6 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 ves- 

 sels, 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, how- 

 ever, nothing has been inferred from such individual points of 

 resemblance. Every cell is not an analogous structure to a ve- 

 getable cell ; and as to the polyhedral form, seeing that it neces- 

 sarily 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 univer- 

 sally 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- 



