The possibility of living without oxygen, in the case of ordinary moulds, is connected with certain morphological modifications which are more marked in proportion as this faculty is itself more developed. These changes in the vegetative forms are scarcely perceptible in the case of _penicillium_ and _mycoderma vini_, but they are very evident in the case of _aspergillus_, consisting of a marked tendency on the part of the submerged mycelial filaments to increase in diameter, and to develop cross partitions at short intervals, so that they sometimes bear a resemblance to chains of conidia. In _mucor_, again, they are very marked, the inflated filaments which, closely interwoven, present chains of cells which fall off and bud, gradually producing a mass of cells. If we consider the matter carefully, we shall see that yeast presents the same characteristics. For instance, what can more closely resemble the mucor of Plates V. and VI. than the _saccharomyces_ of Figs. 33 and 37? Have we not in each case ramified chains of elongated cells or joints, more or less narrowed in the middle, and shorter segments or cells dropping off at the constrictions, and proceeding to bud in the liquid on their own account? Moreover, the less oxygen there is present, the more marked is the tendency to the formation of these budding cells, which isolate themselves and soon drop off. Who could ever imagine, in examining the ferment of _mucor_ represented in Plate VI., that its first germ was the ordinary _mucor_ that is found everywhere, with fine filaments, straight or ramified according to the variety, which send up aerial _hyphae_, terminating in little round heads bearing spores. So was it that in the ferment of Plate XI. we could scarcely recognize the ramified filaments of Figs. 33 and 37. It is a great presumption in favour of the truth of theoretical ideas when the results of experiments undertaken on the strength of those ideas are confirmed by various facts more recently added to science, and when those ideas force themselves more and more on our minds, in spite of a _primâ facie_ improbability. This is exactly the character of those ideas which we have just expounded. We propounded them in 1861, and not only have they remained unshaken since, but they have served to foreshadow new facts, so that it is much easier to defend them in the present day than it was to do so fifteen years ago. We first called attention to them in various notes, which we read before the Chemical Society of Paris, notably at its meetings of April 12th and June 28th, 1861, and in papers in the _Comptes rendus de l’Académie des Sciences_. It may be of some interest to quote here, in its entirety, our communication of June 28th, 1861, entitled, “Influences of Oxygen on the Development of Yeast and on Alcoholic Fermentation,” which we extract from the _Bulletin de la Société Chimique de Paris_:—