Belonging to the very first generation of medical humanists active in Italy at the turn of the sixteenth century, Nicolò Leoniceno was prolific in producing widely-used translations of Galen’s works. By examining the confrontation between the medieval Arabo-Latin tradition and Renaissance humanism in natural philosophy, this article aims to analyze the continuity of a traditional debate as it underwent a transformation via the new elements introduced by the humanist movement. Galen’s newly recovered embryological treatise “On the Formation of the Fetus” profoundly stimulated the debate on the nature of the formative power of the seed. Starting from Galen’s ideas as found in this treatise, Leoniceno composed one of the first “humanist” embryological works, “On Formative Power” (Venice, 1506). This treatise documents the beginning of a shift of the authority on which scholarship was based—from Arabic writers (Avicenna and Averroes) and their Latin followers (Pietro d’Abano and Gentile da Foligno) to the classical Greek sources, including not only the works of Galen himself but also the new Latin translation of Aristotle’s corpus as well as the newly recovered writings of Aristotle’s Greek commentators such as Simplicius and Michael of Ephesus. Leoniceno’s use of these commentators is particularly noteworthy because he was one of the first humanists to make recourse to them in medical and scientific discussions. Moreover, his familiarity with ongoing debates on the soul and intellect by his humanist contemporaries, such as the Florentine Platonists, will also become evident. Certainly, a typical humanist way of doing natural philosophy can be observed in Leoniceno’s text.