The zergspawning pool is a pit of primordial ooze which contains the genetic makeup of the zergling. A core feature of any hive cluster, the spawning pools are all modeled after the first spawning pool on Zerus, the pool of ooze where the very first zerg evolved and consumed essence to evolve.
The spawning pool gives larvae the ability to morph into zerglings and the hatchery the ability to evolve into the lair. The spawning pool is also responsible for evolving the zergling with Metabolic Boost (faster movement) and Adrenal Glands (faster attack).
The spawning pool was visually updated during the first pass of creating final artwork for StarCraft II. The main difference was artistic direction asking for a single large pool rather than the numerous smaller pools seen in the original game. The beating heart in the center was an afterthought to make the design more interesting. It was desired that the intestines have a more "veins pumping look" but the water obscured it too much for this to be implemented.
The research to unlock the ability for zerglings to morph into banelings was once possessed by the spawning pool.