The formation of the relief of cardiac septa occurs in connexion with the development of shape of the external cardiac wall. Changes in the form of the external cardiac wall precede changes in the internal cardiac wall. The originally straight tubular heart is angulated at typical sites during cardiac looping. At the site of angulations the primary circular cross-section is deformed to an oval and is thereby narrowed. This deformation causes a growth of the endocardial and subendocardial tissue along the long sides of the oval to form protrusions into the cardiac lumen. These protrusions represent the anlagen of the fibrous cardiac septa. The site of origin of the fibrous septa is directly dependent on the direction of primary deformation. Origin and site of the ventricular septum results from the locally different directions and patterns of growth in the right and left ventricle. The anlagen of the cardiac septa are not isolated formations, but rather represent a continuous system of protrusions. Depending on the position and form of its components, their free edges grow towards each other and fuse wherever they meet.