The Italian priest and naturalist Spallanzani (1729-1799) was a meticulous and creative experimentalist.  With a greatly improved microscope, he studied the chemical process of digestion, the generation of frogs, and the fertilization of the eggs in various species.  

He examined rotifers (Figs I-VII), describing the internal structure of their trunks and feet, and comparing the ciliated structures on their heads (Fig. I portrays a rotifer with two such structures).  

Spallanzani discovered tardigrades (Fig. VIII), which have a remarkable ability to survive extreme environmental changes.  Their repeated death and revival cycles, now called anabiosis, demonstrated both the surprising resistance of microscopic life and the difficulty of obtaining sterile samples.

Inspired by Redi’s arguments against the spontaneous generation of flies from decomposing meat, Spallanzani became determined to show that microscopic organisms also do not generate spontaneously.  For Spallanzani, all living organisms, large or small, arise from an egg rather than from plastic forces.  To prove this conclusion, Spallanzani demonstrated the power of using control groups with a series of elegant experiments in which he boiled meat broth in flasks under various conditions to see if organisms would appear after a few days. 

Advocates of spontaneous generation reported that even if flasks were brought to a boil, organisms would appear in the broth after a few days.  Spallanzani repeated this experiment, yet pressed further.  If flasks were boiled for an hour or longer, organisms did not appear.  The second group showed that the first experiment had overlooked the resistance of microorganisms.  

Advocates of spontaneous generation reported that even if flasks were sealed with corks, organisms would appear in the broth after a few days. Spallanzani took a step further, sealing the flasks by melting their glass rims onto the corks.  In these tightly sealed flasks no organisms appeared.  Spallanzani concluded that the flasks in the first group, sealed only with corks, regrew organisms due to contamination with outside air.

The History of Science Collections holds other works by Spallanzani, including Saggio di osservazioni microscopiche concernenti il sistema della generazione (ca. 1765), in which he argued against the spontaneous generation theories of Needham and Buffon.