Descent of the heart
The first cells of the cardiac anlage are found in front of the embryonic disk in the cardiogenic plate. Due to the bending of the embryo this anlage experiences a rotation of 180o and at the same time it moves caudally. With 4 weeks the tip of the heart is already at the height of the cervical segment.
In stage 16 (ca. 39th day), it reaches the thoracic vertebrae. The heart sinks further until the tip of a newborn heart comes to lie at the level of the 3rd to 4th intercostal space in relation to the front side of the thorax (i.e., the level of the 8th thoracic vertebra).
In the beginning of the early embryonic stages (stages 8, ca. the 23rd day), the heart forms outside the embryonic disk. It is only with the flexion of the embryo that the heart comes to lie within and ventral to the embryo.
Through the growth of the brain the embryo bends forward progressively and in stage 14 (ca. the 33rd day) it attains its maximum flexion. However, the heart continues to descend further and, at the end of the embryonic period, comes to lie in the thoracic region.