THEORY OF THE CELLS. 211 



menon ; and one which has already been frequently adduced as 

 analogous to assimilation. If a crystal of nitre he placed in 

 a solution of nitre and sulphate of soda, only the nitre crystal- 

 lizes; when a crystal of sulphate of soda is put in, only the 

 sulphate of soda crystallizes. Here, therefore, there occurs just 

 the same selection of the substance to be attracted. 



We observed another law attending the development of the 

 plastic phenomena in the cells, viz. that a more concentrated 

 solution is requisite for the first formation of a cell than for 

 its growth when already formed, a law upon which the differ- 

 ence between organized and unorganized tissues is based. In 

 ordinary crystallization the solution must be more than satu- 

 rated for the process to begin. But when it is over, there 

 remains a mother lye, according to Thenard, which is no 

 longer saturated at the same temperature. This phenomenon 

 accords precisely with the cells ; it shows that a more con- 

 centrated solution is requisite for the commencement of 

 crystallization than for the increase of a crystal already 

 formed. The fact has indeed been disputed by Thomson; 

 but if, in the undisputed experiment quoted above, the crystal 

 of sulphate of soda attracts the dissolved sulphate of soda 

 rather than the dissolved nitre, and vice versa, the crystal of 

 nitre attracts the dissolved nitre more than the dissolved sul- 

 phate of soda, it follows that a crystal does attract a salt held 

 in solution, because the experiment proves that there are de- 

 grees of this attraction. But if there be such an attraction 

 exerted by a crystal, then the introduction of a crystal into a 

 solution of a salt, affords an efficient cause for the deposition 

 of this salt, which does not exist when no crystal is introduced. 

 The solution must therefore be more concentrated in the latter 

 case than in the former, though the difference be so slight as 

 not to be demonstrable by experiment. It would not, how- 

 ever, be superfluous to repeat the experiments. In the in- 

 stance of crystals capable of imbibition, this difference may be 

 considerably augmented, since the attraction of molecules may 

 increase perhaps considerably by the penetrating of the solution 

 between those already deposited. 



We see then how all the plastic phenomena in the cells 

 may be compared with phenomena which, in accordance with 

 the ordinary laws of crystallization, would probably appear if 



