Figure 5.

Panel A is prepared using an episcopic data set from a mouse embryo killed at the end of the 12th day of development. It shows how the aortopulmonary septum, formed by the protrusion from the dorsal wall of the aortic sac, has fused with the distal ends of the outflow cushions (dashed white line). The white arrows with dark borders show the extent of the pericardial cavity. The distal outflow tract is fully divided at this stage. The fused distal cushions have also divided the intermediate part of the outflow tract. Note the appearance of the intercalated cushions (white stars with dark borders) in this middle component. The outflow cushions, however, remain unfused in the proximal part of the outflow tract. Panel B is from an episcopic data set prepared from a human embryo at Carnegie stage 16. It shows how columns of condensed mesenchyme, derived from the cells migrating from the neural crest, occupy the fused distal and the unfused proximal parts of the outflow cushions. They are not involved with separating the intrapericardial arterial trunks, which have already been separated by the aortopulmonary septum, formed from the protrusion of the dorsal wall of the aortic sac.