THEORY OF THE CELLS. 197 
already present in the cell (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. But 
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 power. 
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. We may mention vinous fermentation’ 
' 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. Those 
who do not as yet admit the theory of fermentation set forth by Cagniard-Latour, 
and myself, may take the development of any simple cells, especially of the spores, 
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, 
arseniate of potass, &c. ; and, thirdly, because the principle which excites the process 
of fermentation must be a substance which is again generated and increased by the 
