A cavity in the heart of an embryo, part of which develops into the coronary sinus and part of which is absorbed into the right atrium.
A chamber of the embryonic heart that receives blood from several veins. In the adult heart it becomes part of the right atrium.
In the embryo, a large venous sac just caudal to the developing heart that receives the blood from the umbilical, vitelline, and common cardinal veins. At first, this sac is a separate chamber emptying into the primitive right atrium, but eventually the left part of the sinus venosus forms the coronary sinus (emptying into the right atrium).