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 into 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 

 wav 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 instances 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 



