: an opening between the two embryonic atria of the heart that exists temporarily at the bottom of the developing interatrial septum and is eventually closed off by this septum and replaced by the ostium secundum
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“Ostium primum.” Merriam-Webster.com Medical Dictionary, Merriam-Webster, https://www.merriam-webster.com/medical/ostium%20primum. Accessed 29 May. 2025.
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