To follow the intracellular distribution of calcium in the breast muscles of developing chickens, Ca45 was injected into the albumen of predeveloped eggs. Since the embryos were grown in a radioactive medium, a complete exchange of the isotope for its non-radioactive counterpart in muscles was accomplished. Subcellular particulates of the muscle cells were separated by the method of differential centrifugation. Analysis of the separated fractions showed that in the muscles of the 13-day embryo, when the nuclear-myofibrillar ratio is high, 65 per cent of the muscle calcium is in the nuclei. With the increased synthesis of myofibrils, the nuclear-myofibrillar ratio decreases with a concomitant fall in radioactivity. Thus, calcium was not associated with the developing myofibrils. At the time of hatching, when myofibrils perform physiological work, the highest level of calcium is in the mitochondria. This suggests that the mitochondria play a key role in the physiological activities of calcium in the cell. The microsomal fraction reaches a maximal level of calcium when the adult composition of muscle is attained. Results of investigations on dystrophic muscles show changes in the calcium distribution of the fractions as early as the 3rd week of embryonic development, which are interpreted to indicate an alteration in the protein metabolism of the cell, or an early destruction of muscle tissue. Further, alterations in the calcium content of fractions which seem to regulate the movements of this ion in the cell are discussed. A new technique for homogenizing tissues from embryos of different ages is presented.

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