At the end of the 4th week, the heart consists of undivided chambers. During the next two weeks, important development changes occur, which lead to the formation of the 4-chambered human heart. These changes can be summarized as under:
1) Division of the atrioventricular canal into two canals.
2) Formation of the interatrial septum.
3) Absorption of the right horn of sinus venosus into the wall of the right atrium.
4) Absorption of the primitive pulmonary vein into the wall of the left atrium.
5) Partitioning of the conotruncus.
6) Formation of the interventricular septum.
7) Histogenetic differentiation of the heart wall and formation of the cardiac valves.
It is important to keep in mind, ,once again, that these developmental events do-not necessarily take place in the sequence given above. The
following discussion would make it clear that there is considerable overlap in the timing of various developmental events that bring the internal structure of the heart to an adult-like condition.