Development of the Heart
The superior and inferior ends of the heart tube are fixed by the pharyngeal arches superiorly, and the septum transversum inferiorly. The bulbus cordis and the ventricle grow at a faster rate than other regions, causing the developing heart to bend upon itself and form the U-shaped bulboventricular loop. The atrium and sinus venosus (which has developed the right and left horns of the sinus venosus) come to lie dorsal to the truncus arteriosus, bulbus cordis and ventricle.
An abnormal bending of the heart can lead to dextrocardia.
As the heart elongates and bends, it invaginates slowly into the pericardial cavity. While it is initially suspended from the dorsal wall by a mesentery, the central part of this mesentery soon degenerates, forming a sinus (the transverse pericardial sinus) between the right and left sides of the pericardial cavity. The heart is now only attached at its cranial and caudal ends.
Circulation through the Primordial Heart