The ultrastructure of the developing chick ventricular myocardium was examined between Hamburger-Hamilton stage 12⁻ (15 somites; about 45 hours incubation) and the time of hatching (about 21 days incubation). The myocardium is an epithelial tissue initially containing only developing myocytes. No epicardium is present and no mesenchymal cells such as fibroblasts are seen. By the third day of development, portions of the ventricular endocardium invade the myocardium. After this event and the development of the epicardium from an extramyocardial source, mesenchymal cells are seen within the myocardium. These cells, the first nonmuscular components seen within the myocardium, are probably fibroblasts derived from the endocardium or the epicardium.

The myocardium is honeycombed with large anastomosing channels. The extracellular matrix within these spaces is electron-lucent until the nonmyocardial mesenchymal cells appear. A flocculent component then develops.

Early cardiac myocytes contain few fibrils and large amounts of cytoplasm. The fibrils are not regularly oriented within the young cells. As development proceeds, more fibrils are formed; the cytoplasm decreases and the fibrils become aligned in the mature orderly pattern. Possible mechanisms to explain these phenomena are discussed.