In these works and in the foregoing pages I have tried to show that the multiplication of the egg-cell by division is itself a source of increasing complexity and an active principle in the determination of form, since the products of the division unite to form a higher unity. But in another way the multiplication of cells leads to differentiation among the cells arising from the egg. Although each of these resembles the parent egg, from which they arose by doubling division, yet they differ from it in one point: they are no longer a whole, but have become the subordinate parts of a higher unity, that is, of a higher organism. A cell that is no longer a whole, but the part of a whole, has entered upon reciprocal relations with other cells, and in the functions of its life is limited by these others and by the whole. The further this is carried the more the cell falls short of its independence as an elementary organism, and appears only as a part with its functions subordinate and in dependence upon the whole.[18] Although from the point of view of morphology it has become more and more imperative to regard the cell as the unit of the higher organism, still, from the physiological point of view the higher organisms must be regarded as masses of material acting as wholes, and composed of several grades of structural parts, subordinate in function to the whole, and displaying only a limited division of capacities. And so the cell theory, according to which the cell was exalted unduly as the unit of life, the centre of life, the elementary organism, must take limitation and correction from these wider views. This has already been insisted upon by many physiologists of insight--for instance, by Naegeli (see p. 30), by Sachs, and by Vöchting. 'Cell formation,' declares Sachs (_Physiology of Plants_, p. 73), 'is a phenomenon very general, it is true, in organic life, but still only of secondary significance; at all events, it is merely one of the numerous expressions of the formative forces which reside in all matter, in the highest degree, however, in organic substance.' 'Essentially, every plant, however highly organized, is a continuous mass of protoplasm, surrounded externally by a cell wall and penetrated internally by numerous transverse and longitudinal partitions.' My conception receives strong support from the way in which Vöchting set forth the relations of the cell to the whole: