258 CLASSICS OF MODERN SCIENCE 



Each cell is not capable of producing chemical changes in every 

 organic substance contained in solution, but only in particular ones. 

 The fungi of fermentation, for instance, effect no changes in any 

 other solutions than sugar ; and the spores of certain plants do not 

 become developed in all substances. In the same manner it is prob- 

 able that each cell in the animal body converts only particular constitu- 

 ents of the blood. 



The metaboHc power of the cells is arrested not only by powerful 

 chemical actions, such as destroy organic substances in general, but 

 also by matters which chemically are less uncongenial ; for instance, 

 concentrated solutions of neutral salts. Other substances, as arsenic, 

 do so in less quantity. The metabolic phenomena may be altered in 

 quality by other substances, both organic and inorganic, and a change 

 of this kind may result even from mechanical impressions on the cells. 



Such are the most essential characteristics of the fundamental pow- 

 ers of the cell, so far as they can as yet be deduced from the phenom- 

 ena. And now, in order to comprehend distinctly in what the pecu- 

 liarity of the formative process of a cell, and therefore in what the 

 peculiarity of the essential phenomenon in the formation of organized 

 bodies consist, we will compare this process with a phenomenon of in- 

 organic 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, with- 

 out exception, bounded by plane surfaces. But manifold as is the 

 form of cells, they have very little resemblance to crystals ; round sur- 

 faces predominate, and where angles occur, they are never quite sharp, 

 and the polyhedral crystal-like form of nany cells results only from 



