The Pre-Septate Heart: Weeks 5 and 6

The sinus venosus enters the single atrium caudally at 4 weeks (Figure la). During the 5th week, flexion of the embryo brings the head in close proximity to the tail, resulting in the “comma” embryo in which the sinus venosus joins the atrium dorsally (Figure 2a). The new dorsal position of the sinus venosus is permanent, repre­ sented in the adult by the posterior wall of the R atrium, the venous part of the heart.