In Sicily, reflections on monsters and on «generative stumbles» first appear in a treatise from the second half of the sixteenth century, containing one of the first medicalphysiological descriptions of two-headed births and written in the vernacular by the famous physiologist and anatomist Giovanni Filippo Ingrassia. Ingrassia's contribution to the field of teratology is interwoven with his more important and observable activity in the role of protomedico (chief of the controlling body of all the medical activities in the Kingdom of Sicily), which required him to make important public decisions regarding medical care. It seems certain that Ingrassia's scientific treatise cannot be easily classified along with other teratological descriptions of the second half of the sixteenth century. Rather, a naturalistic sensitivity emerges that is disengaged from transcendental presuppositions. Starting with an analysis of this important work by Ingrassia, this paper then draws comparisons with other teratological treatises.