Inflow Septum and Division of the Atrioventricular Canal

The primitive atrium empties into the primitive left ventricle through the  atrioventricular canal. As development progresses swellings of the endocardial cushions then appear around the edges of the atrioventricular orifice. These are the precursors of the atrioventricular valves and function during this early development as primitive valves. The endocardial cushions grow toward each other and  fuse, separating the atrioventricular canal into two openings which will eventually become the tricuspid and mitral valves. This separation occurs simultaneously.

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