Abstract
The treatise On Formative Power of Ferrara's emblematic medical humanist, Nicolò Leoniceno, is the one of the first embryological monographs of the Renaissance. It shows, at the same time, the continuity of medieval Arabo-Latin tradition and the new elements brought by Renaissance medical humanism, namely through the use of the ancient Greek commentators of Aristotle like Simplicius. Thus this treatise stands at the crossroad of these two currents. The present study analyses the range of Leoniceno's philosophical discussion, determines its exact sources and brings to light premises for the early modern development of the concept of formative force, which will end up in the theory of "plastic nature" at the heart of the Scientific Revolution.