Multiplying afresh in the fermented medium, under the influence of the air, they form a kind of mycodermic film on the liquid surface, or a ring round the sides of the flask, on a level with the liquid. This development might often be mistaken for _mycoderma vini_ or _cerevisiæ_; in reality there is not a single cell of _mycoderma_ formed. If we sow a trace of the new growth in a saccharine medium it will behave exactly as yeast would, budding and multiplying, and setting up fermentation in the liquid. And thus, in spite of its mycodermic aspect, this growth is nothing but yeast, since it gives rise to true alcoholic fermentation; but it is a kind of yeast which, under the foregoing conditions, lives after the manner of fungoid growths, absorbing the oxygen of the air and emitting carbonic acid gas. It appears on the surface of all fermented liquids, especially those which, like beer, contain carbohydrates, and its quantity is the greater, and its action the more rapid, in proportion as it has more perfect access to the air. We have termed this yeast _aërobian ferment_ or _fungoid ferment_. It may easily be understood how this kind of production has escaped notice up to the present time. The conditions of our experiment were, in many respects, novel; a saccharine liquid had never before been caused to ferment by means of pure yeast, absolutely free from foreign germs; a fermented liquid had not previously been exposed to contact with pure air for an indefinite time. On the other hand, all ordinary fermented liquids, when left to themselves in contact with air, are a ready prey to _mycoderma vini_ or _aceti_ at their surface, and then give rise to true fungoid growths. The appearance of these organisms, which always takes place soon, has thus constantly concealed or prevented the development of the true aërobian ferments. In repeating the experiment described any alcoholic ferment may be used, and each one will be found to produce its own peculiar fungoid form of ferment. Another point worthy of notice is that these aërobian ferments, when they put forth buds in the act of fermentation, reproduce the forms of the original ferment, at least apparently so. In this respect they cannot be distinguished, notwithstanding the fact, surprising as it seems, that the two kinds of ferments are not identical. If we operate on a “low” yeast its aërobian ferment will differ physiologically from the ferment from which it sprung, presenting various special peculiarities which are not to be found in the original “low” yeast. In most of our experiments we have found the new aërobian ferment to be similar in its action to “high” yeast, rising to the surface, and producing a beer which possesses a greater fragrance than beer brewed with the identical “low” yeast from which it was derived. Lastly, the properties of an aërobious ferment are not peculiar to first growth, but are hereditary; by repeating the growth of the first aërobian ferment we do not cause them