The author argues that sexual differentiation is established during early embryonic cell divisions, with properties of the ovum's protoplasm predisposing descendant cells toward male or female development. He reviews observations and experiments indicating that factors such as parental nutrition, temperature, and the physical state of the ovum can influence fertilization success and sex outcome. The work surveys prior literature, outlines experimental methods and specific investigations in animals and plants, and proposes techniques for artificially influencing sex ratios while calling for further research and careful methodological practice.