From the time of the Ancient Romans, through the Middle Ages, and until the late nineteenth century, it was generally accepted that life arose spontaneously from non-living matter. Such "spontaneous generation" appeared to occur primarily in decaying matter. For example, a seventeenth century idea for the spontaneous generation of mice required placing sweaty underwear and husks of wheat in an open-mouthed jar. Then, waiting for about 21 days, during which time it was said that the sweat from the underwear would penetrate the husks of wheat, changing them into mice. Although that idea may seem ridiculous today, that and other...
Air could enter the flask, but airborne microorganisms could not - they would settle by gravity in the neck. As Pasteur had expected, no microorganisms grew. When Pasteur tilted the flask so that the broth reached the lowest point in the neck, where any airborne particles would have settled, the broth rapidly became cloudy with life. Pasteur had both refuted the theory of spontaneous generation and convincingly demonstrated that microorganisms are everywhere, even in the air!!! And also because of Pasteurs success, the theory of biogenesis came about that said that living organisms come only from other living organisms.