CELLS. 73 



tendon, bone, &c. It would be wholly toreign to the pur- 

 pose of this work to describe the various tissues which enter 

 into the composition of an animal or plant, and it will be 

 sufficient to describe briefly the structure of a cell. 



The structural elements which compose a typical cell 

 (fig. 20) are the following : — 



I. The Cell-wall. — This is the outer layer or membrane by 

 which the cell is bounded (fig. 20, a). It does not appear 

 to be essential to the existence 

 of a cell, and certainly is not the 

 agent by which cellular activity is 

 manifested. On the contrary, the 

 cell-wall appears to be formed by 

 the transformation, or partial death, 

 so to speak, of the outermost por- 

 tion of the cell-contents. Thus, 

 on the view advocated by that 

 eminent microscopist, Dr Lionel 



Beale, we must regard the cell- Fig. 20. — Four cells from the noto- 



,, 1 /- -1 • 1 chord of the Lamprey, (ireatly 



wall as composed of matter which magnified. (After Todd and 



hj .t 1 11 ^1 •, 1 Bowman.) a Cell-wall; ^ Ccll- 



aS passed through all the vital contents; c Nucleus with nu- 



changes of which it is capable — cieoius. 

 matter which is formed, not formative, or, in other words, 

 matter which is more or less nearly dead. The vital activity 

 of a cell is therefore more or less directly dependent upon 

 the nature of the cell-wall ; and the thicker and more devel- 

 oped the cell-wall becomes, the less efficient is the cell. 

 The actual composition of the cell-wall differs in difi"erent 

 cases. In animal cells it would seem to be of an albumi- 

 nous nature, and it is distinguishable from the cell-contents 

 by being left un tinged by an ammoniacal solution of car- 

 mine. In vegetable cells, the cell-wall is formed of cellulose, 

 and in old cells this is much thickened by the deposition of 

 numerous concentric layers of woody tissue or ligninc on 

 its inner surface. 



2. The Cell-contents. — The materials comprised within the 



