The best known example of a coenzyme in plant tissues is in connection with the activity of the zymase of yeast cells. If yeast juice be filtered through a gelatin filter, the colloidal enzymes which are left behind are entirely inactive in producing fermentation, but may be restored to activity again by mixing with the filtrate. An examination of this filtrate, which contains the coenzyme for zymase, shows that it contains soluble phosphates and some other substance whose exact nature has not yet been determined, both of which are necessary to the activity of the zymase. The phosphates seem to enter into some definite chemical combination with the substrate sugars, while the other coenzyme seems to be necessary in order to make possible the final breaking down of the sugar-phosphate complex by the zymase. This phenomenon of coenzyme relationship is not very frequently observed in plant enzyme studies, probably because the coenzyme (if there be such, in the case which is under observation) usually accompanies the enzyme itself through the various processes of extraction and purification of the material for study. However, care must be taken in all cases when dialysis is employed, to see that a possible coenzyme is not separated from an otherwise active preparation. An entirely different type of phenomenon is that exhibited by "antienzymes." These are found in the various intestinal worms which live in the digestive tracts of animals; and prevent the digestive action of the enzymes of the stomach and intestines upon these worms. Probably similar "antienzymes" are located in the mucous linings of the intestinal tract itself, and serve to prevent the auto-digestion of these organs by the active enzymes with which they are almost continually in contact. The difference between an antienzyme, which protects material which would otherwise be subject to the attack of an enzyme, and an inhibitor, which renders the enzyme itself inactive, is apparent. So far as is known, however, no such substances as antienzymes are present in plant tissues; although the question as to why the proteoclastic enzymes which are elaborated by a given mass of protoplasm do not attack the protoplasm itself, might well be raised. ZYMOGENS It is apparent that, since enzymes are produced by protoplasm for the special needs of any given moment or stage of development, there must be a preliminary stage, or condition, in which they do not exert their characteristic catalytic effect. When in this stage, the compound is known as "proenzyme," or "zymogen." In this stage, it is inactive, but can be made to exhibit its catalytic effect, usually by bringing it into contact with a suitable activator. When once so activated, however, it cannot be returned again to the inactive state.