The gap below it is known as the ostium primum (from Latin 'first opening'), and becomes increasingly small.

Meanwhile, perforations appear in the superior part of the septum primum, forming the ostium secundum (from Latin 'second opening').

The septum primum will eventually form part of the fossa ovalis.

Children born with this defect may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt.

This article incorporates text in the public domain from page 512 of the 20th edition of Gray's Anatomy (1918)