The septation of the arterial pole of the heart in the chick embryo. III. Development of the truncus arteriosus of the heart of chick embryos from 5 1/2 to 7 days of incubation.

At the end of the fifth day of incubation the mesenchymal truncus arteriosus is completely divided into aorta and pulmonary trunk. During the first half of the sixth day the septation process starts in the distal part of the myocardial truncus by the ingrowth of the dense mesenchyme of the aortico-pulmonary septum into the dorsal and ventral-right truncus ridges. During the second half of the sixth day this septation process extends into proximal direction. Proximally to the ingrowing aortico-pulmonary septum the dorsal and ventral-right ridges fuse. In these fused ridges the aortico-pulmonary septum extends proximally for some distance. During the first half of the seventh day the left ventricular outlet is separated from the right ventricular inlet by the downward extension of the right caudal side of the atrioventricular cushion-mass, which continues into the right proximal truncus ridge (at the transition into the distal ventral-right ridge). In the meantime the fusion of the dorsal and ventral truncus ridges continues in proximal direction. This process comes to be completed in the second half of the seventh day by the fusion of the left (dorsal) and right (ventral) truncus ridges up to the level where the left one continues along the free border of the interventricular septum into the right limb of the dorsal extension of the atrioventricular cushion-mass. During the sixth and seventh day the origins of aorta and pulmonary trunk rotate from their right and left positions in younger stages to an almost dorsal and ventral position respectively in older stages. The septum trunci lying in between the left and the right ventricular outlet in the truncus appears to be a straight septum and does not rotate.