In analyzing the structure and functions of the individual cell we regard it as an independent elementary organic unit, but this view is solely a matter of convenience, almost a convention. All the billions of cell's in an adult man are inseparable parts of the single living person. No cell exists as an independent organism in multicellular animals, except the germ-cells, and these only after separation from the gland of origin. Indeed, the biological theory of heredity, already mentioned here, wherein the germ-cell is supposed to carry forward the entire heredity, is now changing toward the view which makes all the somatic cells influence the germ-cells; that is, the body-mass of cells sends on heredity through the germ-cell as the instrument. Adult organisms do not make cells _de novo_. New cells are formed by division from preëxisting cells, but some biologists think the body-cells so affect the new germ-cells as to influence heredity. The cells are organs, nodal points, of a single formative power which pervades the mass of cells as a whole. The protoplasm of each cell is not only in direct apposition with its neighbors, but nearly all biologists are now inclining to the opinion, which Heitzmann proposed in 1873, that division of cell from cell is incomplete in nearly all forms of tissue; and that even where cell-walls are present (an exceptional condition in mammals) they are traversed by strands of protoplasm, by means of which the cells are in organic continuity. The whole body, he contended, is thus a syncytium (a mass of continuous protoplasm stippled with nuclei), with the cells as mere nodal points in an almost homogeneous protoplasmic mass. There are cell-bridges between the sieve-tubes of plants. In 1879 Tangl discovered such connection between the endosperm cells of plants, and later Gardiner, Kienitz-Gerloff, A. Meyer, and many others demonstrated that in nearly all plant tissues the cell-walls are connected by intracellular bridges. Ranvier, Bizzozero, Retzius, Fleming, Pfitzner, and many other observers have found these protoplasmic bridges in animal epithelium. In the skin of a larval salamander they are quite conspicuous. They are known to occur also in smooth muscle-fibre, in cartilage cells, in connective-tissue cells, and in some nerve-cells. Harrison found, in 1908, that in frogs the nerve-fibres develop out of these intracellular bridges. Dendy in 1888, Retzius in 1889, and Palladino in 1890 have shown that the follicle cells of the ovary are connected by protoplasmic bridges, not only with one another, but also with the ovum; and similar connection between somatic cells and germ-cells has been found in a number of plants. Thus even the germ-cell is not independent until it has actually broken away from the gland. A. Meyer holds that both the plant and animal individual are continuous masses of protoplasm, in which the cytoplasmic substance forms a morphological unit, no matter what the cell is. That opinion is