Such an harmonious equipotential system exists only at the beginning of the development of the egg. It is represented by the 8-cell stage of Echinus but not by the 16-cell stage, since, though the 1/16-blastomeres produce gastrulæ (the first larval stage), they do not produce plutei (the second stage). It is represented by the 4-cell stage of Amphioxus but not by the 8-cell stage. It is not exhibited even by the 2-cell stage of the Ctenophore egg. What does this mean? It means that the further development proceeds, the less complete does the “organisation” inherent in any one part of the system become. “The ontogeny assumes more and more the character of a mosaic work as it proceeds” (Wilson). Or perhaps it means, and this is the better way of putting it, that the “organisation,” whatever it may be, depends on size. We see this very clearly in the experiment of cutting in two the blastula of the sea-urchin. If the pieces are of approximately equal size each will form an entire Pluteus larva, but if one of them is below a certain limit of size it will not continue to develop. The “organisation,” therefore, has a certain volume, and this volume is much greater than that of any one of the cells of which the fragment exhibiting it is composed. It is enormously greater than the volume of any group of determinants which we can imagine to represent the different kinds of cells composing the body of the Pluteus larva, and still more enormously greater than the volume of a “molecule” of protoplasm. Now this association of “organisation” and size is of immense philosophical importance, for it does away, once and for all, with the idea that the “organisation” is solely a series of chemical reactions. If it were, one cell of the blastula would contain it, for on the mechanistic hypothesis one cell, the egg-cell, contains it, and this cell can be divided innumerable times and still contain it. The egg is a _complex equipotential system_ (Driesch), which divides again and again throughout innumerable generations, and still contains the “organisation.” It is in vain that we attempt the misleading analogy of the “mass action” of physical chemistry, to show that volume may influence chemical action. In such a mass action what we have is this:-- _A_↓{a} + _B_↓{b} ⇆ _C_↓{c} + _D_↓{d} the letters _A_, _B_ and _C_ standing for chemical substances present, and the letters _a_ and _b_, etc., representing the active masses of these substances. But variations in this active mass affect only the _velocity_ of the reaction. What we have to account for in our blastula experiments is the _nature_ of the reaction, and how can velocity or even nature of reaction affect _form_? If we could show that the form of the crystals deposited from a solution in some reaction depended on the volume of the solution, the analogy would be closer, though even then the difficulties in pressing it would be so enormous as to render it futile to attempt to entertain it.