Answer: incubating

AnswerCrossword Clue
incubating(esp. in a laboratory) Keep (eggs, cells, bacteria, embryos, etc.) at a suitable temperature so that they develop
incubatingDevelop slowly without outward or perceptible signs
incubatingINCUBATE, to warm eggs for hatching
incubating(of a bird) Sit on (eggs) in order to keep them warm and bring them to hatching
incubatingHave an infectious disease developing inside one before symptoms appear