THE CELL AND ITS STRUCTURE. • 19 



the cells become crossed by bars in which the cell- 

 substance assumes different optical properties, the 

 nuclei disappear, and no one would dream the fibre 

 to be composed of cells, had not its early stages 

 been observed. Only a fibre crossed by bars re- 

 mains; these bars have something to do with 

 rendering the muscle contractile, bat as yet it is not 

 known exactly what they are. Besides these cases 

 of old cells in which the nuclei have disappeared, 

 there are rare cases in which no nuclei have ever 

 been found : ^ it remains a moot point whether we 

 are to suppose that a " cell " can exist without having 

 a nucleus at any period of existence, or whether it 

 may be expected that the existence of the nucleus in 

 these forms will be demonstrated by some new method 



' of treatment, or discovered at some stage of growth 

 which has not yet been observed. In some cases the 

 material of the nucleus is perhaps distributed through- 

 out the cell, instead of being concentrated into one 

 mass. 



Be this as it may, it is certain that the nucleus is 



■ only a part of the jelly of the cell, of very much the 

 same kind of stuff, only more active in its nature. 



This stuff, the bit of living jelly which is called a 

 cell, is what is termed protoplasm. Dead proto- 

 plasm (proteid) is found to be a compound of the 

 elements carbon, hydrogen, oxygen, and nitrogen, 



' See p. 140. Non-nucleated unicellular organistns are 

 spoken of as cytods, i.e. cell-like bodies. 



