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 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 solution 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 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 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 erowth. 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 molecules are deposited also between those already present.
