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 cousists, 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 may be 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 is 
formed in a fluid at the expense of a substance held in solu- 
tion by that fluid. The process of crystallization 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 sumple, 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 are deposited also between those already present. 
