THEODOR SCHWANN 253 



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 addi- 

 tion of new molecules to those already present. 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 al- 

 ready 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 when no part of the cell is yet 

 formed. Therefore, the greater the attractive force of the cell is, the 

 less concentration of the fluid is required ; while, at the commence- 

 ment of the formation of a cell, the fluid must be more than concen- 

 trated. But the conclusion which may be thus directly drawn, as to 

 the attractive power of the cell, may also be verified by observation. 

 Wherever the nutrient fluid is not equally distributed in a tissue, the 

 new cells are formed in that part into which the fluid penetrates first, 

 and where, consequently, it is most concentrated. Upon this fact, 

 as we have seen, depended the difference between the growth of or- 

 ganized and unorganized tissues. And this confirmation of the fore- 

 going conclusion by experience speaks also for the correctness of the 

 reasoning itself. 



The attractive power of the cells operates so as to effect the addition 

 of new molecules in two ways, — first, in layers, and secondly, in such 

 a manner in each layer that the new molecules are deposited between 

 those already present. This is only an expression of the fact; the 



