194 THEORY OF THE CELLS. 
cells commence their formation so near to one another that the 
boundaries of the layers forming around each of them meet at 
any spot, a common layer may be formed enclosing the two 
incipient cells. So at least the origin of nuclei, with two or 
more nucleoli, seemed explicable, by a coalescence of the first 
layers (corresponding to the nucleus), and the union of many 
primary cells imto one secondary cell by a similar coalescence 
of the second layers (which correspond to the cell). But the 
further development of these common layers proceeds as though 
they were only an ordinary single layer. Lastly, there were 
some varieties in the progressive development of the cells, which 
were referable to an unequal deposition of the new molecules 
between those already present in the separate layers. In this 
way modifications of form and division of the cells were ex- 
plained. And among the number of the plastic phenomena in 
the cells we may mention, lastly, the formation of secondary 
deposits ; for stances occur in which one or more new layers, 
each on the inner surface of the previous one, are deposited on 
the inner surface of a simple or of a secondary cell. 
These are the most important phenomena observed in the 
formation and development of cells. The unknown cause, 
presumed to be capable of explaining these processes in the 
cells, may be called the plastic power of the cells. We will, in 
the next place, proceed to determine how far a more accurate 
definition of this power may be deduced from these phenomena. 
In the first place, there is a power of attraction exerted in 
the very commencement of the cell, in the nucleolus, which 
occasions the addition of new molecules to those already pre- 
sent. We may imagine the nucleolus itself to be first formed 
by a sort of crystallization from out of a concentrated fluid. 
For if a fluid be so concentrated that the molecules of the 
substance in solution exert a more powerful mutual attraction 
than is exerted between them and the molecules of the fluid 
in which they are dissolved, a part of the solid substance must 
be precipitated. One can readily understand that the fluid 
must be more concentrated when new cells are being formed in 
it than when those already present have merely to grow. For 
if the cell is already partly formed, it exerts an attractive force 
upon the substance still in solution. There is then a cause 
for the deposition of this substance, which does not co-operate 
. 
; 
: 
