In his experiment on spontaneous generation, Louis Pasteur changed only one thing between his experimental groups: whether or not the neck of the flask was broken. That means that his experiment was an Experimentum Crucis.
In 1850's, there was a theory accepted by the scientific community and impulsed by the naturalist Felix Archimede Pouchet, the author of the heterogenesis theory, which said that life can be created by spontaneous generation. His experiments consisted in boiling a broth into a flask and saw how some days later there were microorganisms in the flask.
Louis Pasteur made a similar experiment but changing a few things and he could demonstrate that the the conclusion of Pouchet's experiments were evidently wrong, and he could show a great evidence about his own theory which say that the life only can be produced by another life.
Pasteur's experiment consisted in boiling a broth into swan-necked flask and wait for some month showing the the boiled broth were in the same condition that at the first day, that is without any fermentation nor any presence of microorganisms with the only exception of the U of the shan-necked. The presence of microorganisms in the U place was explained by the contact with the air and dust that came with it, but since the rest of the neck was sterilized these could not be transferred to the broth. Finally, when all the observers were sure about the stability of that situation, Pasteur changed the conditions with the last part of the experiment, that is he broke the neck of the flask and in a few days it was shawn the presence of fermentation and microorganisms presence.
In that way he destroyed the spontaneous generation theory and he oppened the doors to his new theory "Omne vivum ex ovum" to the scientific knowledge with an irrefutable and repeatable experimental probe (Experimentum crucis).
He show the results of the experiment in 1864 in the Sorbona with great success and then it were created the germinal theory of diseases and cellular theory based in these principles, giving beginning to modern microbiology.
The correct answer to this open question is the following.
In his experiment on spontaneous generation, Louis Pasteur changed only one thing between his experimental groups: whether or not the neck of the flask was broken. That means that his experiment was controlled.
When scientists do control experimentation, this means that the scientist can manipulate directly the experiment. The aim is to test just one variable at the time to find specific results. The correct name for the variable being tested is the "independent variable." This allows the scientist to adjust the effects or minimize the effects of the subject matter. That is what Louis Pasteur did in his experiment.
