THEORY OF THE CELLS. 197 



already present in the ecll (assimilation), or such as differ from 

 it in chemical properties. The several Layers grow by assimi- 

 lation, but when a new layer is being formed, different material 

 from that of the previously-formed layer is attracted : for the 

 nucleolus, the nucleus and cell-membrane are composed of 

 materials which differ in their chemical properties. 



Such are the peculiarities of the plastic power of the cells, 

 so far as they can as yet be drawn from observation. lint 

 the manifestations of this power presuppose another faculty of 

 the cells. The cytoblastema, in which the cells are formed, 

 contains the elements of the materials of which the cell is 

 composed, but in other combinations : it is not a mere solu- 

 tion of cell- material, but it contains only certain organic 

 substances in solution. The cells, therefore, not only attract 

 materials from out of the cytoblastema, but they must have 

 the faculty of producing chemical changes in its constituent 

 particles. Besides which, all the parts of the cell itself may be 

 chemically altered during the process of its vegetation. The 

 unknown cause of all these phenomena, which we comprise 

 under the term metabolic phenomena of the cells, we will 

 denominate the metabolic potver. 



The next point which can be proved is, that this power is 

 an attribute of the cells themselves, and that the cytoblastema 

 is passive under it. Vie may mention vinous fermentation 1 



1 I could not avoid bringing forward fermentation as an example, because it is 

 the best known illustration of the operation of the cells, and the simplest represen- 

 tation of the process which is repeated in each cell of the living body. Th< 

 who do not as yet admit the theory of fermentation set forth by Cagniard-Latonr, 

 and myself, may take the development of any simple cells, especially of the Bpores, 

 as an example; and we will in the text draw no conclusion from fermentation 

 which cannot be proved from the development of other simple cells which grow 

 independently, particularly the spores of the inferior plants. We have every con- 

 ceivable proof that the fermentation-granules are fungi. Their form is that of fungi; 

 in structure they, like them, consist of cells, many of which enclose other young cells. 

 They grow, like fungi, by the shooting forth of new cells at their extremities; they 

 propagate like them, partly by the separation of distinct cells, and partly by the gene- 

 ration of new cells within those already present, and the bursting of the parent-cells. 

 Now, that these fungi are the cause of fermentation, follows, first, from the constancy 

 of their occurrence during the process; secondly, from the cessation of fermentation 

 under any influences by which they are known to be destroyed, especially boiling heat, 

 arseniatc of pota8S, &C. ; and, thirdly, because the principle which excites the pro, 

 of fermentation must be a substance which is again generated and in< by the 



