A very large number of ferments are already known. Each has its own specific work to do: “To every fermentable substance is fitted a ferment, as a key to a lock.” It will be understood, from what has been already said regarding our inability to determine the composition of any ferment, that we cannot say whether or not these various ferments differ one from another in chemical constitution. They are classified according to their action, and not according to their nature. Those which build up are termed “synaptases” (συνάπτω, I unite); those which decompose, or hydrolyse, “diastases” (διάστασις, separation). The termination “ase” is added to the name of the substance upon which the ferment acts, except in cases in which other terms have already become so general as not to be displaceable: amylase, hydrolysing starch; sucrase, inverting cane-sugar; protease, hydrolysing proteins. Unfortunately, there is little uniformity in this nomenclature; amylopsin, invertin, pepsin, are terms used as often as those terminating in “ase.” As a distinguishing termination, “in” or “sin” is less desirable than “ase,” owing to the fact that it has been appropriated already as the termination of the names of albuminoids—_e.g._, gelatin, chondrin, mucin. The various ferments are substances which protoplasm sets aside for specific purposes. Primitively, contact with the substance to be fermented determined the nature of the ferment assigned to the task. There are reasons for thinking that protoplasm still retains its power of making a suitable response; cases may be cited in which the lock presented to protoplasm shapes the wards of the key. In such cases the fermentable substance provokes the formation of the ferment. But, for the most part, in situations where particular ferments are regularly needed, protoplasm has acquired the habit of making such ferments and no others. The cells of salivary glands accumulate ptyalin, the cells of gastric glands accumulate pepsin, during the intervals between meals.