202 THEORY OF THE CELLS. 



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 pre- 

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

 were present in another combination. They therefore require 

 no extraneous 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 circumstances, 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 in- 

 crease. 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 j)henomena 

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

 the points of resemblance between them should not be over- 

 looked. They agree in this important point, that solid bodies 

 of a certain regular shape are formed in obedience to definite 

 laws at the expense of a substance contained in solution in a 

 fluid; and the crystal, like the cell, is so far an active and posi- 

 tive agent as to cause the substances which are precipitated to 

 be deposited on itself, and nowhere else. We must, therefore, 

 attribute to it as well as to the cell a power to attract the sub- 

 stance held in solution in the surrounding fluid. It does not 

 indeed follow that these two attractive powers, the power of 

 crystallization — to give it a brief title — and the plastic power 

 of the cells are essentially the same. This could only be ad- 

 mitted, if it were proved that both powers acted according to 

 the same laws. But this is seen at the first glance to be by 

 no means the case : the phenomena in the formation of cells 



