Initially septum primum arises from the roof of atrial
chamber.
A septum present in the atrioventricular canal is called septum
intermedium , in which septum primum grows downward.
There forms a formen called foramen primum , it is also called
ostium primum. Through which blood flows from right atria into left
atria.
later the fusion of septum primum with septum intermedium
causing closure of foramen primum.
Foramen secundum is formed in te upper part of septum
primum.
Fossa ovalis is a small depression on the right atrium in the
interatrial septum at the time of child birth .It is the remnant of
foramen ovale during development.
Hence blood doesnot flow from right atria to left atria or vice
versa