[24] Engel, “Over enkele origineelle teekeningen,” 46–55. In tracking down the drawings, it was discovered that the Amsterdam Zoological Museum no longer exists and the collection was transferred to several places, including the special collections at University of Amsterdam Library, Artis Library, and the Naturalis Biodiversity Center, among possibly others. None of the abovementioned institutions have identified the sketches as part of their collections. However, it is possible that they are in the restricted parts of the institutions’ archives that are not available at the moment.