The present essay analyzes an eighteenth-century phase of the querelle des monstres and highlights two main points. 1) As the cases of Lémery and Winslow demonstrate, in the period when preformation was the dominant view, the dispute over the origin of monsters carried into the very field of preformation the contrast which had originally opposed it to the now defeated model of epigenesis, namely the alternative between mechanical genesis and pre-existence of the monstrous form itself. 2) One of the most important episodes in the shift of teratology from a primarily theological or metaphysical issue to a purely natural one was due to Albrecht von Haller. Haller shifted the dispute from anatomy to embryology; and it is on an embryological base and not on metaphysics that he built his own demonstration of the original nature of the monster. He was furthermore the only scientist of authority who dealt with teratology from an epigenetic standpoint. His numerous changes of view in the field of embryology did in fact never affect his early adherence to the thesis of original monstrosity.