What did retard physiological discovery was the lack of progress made by chemistry and physics, and this may have been the result of the Stahlian phlogistic hypothesis. However this may be, it seems clear that it was the discoveries of the great chemists of the close of the eighteenth century that again introduced mechanistic views into physiology. With the discoveries of Lavoisier and his successors the latter science acquired new methods of research and the older working hypotheses were re-introduced. There has been no recession from this position during the nineteenth century. Mechanistic biology culminated in the writings of Huxley and Max Verworn and received a new accession of strength almost in our own day in the modern discoveries of physical chemistry; and when physiology became truly a comparative science, and embraced the lower invertebrates, it became perhaps most mechanistic--witness the writings of Jacques Loeb. Of far greater philosophical importance than the physico-chemical investigation of the functioning of individual organisms has been the essentially modern experimental study of embryological processes. The former deals essentially with the _means_ of growth, reproduction, and so on. We can no longer doubt that the changes which we can observe taking place in the organism, either the developing embryo or the fully formed animal, are, in the long run, physico-chemical changes; and in ultimate analysis we cannot expect to find anything else than processes of this nature. But physiological investigation has failed to provide anything more than this analysis. If we watch the development of the egg of an animal into a larval form, and continue to trace the metamorphosis of the larva into the perfect animal, we cannot fail to conclude that, beside the individual physico-chemical reactions which proceed, there is also _organisation_. The elementary processes must be _integrated_. There must be a due order and succession in them. In studying developmental processes, in considering the developing organism _as a whole_, we are impressed above all else with the notion that not only do physico-chemical reactions occur, but that these are _marshalled_ into place, so to speak. When we attempt to make a description of this integration of those ultimate processes which we can describe in terms of physical chemistry, physiology fails us. “At present,” says Morgan, “we cannot see how any known principles of chemistry or of physics can explain the development of a definite _form_ by the organism or by a piece of the organism.” It is true that we can attempt to imagine a physico-chemical mechanism which is the organisation of the developing embryo; but this must be a logically constructed mechanism, not only incapable of experimental verification, but which can also be demonstrated, purely by physical arguments, to be false. This conclusion may, without exaggeration, be said to be that of modern experimental embryology.