The heart tube hosts progenitors of the conus, right ventricle, left ventricle, atrioventricular canal, and atrial appendages, stemming from both first and second heart field cells. Looping (day 23 in humans) begins with elongation of, the primitive heart tube, along with its simultaneous bending toward the right, into a C-shaped structure. The ventral surface of the heart tube forms the right outer curvature of the C-shaped heart. The inner curvature of the C-shaped heart is primarily derived from the dorsal side of the straight tubular heart, released with the rupture of the dorsal mesocardium . With continued, bilateral elongation, the heart tube takes on an S-shaped configuration. Completion of cardiac looping brings the four presumptive chambers of the future heart into their correct spatial orientation in relation to one another.