The Spawning Pool is the first building Zerg that can be morphed in a new Starcraft match, after the Hatchery. Construction of the Spawning Pool unlocks the ability of the Hatchery to morph Zerglings.
Two upgrades are available at the Spawning Pool, both benefiting the Zergling: Metabolic Boost to enhance its movement and Adrenal Glands to enhance its attack. The latter, Adrenal Glands, first requires a Hive before being able to be researched. The Spawning Pool also enables the production of the other buildings in the Zerg tech tree.
In all race match-ups, the Spawning Pool is the first building to morphed by Zerg. The Metabolic Boost speed upgrade is generally also researched early on in all match-ups, enabling "Speedlings" to be more effective in both offensive and defensive purposes. In the ZvZ match-up, which rarely progress to the Hive stage, the Adrenal Glands upgrade is very uncommon; in the ZvT and ZvP match-ups, they are among the earliest upgrades researched by the Zerg player in the Hive stage as they significantly enhance the effectiveness of Zerglings, which are often referred to as "Cracklings".
See also Zergling