The relation here described, of the catalytic stimuli to the production of the metamorphic processes, leads us to the distinctions between primary and secondary effects of stimulation. Should the general fact be established, which has up to now only been pointed out in individual cases, that all the metamorphic processes are merely secondary results of primary alterations in rapidity of individual metabolic constituent processes, _then the primary reactions of every stimulus would consist purely in the excitation or depression of the directly concerned constituent_. Whether or not, as may be assumed, this primary effect of stimulation applies to _all_ stimuli, is a question which only the future can answer. The metamorphic processes are not, however, the only secondary effects of stimulation. The influence of long-continued excitation of the functional constituent processes upon the entire cytoplastic metabolism can be looked upon as a secondary response. Therefore, they may be considered as a _secondary_ effect of stimulation which, in contrast to this _primary excitation_, may be called the _secondary excitation_. Further: While the secondary excitation and metamorphic processes are generally produced by the continued existing effects of weak stimulation, we also observe as the result of a stimulus of short duration or frequently repeated at brief intervals, but otherwise not exceeding the physiological limits of intensity, a secondary effect, which plays a very important part in the activity of the organism. I refer to fatigue. Here a secondary depression is developed in connection with the primary excitation, for fatigue of a living organism must be characterized as a depression of activity. This case shows that we have to distinguish between a _primary depression_, as for example, produced by temperature reduction, withdrawal of food, deficiency of oxygen, etc., which occurs as a direct effect of stimulation, and _secondary depression_, which as in fatigue is an _indirect_ result of primary excitation.