[6] This difficulty is also felt by mechanistic writers like Child, who on page 12 of his recent book on _Senescence and Rejuvenescence_ (Chicago, 1915) makes the following remarks: “These theories of Weismann do not account satisfactorily for the peculiarly constant course and character of development and morphogenesis. If we follow them to their logical conclusion, which their authors have not done, we find ourselves forced to assume the existence of some sort of controlling and co-ordinating principle outside the units themselves and superior to them. If the units constitute the physicochemical basis of life, as their authors maintain, then this controlling principle, since it is an essential feature of life, must of necessity be something which is not physicochemical in nature. In short these theories lead us in the final analysis to the same conclusion as that reached by the neovitalists. If we are not content to accept this conclusion we must reject the theories.” These last sentences do not exhaust all the possibilities, since the writer is trying to show in this book that the widest acceptance of the chromosome theory of heredity is compatible with a consistent physicochemical conception of the organism as a whole. In any case, we can state today that the cytoplasm contains the rough preformation of the future embryo. This would show then that the idea of the organism being a mosaic of Mendelian characters which have to be put into place by “supergenes” is unnecessary. If the egg is already the embryo in the rough we can imagine the Mendelian factors as giving rise to specific substances which go into the circulation and start or accelerate different chemical reactions in different parts of the embryo, and thereby call forth the finer details characteristic of the variety and the individual. The idea that the egg is the future embryo is supported by the fact that we can call forth a normal organism from an unfertilized egg by artificial means; while it is apparently impossible to cause the spermatozoön to develop into an organism outside the egg.