The bulbus cordis and its septa in Sphenodon punctatus

Two embryos of Sphenodon punctatus , measuring 17.4 mm and 25.2 mm in length, were used in examining the arrangement of the septa and associated vessel pattern in the bulbus cordis. As in other reptiles, two septa subdivide most of the bulbus into three arterial vessels. The aortico‐pulmonary septum passes through an angle of about 160° in its descent toward the ventricle. The aortic septum describes an angle of about 150°. Both partitions terminate anterior to the level of the ventricle, leaving an undivided remnant of the bulbus cordis from which the arterial vessels spring. When compared to other reptilian embryos whose bulbi have been studied in detail (i.e., Chrysemys and Aristelliger ), Sphenodon shows similarities in the proximaldistal arrangement of the arterial apertures, the mode of descent of the bulbar septa, and the endocardial cushions, which comprise the bulbar septa. The rotation of the septa, however, is substantially different from that observed in the turtle and the lizard so far studied. The aortico‐pulmonary and aortic septa in Sphenodon spiral through angles greater than those in Chrysemys and Aristelliger . This pattern in Sphenodon appears to represent a primitive phylogenetic feature in terms of the evolution of the bulbus in reptiles.