In this video, we're going to do a side by side comparison of John Needham's and Lazaro Spallanzani's experimental design. And once again, Lazaro Spallanzani's experiments differed from John Needham's experiments in two significant ways. Number 1, Lazaro Spallanzani completely sealed the flasks by melting the flasks closed before he boiled the broth in the flask. And this is important because this helped ensure that no microbes could enter into the broth after boiling the broth. Now the second significant way that Spallanzani's differed from John Needham's is that Spallanzani boiled the broth for a significantly longer period of time. A longer boiling time helps to ensure that all of the microbes were killed by the heat, whereas shorter and brief boiling like what Needham did may not necessarily kill all of the microbes, and so there may still be some lingering microbes in the broth.

Now if we take a look at our image down below, notice that the top half of the image is focused on John Needham's experiment. Whereas the bottom half of the image is focused on Lazaro Spallanzani's experiment. And so, focusing on John Needham's experiment first, notice that Needham did not seal the flask by melting it closed and so the flask remained open to the environment. Also, he only boiled his broth for a brief period of time, a short period of time. Short boiling times may not necessarily kill any microbes that might be in the broth originally. And so there could be some lingering microbes even after a short period of boiling. Also, he allowed the broth to cool for a short period of time, which allowed for the opportunity for microbes to enter the flask from the air and contaminate the flask after boiling. And so where Needham thought that his flask was going to be sterile, it turns out that he was incorrect because there could have been lingering microbes or the microbes could have contaminated the flask from the air.

Also, John Needham sealed his flasks by using a cork, which is porous. A porous cork, meaning that it has pores in it and has holes in it, allows for microbes to pass through the cork and enter into the flask. And so here we're showing you how microbes can enter through the porous cork and still contaminate the flask. And so where John Needham saw microbial growth and attributed it to spontaneous generation, he was incorrect. The microbes could have come from being either lingering microbes after short boiling times or microbes that simply just entered into the flask from the air.

Now down below, we're showing you Spallanzani's experiment, in which you can see he sealed his flask by melting the glass closed. Melting the glass helped to prevent contamination throughout his experiment. Then he boiled his broth for a significantly longer period of time, which helped to ensure that all of the microbes that may have been existing in the broth were killed and that his broth was actually sterile. And so what you can notice is that in Spallanzani's experiments, the microbes were absent. And because the microbes were absent, he suggested that spontaneous generation was not the way for microbes to develop. And so you can see there is no spontaneous generation here.

And once again, even after Spallanzani's experiment, some people still remained skeptical of this idea of biogenesis, that life only comes from life and that life cannot spontaneously generate. They remained skeptical even after Spallanzani's experiment. And so it was not until almost 100 years later after Spallanzani's experiment that another French scientist by the name of Louis Pasteur, was able to confirm Spallanzani's findings using another simple experiment, with a custom flask. And so we'll get to talk about Louis Pasteur's experiment in our next lesson video. But for now, this here concludes our lesson comparing John Needham's and Lazaro Spallanzani's experimental design, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.