The chief organised ferments are the yeast-plant, which produces alcohol and carbonic acid from grape sugar, and various kinds of bacteria, one of which produces butyric, another lactic, and another acetic fermentation. Both yeast and bacteria belong to the lowest class of plants, the protophytes. To this class also belong moulds, the action of drugs upon which is sometimes important, inasmuch as moulds give rise to some skin diseases.

Yeasts, moulds, and bacteria have been variously classified by different authors, and the classification is apt to undergo changes as our knowledge of the life-history of these different organisms increases.

At present it is not certainly known whether the various.

1 Just after this there is unfortunately a blank in my notes, but Professor Kuhne has kindly supplied the deficiency, and informs me that he was then speaking of slices taken from the external surface of the stomach, and therefore containing the lower ends only of the gastric glands.

2 Liversedge (Nov. 1872), Journ. of Anat. and Physiol., Nov. 1873, p. 23.

3 Heidenhain, Pflilger's Archiv, Bd. xi. p. 557.

kinds of bacteria, for example, are generically or specifically different, or whether they can, by altered cultivation, be transformed into one another or not.

Koch, who has cultivated them by the dry process on gelatine instead of in liquid, and has thus been able to avoid admixture of different kinds of bacteria, has come to the conclusion that each kind possesses distinctive characters; but Klein has shown that, even when cultivated in this way, bacteria may vary much in form. Thus the bacillus anthracis may form torula-like cells, from which ordinary bacilli are again produced.

The numerous names used in treatises on the subject of organised ferments are apt to lead to confusion, hence some of the names are given here simply for the purpose of reference.

Thus Brefeld's classification is :-(1) Phycomycetes = algoid fungi; (2) Mycomycetes = true higher fungi; (3) Myxomycetes = gelatinous fungi; (4) Blasto-mycetes = yeast fungi; (5) Schizomycetes=bacteria.

The classification into yeasts, moulds, and bacteria which I have followed may not be botanically correct, but it is convenient for our present purpose.