194 PASTEUR: THE HISTORY OF A MIND 



"In the experiments conducted as I have just de- 

 scribed, the yeast which comes into existence, and which 

 very promptly causes an active alcoholic fermentation, 

 is introduced originall}^ by the atmospheric air, which 

 allows the germs to fall either upon the mycoderma 

 pellicle or the objects which are used during the succes- 

 sion of manipulations. Two circumstances of these 

 experiments gave me warning of the existence of this 

 cause of error. It sometimes happened that I found 

 among the cells of the mycoderma in the bottom of 

 the flasks where I had submerged the flowers of wine, 

 some large spherical cells of Mucor mucedo or racemosus, 

 yeastlike cells with which we shall soon become familiar 

 in studying this curious mold. Since there is present 

 Mucor mucedo or 7'acemosus, although I had sown only 

 Mycoderma vini, it must be, I said to myself, that one 

 or several spores of this Mucor have been introduced 

 by the ambient air. Now, if the air brings the spores 

 of Mucor into my operations, why would it not bring the 

 cells ot yeast, especially in my laboratory? Further, 

 more, it happened that in a number of the experiments 

 which I repeated many times, under the pressure of my 

 doubts, and in which I did not grow weary of searching 

 for this desirable transformation which accorded so well 

 with the physiological theory of fermentation to which 

 I had been led, some had a negative result, that is to say 

 the transformation of the mycoderma into yeast did 

 not occur, although the conditions were as similar as 

 one could wish to those of the experiments in which I 

 saw it take place. Why, thought I, this inactivity 

 in the cells of the mycoderma? Even in the most 

 favorable cases of the supposed transformation it hap- 

 pened without doubt that a multitude of cells of Mi/co- 

 derma vini did not become yeast cells, but how admit 

 that among the billions of submerged cells all were 



