interatrial septum atrial septum primum http://upload.wikimedia.org/wikipedia/commons/c/c7/Gray465.png Wikipedia:Septum_primum septum primum EMAPA:17013 In the developing heart, the cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. The increasingly smaller gap below it (before it fuses with the endocardial cushion) is known as the ostium primum. The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely. Meanwhile, perforations appear in the superior part of the septum primum, forming the ostium secundum. This will eventually form part of the fossa ovalis. UBERON:0004157 NCIT:C34294 EHDAA2:0001825 FMA:63909 interatrial septum primum SCTID:362017007 EHDAA:1900 atrial septum primum part_of IAS in EHDAA2 The first of the two membranous sheets which partially separate the two atria during embryonic development. [TFD][VHOG] UBERON:0004154 The tetrapod clade develops a complete atrial septum and loses the fifth aortic arch altogether.[well established][VHOG] VHOG:0000020 UMLS:C0225837