In examining the saliva for lymph-cells, it is probable that some much larger objects may have been noticed of irregular polygonal outline and with a well-marked nucleus. These are cells from the lining _epithelium_ of the mouth, and only differ from those of the _epidermis_ of skin in their form and size (Fig. 3). Note the markings caused by the pressure of overlapping cells. In these cells the wall is formed of keratin or horny tissue, which takes the place of the cellulose of the yeast. [Illustration: FIG. 4.--_Penicillium glaucum_, a common green mould.] Other simple forms of cell are those of _Saccharomyces mycoderma_ or _torula_ which forms a skin on the surface of old liquors, and which much resembles a small yeast; and of the various ferments which are found in liquors, bates and drenches, which will be more fully described in the chapter following. Many of these, such as the acetic and lactic ferments, which, like all other _bacteria_, multiply by division, do not separate, but remain connected in chains or chaplets, like a string of beads. From these, the step is not a long one to the _hyphæ_ or stems of the higher moulds, which are too frequently found on leather which has been slowly dried, and which consist simply of tubular cells which elongate and divide by the formation of _septa_ or cross-partitions, and thus build up a complicated plant-structure (Fig. 4). As we proceed higher in the scale of plant and animal life, the forms and products of the cells become more varied, and instead of one single cell, fulfilling all the functions of the plant or animal, each class of cell has its own peculiar duties and properties, while all work together for the maintenance of the complex structure of which they form a part. CHAPTER IV. _PUTREFACTION AND FERMENTATION._ The chemical changes produced by the unicellular plants, such as yeasts and bacteria, to which allusion has been made in the last chapter, are known as fermentation and putrefaction, and are of such importance to the tanner, both for good and evil, that the subject must be treated in some detail. No scientific distinction exists between fermentation and putrefaction, though it is customary to restrict the latter term to those decompositions of nitrogenous animal matter which yield products of disagreeable smell and taste. The organisms which are the cause of both fermentation and putrefaction are known by the general term of “ferments.” This term has also been extended in recent years so as to include the so-called “unorganised ferments” (enzymes, zymases) which are active products secreted by the “organised ferments” or living organisms. These latter are again divided into three classes:-- 1. Moulds. 2. Yeasts (Saccharomycetes). 3. Bacteria. The members of one class are distinguished from those of another by their form, and, more especially, by the substances they produce during their life-history. All three classes are now considered to be fungi.