The primitive atrium is a stage in the embryonic development of the human heart.

For a time the atria communicate with each other by an opening, the primary interatrial foramen, below the free margin of the septum.

The limbus fossæ ovalis denotes the free margin of the septum secundum.

Subsequently, the common trunk and the two vessels forming it expand and form the vestibule or greater part of the atrium, the expansion reaching as far as the openings of the four vessels, so that in the adult all four veins open separately into the left atrium.

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