The answer is septum.
Frog hearts have 3 chambers - one ventricle and two atria. The left atrium receives oxygenated blood, and the right atrium receives deoxygenated blood and they both empty into the ventricle. Since there is only one ventricle, oxygenated and deoxygenated blood will mix. If there would be a septum in the ventricle, similar to those in lizards, less mixing will occur. Anyway, in frog's ventricle, there is a ventricular fold that prevents complete mixing of oxygenated and deoxygenated blood.