DEVELOPMENT OF THE HUMAN HEART 



2G5 



blended. In No. 163 the blending of the cushions is complete 

 (fig. 10) and the permanent foramen ovale is fully established. 

 Together the united endocardial cushions form a cubical plug 

 which blocks the center of the atrial canal leaving a channel on 

 either side. It also projects into the left ventricle and is attached 

 to its walls forming the two papillary muscles. The atrial canal 

 is divided into two canals which now form the right and left ostia. 

 The two cushions give rise to the medial cusps of the bicuspid 

 and tricuspid valves, that is, the medial cusp of the tricuspid 

 valve and the anterior cusp (B.N. A.) of the mitral valve; the 

 right halves of each cushion make the former and the left halves 

 the latter. 



Fig. 10 Coronal section. Embryo 9 mm. long (No. 163). X 40. 



The septum aorto pulmonale is still incomplete in the speci- 

 mens described, and the interventricular foramen is wide open, 

 but the septum of the ventricle is well formed, extends upward 

 and blends with the posterior endocardial cushion behind. Be- 

 hind the posterior endocardial cushion a marked band of muscle 

 extends from the wall of the sinus venosus to the interventricular 

 foramen where it spreads over the inner walls of the two ventricles 

 as shown in fig. 9. This is the sino- or atrio-ventricular bundle 

 (bundle of His). These structures I shall describe in the heart of 

 embryo No. 353 which is an unusually well preserved specimen, 

 perfect in every respect and of the right stage of development 

 for this purpose. 



