THEORY OF THE CELLS. 211 
menon, and one which has already been frequently adduced as 
analogous to assimilation. If a crystal of nitre be 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 Thénard, 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 versd, 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 
