This paper focuses on the analysis and contextualization of an eighteenth-century experimental notebook that contains a fundamental discovery: the division of infusoria, reinterpreted a century later as mitosis. This unknown research by the young Genevan patrician Horace-Bénédict de Saussure took place in a dynamic European context, encouraged in particular by his uncle Charles Bonnet. Based on an analysis of the diary of microscopic experiments, combined with correspondence between scientists, a model of scientific discovery as a process from its construction to its dissemination is proposed, reconciling logical, social and linguistic factors. Finally, the article shows that, contrary to the view of classical historians of microscopy that the eighteenth century was devoid of fundamental research, it was a period of cutting-edge microscopic research.