162 VIGNETTES FROM INVISIBLE LIFE. 



being that in the Diatoms and such like forms this 

 process affects or rather, involves their whole 

 being or individuality, whereas in complex organisms 

 it is only a process of growth, a multiplication of 

 cells from which the whole compound structure is 

 built up. 



Self -division in the Diatom, however, is not the 

 simple and easy process which it is generally seen 

 to be in most other minute existences ; and for this 

 reason, that it is invested with a material that does 

 not grow pari passu with it. In other words, 

 the valves are composed of pure silex, and inca- 

 pable of participating in the process. 



These two silicious valves bound the inner life 

 substance, which latter grows and expands, and 

 would undoubtedly burst its shelly investment if 

 there were no other means of overcoming its resist- 

 ance. This is furnished by the zones, which, as 

 before observed, overlap each other, apparently for 

 this very purpose ; for when the pressure of growth 

 from beneath is brought to bear against them, they 

 are able to yield to at least the extent of the overlap 

 without exposing the soft cell substance beneath. In 

 this way room is given for the living cell to grow, 



