Kinetics of accumulation of creatine phosphokinase activity in developing embryonic skeletal muscle in vivo and in monolayer culture

1. 1. The creatine phosphokinase content of developing chick muscle in vivo and in vitro has been measured using the firefly ATP assay method of Strehler and Totter. 2. 2. The concentration of creatine phosphokinase in chick leg musculature increases with a constant doubling time between the 11th and 22nd day of incubation. The doubling time is slightly longer than one day at 37 °C. 3. 3. The concentration of creatine phosphokinase in developing monolayer culture of chick muscle increases much more slowly under our experimental conditions (36.5 °C). 4. 4. In tissue cultures this enzyme is not detectable until after the appearance of multinuclear muscle elements.