Definition: "A sheet of extracellular matrix that separates the outer myocardial cell layer from the inner endocardial tube in the early embryonic heart. As the heart is going through the looping stages, this extracellular matrix/cardiac jelly starts to thicken in two regions, the atrioventricular canal region and the ventricular outflow tract region. These two discrete swellings of cardiac jelly are known as cardiac cushions. The cardiac jelly plays a central role in the septation of the heart and comprises most of the thickness of the heart wall at early stages, but disappears as the heart differentiates."