This chapter explains the concept to investigate human organs and provide scientific descriptions of sperm and eggs, and to explain complex processes such as pregnancy and foetal growth. Enlightenment scientists believed that the man's seeds carried the life-force, soul, or form depending on which term they used. Two main positions can be discerned which coincide roughly with the two main theories of procreation. The supporters of the epigenesis theory envisaged foetal development in the womb as a gradual evolution from formless matter to an animal shape to a human form. The competing theory, preformation, held that a pre existing human individual existed in male sperm or in the female egg. The bodily location of the soul was a contested issue among medieval theologians, the New Testament and patristic writings. Descartes did in fact present an intermediary solution by locating the soul in a tiny structure in the interior of the brain, the pineal gland.