Full Idea
Unlike earlier writers (such as Fraenkel), Zermelo clearly allows that there might be ur-elements (that is, objects other than the empty set, which have no members). Indeed he sees in this the possibility of widespread application of set-theory.
Gist of Idea
Zermelo allows ur-elements, to enable the widespread application of set-theory
Source
Michael Hallett (Introduction to Zermelo's 1930 paper [1996], p.1217)
Book Reference
'From Kant to Hilbert: sourcebook Vol. 2', ed/tr. Ewald,William [OUP 1996], p.1217