Normal stages of intracardiac development of the mouse heart are illustrated by a collection and analysis of two complementary SEM views, the septal and parietal halves of the right ventricle and conotruncus. These views are particularly suitable for understanding the septation of the outflow tract of the heart. They represent references for the studies of the pathogenesis of conotruncal malformations. The analysis is based on 90 hearts between 11ed and 15ed at intervals of 8 hours with an additional stage at 16ed. They were prepared by perfusion fixation, microdissection, and critical point drying and were examined in SEM. The following main features of the intracardiac morphogenesis were observed: (1) the presence of two spirally positioned conotruncal ridges, their disto-proximal fusion, and the formation of the semilunar valves at their distal part; (2) the trabecular transformation of the conotruncal wall; and (3) the presence of two different interventricular foramina (FIV II and FIV III), with the final closure of FIV III at 14edl6h-15ed.

As a result of these observations, our description of the outflow tract septation and its interpretation are different from other conceptions based on mechanisms such as bulbar shift, bulbar absorption, torsions, or transfer of the aorta into the left ventricle.