THEODOR SCHWANN 259 



mechanical causes. The structure too of cells and of crystals is dif- 

 ferent. Crystals are solid bodies, composed merely of layers placed 

 one upon another ; cells are hollow vesicles, either single, or several 

 inclosed one within another. And if we regard the membranes of 

 these vesicles as layers, there will still remain marks of difference be- 

 tween them and crystals ; these layers are not in contact, but contain 

 fluid between them, which is not the case with crystals ; the layers in 

 the cells are few, from one to three only ; and they differ from each 

 other in chemical properties, while those of crystals consist of the same 

 chemical substance. Lastly, there is also a great difference between 

 crystals and cells in their mode of growth. Crystals grow by appo- 

 sition, the new molecules are set only upon the surface of those al- 

 ready deposited, but cells increase also by intussusception, that is to 

 say, the new molecules are deposited also between those already 

 present. 



But greatly as these plastic phenomena differ in cells and in crystals, 

 the metabolic are yet more different, or rather they are quite peculiar to 

 cells. For a crystal to grow, it must be already present as such in the 

 solution, and some extraneous cause must interpose to diminish its 

 solubility. Cells, on the contrary, are capable of producing a chemical 

 change in the surrounding fluid, of generating matters which had not 

 previously existed in it as such, but of which only the elements were 

 present in another combination. They therefore require no extrane- 

 ous influence to effect a change of solubility ; for if they can produce 

 chemical changes in the surrounding fluid, they may also produce such 

 substances as could not be held in solution under the existing circum- 

 stances, and therefore need no external cause of growth. If a crystal 

 be laid in a pretty strong solution, of a substance similar even to itself, 

 nothing ensues without our interference, or the crystal dissolves com- 

 pletely : the fluid must be evaporated for the crystal to increase. If a 

 cell be laid in a solution of a substance, even different from itself, it 

 grows and converts this substance without our aid. And this it is 

 from which the process going on in the cells (so long as we do not 

 separate it into its several acts) obtains that magical character, to 

 which attaches the idea of Life. 



From this we perceive how very different are the phenomena in the 

 formation of cells and of crystals. Meanwhile, however, the points of 

 resemblance between them should not be overlooked. They agree in 



