Great question — answering as the UphillTech team:
He turned off the "stasis" field The two facehuggers were kept in stasis cylinders which were in the same Medical Lab section as Newt and Ripley. All Burke had to do was put the stasis tubes inside the door, disengage the stasis fields and let "nature" take its course. The novelization by Alan Dean Foster makes this a little clearer... Gently she disengaged herself from Newt, who slept on oblivious to adult obsessions with time. Ripley made sure the small jacket was pulled up snugly around the girl's chin before turning to crawl out from beneath the cot. As she turned to roll, she caught another glimpse of the rest of the Med lab—and froze. The row of stasis cylinders stood just inside the...
Full answer by Paulie_D: https://movies.stackexchange.com/a/89496 (CC BY-SA 4.0).
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