One of the founders of the Museum Vrolikianum, Professor Willem Vrolik (1801-1862), was very interested in teratology, especially in a congenital malformation termed cyclopia. In 1834 he published a paper on cyclopia. This work was mainly based on studies of cyclopic specimens present in the collection of his father, Professor Gerardus Vrolik. In this study he proposed a classification system for cyclopes, in which he divided them into five main types. This study also formed the basis for the chapters on cyclopia in his Handbook of pathological anatomy (1842-1844) and his Tabulae ad illustrandam embryogenesin hominis et mammalium (1844-1849). In these studies the specimens of cyclopes of man and mammals, still present in the collection of the Museum Vrolik in the Department of Anatomy and Embryology of the University of Amsterdam, were described and illustrated with beautiful lithographs. The collection consists of five human cyclopes and nineteen other cyclopic mammals. These mammals are pigs, lambs and a cat.