DNA methylation is an epigenetic modification that plays an important role in the proper development and functioning of an organism. The DNA methylation level is species-, tissue- and organelle-specific, and the methylation pattern is determined during embryogenesis. A correlation between methylation and age is also observed. Epigenetic phenomena are an enormously interesting research subject, not only from the perspective of pure science, but also due to their possible applications in medicine. The aim of this study was to determine the global DNA methylation level in relation to the developmental stage of the embryo. The global level of 5-methylcytosine in the DNA during pulmonary respiration was found to be higher than during allantoic respiration. The analysis shows a clear dependence between the stage of individual development and the global DNA level of 5-methylcytosine. In the future, methylation level may be a determinant of age and perhaps even a tool for predicting life expectancy. Abnormalities in the methylation process result in premature ageing at the cellular and individual level.