No cardiac muscle is formed from a functional syncytium; it is the other way around. Cardiac muscle within the walls of the heart form a functional syncytium. A syncytium is a mass of cells that have merged together. The muscle cells in the cardiac syncytium are derived from the mesoderm.
Most syncytia are composed of a single mass of protoplasm with multiple cell nuclei. The mass is the result of surrounding cells fusing together into one larger cell. In cardiac tissue, the individual muscle cells do not fuse this way. Instead, they are linked together to form a mass of tissue that functions as if it were one large cell, which is why it is called a functional syncytium.