_“Coagulases” or coagulating enzymes_ as the rennet (lab, chymosin) which curdles milk; fibrin ferment (thrombin, thrombase) which causes the coagulation of blood. These apparently act by splitting up a substance in the fluids mentioned, after which splitting one of the new products formed combines with other compounds present (usually a mineral salt, and in the cases mentioned a calcium salt) to form an insoluble compound, the curd or coagulum. _Another variety is the “activating” enzymes or “kinases”_ such as the enterokinase of the intestine. The action here is a splitting of the _zymogen_ or mother substance or form in which the enzyme is built up by the cell so as to liberate the active enzyme. Of a character quite distinct, from the splitting enzymes are II. The _zymases_. Their action seems to be to cause a “shifting on rearrangement of the carbon atoms” so that new compounds are formed which are not assumed to have been constituents of the original molecule. Most commonly there is a closer combination of the carbon and oxygen atoms, frequently even the formation of CO₂ so that considerable energy is thus liberated. Examples are the _zymase_ or _alcoholase_ of yeast which converts sugar into alcohol and carbon dioxide; C₆H₁₂O₆ = 2C₂H₆O + 2CO₂: also _urease_, which causes the change of urea into ammonia and carbon dioxide. Another common zymase is the _lactacidase_ in lactic acid fermentation. III. _Oxidizing enzymes_ also play an important part in many of the activities of higher plants and animals. Among the bacteria this action is illustrated by the formation of nitrites, nitrates and sulphates and the oxidation of alcohol to acetic acid as already described. IV. _Reducing enzymes_ occur in many of the dentrifying bacteria and in those which liberate H₂S from sulphates. A very widely distributed reducing enzyme is “catalase” which decomposes hydrogen peroxide. As previously stated, most of the physiological activities of bacteria are due to the enzymes that they produce. It is evident that for action to occur on substances which do not diffuse into the bacterial cell--starches, cellulose, complex proteins, gelatin--the enzymes must _pass out_ of the bacterium and consequently may be found in the surrounding medium. Substances like sugars, peptones, alcohol, which are readily diffusible, may be acted on by enzymes _retained within_ the cell body. In the former case the enzymes are spoken of as extra-cellular or “_exo-enzymes_,” and in the latter as intra-cellular or “_endo-enzymes_.” The endo-enzymes and doubtless also the exo-enzymes may after the death of the cell digest the contents to a greater or less extent and thus furnish substances that are not otherwise obtainable. This process of “self-digestion” is known technically as “_autolysis_.”