In the rebuilding process, former one- and two-story wood structures were replaced by modern five- and six-story buildings of concrete and steel in the European style. Straight new motorways replaced twisting narrow streets. The first underground subway system opened in 1927 and a new airport in 1931. At 6.36 million, the city's population in 1935 was larger than before the earthquake; it was nearly as large as London or New York. The "secondary centers" or "satellite cities" (fukutoshin) of Shinjuku, Shibuya, and Ikebukuro grew rapidly. They and the entire surrounding Tokyo prefecture was incorporated into the city in 1943.