interatrial septum atrial septum secundum NCIT:C34295 VHOG:0000021 The septum secundum, semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and ostium secundum. Shortly after birth it fuses with the septum primum, and consequently the foramen ovale is closed, but sometimes the fusion is incomplete and the upper part of the foramen remains patent. The limbus fossae ovalis denotes the free margin of the septum secundum. The tetrapod clade develops a complete atrial septum and loses the fifth aortic arch altogether.[well established][VHOG] UMLS:C0225838 Wikipedia:Septum_secondum UBERON:0004158 EHDAA:3406 EHDAA2:0001827 interatrial septum secundum UBERON:0004155 SCTID:308850005 septum secundum The second of the two membranous sheets which partially separate the two atria during embryonic development. [TFD][VHOG] FMA:63910 EMAPA:17864 Missing definition for term UBERON:0004155. Please replace it using linked reference.