Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont , a seventeenth century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks.
In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers Figure 1.
Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.
Figure 1. Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container. In , John Needham — published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures.
He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes. This suggested that microbes were introduced into these flasks from the air. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation Figure 2. Figure 2. The debate over spontaneous generation continued well into the nineteenth century, with scientists serving as proponents of both sides.
To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem. Louis Pasteur, a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. Although such a concept may seem laughable today, it is consistent with the other widely held cultural and religious beliefs of the time.
The first serious attack on the idea of spontaneous generation was made in by Francesco Redi, an Italian physician and poet. 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. This was one of the first examples of an experiment in the modern sense, in which controls are used. In spite of his well-executed experiment, the belief in spontaneous generation remained strong, and even Redi continued to believe it occurred under some circumstances.
The invention of the microscope only served to enhance this belief. Microscopy revealed a whole new world of organisms that appeared to arise spontaneously. It was quickly learned that to create "animalcules," as the organisms were called, you needed only to place hay in water and wait a few days before examining your new creations under the microscope. When Jean Baptiste Lamarck proposed his theory of evolution, to reconcile his ideas with Aristotle's Scala naturae , he proposed that as creatures strive for greater perfection, thus move up the "ladder," new organisms arise by spontaneous generation to fill the vacated places on the lower rungs.
In - , John Needham, a Scottish clergyman and naturalist showed that microorganisms flourished in various soups that had been exposed to the air. He claimed that there was a "life force" present in the molecules of all inorganic matter, including air and the oxygen in it, that could cause spontaneous generation to occur, thus accounting for the presence of bacteria in his soups. He even briefly boiled some of his soup and poured it into "clean" flasks with cork lids, and microorganisms still grew there.
A few years later - , Lazzaro Spallanzani, an Italian abbot and biologist, tried several variations on Needham's soup experiments. First, he boiled soup for one hour, then sealed the glass flasks that contained it by melting the mouths of the flasks shut. Soup in those flasks stayed sterile. He then boiled another batch of soup for only a few minutes before sealing the flasks, and found that microorganisms grew in that soup.
In a third batch, soup was boiled for an hour, but the flasks were sealed with real-cork corks which, thus, were loose-fitting enough to let some air in , and microorganisms grew in that soup. Spallanzani concluded that while one hour of boiling would sterilize the soup, only a few minutes of boiling was not enough to kill any bacteria initially present, and the microorganisms in the flasks of spoiled soup had entered from the air.
This initiated a heated argument between Needham and Spallanzani over sterilization boiled broth in closed vs. Needham claimed that Spallanzani's "over-extensive" boiling used to sterilize the containers had killed the "life force. In the minimally-boiled flasks, he felt the boiling was not severe enough to destroy the life force, so bacteria were still able to develop. By , the debate had become so heated that the Paris Academy of Sciences offered a prize for any experiments that would help resolve this conflict.
The prize was claimed in by Louis Pasteur, as he published the results of an experiment he did to disproved spontaneous generation in these microscopic organisms. Observation s : From Needham's and Spallanzani's experiments, it was known that soup that was exposed to the air spoiled — bacteria grew in it.
Containers of soup that had been boiled for one hour, and then were sealed, remained sterile. Boiling for only a few minutes was not enough to sterilize the soup. Pasteur had previously demonstrated that the dust collected by drawing air through a cotton ball contained large numbers of bacteria, hence he knew that bacteria were present in the air and could be filtered out by using a cotton ball.
He also knew that bacteria would settle out on the walls of a long, bent, glass tube as air was passed through it. Question: Is there indeed a "life force" present in air or oxygen that can cause bacteria to develop by spontaneous generation? Is there a means of allowing air to enter a container, thus any life force, if such does exist, but not the bacteria that are present in that air?
Hypothesis: There is no such life force in air, and a container of sterilized broth will remain sterile, even if exposed to the air, as long as bacteria cannot enter the flask. Prediction: If there is no life force, broth in swan-neck flasks should remain sterile, even if exposed to air, because any bacteria in the air will settle on the walls of the initial portion of the neck. Broth in flasks plugged with cotton should remain sterile because the cotton is able to filter bacteria out of the air.
Testing: Pasteur boiled broth in various-shaped flasks to sterilize it, then let it cool. As the broth and air in the containers cooled, fresh room air was drawn into the containers. None of the flasks were sealed — all were exposed to the outside air in one way or another. This allowed air to enter these flasks, but the long, swan neck or the cotton balls filtered out any bacteria present in that air. He subsequently broke the long necks off some of the swan-neck flasks. According to one freshman biology text, some of his original flasks, on display in France , still are sterile.
Data: Broth in flasks with necks opening straight up spoiled as evidenced by a bad odor, cloudiness in previously clear broth, and microscopic examination of the broth confirming the presence of bacteria , while broth in swan-neck flasks did not, even though fresh air could get it. Broth in flasks with cotton plugs did not spoil, even though air could get through the cotton. If the neck of a swan-neck flask was broken off short, allowing bacteria to enter, then the broth became contaminated.
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