Septation of the cardiac truncus arteriosus in the normal rat embryo was described and illustrated by three-dimensional reconstruction from serial sections through the developing outflow tract. Tracings of selected structures were digitized on an Apple II-plus microcomputer, relayed through a central computer facility, and displayed on a Hewlett-Packard 9845C graphics computer. Rotation and dissection of the color images allowed a kinetic description of the principal features of truncal septation and comparison with analogous events in the chick.

A complex of tissue structures appeared in the downstream, distal truncus at 13 days of gestation and moved toward the ventricle(s) through the following day. As in the chick, the forming semilunar valves remained within 200 μm of the cephalic margin of the myocardial sleeve as they rotated and descended toward their definitive position against the heart. Mesenchymal condensations formed within the intervening aorticopulmonary (AP) septum and remained in close association with the principal bifurcation of the spiralling blood lumens, passing between the valves and descending along the conus septum. As the conus septum fused at 15 days of gestation, three outlets for cardiac outflow were seen: the major left and right streams to the aortic and pulmonary valves, respectively, and an apparently dwindling stream which crossed from the right ventricle behind the conus septum to exit through the aorta. In the chick, but not in the rat, nerve bundles invaded the distal truncus during septation to form the cardiac plexus.