In the developing heart, the cavity of the primitive atrium

Primitive atrium

The primitive atrium is a term used to describe a stage in the embryonic development of the human heart. It grows rapidly and partially encircles the bulbus cordis; the groove against which the bulbus cordis lies is the first indication of a division into right and left atria.The cavity of the...

 becomes subdivided into right and left chambers by a septum

Septum

In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

, 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

Ostium primum

In the developing heart, the atria initially communicate with each other by an opening between the free edge of the septum primum and the AV cushions, known as the primary interatrial foramen or ostium primum , below the free margin of the septum.-Closing of ostium primum:This opening is closed by...

 (ie. "the first opening"). 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

Ostium secundum

The ostium secundum is a foramen in the septum primum.It should not be confused with the foramen ovale, which is a foramen in the septum secundum.-Clinical significance:...

 (ie. "the second opening"). This will eventually form part of the fossa ovalis

Fossa ovalis

Fossa ovalis can refer to:* Fossa ovalis , also called the saphenous opening* Fossa ovalis , an embryonic remnant of the foramen ovale...

.

External links

• Overview at um.edu.mt
• Diagram at lww.com - "Atrial Partitioning"