DEVELOPMENT OF THE HUMAN HEART 



269. 



The anatomy of the heart of embryo No. 353 may serve as a 

 basis to describe the final closure of the interventricular opening 

 and the formation of the membranous septum. In this specimen 

 the septum aorto pulmonale has grown down to the interventric- 

 ular foramen and blends with the right tip of the anterior endo- 

 cardial cushion which is lodged in the foramen. The aortic sep- 

 tum, also extends to the lateral side of the right venous ostium 

 and the posterior cushion extends down the anterior border of 

 the ventricular septum. So the interventricular foramen is 



Fig. 12 Coronal section. Embryo 13 mm. long (No. 175). X 40. 



bounded above by the union of the septum aorto pulmonale and 

 the anterior cushion, in front by the septum aorto pulmonale, 

 behind by the extended portion of the posterior cushion and 

 below by the muscle of the ventricular septum. 



In embryos 109 (10.5) and 317 (12.5) the ventricular septum 

 is much more developed than in the stage just described. In 

 these two specimens the fibrous tissue forming the septa and 

 valves is much as in No. 353, but the muscle wall has grown up 

 and nearly closes the interventricular foramen. However, there 



