Adult male chickens have a greater muscle mass than adult female chickens. Available data suggests that this dimorphism is a result of events that occur during embryonic myogenesis. Previous results from our laboratory have established that at day 12.5 of embryonic development (E12.5), male embryos have a greater mass of breast muscle than female embryos, but that there is no significant difference in male and female muscle mass at other stages of development. The objective of this research was to characterise morphological difference between male and female embryonic muscle and identify possible factors involved in this process. Here, we have further characterised this E12.5 morphological difference, and found that male myofibres are larger than female myofibres but myofibre density and PAX7 positive cell density is lower in males than in females. We also measured the expression level of MuSK, a key gene in neuromuscular junction formation, and found that MuSK is expressed at higher levels in male chicken breast muscle than in female chicken breast muscle. This may result in a greater degree of neuromuscular junction formation and lead to more myofibres surviving in male embryos.