Abstract
Cardiac tissue from rat embryos at 10, 11, 12, 13, 14, 15, days of gestation and newborn rats were processed and studied under the electron microscope. A unique observation in the developing myocardium is detection of the simulataneous occurrence of mitosis and synthesis of muscle‐specific contractile proteins (actin and myosin filaments) in the same cells. Mitotic cells containing sites of myofilaments are discernible throughout the gestational period covered in this investigation, including the myocardium of new born. Unlike skeletal muscle, in the myocardium of rat embryos the onset of fibrillogenesis is not correlated with cessation of mitotic activity.

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