other substances than those essential to the reaction. It is a well-known fact that the slightest contamination may influence the reaction to a very great extent. It may either fail altogether, or be retarded, or may even be diverted into quite a different direction. We meet with great difficulties if we have to follow up several reactions in one and the same medium. Intermediate products mav act, one upon the other, to such an extent that we arrive at a series of final products whose origin it would be extremely difficult to account for. Now, if in an animal organism the separate processes were not regulated in a very strict manner, and if, for instance, the blood did not receive substances which are in harmony with it, that is, always transformed in a definite and regular manner, it would be difficult for us to understand how the separate secretions always attain their aims in a very certain way, and how they are able locally to attack particular metabolisms, and either retard, or hasten, or initiate them.
There is not the slightest doubt that the course of this metabolism, as well as the inter-relations of the cells of a particular organ, is only imaginable under the supposition that the metabolism of the whole organism is regulated in the most precise way, not only quantitatively, but also qualitatively. We are bound to imagine that, in the work of the cells, the same stages of decomposition recur regularly, and
