Complex sentence: Lazzaro Spallanzani repeated the experiments but this time covering the containers which contained the gravy and wheat (which Needham did not do), and the living organisms which appeared in Needham's original experiments did not appear.
Split into simpler sentences: Lazzaro Spallanzani repeated the experiments but this time covered the containers which contained the gravy and wheat, which Needham did not do. The living organisms which appeared in Needham's original experiments did not appear.