Hive

The hive is the primary structure of the zerg and is the key that unlocks the final stages of their evolution. Protected by a dense exoskeleton and granted full access to the Overmind's pool of knowledge, it continues to provide the same functionality that it has since it was a hatchery (birthing larvae, maintaining creep, and digesting minerals/gas). Once the hive is present, the being commanding it has the potential to unleash the full fury of the zerg.

The genetic model for a hive is extremely complex, and would be indecipherable by any terran scientist.

StarCraft II
The hive reappears in StarCraft II. The hive can set two rally points, one ("Y") for most units and one ("G") for drones. The does not get a rally point.

Each individual egg can be separately rallied; this overrides rally points set for the hive.

Development
The hive for StarCraft II was based off the lair in StarCraft, in order to make it look more 'evil'. There were also plans to change the middle maw portion more gradually as it evolved from a hatchery, but there wasn't enough time to do so.