Louis Pasteur was responsible for the development of the swan knecked flask experiment which effectively disproved the previously accepted theory that cells were spontaneously generated from non-living matter (abiogenisis- without reproduction of cells).
Pasteur was alive from 1822-1895 and during that time people debated the issue with reference to how bacteria and mould could just "appear" on food and drink.
Many believed that just because you boiled some broth and covered it to show that no bacteria grew did not prove that bacteria came flying in through the air,because boiling killed the "active substace" and air was also needed for spontaneous generation of bacteria.
So pasteur developed the swan-knecked flask which allowed air in, but trapped any microorganisms in the bend of the kneck, preventing them from reaching the broth.
He added beef broth to these swan-necked flask, boiled them to kill any microorgansms that may already be present, and left them sitting. Months and months later there was still no growth of bacteria.
But when he broke the kneck of one flask, bacteria grew in the broth within days. Also in another flask, he tilted itover so broth could pour into the bend of the neck then he straightened it again. This flask also had bacterial growthwithin a few days.
The experiment showed that if bacteria -already living cells- were allowed into the broth, then more bacteria could grow, but if they were trapped in the neck and never got to the broth, the broth remained clear and free from bacteria.
It is said there is still a Louis Pasteur swan-necked flask with broth in it, with no bacterial growth in it, this many years down the track.