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Development of interatrial septum

Through all stages of development of fetus blood shunts from right atria to left atrium so it bypass lungs.
Inter atrial septum is developed from  Septum Primum and Septum Secundum
Septum Primum
Septum Primum is a crescent shaped membranous extension of dorso-cranial wall. 
It will grows downward towards endocardial cushions
The opening between the Septum Primum and endocardial cushions is foramen primum (ostium primum), which  serves as a shunt.
A series of perforations will develop in cranial end that coalesce to form foramen secundum (ostium secundum)
In the later stages of development the septum primum fuses with endocardial cushions – this will obliterates foramen primum
Septum Secundum
Septum Secundum is a crescent shaped septum begins to form to the right side of of septum primum
Grows as septum primum downwards, it does not fuse with endocardial cushion, the opening is called foramen ovale
Because of the arrangement of the two septa it act as one-way valve.