.54 THE CELL. 



nerve-fibres, etc., though fibres are not always cells. A few ex- 

 amples illustrating such a series are shown in Fig. 22. Towards 

 their ends elongated cells may be angular, rounded, attenuated, 

 or branched. If the cells are extended in two directions instead 

 of one, flattened or scale-like forms are produced, which are often 

 joined by their edges into a tissue resembling a pavement (Fig. 

 23). Seen from their fiat surfaces, such cells may have the most 

 diversified outlines angular, wavy, toothed, irregularly branch- 

 ing, etc. as shown in Fig. 24, a. "Whether elongated, flattened, 

 or approximately equilateral, cells may be produced into various 

 projections, which give rise to such a diversity of forms that a 

 volume would be required to describe them. These projections 

 may be few and inconspicuous ; but from this condition there is 

 every possible gradation leading to cells which display innumer- 

 able lobes, branches, or filaments, and these may even be many 

 times longer than the main body of the cell, sometimes forming 

 the most complicated networks, or systems of branches. A vein- 

 few of these different forms are shown in Fig. 24. For other ex- 

 amples the student is referred to the various figures throughout 

 this book. 



Variations in Structure. The variations in structure may af- 

 fect the cell- substance, the membrane, or the nucleus; and in 

 many cases one or more of these parts may be entirely wanting. 

 The membrane is frequently absent during a part or the whole 

 of the life of the cell. On the other hand, the cell-substance and 

 nucleus often disappear from old cells, for example in the case 

 of wood (Fig. 3), leaving a cavity filled with air or water. Such 

 cells scarcely merit the name, being really the dead remains of 

 cells; though it is a curious fact that the word cell was first ap- 

 plied to precisely such structures (p. 48). It is still an open ques- 

 tion, whether the nucleus is ever really wanting in living cells, 

 though it is often very difficult to see, and its existence has been 

 denied in a number of the lower forms of life. 



It is hardly worth while to make at this point more than a 

 brief mention of the manifold special variations in the structure 

 of cells, since these will be better understood in the course of 

 future study. 



Cell-substance. In the last chapter some account lias been 

 given of the deposition of lifeless matter and of other changes 

 taking place in the cell-substance. Most of these modifications 



