Figure 17.

This sagittal section is from a human embryo at Carnegie stage 18, just before the completion of cardiac septation. Note that the distal outflow tract has now separated into the intrapericardial components of the pulmonary trunk, which is sectioned, and the aorta, which cannot be seen in this section. The arterial roots at this stage, however, are still encased within the muscular wall of the proximal outflow tract. The sinutubular junction, shown by the arrow, is formed at the site of the initial dog-leg bend (see fig 15). Note the cushions fusing to form the aortic leaflet of the mitral valve, which at this stage is still separated by the musculature of the inner heart curvature from the developing leaflets of the aortic valve. Note also the left superior caval vein (LSCV) now incorporated within its own wall within the left atrioventricular groove.