These chambers contain alien foetuses. The design of these containers seems to suggest that the aliens which use this process rely completely on laboratory reproduction. The rich nutrients ensure rapid development of the foetus. This factory line system could generate thousands of alien clones in a short space of time. The process could be easily adapted for human reproduction, or alien-human hybrids.
This is an unused feature that can't be researched during a normal game since you will never retrieve it from tactical missions. This research can only be done by editing one of the saved game files and assigning 1 unit of Alien Reproduction to the stores of one of your bases.
Although there are no hybrids appearing on X-COM they appear later on Apocalypse.