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FRANKLIN P. MALL 



In 383, 113 and 163 the foramen ovale II has just formed, being 

 smallest in the first and largest in the last. The cushions are well 

 developed in No. 383; the anterior reaches to the septum primum 

 and the posterior to the opening of the sinus into the right atrium. 

 There is a large space between them connecting the two atria. 

 High up in the atria the septum primum is broken though forming 

 the foramen ovale II, as shown by Born. The arrangement of 

 the endocardial cushions with the space between them and the 



Fig. !> Sagittal section. Embryo S nun. long (No. 113). X 40. 



foramen ovale II is well shown in No. 113 (fig. 9). Both cushions 

 now course to the sinus venosus and are blended with the con- 

 nective tissue above it. Behind the cushions there is a muscle 

 strand from the sinus to the ventricle which marks the position 

 of the atrio- ventricular bundle. The thin septum primum reaches 

 to the two cushions, and high up it is perforated by an opening 

 with sharply defined walls. Between the anterior and posterior 

 cushions well within the atrial canal the two atria still communi- 

 cate with each other; in this region the cushions are as yet not 



