Here we have a most elaborate apparatus, the delicate structure of which we are just beginning to appreciate through our most powerful microscopes, but whose significance is rather a matter of conjecture than knowledge. Its intricate structure corresponds to the very complicated functions of the mind. Nevertheless, this elementary organ of psychic activity—of which there are thousands in our brain—is nothing but a single cell. Our whole mental life is only the joint result of the combined activity of all these nerve-cells, or soul-cells. In the centre of each cell there is a large transparent nucleus, containing a small and dark nuclear body. Here, as elsewhere, it is the nucleus that determines the individuality of the cell; it proves that the whole structure, in spite of its intricate composition, amounts to only a single cell. Fig.8 Unfertilised ovum of an echinoderm. Fig. 8—Unfertilised ovum of an echinoderm (from _Hertwig_). The vesicular nucleus (or “germinal vesicle”) is globular, half the size of the round ovum, and encloses a nuclear framework, in the central knot of which there is a dark nucleolus (the “germinal spot”). In contrast with this very elaborate and very strictly differentiated psychic cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has hardly any structure at all. But even in the case of the ovum we must infer from its properties that its protoplasmic body has a very complicated chemical composition and a fine molecular structure which escapes our observation. This presumed molecular structure of the plasm is now generally admitted; but it has never been seen, and, indeed, lies far beyond the range of microscopic vision. It must not be confused—as is often done—with the structure of the plasm (the fibrous network, groups of granules, honey-comb, etc.) which does come within the range of the microscope.