The main reason for this lies in the embryonic development. During the embryonic development blood flows from the right atrium to left atrium through foramen ovale and from atrium to ventricle via atrioventricular channel.
造成这种情况的主要原因在于胚胎发育。在胚胎发育过程中，血液通过卵圆孔从右心房流向左心房，通过房室通道从心房流向心室。
The wall of atrioventricular channel contains endocardial cushion. This endocardial cushion have the primordial cells which forms the cusps of atrioventricular valves.
房室通道壁包含心内膜垫。这个心内膜垫具有形成房室瓣尖的原始细胞。
During developmental stage blood flows from right atrium to left atrium due to pressure gradient. This pressure gradient is created due to following reasons: 1) The right atrium receives blood from both the venacava as well as Umblical vein(which is obliterated in adults). 2) Left atrium receives blood only from pulmonary veins. The amount of blood in pulmonary circuit is low during intrauterine life due to very high vascular resistance as the lungs have not yet expanded. Due to this more pressure develops in right side than the left.
在发育阶段，由于压力梯度，血液从右心房流向左心房。产生这种压力梯度是由于以下原因： 1） 右心房接收来自 venacava 和 Umblical 静脉（在成人中被抹去）的血液。2） 左心房仅接收来自肺静脉的血液。由于肺尚未扩张，血管阻力非常高，因此宫内肺循环中的血量很低。因此，右侧产生的压力比左侧大。
The flow of blood from right atrium to left atrium leads to bulging of central endocardial cushion towards left. Due to this bulging, the third cusp of mitral valve that would have developed gets fused.
血液从右心房流向左心房导致中央心内膜垫向左凸出。由于这种凸起，本应形成的二尖瓣的第三个尖瓣被融合。