This chapter provides new insights into the developments that gave rise to the present-day understanding of gametes. Examining nineteenth-century research on reproductive substances, cells and heredity, it reassesses Thomas Laqueur’s view that in the modern era egg and sperm became a synecdoche for two incommensurable sexes. It argues that not only ideas stressing the difference between the sexes, but also views that presumed their similarity and equality, shaped the modern concept of germ cells. Moreover, by scrutinizing changing views on and debates over sperm from the late eighteenth to the late nineteenth century, this chapter explores the intricate history of ideas about the male role in reproduction. Well into the nineteenth century, the opinion prevailed that men did not provide anything material to the embryo. The demise of this view from the 1840s onwards, and the conceptualization of the male contribution in material and quantitative terms, was a decisive historical turning point which led biologists to describe eggs and sperm not only as entities embodying sexual difference, but also as cells containing equal shares of the hereditary material.