, 422 and 2 the lumen of the bulb is °° -shaped, the 

 endocardial cushions are much more pronounced than before; 

 they are ready to fuse as is indicated in figs. 7 and 8. The sep- 

 tum primum and the interventricular septum are well marked. 

 In 397 the septum primum is very thin above so that it is uncer- 

 tain whether or not it has broken through to form the foramen 

 ovale II. 



