Amsterdam offers a wealth of urban planning and architecture from the pre- and post-war periods. The south of the city in particular is a veritable open-air museum of modernism.  Most of the buildings were designed by architects of the expressionist Amsterdam School, but there are also some impressive examples of Functionalism — most notably the building of the Open-Air School by Jan Duiker. In the 1950s, Functionalism was replaced by Structuralism. The best-known icon of this school was created by Aldo van Eyck: an orphanage for 125 children. Twenty years later, Herman Hertzberger further developed the typical spatial construction of structuralism in his design for the Apollo Schools.