INTRODUCTION.
tains an independent existence. Examples, however, also occur in plants, where the cells coalesce, and this not merely with regard to their walls, but the cavities also. Schleiden has found that in the Cacti, the thickened walls of several cells unite to form a homogeneous substance, in which only the remains of the cell-cavities can be distinguished. PI. I, fig. 3, represents such a blending of the cell-walls observed by Schleiden. The entire figure is a parent cell, with thickened walls, in which four young cells have formed, the walls of which are likewise thickened and have coalesced with each other, as well as with those of the parent cell; so that only the four cavities remain with their nuclei in a homogeneous substance. The spiral vessels, and, according to Unger, the lactiferous vessels also, afford examples of the union of the cavities of several cells by the absorption of the partition walls.
After these preliminary remarks we pass on to animals. The similarity between some individual animal and vegetable tissues has already been frequently pointed out. Justly enough, however, nothing has been inferred from such individual points of resemblance. Every cell is not an analogous structure to a vegetable cell; and as to the polyhedral form, seeing that it necessarily belongs to all cells when closely compacted, it obviously is no mark of similarity further than in the circumstance of densely crowded arrangement. An analogy between the cells of animal tissues and the same elementary structure in vegetables can only be drawn with certainty in one of the following ways: either, 1st, by showing that a great portion of the animal tissues originates from, or consists of cells, each of which must have its particular wall, in which case it becomes probable that these cells correspond to the cellular elementary structure universally present in plants; or, 2dly, by proving, with regard to any one animal tissue consisting of cells, that, in addition to its cellular structure, similar forces to those of vegetable cells are in operation in its component cells; or, since this is im- possible directly, that the phenomena by which the activity of these powers or forces manifests itself, namely, nutrition and growth, proceed in the same or a similar manner in them as in the cells of plants. I reflected upon the matter in this point of view in the previous summer, when, in the course of my re-
