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thymidine3H do not bind fluorescein-labeled antibody to myosin. All of the studies which form the basis of these generalizations about myofibril differentiation (1-7) have been done with skeletal muscle . Yet, it is difficult here to separate cell fusion, ending of DNA synthesis, and initiation of myosin synthesis because the three processes occur concurrently. In cardiac muscle, however, the differentiating cells do not fuse. Thus, by studying cardiac muscle, we thought that we could judge whether an obligatory relationship exists between cessation of DNA synthesis and onset of myosin synthesis, or whether it is cell fusion in skeletal muscle that turns off DNA synthesis . Shafiq et al., in an electron microscope study of skeletal muscle and cardiac muscle from 3-wk old rats, reported that no mitotic figures were found in either the skeletal muscle fibers or in myocardial cells containing myofibrils (8) . Manasek, however, in the same year, reported mitotic figures in electron micrographs of chick cardiac muscle cells that already contained myofibrils (9) . Using cinematography, Mark and Strasser (10) noted Within the past decade, numerous investigators have contended that neither DNA synthesis nor mitosis can occur in cells which have synthesized specialized or so-called "luxury" proteins (1-8) . In particular, the idea has been widely promulgated that once a muscle cell has synthesized myofilaments, it is no longer capable of replicating its DNA or undergoing mitosis. Stockdale and Holtzer, for example, working on 10 day old chick embryonic thigh muscle in vitro and 3-4-day old chick somites in vivo, concluded that myoblasts which are synthesizing contractile proteins do not synthesize DNA and do not divide (1) . Okazaki and Holtzer (2, 3) stated that nuclei regulating the synthesis of myosin cannot replicate DNA. Radioautographic studies indicated that the decision to translate for myosin and to withdraw from the cell cycle was acted on in the same GI in a myoblast that synthesized DNA for the last time about 8 hr previously (4-6) . Coleman and Coleman (7) also observed that presumptive skeletal muscle cells which incorporate

DEOXYRIBONUCLEIC ACID SYNTHESIS AND MITOSIS IN DIFFERENTIATED CARDIAC MUSCLE CELLS OF CHICK EMBRYOS