work on the section between "invalides" and "croix-rouge" began in 1913 and ended on 18 march 1920. however the prospect of low revenues that would certainly result from such a small section serving only neighborhoods of little activity caused the compagnie du chemin de fer métropolitain de paris (cmp) to delay the construction and postpone the opening and what would surely be a resulting deficit as far back as possible. finally, after being required by the city to do so, the company opened the line for use on 30 december 1923. the new line 10 consisted of six stations, each of which with a vaulted ceiling. the line borrowed the "rue de four", the "rue de sèvres", and the "boulevard des invalides" with a very pronounced bend at the station of "duroc".