When a cell divides, a very complex train of events usually occurs. This process of “Mitosis” exhibits many variations of detail, and without actual demonstration it is rather difficult to explain clearly. But its essential feature is evidently the exact halving of all the structures in the cell which is about to divide. In the ordinary cell which is not going to divide immediately, the chromatin is diffused throughout the nucleus as very numerous fine granules, recognised only by their staining reactions. They may be concentrated at some part of the nucleus, so that a division through a plane of geometrical symmetry of the cell would not, in general, exactly halve the chromatin. Prior to division, therefore, this substance becomes aggregated as granules lying along a convoluted filament of a substance called “linin,” which is characterised principally by the fact that _it does not stain with the dyes that stain the chromatin_. The filament breaks up into short rods, called Chromosomes, and these rods become arranged in the equator of the nucleus. The rods then split longitudinally, and one-half of each moves towards one pole of the nucleus, the other half moving towards the other pole. Various other modifications of the cell and nucleus occur concomitantly with these changes, but the essential thing that happens seems to be the halving of all the structures of the cell, and this is the simplest explanation of the phenomena of mitotic cell division. Two daughter-cells are then formed by the division of the mother-cell, and each of these daughter-cells receives one-half of each of the chromatin granules that were contained in the mother-cell. The chromosomes, or “Idants,” are seen to consist of discrete granules, and these are (generally) the bodies known as the “Ids.” The id cannot be resolved by the microscope into any smaller structures: it lies on the limits of aided vision; but the hypothesis assumes that it is composed of parts called “Determinants,” and the determinants are further supposed to consist of “Biophors.” The biophors are the ultimate organic units or elements, and they are of the same order of magnitude as chemical molecules. We must suppose them to be more complex than a protein molecule, and the latter contains many hundreds (at least) of chemical atoms. Now it is possible to calculate the number of atoms contained in a particle of the same size as the id: such a calculation may be made by different methods, all of them yielding concordant results. This calculated number of atoms may be less than that which we must suppose to be present in the biophors, of which the hypothetical id is composed![28] [28] “But,” says Weismann, referring to an objection of this nature, “it should rather be asked whether the size of the atoms and molecules is a fact, and not rather the very questionable result of an uncertain method of investigation.”