THEORY OF THE CELLS. 199 
contents of the cell are different from those which are produced 
by change in the external cytoblastema. What is the cause 
of this difference, if the metamorphosing power of the cell- 
membrane be limited to its immediate neighbourhood merely ? 
Might we not much rather expect that converted substances 
would be found without distinction on the inner as on the 
outer surface of the cell-membrane? It might be said that the 
cell-membrane converts the substance in contact with it without 
distinction, and that the variety in the products of this con- 
version depends only upon a difference between the convertible 
substance contained in the cell and the external cytoblastema. 
But the question then arises, as to how it happens that the 
contents of the cell differ from the external cytoblastema. If 
it be true that the cell-membrane, which at first closely sur- 
rounds the nucleus, expands in the course of its growth, so as 
to leave an interspace between it and the cell, and that the 
contents of the cell consist of fluid which has entered this 
space merely by imbibition, they cannot differ essentially from 
the external cytoblastema. I think therefore that, in order to 
explain the distinction between the cell-contents and the ex- 
ternal cytoblastema, we must ascribe to the cell-membrane not 
only the power in general of chemically altering the substances 
which it is either in contact with, or has imbibed, but also of 
so separating them that certain substances appear on its inner, 
and others on its outer surface. The secretion of substances 
already present in the blood, as, for instance, of urea, by the 
cells with which the urinary tubes are lined, cannot be ex- 
plained without such a faculty of the cells. There is, however, 
nothing so very hazardous in it, since it is a fact that different 
substances are separated in the decompositions produced by 
the galvanic pile. It might perhaps be conjectured from this 
peculiarity of the metabolic phenomena in the cells, that a 
particular position of the axes of the atoms composing the cell- 
membrane is essential for the production of these appearances. 
Chemical changes occur, however, not only in the cytobla- 
stema and the cell-contents, but also in the solid parts of 
which the cells are composed, particularly the cell-membrane. 
Without wishing to assert that there is any intimate connexion 
between the metabolic power of the cells and galvanism, I may 
yet, for the sake of making the representation of the process 
