These migrated cells form the "swellings" called the endocardial cushions seen in the heart tube.

Upon sectioning of the heart the atrioventricular endocardial cushions can be observed in the lumen of the atrial canal as two thickenings, one on its dorsal and another on its ventral wall.

These thickenings will go on to fuse and remodel to eventually form the valves and septa of the mature adult heart.

A problem in endocardial cushion development or remodeling is thought to be associated with atrioventricular septal defect.

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