the common atrial chamber?

How do ** defects develop? Embryology.

All ** septal defects result from some degree of incomplete fusion of the endocardial cushions during the 5th week of fetal development. There are four endocardial cushions, which originate from the primitive mesoderm to form the atrioventricular (**) valves, atrial and ventricular septum. The atrial portion of the ** septum develops from the growth of the endocardial cushion tissue toward the posterior wall of the common atrial chamber. The ventricular portion of the ** septum results from the growth of the endocardial cushion tissue toward the apex of the heart. As the endocardial cushion tissue continues to grow to the right and the left, it results in the formation of the tricuspid and mitral valves.

房室间隔缺损是如何发展的？胚胎学。

所有房室间隔缺损都是由于胎儿发育第5周时心内膜垫不完全融合所致。有四个心内膜垫，它们起源于原始的中胚层，形成房室瓣、心房和室间隔。房间隔的心房部分由心内膜垫组织向共同心房后壁生长发育而来。房室间隔的心室部分是心内膜垫组织向心尖部生长的结果。随着心内膜垫组织不断向右和向左生长，导致三尖瓣和二尖瓣的形成。

the common atrial chamber共同心房?啥意思？