Axial twist theory
Scientific theory in vertebrate development Axial twist Schema of the proposed development of the axial twist. Developmental phases are (from top to bottom): (1) the embryo turns on its left side; (2) the anterior head grows in the same direction, but the rest of the body grows oppositely into a twist. So that ultimately (3) external bilateral symmetry is regained. Note that there is no evolutionary pressure for internal symmetry so the heart (and other organs) remain asymmetric. Details System