this is the scintifical explination fromhttp://www.asa3.org/ASA/topics/Origin%20of%20Life/spontaneous_generation.html

The first serious attack on the idea of spontaneous generation was made in 1668 by Francesco Redi, a natural philosopher to the Tuscan court that had been the patron of Galileo. At that time, it was widely held that maggots arose spontaneously in rotting meat. Redi believed that maggots developed from eggs laid by flies. To test his hypothesis, he set out meat in a variety of flasks, some open to the air, some sealed completely, and others covered with gauze. As he had expected, maggots appeared only in the open flasks in which the flies could reach the meat and lay their eggs.