Assimilator

The assimilator is a protoss building used for harvesting vespene gas. Although materials were usually warped in from Aiur, vespene is still required for these materials to be produced and the creation of warp rifts.

StarCraft
An assimilator must be warped in on top of a vespene geyser before probes can collect vespene gas from it, packaging the gas for collection.

StarCraft II
The assimilator remains much the same as its predecessor in StarCraft II. The assimilator no longer yields any gas when it is depleted.

Development
The assimilator's StarCraft II form went through a few revisions before the designers decided upon the current version in-game. The intention for the update was to showcase the functionality a little better by actually seeing the little gas cubes being manufactured inside. The final form is effectively a dome that covers a vespene geyser, with a crater visible inside.