THEORY OF THE CELLS. 201 



tinctly in what the peculiarity of the formative process of a 

 cell, and therefore in what the peculiarity of the essential 

 phenomenon in the formation of organized bodies consists, we 

 will compare this process with a phenomenon of inorganic 

 nature as nearly as possible similar to it. Disregarding all 

 that is specially peculiar to the formation of cells, in order to 

 find a more general definition in which it maybe included with 

 a process occurring in inorganic nature, we may view it as a 

 process in which a solid body of definite and regular shape i - 

 formed in a fluid at the expense of a substance held in solu- 

 tion by that fluid. The process of ciystallization in inorganic 

 nature comes also within this definition, and is, therefore, the 

 nearest analogue to the formation of cells. 



Let us now compare the two processes, that the difference 

 of the organic process may be clearly manifest. First, with 

 reference to the plastic phenomena, the forms of cells and 

 crystals are very different. The primary forms of crystals 

 are simple, always angular, and bounded by plane surfaces ; 

 they are regular, or at least symmetrical, and even the very 

 varied secondary forms of crystals are almost, without exception, 

 bounded by plane surfaces. But manifold as is the form of 

 cells, they have very little resemblance to crystals ; round 

 surfaces predominate, and where angles occur, they are never 

 quite sharp, and the polyhedral crystal-like form of many cells 

 results only from mechanical causes. The structure too of cells 

 and of crystals is different. 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 between them and 

 crystals ; these layers are not in contact, but contain fluid be- 

 tween 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 con- 

 sist of the same chemical substance. Lastly, there is also a 

 great difference between crystals and cells in their mode of 

 growth. Crystals grow by apposition, the new molecules are 

 set only upon the surface of those already deposited, but cells 

 increase also by intussusception, that is to say, the new mole- 

 cules arc deposited also between those already present. 



