Modified incubation factors affect skeletal musculature development of broiler chickens during embryonic and postnatal periods of development. These changes appear to have great influence on the proliferation of myoblasts and muscle growth after hatching. In this study, the histomorphological and -morphometric parameters of broiler chicken leg musculature, after thermal and light treatments during embryogenesis, were examined. The applied treatments included thermal manipulation (exposure of

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... tilized eggs to increased temperature), light manipulation (exposure of fertilized eggs to monochromatic green light) and simultaneous thermal and light manipulations. Following the specific parameters that were observed, it was determined that in the late postnatal period of development, treated groups have a larger diameter and smaller nucleocytoplasmic ratio of muscle cells compared to those of the control group, in which fertilized eggs were incubated at a constant temperature of 37.8°C, without light. Diameter was increased by 4.20%, 3.77% and 4.55% on day 42 after hatching in thermal, light and combined thermal and light treated groups, respectively, compared to the control. The nucleocytoplasmic ratio was increased by 37.5% on day 42 after hatching in the control group compared to all treated groups. The volume density of the muscle connective tissue was approximately at the same level between the groups. It can be concluded that thermal and light treatments stimulate the proliferation of myoblasts and enhance development and growth of leg muscles in broiler chickens.