B747-400 dumps fuel and returns to departure airport when landing gear fails to retract after takeoff.

Narrative

I was flying the takeoff from ZZZ. I called for the gear up and the Captain raised the gear handle and it would not go past the off position. The gear tilt message was displayed and the gear disagree message was displayed shortly there after. Captain directed me to continue to fly the aircraft and talk on the radios. Captain and the other First Officer ran the gear tilt procedure and then the gear disagree procedure. Prior to the procedure we cleaned up the aircraft and flew at 255 KTS and told ATC we would like to stay at 5000 FT and have vectors over the water so we could run the checklists. After talking to maintenance through dispatch; we agreed to dump fuel and return to ZZZ. Gear handle was placed in down position per the checklist and checked normal. ATC declared an emergency for us and fire equipment was standing by. Captain took control of the aircraft for landing and it was uneventful. Aircraft was checked on taxiway by maintenance and fire fighters. We were cleared to taxi to the gate.