Blood making it into the right ventricle can readily cross into the left ventricle as the interventricular septum has not yet formed to separate the right ventricle from the left ventricle. Blood entering the Pulmonary arteries from the right ventricle can cross directly into the aorta via a connection called the ductus arteriosis . This connection normally closes a few days after birth. Aside from having a single ventricle, you can see that nothing separates the atria from the ventricles. They form a continuous canal - the atrioventricular canal .
Most people believe they contribute little to no tissue to the actual valve leaflets.
However, their position within the atrioventricular canal is of importance. They also fuse during the 4th week of development to form a ring of tissue from which the valves will develop. The second figure shows how the atrioventricular valves develop. Over time the vetricular tissue is 'hollowed out' so the mesenchymal tissue overhangs the receding myocardium. Degenerating ventricular muscle forms the chordae tendineae - thick stringy attachments which connect the valve leaflets to the papillary muscles of the ventricular wall.