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. [ ]

Synonyms: atrial septum primum interatrial septum primum septum primum

This is just here as a test because I lose it

Term information

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external definition

The first of the two membranous sheets which partially separate the two atria during embryonic development. [TFD][VHOG]

external ontology notes

part_of IAS in EHDAA2

has alternative id


homology notes

The tetrapod clade develops a complete atrial septum and loses the fifth aortic arch altogether.[well established][VHOG]



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