As for the success of M. Trécul’s experiments on the _penicillium_, we have no doubt that that gentleman has sown germs of yeast or _torulæ_, which bear so striking a resemblance to yeast, at the same time that he sowed spores, since he took his spores from sporangia of _penicillium_ that had been exposed to contact with ordinary air. The conditions under which M. Trécul conducted his experiment rendered it difficult for the spores—although, relatively, much more numerous than the contaminating germs of ferment with which they happened to be associated—to make way against these latter, since the spores were unable to continue their development in a medium that was deprived of oxygen. On the other hand, the cells of ferment might easily have multiplied to such an extent as to make the discovery of the spores that had been sown a matter of some difficulty, since those spores would have been lost amongst the great number of cells of ferment. This was, probably, one of the causes which led to the mistaken notion that such spores underwent conversion into cells of ferment. Now although botanists describe several varieties of _penicillium glaucum_, we do not suppose that the cause of the difference between our results and those obtained by M. Trécul can be attributed to our having operated upon a different variety of _penicillium_ from that which he used. Supposing that there had been a great difference between our two varieties, still M. Trécul declares that he has realized the phenomenon with numerous varieties of this fungus. M. Trécul expresses himself as follows: “I have used spores of _penicillium_ of several varieties in these experiments: Firstly, thick, green, elliptic spores of a variety of _penicillium_ that grows on lemons; secondly, elliptic spores of a bluish colour and smaller than the preceding, of another variety of _penicillium_ found on lemons; thirdly, spherical spores of the variety termed _penicillium crustaceum_; fourthly, spores of the _penicillium_ that develops on the yeast of beer.[61]” This criticism of M. Trécul’s opinions was written on the occasion of a discussion at the Academy, after we had been induced to read over again the remarks which he had published on the subject. We were so impressed by the positive manner in which his conclusions were stated, that we asked ourselves, once more, which of us could be mistaken, and once more, also, we applied ourselves to fresh experiments, which we conducted with every possible precaution, following, as far as we could without falling into the errors of which we accuse the learned botanist, the mode of procedure that he adopted. As his descriptions struck us as being at times insufficient, we resolved to ask him for certain explanations _vivâ voce_ (November 3rd, 1873), which he gave us with the greatest willingness.