sentences:  Under Rebay's guidance, Guggenheim sought to include in the collection the most important examples of non-objective art available at the time by early modernists such as Rudolf Bauer, Rebay, Kandinsky, Piet Mondrian, Marc Chagall, Robert Delaunay, Fernand Léger, Amedeo Modigliani and Pablo Picasso.By the early 1940s, the foundation had accumulated such a large collection of avant-garde paintings that the need for a permanent museum building had become apparent.In 1943, Rebay and Guggenheim wrote a letter to Frank Lloyd Wright asking him to design a structure to house and display the collection.