The anlage of the heart develops rapidly. At the same time, the pericardial cavity enlarges greatly, during which time the primordia of the pleural cavities (pericardioperitoneal canals) remain very small. In parallel with its differentiation, the heart becomes shifted further caudally (descent of the heart). This descent changes the course of the affluent veins, particularly that of the common cardinal veins that now run from cranio-lateral to caudo-ventral towards the sinus venosus.