Zerg egg

The Zerg egg is the basis of all Zerg units other than Zerg Larvae. They exist only for a short time: they are created when a Larva is given an order to morph into a unit and burst to give burst to the ordered unit.

Overview
All Zerg units are morphed from the Zerg Larvae. When one is given an order to morph into a unit, it encases itself in a soft yet extremely protective cocoon that is very difficult to destroy, entering its pupal stage as it transmogrifies its own body into the ordered unit. When the morphing is complete, the egg ruptures and the unit bursts out of it.

Game unit
The egg has no upgrades that influence them, as their sole reason for existance is to builld units and disappear, and nothing more. However, unlike in other races, Zerg units can be killed individually before they are constructed, as the egg can be destroyed when it suffers a constrated heavy fire. Its extremely heavy armor that it retains from its larval form contributes in protecting the unit inside.

StarCraft II
In StarCraft II, all Zerg units (other than spawned units like Broodlings and Infested Marines) are still morphed from Larvae, briefly turning into an egg as they are 'trained'.