According to the search results, the optimal day of embryonic development to dissect motor neurons for cell culture in rats is embryonic day 14 (E14) . At this stage, the motor neurons have been born but have not yet undergone extensive axonal extension or developmental apoptosis. Dissecting motor neurons at this stage allows for the isolation of large motor neurons with high survival and purity. However, it is important to note that different protocols may have slightly different recommendations, and it is always best to consult the specific protocol being used for the most accurate information.