the molecule into larger fragments, while in the second phase these fragments are split up through oxydation into the formation of carbon dioxide and water. We can then say with certainty that in the first stage only a comparatively _small_ amount of energy production occurs, for energy production by enzymic processes of this kind is never great; the second phase, on the other hand, must be associated with a very considerable energy production, for by the addition of oxygen and the formation of carbon dioxide and water the strongest affinities possible are combined. With this assumption in certain cases, as, for instance, in the sudden production of energy in muscle contraction, which necessarily occurs in the purely oxydative phase of the whole process, the view is forced upon us, that, in these cases, the entrance of oxygen into the molecule from the very beginning, even the first impact, produces oxydative decomposition of the whole molecule. The view that, in the reactions of warm-blooded animals, which occur with great rapidity and considerable energy production, the oxygen primarily explosively breaks up the whole carbon chain, certainly presents no more difficulties than the supposition that the simpler substances are attacked secondarily, provided sufficient oxygen be present. This method would be obviously the simplest. This is, however, mere speculation and a definite decision between the two possibilities cannot be made at present. However, whether the process takes place in two phases, an anoxydative and an oxydative, or simply in an oxydative phase, in _any case, the sudden discharge of energy in the aërobic organism set free by the stimulus, is brought about by the addition of oxygen_. This is a highly important fact and as such requires the most thorough confirmation, and is best accomplished by the investigation of the state of excitation of aërobic substances on the withdrawal of oxygen. Experience gained by observation in this respect on a great number of living substances shows that excitability decreases upon the withdrawal of oxygen. In this connection I should like to cite some particularly significant instances. [Illustration: A B Fig. 10. _Rhizoplasma kaiseri._ A--Under normal conditions. B--In an atmosphere of pure hydrogen.] During a sojourn at the Red Sea in 1894–95 I was able to establish this dependence in the single-celled organism, the _Rhizoplasma Kaiseri_, a large naked orange-colored rhizopod. (Figure 10, A.) Mechanical stimulation, which under normal vital conditions of these organisms brings about contraction in the long-branched pseudopods, becomes ineffective with a cessation of the movement of protoplasm, when oxygen is removed and is replaced by a stream of hydrogen. (Figure 10, B.) With renewed introduction of oxygen there is a return of the protoplasmic movement and entire recovery takes place.