It is semilunar in shape, and grows downward from the upper wall of the atrium immediately to the right of the septum primum and ostium secundum.

At the end of the fifth week of development, the septum secundum grows from the upper wall of the primitive atrium.

[1] Before birth, it does not fuse with the septum intermedium, leaving a gap to form the foramen ovale.

[1] The fossa ovalis denotes the free margin of the septum secundum after birth.

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