Fig. 7.

Models for spiral septation of the outflow tract. The arrows in panel A indicate the fusion of opposite distal and proximal cushions to form spiralling longitudinal ridges, as shown in panel B. Others, however, argue that the spiralling ridges exist through the length of the outflow tract from the outset (see text for discussion). Be that as it may, having fused to form a spiralling septum, it is argued that ‘detorsion’ (panel C) is required to produce the definitive septum (panel D).